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I . REPORT NO. 2. G O V E R N M N T ACCESSION NO. 3. RECIP IENT 'S CATALOG NO.

NASA CR-2360

Survey on Ef fec t of Surface Winds on A i r c r a f t Design and December 1973 4. TITLE AND SUBTITLE 5. REPORT DATE

Operation and Recommendations f o r Needed Wind Research 6. PERFORMlNG ORGANIZATION CODE

L i

7 . AUTHOR(S) John C. Houbolt

9. PERFORMING ORGANIZATION NAME AND ADDRESS I Aeronaut ical Research Associat ion of Pr ince ton , Inc. 50 Washington Road Pr ince ton , New Je r sey 08540

1 2 . SPONSORING AGENCY NAME AND ADDRESS

National Aeronautics and Space Administration Washington, D. C. 20546

8. PERFORMING ORGANIZATION REPOR r I

ARAP 194 1 0 . WORK UNIT NO.

1 1 . CONTRACT OR GRANT NO.

NAS8-28136 (3. TYPE OF REPORi' & PERIOD COVEREI -

Dee. 1971 - Aug. 1973 Contractor Final

1.a. SPONSORING AGENCY CODE

1 7 . KEY WORDS

A i r c r a f t Response Atmospheric Turbulence Wind Shear A i r c r a f t Design and Operations

I 15 . SUPPLEMENTARY NOTES

This r e p o r t w a s prepared under t h e t echn ica l monitorship of t h e Aerospace Environment Divis ion, Aero-Astrodynamics Laboratory, NASA-Marshall Space F l igh t Center.

Resul ts are presented of a survey of t h e e f f e c t of environmental s u r f a c e winds and gust : on a i r c r a f t des ign and opera t ion . A l i s t i n g of the very l a r g e number of problems t h a t a r e encountered i s given. At ten t ion is ca l l ed t o t h e many s t u d i e s t h a t have been made on s u r f a c e winds and g u s t s , but development i n the engineer ing app l i ca t ion of t h e s e r e s u l t s t o ae ronau t i ca l problems is pointed ou t t o be s t i l l i n t h e embryonic s t age . Control of t h e a i r c r a f t is of paramount concern. t i o n i n s imula t ion s t u d i e s of a i r p l a n e opera t ion and c o n t r o l are d iscussed , and an attempt i s made t o i d e n t i f y t h e i r main gaps o r de f i c i enc ie s . c i t e d . The need f o r b e t t e r exchange between t h e meteorologis t and t h e ae ronau t i ca l engineer i s d iscussed . made. Needed r e sea rch e f f o r t t h a t w a s ind ica ted by t h e survey is recomended. Key d e f i c i e n c i e s a r e : a l a c k of knowledge of s p a t i a l c o r r e l a t i o n func t ions t h a t are c r i t i - c a l t o a i r c r a f t response, l a c k of development o r understanding of unusual and severe wind shea r and c r o s s winds, and inadequate modeling.

16. ABSTRACT

Mathematical models and t h e i r app l i ca

Key r e fe rence material i s

Suggestions f o r improvements i n t h e wind and gus t models are

18. 0 I STR IB U T I ON STATEMENT

20

19 . SECURITY CLASSIF . (d thL rmpd) 20. SECURITY CLASSIF. (ofthi* P e e )

Unclas s i f i ed Unclass i f ied

2 1 . NO. OF PAGES 22. PRICE

Domestic, s3.75 79 Foreign, $6.25

FOREWORD

The research repor t ed he re in w a s supported by NASA Contract NAS8-28136. Dr. George H. F i c h t l of t h e Aerospace Environment Divis ion w a s t h e s c i e n t i f i c monitor , and support w a s provided by Mr. John Enders of t h e Aeronaut ical Operating Systems Divis ion , Of f i ce of Advanced Research and Technology, NASA Headquarters.

The mot iva t ion f o r t h e p repa ra t ion of t h i s r e p o r t w a s t h e need of a d e f i n i t i v e s ta tement of t h e requirements of t h e ae ronau t i ca l and aerospace communities re la t ive t o low level ( a l t i t u d e s %600 m) wind i n p u t s f o r t h e des ign and s a f e opera t ion of ae ronau t i ca l systems. and atmospheric s c i e n c e communities i n the development of programs aimed a t t h e development of low level wind i n p u t s f o r ae ronau t i ca l a p p l i c a t i o n s by i d e n t i f y i n g t h e d e f i c i e n c i e s that exist i n our information. It is a l s o hoped t h a t t h i s r e p o r t w i l l h e l p t o stimu- l a t e t h e needed d ia logue between t h e engineer ing and atmospheric s c i e n c e communities relative t o t h e needs and a p p l i c a t i o n of n a t u r a l environment i n p u t s t o engineer ing problems.

<

It i s hoped t h a t t h i s r e p o r t w i l l a i d t h e ae ronau t i ca l

Although t h i s r e p o r t i s intended t o i d e n t i f y t h e d e f i c i e n c i e s i n low level wind i n p u t s f o r t h e next t e n y e a r s , i t is be l ieved that i t should be updated i n approximately f i v e yea r s from now t o inco rpora t e t h e p o s s i b l e unforeseen needs which w i l l undoubtly arise due t o developments and advances i n t h e s ta te-of- the-ar t i n t h e des ign and ope ra t ion of a e r o n a u t i c a l systems. gu ide the atmospheric s c i e n t i s t i n h i s r e sea rch program planning s o as t o s a t i s f y t h e s e needs.

This hopefu l ly w i l l

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AC KNOWUDGMENT

The a u t h o r wishes to t hank t h e many i n d i v i d u a l s he c o n t a c t e d f o r hav ing he lped i n making t h i s su rvey p o s s i b l e - f o r t he t ime they s p e n t with h i m , f o r the i-iotiol-is i h e y expres sed , and f o r t h e h e l p t h e y gave i n f e r r e t i n g o u t t h e many key r e f e r e n c e s t u d i e s . ( O r g a n i z a t i o n s and i n d i v i d u a l s c o n t a c t e d a r e i n d i c a t e d i n a n Appendix.)

A p p r e c i a t i o n i s expres sed t o D r . George F i c h t l of NASA- Marsha l l Space F l i g h t C e n t e r , Aerospace Environment D i v i s i o n , f o r h i s t hough t s and s u g g e s t i o n s , and g e n e r a l gu idance and encouragement he gave d u r i n g t h e c o u r s e of t h i s p r o j e c t .

A p p r e c i a t i o n i s a l s o expres sed t o Mrs. L o i s Ridgway f o r her c h e e r f u l e f f o r t s i n t y p i n g t h i s r e p o r t .

iv

TABLE OF CONTENTS Page INTRODUCTION ............................................. 1

QUESTIONS FORMING BASE OF SURVEY ......................... 3

CITED PROBUMS ASSOCIATED WITH SURFACE WINDS AND GUSTS ... 5 Ground-type Operations. Inc luding Parking. Taxiing. and Hovering ........................................ 5 Approach and Landing (Cross-winds and wind s h e a r ) ... Carrier Operat ions .................................. 9

7

Sensors. Ins t ruments ................................ 10

Measurements ........................................ 11 Turbulence Models and Simulat ion .................... 11 S p e c i f i c a t i o n s and C r i t e r i a ...,..................... 13

STUDIES OF THE LOW ALTITUDE WINDS AND GUSTS .............. 14

MODELS OF SURFACE WINDS AND GUSTS ........................ 15

APPLICATION OF MODELS I N AIRCRAFT STUDIES ................ 20 Simula tor o r f l i g h t s t u d i e s ......................... 20 Carrier ope ra t ion ................................... 21

Wind t u n n e l p r o j e c t s ................................ A n a l y t i c a l s t u d i e s .................................. 21

22

SELECTED REFERENCES REPRESENTING OVERVIEW OF GROUND WINDS PROB.MS ........................................... 22

THE METEOROLOGIST AND THE AERONAUTICAL ENGINEER .......... 23

SUGGESTED FORM OF MODELS ................................. 27

PROBABILITY CONSIDERATIONS IN MODELS ..................... 31

CONCLUSIONS AND RECOMMENDATIONS .......................... 33

APPE.IX ................................................. 37

REFERENCES Grouping of References .............................. 41 Reference L i s t ...................................... 42

F I G U ~ S .................................................. 65

V

LIST OF FIGURES

FIGURE 1.

FIGURE 2.

FIGURE 4.

FIGURE 5.

FIGURE 6 .

FIGURF: 7.

FIGURE 8.

FIGURE 9.

FIGURE 10.

Spectrum of Wind Speed a t Brookhaven, New York, U . S . A .

Typica l Mean P r o f i l e s over T e r r a i n s of D i f f e r i n g Roughness

Average Values of Cedar E i l l Tower Wind P r o f j l e s f o r Combined Unstable-Neutral Lapse Rate

Examples of "Nonclassical" Wind P r o f i l e s

Gust Spectrum Funct ion f o r w

Nondimensional w Spec t r a - Neutra l Case

Nondimensional w Spec t r a - Two Unstable Cases

Two-Argument C o r r e l a t i o n Funct ion of u i n Time and Space

Predominant Force and Moments tha t Act on an Airplane i n Turbulence

Geometry Involved i n Consider ing the C o r r e l a t i o n of Turbulence V e l o c i t i e s between Two P o i n t s f o r A i r c r a f t F l i g h t Pa th no t Aligned with Wind D i r e c t i o n

Page

65

66

67

68

69

70

71

72

73

74

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SURVEY ON EFFECT OF SURFACE WINDS ON AIRCRAFT DESIGN AND OPERATION

AND RECOMMENDATIONS FOR NEEDED WIND RESEARCH

John C . Houbo l t

INTRODUCTION

Much work has been done on the problem of t h e f l i g h t of a i r c r a f t i n atmospheric tu rbulence f o r f l i g h t l e v e l s above the p l a n e t a r y boundary l a y e r , tha t i s , f o r a l t i t u d e s above 300 t o 600 meters (1000 - 2000 f t ) . t h r e e main l i n e s : c o l l e c t i o n and a n a l y s i s of tu rbulence data, mathematical modeling of t h e turbulence environment, and on t h e development of methods f o r e s t a b l i s h i n g t h e s t r u c t u r a l response and design of a i r c r a f t due t o th is turbulence . References 1-25 r e p r e s e n t a few of the many r e p o r t s that may be found r e l a t i v e t o t h e s e s t u d i e s .

E f f o r t has been concent ra ted a long

By c o n t r a s t , the e f f o r t that has been spent on winds and g u s t s i n t h e su r face boundary layer ( a l t i t u d e s up t o - 30 t o 90

mendous amount of work has been done on measuring the environmental winds and g u s t s i n these lower a l t i t u d e s , i n a s s o c i a t i n g and c o r r e l a t i n g t h e evidence w i t h meteorological measurements, and i n developing mathematical d e s c r i p t i o n s . t h e eng inee r ing a p p l i c a t i o n of t hese s u r f a c e wind and gus t s t a t i s t i c s t o a i r c r a f t design and opera t ions problems, however, a r e f o r the most p a r t s t i l l i n a formative s t a g e .

loo-300 f t ) and through t h e p l a n e t a r y l a y e r (-- 300 t o meters 1.000 m 1000-3000 f ) ) i s markedly -d i f f e ren t i n na tu re . A t r e -

Developments i n

Environmental su r f ace winds and gusts a r e known t o have a s t r o n g i n f l u e n c e on t h e design and opera t ion of aerospace ve- h i c l e s . A t c r u i s i n g a l t i t u d e s gus t s a r e of main concern; winds e n t e r p r i m a r i l y only i n t h e way they a f f e c t nav iga t ion . A t the s u r f a c e , however, winds and gus t s a re both of s p e c i a l concern t o a i r c r a f t o p e r a t i o n s . A few recognized examples may i l l u s t r a t e i n gene ra l some of t h e problems that arise. Cross winds r e p r e s e n t a hazard du r ing approach and l and ing and may even prec lude landing . Gust encounter i s most p reva len t and can be very severe near t h e s u r f a c e . l e a d s t o undershoot o r overshoot of the touchdown a rea , and i n some c a s e s causes t h e l and ing t o be abor ted or even imposs ib le .

Wind shear complicates t h e approach t o l and ing and

The winds and g u s t s no t on ly a f f e c t a i r c r a f t s t r u c t u r a l des ign and passenger comfort , bu t i n p a r t i c u l a r cause s e r i o u s a i r c r a f t c o n t r o l problems ., C a r r i e r ope ra t ions a re in f luenced s t r o n g l y by t h e winds. I n s p e c i a l miss ions , such as high-speed, low-level p e n e t r a t i o n , l i m i t a t i o n s o f t e n a r i s e due t o winds or g u s t s because of excess ive load ing on t h e p i l o t o r d e t e r i o r a t e d a i r c r a f t p o i n t i n g or aiming accuracy.

design problems, even i n qu ie scen t a i r . OperatJon i n the presence of ground winds aggrava te cons iderably these a l r e a d y severe problems. t h e i r ope ra t ion because of excess ive winds. Rocket-propelled veh ic l c s launched from open pads a r e o f t e n designed i n part by quasi-s teady winds encountered p r i o r t o launch, and f i x e s must o f t e n be made t o the c o n f i g u r a t i o n t o prevent o r minimize s e l f - induced type i n s t a b i l i t i e s , such as a s s o c i a t e d w i t h vor tex shedding. The pre launch wind l o a d problem has i n f a c t been s tudied r a t h e r ex tens ive ly , and t h e r e i s now probably a b e t t e r understanding of t h e problem a r e a than any o the r of t he ground winds problems of a i r c r a f t and space v e h i c l e s ; see r e f e r e n c e 26 and the r e fe rences c i t e d i n r e fe rence 27.

For VSTOL a i r c r a f t and h e l i c o p t e r s , s i a b i l i t y a i id coi-iti-211 2i;d k;andling q w d i t i e s r e p r e s e n t s c c u t e

Small s ea rch or survey a i r c r a f t a r e l i m i t e d i n

A s an a s i d e , i t i s mentioned t h a t ground winds and g u s t s a r e of concern i n many a r e a s bes ide aerospace endeavors. Winds o f t e n l i m i t t r a f f i c ope ra t ions on t u r n p i k e s or high-speed road systems. The wind "whi s t l i ng through t h e mountains" i s of concern t o high- speed r a i l t r a n s p o r t a t i o n systems. paramount concern i n t h e des ign of c e r t a i n type b r i d g e s . wise, t h e design and s t r u c t u r a l response of b u i l d i n g s and

Boating ope ra t ions a r e hampered by t h e winds. The d i s p e r s i o n of contaminants and p o l l u t a n t s due t o winds and g u s t s has become of i n c r e a s i n g concern i n r e c e n t yea r s . formation of sand dunes by winds commands i n t e r e s t , r e f e r e n c e 34.

I n s p i t e of t h e f a c t t h a t s u r f a c e winds r e p r e s e n t a r e a l f a c t o r i n t h e design and o p e r a t i o n of aerospace veh:i.cles, t h e r e i s no u n i f i e d documentation on t h e i r t r e a t m e n t . Each problem a r e a i s t r e a t e d somewhat independent ly of t h e o t h e r , and i n p u t winds d e s c r i p t i o n may vary widely, bo th wi th r e s p e c t t o mathe- m a t i c a l modeling and degree of accuracy . veh ic l e , the d e s c r i p t i o n may va ry accord ing t o t h e problem be ing t r e a t e d . For f l i g h t s imula t ion , or t h e des ign of t he t h r u s t vector c o n t r o l system, f o r example, d e t a i l e d t ime h i s t o r i e s of wing i n p u t s may be used. as s t r u c t u r a l e x c i t a t i o n due t o g u s t s , a power s p e c t r a l r e p r e - s e n t a t i o n may be p r e f e r r e d .

understanding of how winds e n t e r i n t o t h e des ign and o p e r a t i o n of aerospace v e h i c l e s and, f o r t h e f u t u r e , t h e p i c t u r e i s even more nebulous. Various q u e s t i o n s e x i s t . What a r e t h e c u r r e n t problems t h a t a r e brought about by s u r f a c e w i n d s and g u s t s , and

For yea r s winds have been of Like-

s t r u c t u r e s due t o winds i s of c e n t r a l concern, r e f e r e n c e s 28-33.

Even t h e s tudy of t h e

Even f o r a given

Yet f o r o t h e r response problems, such

I n gene ra l , t h e r e i s no c l e a r o v e r a l l p i c t u r e o r i n t e g r a t e d

h

I

i

what new problems a r e l i k e l y t o become of importance i n t h e f u t u r e ? What should t h e engineer know about winds and g u s t s t o so lve t h e s e problems? What mathematical models a r e appropr i a t e? These ques t ions appear worthy of eva lua t ion . I n t u r n , f u r t h e r ques t ions a r i s e . A r e the c u r r e n t needs of t h e engineer w i t h r e s p e c t t o su r face wind problems being s a t i s f i e d and, i f n o t , what should be done t o s a t i s f y these needs? I n a d d i t i o n , wind and gus t in format ion i s u s u a l l y c o l l e c t e d by me teo ro log i s t s who may have l i t t l e knowledge or i n t e r e s t of what t h e aerospace engineer needs. Perhaps t h e most appropr i a t e data a r e not be ing c o l l e c t e d ; perhaps the most s u i t a b l e data f o r eng inee r ing use a r e no t be ing presented t o t h e engineer . Improvements i n communication between the aerospace engineer and t h e meteorolo- g i s t may be needed s o t h a t the needs of t h e engineer a r e b e t t e r s a t i s f i e d .

I n an a t tempt t o i d e n t i f y and eva lua te t h e s e var ious problems more c l e a r l y , a survey w a s undertaken; s p e c i f i c a l l y , t h e survey had t h e o b j e c t i v e of making a c r i t i c a l e v a l u a t i o n of our knowledge of s u r f a c e winds and gusts and t h e i r d e s c r i p t i o n , and how they in f luence and a r e taken i n t o account i n t h e design and ope ra t ion of aerospace veh ic l e s .

The survey cons i s t ed of a l i t e r a t u r e sea rch and, i n p a r t i c u - lar , pe r sona l v i s i t s t o members of var ious groups, i nc lud ing indus t ry , u n i v e r s i t i e s , and government agencies , see Appendix. The c r o s s s e c t i o n of aerospace vehic les w a s included as much as p o s s i b l e , i nc lud ing rocket-powered veh ic l e s , convent iona l and s p e c i a l purpose a i r c r a f t , and h e l i c o p t e r s and VSTOL a i r c r a f t . Needs and requirements f o r now and up t o t e n yea r s i n the f u t u r e were sought . Emphasis w a s placed more on a i r c r a f t - t y p e veh ic l e s than on launch veh ic l e s , bu t imp l i ca t ions w i t h r e s p e c t t o space s h u t t l e ope ra t ions were a l s o included.

QUESTIONS FORMING BASE OF SURVEY

I n a n a t tempt t o fe r re t out and promote d i scuss ion on the problems that a r e brought about b y environmental su r f ace winds and g u s t s , ques t ions of t h e fo l lowing type were p re sen ted t o t he va r ious i n d i v i d u a l s and groups that were con tac t ed .

For t h e veh ic l e :

1. What problems a r i s e and m i g h t a r i s e as a r e s u l t of s u r f a c e winds, and i n what ways does t h e engineer need wind and gust informat ion?

2.

3. To w h a t e x t e n t i s des ign inf luenced? Operation?

What response c a l c u l a t i o n s a r e made?

3

For t h e i n p u t :

4. How are the s u r f a c e winds c h a r a c t e r i z e d and modeled?

5. I s informat ion of adequate form?

With r e spec t t o adequacy:

6 . What c r i t e r i a and requirements e x i s t c u r r e n t l y ?

7. Are c u r r e n t needs of t he engineer be ing s a t i s f i e d ?

8. What a r e t h e gaps and d e f i c i e n c t e s ?

9. I f needs a r e no t be ing s a t i s f i e d , what should be ob t a i n e d?

10. What programs should be undertaken t o o b t a i n i n f o r - mation t o s a t i s f y needs?

For t h e me teo ro log i s t :

11. I s t h e meteorology community acquain ted w i t h the needs cr?f t h e aerospace engineer?

1 2 . What informat ion should t h e me teo ro log i s t be o b t a i n i n g t o he lp the engineer , t o d e f i n e t h e e a r t h ' s boundary l a y e r b e t t e r ?

There was an at tempt t o emphasize c e r t a i n a r e a s of t h e a i r - c r a f t opera t ion problems t h a t were envis ioned t o be most s i g n i - f i c a n t ; s p e c i f i c a l l y :

1) On the r o l e of S p e c i f i c a t i o n s and C r i t e r i a

2 ) Handling Q u a l i t i e s and S t a b i l i t y and Cont ro l a s p e c t s

3) The presence of a p i l o t i n t h e loop or t h e use of automatic approach c o n t r o l

4) It i s of i n t e r e s t t o no te t h a t even though t h e q u e s t i o n s

l i s t e d i n t h i s s e c t i o n were p re sen ted t o the va r ious i n d i v i d u a l s contac ted as a means f o r s t a r t i n g a d i s c u s s i o n , t h e q u e s t i o n s were soon bypassed. Very quick ly , t h e i n d i v i d u a l s or group contac ted began to p r e s e n t ground winds and g u s t problems as they s e e them, or as rep resen ted by t h e i r exper ience . It i s mainly on t h i s spontaneous response that t h i s r e p o r t i s based .

The r o l e and use of s i m u l a t o r s

4

I C I T E D PROBIEMS ASSOCIATED WITH SURFACE WINDS AND GUSTS

This s e c t i o n p r e s e n t s a l i s t i n g , almost verbatim, of t he I var ious problems t h a t were c i t e d by i n d i v i d u a l s contac ted i n t h e

survey. It w a s f e l t t h a t such a l i s t i n g i s a d i r e c t , simple way of b r i n g i n g out c l e a r l y t h e va r ious problems t h a t a r e considered s i g n i f i c a n t and what c e r t a i n no t ions t o w a r d s t hese problems a r e . The main screening t h a t has been done is t o sepa ra t e t h e comments i n t o t h e seven b a s i c c a t e g o r i e s t h a t a r e i n d i c a t e d . It w i l l be seen t h a t t h e su r face winds and g u s t s problems encountered a r e d ive r se and ex tens ive , and a r e more i n number than one might have imagined. It w i l l a l s o be seen t h a t many of t he problems a r e not even def ined we l l as y e t , t h a t much more informat ion i s needed about t h e inpu t su r face winds and gusts, and tha t development of d e f i n i t i v e design procedures - on input modeling, on engineer ing-type a p p l i c a t i o n s , on design impl i ca t ions - a r e very much needed. References 35, 36 and 37 a l s o p r e s e n t d i scuss ions of t h e o v e r a l l na tu re of t h e ground winds problem and a r e suggested for reading .

I

A Ground-Type Operat ions, Inc luding Parking, Taxi ing, and Hovering

1.

2 .

3.

4.

5.

6.

7 .

The m i l i t a r y i s concerned w i t h t h e e f f e c t of ground winds on - a l l equipment; s tandards a r e p r e s e n t l y be ing r e v i s e d .

Tornadoes and hu r r i canes p r e s e n t a i r c r a f t f l y - away problems a t a l l a i r p o r t s i n t h e i r p a t h s .

Stops or snubbers must be designed i n t o tabs and e l e v a t o r s t o prevent banging" due t o t a i l winds; improved f a t i g u e l i f e i s a l s o obtained.

TI

Gusts a r e a concern f o r cargo doors; motors a r e designed t o s t a l l f o r winds g r e a t e r t han 33.4 m / s (65 k n t s ) .

Canopy des ign and wing f o l d design i s inf luenced by winds; l ocks must be secured i n many configu- r a t i o n s , f o r example, f o r winds g r e a t e r than 20.6 m / s (40 k n t s ) .

For 18 m / s (35 knts) winds and i n a hovering cond i t ion , c o n t r o l power i s the key problem.

Taxi ing from t h e l e e p r o t e c t i o n of a b u i l d i n g i n t o a s t r o n g wind r e p r e s e n t s a hazard.

8 . Winds and gus t s blowing ac ross the f a c e of l a r g e f a n engines may lead t o l a r g e b l ade s t r e s s e s .

9. The r e c i r c u l a t i o n or r e i n g e s t i o n of ho t engine gases l ead ing t o reduced engine power or compressor s t a l l r ep resen t s a severe problem f o r many VSTOL conf igu ra t ions , r e f e rence 3 8 ; winds a f f e c t t h i s probiem g r e a t l y .

10. The Har r i e r (a f i x e d wing, vec tored- thrus t VSTOL a i r c r a f t ) gene ra l ly t akes of f i n t o the wind, bu t if no wind, take-off i s made w i t h a forward roll of 4.5 i n / s (10 mph) t o avoid i g g e s t i o n problems. Also, t h i s a i r c r a f t has only 8 f o r t a i l c l ea rance , so gus t s can c r e a t e q u i t e a hazard at ope ra t ions near t h e ground.

11. Winds p resen t a problem t o r o t o r b lades i n shut - down cond i t ion ; "gust l o c k s " must be added i n many conf igu ra t ions .

1 2 . Parked r o t o r s must be t i e d down t o prevent auto- r o t a t i o n due t o winds; f l a p p i n g may be severe i f c o n t r o l s a r e not l e f t i n a c o r r e c t p o s i t i o n .

1 3 . Blade f l a p p i n g may be excess ive dur ing h e l i c o p t e r runup i n presence of winds; s t o p s or fuse l age may be h i t . Blade f l a p p i n g i s i n gene ra l aggravated by winds. Some h e l i c o p t e r s have placarded use i n winds; f o r example, i n winds g r e a t e r t han 1 1 . 2 m / s (25 mph), c y c l i c c o n t r o l must be d isp laced t o cause r o t o r t o be t i l t e d i n t o the wind; or c o l l e c - t i v e p i t c h must no t be used t o s top r o t o r when winds a r e blowing.

14. Gusts and winds are t o o s t r o n g a t t imes f o r c e r t a i n h e l i c o p t e r s , such as t h e ~ 5 6 ~ ; l i m i t s are, however, r e a l l y no t known.

15. Hel i cop te r s must n o t hover near open hanger doors

16.

due t o se l f -gene ra t ed d e s t a b i l i z i n g wind c u r r e n t s .

Deflected s l i p s t ream c o n f i g u r a t i o n s of low-wing loading are very s u s c e p t i b l e t o winds; d i f f e r e n t c o n t r o l systems, such as s p o i l e r s f o r roll, re- a c t i o n j e t s f o r yaw, appear a n e c e s s i t y .

17. The inboard l a n d i n g c o n f i g u r a t i o n of a th ree - fan STOL c o n f i g u r a t i o n w a s hard t o handle i n t h e presence of winds, t i p p e d over a t t imes; outboard l and ing c o n f i g u r a t i o n by c o n t r a s t w a s considered very good.

6

18. T i l t wing a i r c r a f t may have var ious b lade i n - s t a b i l i t i e s ; winds may be expected t o aggravate these i n s t a b i l i t i e s .

19. A r r e s t i n g equipment may be r equ i r ed in. e l e v a t e d STOL p o r t s because of winds.

d i f f e r e n t r e l a t i v e wind and water movements is hazardous.

I1 20. Taxiing" up t o a dock i n a seaplane w i t h

B. Approach and Landing (Cross-winds and wind shear)

encountered dur ing approach and l and ing pose s i g n i f i c a n t design and ope ra t iona l problems t o a l l a i r c r a f t , b u t f o r VSTOL conf igu ra t ions they a r e of c r i t i c a l concern ( s e e geferences 39 and 40 as examples).

2. If t h e c r o s s winds are too l a r g e , a i r c r a f t may be blown o f f the runway; some STOL c o n f i g u r a t i o n s a r e much more s u s c e p t i b l e than o t h e r s .

1. I n genera l , c r o s s winds, wind shea r , and g u s t s *

3. Control power, e s p e c i a l l y r o l l power, i s perhaps the i tem of major concern dur ing take-off and landing .

4. The c o n t r o l power requi red t o handle ground winds and gusts i s r e a l l y no t known, e s p e c i a l l y f o r low- wing load ing high-aspect r a t i o STOL conf igu ra t ions .

should be inc luded . With t h e thousands of d i f f e r e n t c r o s s winds that a r e t o be found throughout the world, what types should be considered f o r design, and what c h a r a c t e r i s t i c s a r e s i g n i f i c a n t i n the i r d e s c r i p t i o n ( r e f e r e n c e 41) .

6. There i s a need f o r a cross-wind r a t i o n a l e f o r approach and l and ing i n a r e a s where t h e r e i s a high d e n s i t y of bu i ld ings and o t h e r o b s t r u c t i o n s , s i n c e c r o s s winds may be a l t e r e d i n t o d i s c r e t e and unusual p a t t e r n s .

S t a b i l i t y d e r i v a t i v e s must be such t o give des i r ed c o n t r o l a t low speeds; there u s u a l l y i s much d i f f i c u l t y i n c a l c u l a t i n g these d e r i v a t i v e s , and o f t e n i n order t o achieve des i r ed Values, performance may be compromised.

5. Cross-wind informat ion i s not adequate; g u s t s

7 .

8 . Because wind shear a f f e c t s t he power t o be used, and because s t a b i l i t y d e r i v a t i v e s a r e inf luenced by power, t h e r e i s a dependence of s t a b i l i t y d e r i v a t i v e s on wind shear f o r n e a r l y a l l powered- type l i f t systems.

9. With some conf igu ra t ions , f l i g h t i s not made if cross winds exceed 15.4 m / s (30 k n t s ) because of d i f f i c u l t y wi th the drag l and ing parachute ; i n o the r i n s t a n c e s , t he parachute i s r e l e a s e d a f t e r l and ing t o avoid dragging a i r c r a f t .

10. Air-cushion l and ing systems a r e considered by many t o be i d e a l f o r cross-wind l and ings ( r e f e r e n c e 42 ) .

t o be a s t r o n g f a c t o r as t o why t h e STOL s t r i p p r o j e c t i n Manhattan was stopped.

1 2 . The t h r u s t - d r a g c o n t r o l system s t u d i e d on a n O t t e r a i r c r a f t was judged t o have worked very w e l l i n combating the wind shear problem.

11. Severe c r o s s winds 35 days of a year a r e be l ieved

13. Inve r s ions o r o t h e r meteoro logica l changes between day and n igh t may cause a s i g n i f i c a n t change i n t h e wind v e l o c i t y p r o f i l e . In s t ead of t h e c h a r a c t e r i s t i c monotonic i n c r e a s e i n v e l o c i t y wi th a l t i t u d e , some p r o f i l e s have been found wherein t h e v e l o c i t y peaks (a low- leve l j e t s t ream) , t hen f a l l s of f wi th i n c r e a s i n g a l t i t u d e , even r e v e r s i n g d i r e c t i o n s i n some i n s t a n c e s . The l and ing problem a s s o c i a t e d wi th wind shear i s t h u s g r e a t l y aggrava ted . Much more informat ion i s needed on t h e var ious p r o f i l e s t h a t may be encountered - t h e i r d e t a i l and as t o when and where they occur .

14. I n wind and gus ty cond i t ions , i t may be b e t t e r t o l e t a h e l i c o p t e r " f l y i t s e l f " ; p i l o t s o f t e n aggrava te t h e motions. Some opin ion w a s expressed t h a t h e l i c o p t e r s a r e hard t o f l y i n c r o s s winds and g u s t s ; o t h e r s expressed the c o n t r a s i n g observa t ion tha t h e l i c o p t e r s a r e n ' t a f f e c t e d g r e a t l y by winds or g u s t s .

15. The S t a b i l i t y Augmentation System (SAS) on h e l i - c o p t e r s i s cons idered t o r e p r e s e n t a l s o a good gus t a l l e v i a t o r .

16. The approach t o l a n d i n g wi th a h e l i c o p t e r on t h e top of a t a l l b u i l d i n g i n t h e presence of winds i s considered t o be a problem.

8

17

18 . 19

20.

21 .

22.

23.

24.

25.

26 .

Ground handl ing, cross-winds, automatic approach systems r ep resen t problem a r e a s of unknown scope f o r a l l remotely p i l o t e d veh ic l e s .

The winds and g u s t s that should be used i n l and ing s t u d i e s of t he space s h u t t l e are unknowns.

Gust a l l e v i a t i o n systems a r e considered important t o STOL conf igu ra t ions which may c r u i s e a t M = 0.7 t o 0.8 2000 f t I80 knts].

Simulat ion s t u d i e s of advanced t r a n s p o r t con- f i g u r a t i o n s (6 engines , 6 f a n s ) i n d i c a t e that winds and gus t s i g n i f i c a n t l y degrade approach c o n t r o l .

b u t are a b l e t o l a n d on 610 m s t r i p s a t approach speeds of 41.2 m / s

S t u d i e s of the D O 3 1 c o n f i g u r a t i o n i n d i c a t e c o n t r o l i s r e l a t i v e l y unaf fec ted by g u s t s , bu t why t h i s m i g h t be so i s no t known.

The Breguet and Harrier have h a d t r o u b l e i n some cross-wind landings .

The A-7 i s not flown if t h e c r o s s winds exceed 10.3 m / s (20 k n t s ) ; t h e A-8 i s r e s t r i c t e d f o r c r o s s winds g r e a t e r t han 9.3 m / s (18 k n t s ) .

The X-14 was n o t flown i n winds above 2.6 m / s ( 5 knts).

Low-wing load ing a i r c r a f t (U-2) a r e known t o be d i f f i c u l t t o f l y i n gus ty winds.

Winds and gusts seve re ly r e s t r i c t crop dus t ing ope ra t ions .

C . C a r r i e r Operations

1. C a l m winds or winds g r e a t e r t han 15.4 m / s (30 knts) are a hazard i n a i r c r a f t c a r r i e r l and ing ope ra t ions . For calm cond i t ions t h e c a r r i e r must steam a t 15.4 m / s ( 3 0 k n t s ) , forming thereby a s t r o n g turbu- l e n t burb le behind t h e c a r r i e r . For winds g r e a t e r t han 15.4 m / s (30 k n t s ) , s t ee rage way becomes a problem; the s e a s a r e u s u a l l y rough i n t h e s e cond i t ions , c a r r i e r motions become a b i g f a c t o r , and c a r r i e r s t ee rage may be r e s t r i c t e d because of o the r s h i p s i n proximity.

2. Extensive s t u d i e s of wakes behind c a r r i e r s have been made. (Spec i f i c r e fe rences a r e discussed i n a l a t e r s e c t i o n . ) 9

D.

3. An angle of a t t a c k approach i s u s u a l l y used. When g u s t s a r e p r e s e n t , an increment of 1 / 2 t he maximum gus t v e l o c i t i e s i s added t o t h e nominal approach speed of 1.18 Vs , where Vs denotes s t a l l speed.

4. The angle of a t t a c k approach g ives d i f f i c u l t y when s t r o n g drafts are p r e s e n t . Navy i s making ex- t e n s i v e s tudy us ing q (dynamic p r e s s u r e ) and power c o n t r o l f o r d i f f i c u l t s i t u a 2 i o n s . A ma t r ix has been e s t a b l i s h e d t o i n d i c a t e when cond i t ions are s a t i s f a c t o r y t o use the power c o n t r o l system; p i l o t t akes over when cond i t ions a r e exceeded.

Sensc)Ts, Ins t ruments

1.

2 .

3 .

4.

5.

6.

I n c r o s s winds and wind shea r cond i t ions , t he cues given t o t h e p i l o t a r e very impor tan t .

Disp lays g iv ing informat ion on the n a t u r e of t h e c r o s s winds o r wind shears a r e d i f f i c u l t t o make. Much work i s needed t o develop senso r s , i n s t r u - ments, and d i s p l a y s t o d e t e c t c r o s s winds and wind shea r s , and i n t u r n t o i n d i c a t e what should be done t o coun te rac t t h e s e i n p u t s ( r e f e r e n c e s 43 and 44) .

Good sens ing systems and d i s p l a y s a r e needed t o i n d i c a t e t h e winds r e l a t i v e t o t h e a i r c r a f t . With t h e a i r f low over the a i r c r a f t , a l l forms of down- wash and l o c a l a i r f low d i s tu rbances , i t i s very d i f f i c u l t t o e s t a b l i s h how t h e wind i s blowing r e l a t i v e t o t h e a i r c r a f t .

Work i s needed t o develop in s t rumen t s beyond t h a t used i n f i x e d wing a i r c r a f t , such as b e t t e r a l t i - meters , t op of t rees i n d i c a t o r , r e l a t i v e wind i n d i c a t o r (as mentioned i n i t e m 3 ) . Since most a i r speed i n d i c a t o r s d o n ' t g ive in fo rma t ion u n t i l a r e l a t i v e speed of 15.4 m / s ( 3 0 k n t s ) i s reached, t h e r e i s a l s o a need f o r a r e l a t i v e a i r speed i n d i c a t o r f o r speeds between 0 and 15.4 m / s (30 k n t s ) . A t p r e s e n t , p i l o t s must r e l y on v i s u a l cues f o r t h i s range. Proper d i s p l a y t o t h e p i l o t i s cons idered most impor tan t .

I n t h e use of t h e new microwave l a n d i n g system, what a r e t h e parameters that should be monitored.

The H a r r i e r a i r c r a f t must be flown w i t h c a r e i n c r o s s winds. The use of a s i d e - s l i p vane, i n the form of a v i s u a l d i s p l a y t o t h e p i l o t and a l s o through an automatic c o n t r o l i n t h e rudder c o n t r o l system, i s be ing s tud ied a t p r e s e n t as a p o s s i b l e means f o r improving f l i g h t s a f e t y i n cross-wind s i t u a t i o n s .

10

E. Measurements

1. There i s a l a c k of s t a n d a r d i z a t i o n i n t h e var ious wind and gust measurements t h a t a r e made. Aspects inc lude time of averaging, h e i g h t s of measurements, l o c a t i o n ( t h a t i s , a r e measurements i n a f r e e space or i n t h e l e e of a b u i l d i n g ) , d i r e c t i o n a l p r o p e r t i e s of t h e winds.

LOCAT and other tower data appear t o apply mainly t o t h e so-ca l led "good day"; stormy days a r e not inc luded i n genera l .

More measurement should be made on " d i s c r e t e g u s t " e f f e c t s behind obs t ac l e s .

B e t t e r wind r e p o r t i n g a t a i r p o r t s i s needed ( r e fe rence 45).

Measurements a r e needed t o i n d i c a t e when c r i t i c a l cond i t ions a r i s e . i s needed a t times b y t h e p i l o t s . An example, as obta ined from a FAA spokesman, i l l u s t r a t e s the need. I n a s h o r t pe r iod of time near the beginning of 1973, 9 abor ted landing a t t empt s were made a t t h e J F K a i r p o r t ; 3 of these involved a s i n g l e 747 a i r - c r a f t , which f i n a l l y d ive r t ed 1500 miles t o make a landing . which wind d i r e c t i o n a c t u a l l y changed w a s l a t e r diagnosed t o be the cause of t h e t r o u b l e .

2 .

3.

4.

5. Addi t iona l in format ion on shear

A very unusual wind shear cond i t ion i n

F. Turbulence Models and Simulat ion

1. It i s unanimous that b e t t e r wind and gust models are needed f o r use i n a n a l y t i c a l and s imula t ion s t u d i e s ; t h e r e i s at t h e same time a b i g d i s - agreement as t o what i s needed.

Turbulence and wind i n p u t s a r e considered by some t o be e s s e n t i a l o r t he most important i n g r e d i e n t i n s imula t ion s t u d i e s of a i r c r a f t approach and l and ing . Many f e l t , i n f a c t , t h a t the kind of modeling made w i l l probably "make o r break" a STOL system t h a t i s under cons ide ra t ion .

2 .

3. Gust models based on high a l t i t u d e turbulence are i n gene ra l found t o be t o o severe and u n r e a l i s t i c . Based on p i l o t r e a c t i o n s from bo th f i x e d base and f l i g h t s imula t ion s t u d i e s , much lower values of gus t s e v e r i t y and i n t e g r a l s c a l e a r e i n d i c a t e d r e l a t i v e t o those o f t e n quoted. Values of 1 m / s

11

3 f p s f o r r m s s e v e r i t y , i n s t e a d of 1.8 or 2.4 m / s 16 o r r a t h e r t han 760 m (2500 f t ) , were found t o be much more r e a l i s t i c .

f p s ) , and L = 60 - 90 m (200-300 f t )

4. Some f e l t t h a t any we l l s p e l l e d out s e t of models

5. Some f e l t t h a t a " s t a b l e " of wind and gus t models were needed, dependent on a p p l i c a t i o n ; t h e thought w a s a l s o expressed that perhaps only the more severe half of any measured s e t of w j n d p r o f i l e s should be used f o r modeling purposes .

would be accep tab le .

6. Some f e l t t h a t perhaps a s o p h i s t i c a t e d model was not r e a l l y needed; t h e a l t e r n a t i v e suggested w a s t o p i c k some of t h e more severe w i n d s and gus t cond i t ions t h a t might be encountered and t o s tudy whether the a i r p l a n e could be flown or no t , or a t l e a s t f i n d out what problems m i g h t be i n d i c a t e d .

7. Gaussian-type gus t i n p u t s were no t cons idered r e a l i s t i c by some p i l o t s . Nons ta t ionary g u s t e f f e c t s were cons idered a most e s s e n t i a l i t em. Pa tch iness , i n f a c t , i s o f t e n s imulated simply by t u r n i n g t h e gus t gene ra to r on and o f f .

d i f f e r e n t . A l l s p e c i f i c a t i o n s or g u i d e l i n e s are considered i n need of improvement. N o d i s t i n c t i o n i s made between convent iona l , STOL, or h e l i c o p t e r a i r c r a f t . The model and c o n f i g u r a t i o n i n t e r f a c e may be impor tan t ; models, f o r example, should perhaps be t a i l o r e d t o the c o n f i g u r a t i o n . I n a broader con tex t , three b a s i c n o t i o n s a r e of concern:

8. Models i n va r ious s p e c i f i c a t i o n s a l l appear

1) A r e gus t and wind models c o n f i g u r a t i o n - dependent ?

2 ) The a b i l i t y f o r a given des ign group t o u t i l i z e a model i s an impor tan t factor..

3) The des ign group phi losophy a l s o has an important b e a r i n g w i t h r e s p e c t t o what models a r e used.

9. Many ques t ions about t h e d e t a i l of t h e models need

1) Are r e s u l t s a f f e c t e d g r e a t l y by d i f f e r e n t

2 ) I s t h e use of d i f f e r e n t i n t e g r a l s c a l e va lues

s tudy:

s p e c t r a l shapes?

f o r t h e u , v , and w components impor tan t?

12

1

10.

11.

12.

I s it necessary t o cons ider c o r r e l a t i o n e f f e c t s between the u , v , and w components?

How important a r e the c r o s s c o r r e l a t i o n e f f e c t s t h a t might a r i s e i n t h e low a l t i t u d e turbulence as a r e s u l t of o b s t r u c t i o n s such as trees, c l i f f s , o r b u i l d i n g s , as c o n t r a s t e d t o the use of i so t rop ic - type turbulence?

A r e geographical and seasona l changes i n s p e c t r a l conten t s i g n i f i c a n t ?

What are r e a l i s t i c environmental and load l i m i t s ?

Is i t important t o model d i s c r e t e @st e f f e c t s such as might arise behind o b s t a c l e s ?

The f l o w behind an a i r c r a f t c a r r i e r has been s t u d i e d ex tens ive ly and some e x c e l l e n t models of t h e approach environment has been made .

G. S p e c i f i c a t i o n s and C r i t e r i a . - A few comments a r e made he re w i t h r e s p e c t t o t h e s ta tements and g u i d e l i n e s t h a t a r e s e t f o r t h i n s p e c i f i c a t i o n and c r i t e r i a documents r e l a t i v e t o the t rea tment of s u r f a c e winds and gus ts , r e f e r e n c e s 46-61. Not many s t a t emen t s were o f f e r e d by t h e i n d i v i d u a l s contac ted , b u t t h e few that were made a r e l i s t e d he re . The s t a t emen t s a r e no t intended t o be c r i t i c a l of t h e s p e c i f i c a t i o n s t h a t e x i s t . Rather, the l a c k of d e t a i l e d coverage t h a t e x i s t s i n var ious s p e c i f i c a t i o n s may be concluded as due t o t h e f a c t that models, procedures and techniques f o r handl ing ground winds and g u s t s a r e s t i l l i n t h e embryonic s t age of development. With few except ions , such as r e f e r e n c e 46 and c e r t a i n i n t e r n a l NASA documents, gu ide l ines f o r hand l ing gusts, i f presented a t al l , a r e based mostly on high a l t i t u d e turbulence models. Even these models a r e s u b j e c t t o q u e s t i o n . develop r e a l i s t i c l ow-a l t i t ude turbulence models. and a t t e m p t s t o feed back results of t h e s e a p p l i c a t i o n s t o develop b e t t e r i n p u t models, are few. s e t down d e f i n i t e , w e l l though out , proven and accep tab le l o w - a l t i t u d e turbulence models and a p p l i c a t i o n procedures a t the moment, and i t i s thus no wonder t h a t models cannot be s t a t e d i n s p e c f f i c a t i o n s . Observat ions o f f e red by i n d i v i d u a l s went as fo l lows :

Only a r e l a t i v e l y few a t tempts have been made t o Appl ica t ions ,

It i s t h u s not p o s s i b l e t o

1) The s p e c i f i c a t i o n s give mainly only gene ra l s ta tements w i t h r e s p e c t t o handl ing gusts and winds, u s u a l l y i n terms of c o n t r o l motions, s u c h as "con t ro l motion should not be excess ive when winds and g u s t s a r e p r e s e n t ; " or "hover i s t o be made w i t h c o l l e c t i v e p i t c h 11 not exceeding 5 1 i n . calm a i r (< 1.5 m / s o r < 3 k n t s ) .

Each new Request f o r Proposal (RFP) has s p e c i f i c r e - quirements s p e l l e d out , u s u a l l y p e r t a i n i n g t o c o n t r o l r a t e s or o t h e r c o n t r o l c h a r a c t e r i s t i c s ; no s p e c i f i c wind or gus t requirements are i n d i c a t e d .

For c a r r i e r ope ra t ions , s ta tements such as t h e fo l lowing appear: "Operation i n the i s l a n d wake should be kept t n a m5E5xm," ?iover. behind t h e c a r r i e r should be avoided," "launch and recovery should be made i n r e l a t i v e wind l e s s than 23 m / s (45 k n t s ) . "

STUDIES OF THE LOW ALTITUDE WINDS AND GUSTS

The a b i l i t y t o apply informat ion on environmental s u r f a c e winds and gus t s t o eva lua te des ign and o p e r a t i o n a l problems of a e r o n a u t i c a l systems, and t o i n t e r p r e t results meaningful, i s a t p r e s e n t somewhat l i m i t e d . Methods and procedures a r e s t i l l i n t h e developmental s t age , and .on ly r e c e n t l y are ques t ions be ing r a i s e d with r e s p e c t t o whether t h e proper type of in format ion i s be ing c o l l e c t e d . I n s p i t e of t h i s s i t u a t i o n wi th r e s p e c t t o a e r o n a u t i c a l problems, t he amount of s tudy t h a t has been made of su r face winds and g u s t s i s s t agge r ing . T h i s s e c t i o n p r e s e n t s a sampling of t h e many r e f e r e n c e s t h a t may be found r e p o r t i n g these s t u d i e s . It i s recognized tha t i t i s no t p r a c t i c a l t o s tudy a l l of t he r e f e r e n c e s i n d i c a t e d ; as an a i d t o t h e r eade r , t h e r e f o r e , a few key papers which p r e s e n t r e p r e s e n t a t i v e i n f o r - mation w i l l be s i n g l e d out i n a l a t e r s e c t i o n of t h i s paper . A s an a r b i t r a r y breakdown, t h e r e f e r e n c e s l i s t e d h e r e i n on environmental s t u d i e s are given i n t h r e e groups.

Books.- References 62 through 67 r e p r e s e n t a sampling of t h e e x c e l l e n t textbooks t h a t may be found r e l a t i n g t o low-level atmospheric tu rbulence .

Measurement techniques.- A sampling i n d i c a t i n g t h e types of s t u d i e s that have been made on senso r s and measurement techniques i s given by r e f e r e n c e s 68 through 85.

Data ga the r ing and a n a l y s i s and b a s i c me teo ro log ica l s t u d i e s . - S t u d i e s of t he su r face wind and gus t environment may be c l a s s i f i e d broadly i n t o two types . One i s a s s o c i a t e d wi th t h e sampling and a n a l y s i s of s t a t i s t i c a l d a t a on t h e winds and g u s t s . The o the r s tend t o be more t h e b a s i c r e s e a r c h type , wherein the a t tempt i s t o c o l l e c t t h e s t a t i s t i c s on the winds and g u s t s and va r ious meteoro logica l parameters as w e l l , t o ana lyze t h e data i n terms of t h e meteorology, and t h e n t o t r y t o c o l l a p s e a l l t h e data i n t o simple gene ra l i zed o r u n i v e r s a l form. References 86 through 2 2 1 r ep resen t t h e s e s tud ie s .

14

Some c u r r e n t measurement programs.- Mention is-made he re of a f e w measurement programs that a re under cons ide ra t ion o r t h a t may have been s t a r t e d . These programs a r e mentioned simply as e f f o r t s t o keep i n mind which poss ib ly m i g h t have some inpu t toward understanding the s u r f a c e winds and g u s t s problem b e t t e r .

1. A l a r g e j o i n t e f f o r t by the Environmental Sciences Serv ice Adminis t ra t ion (ESSA) and t h e Canadiark measuring winds over Lake E r i e , bo th h o r i z o n t a l l y and v e r t i c a l l y ; main ob jec t ive i s t o t r y t o m d e r s t a n d weather p a t t e r n s b e t t e r .

A l i n e of 15 towers , spaced a t 300 m (1000 f t ) , a long t h e 4600 m (1500 f t ) runway a t Edwards A i r Force Base Towers a r e t o be 90 m (300 f t ) of f t he c e n t e r l i n e of t h e runway; anemometers a t 3 m (10 f t ) e l e v a t i o n w i l l measure u and v components. The s tudy i s f o r o p e r a t i o n a l purposes, no t r e sea rch .

Minnesota t o ob ta in information up t o the 1 kilometer a l t i t u d e (sponsor AFCRL) . Three ins t ruments a r e t o be mounted on the t e t h e r a t low a l t i t u d e s t o measure su r face f r i c t i o n ; f i v e a d d i t i o n a l sensors are t o be used a t t h e h igher a l t i t u d e s .

2 .

3. Tethered ba l loon experiment t o be conducted i n

4. A FAA program be ing conducted by B a t t e l l e , Northwest, i n t h e S e a t t l e a r e a , One of t h e o b j e c t i v e s of t he program i s t o a t tempt , by tower measurements, t o i d e n t i f y t h e meteoro logica l in format ion of s i g n i f i c a n c e t o VSTOL a i r c r a f t and t h e i r opera t ion .

MODELS OF SURFACE WINDS AND GUSTS

Considerable e f f o r t has been made t o der ive mathematical models of the su r face winds and gus t s f o r use i n aerospace eng inee r ing a p p l i c a t i o n s . It i s observed, however,. t ha t only a r e l a t i v e l y few i n d i v i d u a l s have t r i e d t o u t i l i z e these models, o r a r e even familiar with them. There appears t o be g r e a t e r f a m i l i a r i t y w i t h t he h igher a l t i t u d e gus t models and as a consequence, t hese models have o f t e n been app l i ed , inappropr i - a t e l y , t o s tudy a i r c r a f t problems a t boundary l a y e r a l t i t u d e s ; r e s u l t s ob ta ined have u s u a l l y been judged as be ing There i s t h u s a need f o r educat ion on the need f o r and use of s u r f a c e l a y e r models.

References 46, 47, 114, 160, and 222 through 238 a r e r e p r e s e n t a t i v e of s t u d i e s d e a l i n g w i t h t h e development of sur- f a c e l a y e r models. It i s not t he i n t e n t of t h i s survey t o give an in -dep th review of these models, s i n c e t h i s has been done i n p a r t e lsewhere; r e fe rence 46 gives a very ex tens ive coverage of

11 u n r e a l i s t i c . "

var ious wind and gus t models; r e f e r e n c e s 234 and 235 p r e s e n t e x c e l l e n t reviews. It i s f e l t app ropr i a t e , however, t o i n d i c a t e t h e i r nature. The f i g u r e s t h a t a r e p re sen ted are in tended t o be p i c t o r i a l and not d e f i n i t i v e i n n a t u r e . Thus, n o t t oo much a t t e n t i o n should be pa id t o t h e u n i t s o r t h e nomenclature used Dif fe rences i n u n i t s and i n nomenclature r ep resen t , i n f a c t , a major reason why it i s p r e s e n t l y d i f f i c u l t t o compare one model t o another

Figure I, t aken from re fe rence 235, and presented i n many forms elsewhere, i s a vers ion of a popular bu t t y p i c a l presen- t a t i b n on the spectrum of wind speed t h a t has been obtained from tower measurements over a long time obse rva t ion . The p o r t i o n Of t h e curve showing peaks i n t h e per iod range of one week arid one day i s a s s o c i a t e d wi th the winds; t h e s e winds are quas i - s teady i n na ture t o t h e a i r p l a n e and c r e a t e t h e c r o s s wind and wind shear problems encountered dur ing take-off and l and ing . The broad dip i n the pe r iod range of one hour has been termed t h e spectrum gap. The p o r t i o n i n the pe r iod range of one minute t o one second i s a s s o c i a t e d w i t h t he atmospheric g u s t s or tu rbu- l ence t h a t has been c r e a t e d i n t h e main by the winds. Th i s tu rbulence , which a c t s i n superimposed f a sh ion t o t h e winds, g ives r ise t o t h e r a p i d l y f l u c t u a t i n g d i s tu rbances t h a t a r e encountered dur ing take-off and l and ing .

I n c o n t r a s t t o a n e g l i g i b l e c o r r e l a t i o n between the u and w tu rbulence components a t h igher a l t i t u d e s , t h e r e i s a s t r o n g c o r r e l a t i o n between u and w i n t h e su r face boundary l a y e r , l e a d i n g t o the d e f i n i t i o n

u z = 2 / - u w and a sur face s t r e s s

- where uw r e p r e s e n t s a time average of t h e product uw The value u i s used e x t e n s i v e l y i n models as a b a s i c r e f e r e n c e va lue . -l

I n t h e s u r f a c e boundary l a y e r , and f o r n e u t r a l a tmospheric s t a b i l i t y cond i t ions , t h e mean wind v e l o c i t y p r o f i l e U as a func t ion of he igh t z i s found t o be r ep resen ted w e l l by t h e e quat ion

- _ U -Tin- 1 Z

z O U z

16

where k i s von K & r m d n l s cons t an t (=: 0.4), and zo i s a roughness l eng th , which i s . r o u g h l y 1/30 the dimension of t he t y p i c a l roughness p a r t i c l e ( r e fe rences 4.6 and 235). n e u t r a l cond i t ions , a modi f ica t ion t o t h i s equat ion i s employed u t i l i z i n g a un ive r sa l - type func t ion which i s dependent on s t a b i l i t y cond i t ions , r e f e rence 65. l a y e r a power l a w p r o f i l e i s o f t e n used, of t h e type

For non-

For the e n t i r e boundary

a u, ( 3 )

where U1 i s the v e l o c i t y a t some r e f e r e n c e he igh t

r e f e r e n c e s 32 and 235. der ived from t h i s equat ion . Peak wind p r o f i l e s a t s e l e c t e d l o c a t i o n s , i n c o n t r a s t t o mean p r o f i l e s , are t r e a t e d e x t e n s i v e l y i n r e f e r e n c e 46 by an equat ion similar t o equa t ion (3) ; gust f a c t o r s , t h e r a t i o of peak winds to mean winds, are a l s o p re - s en ted .

value c1 i s dependent on zo and Figure 2 i n d i c a t e s example p r o f i l e s as

The mean v e l o c i t y p r o f i l e i s of concern i n t h e approach t o landing , s i n c e i t r e q u i r e s t h e p i l o t t o make power and a t t i t u d e changes i n o rde r t o main ta in a given a i r speed. F l u c t u a t i o n s about t h e mean p r o f i l e are perhaps of even g r e a t e r concern, s i n c e they l e a d t o hazardous f l i g h t cond i t ions ( l i k e l i h o o d of s ta l l , c o n t r o l d i f f i c u l t y i n genera l ) and i n t u r n t o l a r g e d i s - p e r s i o n s i n t h e touchdown p o i n t , o r even abor ted landings . F igu re 3, t aken from reference 237, i n d i c a t e s t h e magnitude of t h e s e f l u c t u a t i o n s t ha t a r e p o s s i b l e . I n many i n s t a n c e s unusual v e l o c i t y p r o f i l e s of the type i n d i c a t e d i n F igure 4 develop; t h e approach and l and ing problems f o r - t h e s e unusual p r o f i l e s a r e i n gene ra l much more severe than t h e s t a t i s t i c a l average case . L i t t l e i s known,however, w i t h r e s p e c t how t o a n t i c i p a t e o r d e t e c t t h e s e p r o f i l e s , and how t o cope w i t h them when encountered . Much s tudy on t h e s e odd o r unusual p r o f i l e s i s t h e r e f o r e needed t o e s t a b l i s h r e p r e s e n t a t i v e s e t s of p r o f i l e s , t o unders tand the i r cause and how t o d e t e c t them, and what o p e r a t i o n a l procedures should be followed, i f p o s s i b l e , when they are de tec t ed .

wind f lows a t an angle t o t h e runway, have a l s o been developed, r e f e r e n c e s 41, 59 and 60. The more unusual o r severe c r o s s winds have not r ece ived much a t t e n t i o n , however. A s an example, if an o b s t r u c t i o n i n t e r v e n e s such as a group of t r e e s , o r a h i l l , o r a b u i l d i n g , t hen sharp, d i s c r e t e - t y p e d i s tu rbances may develop i n the c r o s s wind, which may cause s e r i o u s c o n t r o l d i f f i c u l t y ; a r a p i d change i n wind d i r e c t i o n w i t h a l t i t u d e i s

Cross-wind models, a s soc ia t ed with the f a c t that t h e mean

* another example. I n genera l , more s tudy i s needed of c r o s s winds r e l a t i n g t o t h e i r magnitude and d i r e c t i o n a l v a r i a b i l i t y , and on the form they may t a k e .

Modeling of t h e turbulence i s u s u a l l y m a d e i n s p e c t r a l form, a l though d i s c r e t e gus t models cont inue a l s o t o be examined, r e f e r e n c e s 11 and 226. Some r e p r e s e n t a t i o n s a r e based on t h e so-called vnr, K&m& fsrm, glvei? by tile eyuakions

nr

2 o W 2Lw

Other r e p r e s e n t a t i o n s use t h e Dryden form

2 2Lu 1 @,(a) = ou - 2 2 7r 1 + LUR (7)

2 OV 7J

* The shear s t r e s s e s a c t i n combination w i t h t h e C o r i o l i s Forces due t o t h e e a r t h ' s r o t a t i o n and cause a sys t ema t i c d e f l e c t i o n of t he mean wind; r e s u l t s of a f a i r l y s imple theo ry by Ekman a r e commonly r e f e r r e d t o as t h e Ekman S p i r a l .

18

Lo I n t h e s e equat ions R = - where LU i s c i r c u l a r f requency and V i s v e l o c i t y . I n h igh a l t i t u d e models t h e wind speed i s t a c i t l y ignored and V i s normally taken as the speed of t he a i r c r a f t . For low a l t i t u d e s i t u a t i o n s , p a r t i c u l a r l y w i t h VSTOL a i r c r a f t , t h e a i r p l a n e speed may be of t h e same order as t h e wind speed, even lower, and thus V must be def ined d i f f e r e n t l y ; only a few seem t o recognize t h i s f a c t . S p e c i f i c a l l y , the r e l a t i v e v e l o c i t y between the a i r c r a f t and t h e wind should be used f o r V as a good approximation. A gene ra l i zed c o r r e l a t i o n d i s t a n c e w i l l be proposed i n a l a t e r s e c t i o n of t h i s r e F c r t which i n h e r e n t l y i n c l u d e s cons ide ra t ion of bo th t h e w l $ 7 and a i r c r a f t v e l o c i t i e s .

v y

T h e o r e t i c a l l y , and because b e t t e r agreement i s found wi th measured r e s u l t s , equa t ions (4)-(6) a r e p r e f e r r e d model choices . Since equat ions (7 ) - (9 ) r ep resen t r a t i o n a l func t ions , however, t hey can be r ep resen ted much e a s i e r i n s imula tor s t u d i e s ; that i s , a white no i se genera tor w i t h simple f i l t e r s can be used t o d u p l i c a t e t h e i r s p e c t r a l makeup, r e fe rences 47 and 234. Dryden equat ions a r e t h e r e f o r e u s u a l l y chosen f o r use i n simu- l a t o r s t u d i e s .

The

A common way t o express the r m s tu rbulence s e v e r i t y va lues o U Y o v , o w , i n t h e e a r t h ' s boundary l a y e r i s i n terms of the f r i c t i o n v e l o c i t y uT , equat ion (1) . Representa t ive va lues a r e given i n r e f e r e n c e s 46 and 235. A s an example, t h e fol low- i n g va lues a r e i n d i c a t e d below a t an a l t i t u d e of about 20 m

ow = 1.3 U T

A s t h e a l t i t u d e i n c r e a s e s t h e cons t an t s i n these r e l a t i o n s approach one another , s p e c i f i c a l l y a value of about 1.25. Above t h e boundary l a y e r t h e f r i c t i o n v e l o c i t y uT f a l l s t o zero. A t such a l t i t u d e s , equa t ions (10)-(12) imply t h a t t h e r m s va lues f o r the tu rbu lence components a l s o vanish. The mechanism f o r t u rbu lence gene ra t ion i s d i f f e r e n t , however, and equa t ions of t h e type given by equat ions (10 ) - (12 ) do not apply. l e n c e t ends t o i s o t r o p y , w i t h ou , ov and ow a l l about e qual .

The turbu-

I n t e g r a l s c a l e values Lv , Lu , L , f o r t h e boundary l a y e r a r e given i n r e fe rences 46 and 235. va lues a r e observed t o inc rease with a l t i t u d e up t o about 300 m (1000 f t ) ; above t h i s a l t i t u d e the s c a l e values seem t o be c o n s t a n t . It should be noted t h a t t h e s c a l e va lues t ha t a r e

I n genera l , t h e s c a l e

derived from data a r e q u i t e s e n s i t i v e t o t h e d e r i v a t i o n technique used, or on s p e c t r a l f u n c t i o n choice that i s m a d e , s ee r e fe rence 25. Thus, c a r e should be exe rc i sed i n comparing the s c a l e values obta ined by d i f f e r e n t i n v e s t i g a t o r s .

I n an a t tempt t o t a k e i n t o account t h e time de lay between wing and t a i l gus t encounter , and the nonuniformity of t h e gus t s i n t h e spanwise d i r e c t i o n , s p e c t r a models of "equiva.lent" airp!la.m angular v e l o c i t i e s have been advanced, r e fe rence 47. The problem i s e s s e n t i a l l y t h a t of t a k i n g i n t o account t h e c o r r e l a t i o n of t h e gust v e l o c i t i e s t h a t a c t over one p o r t i o n of t he a i r c r a f t w i t h the g u s t s that a c t over another p o r t i o n . I n genera l , t h e models t h a t have been advanced a r e n ' t adequate , and much more s tudy of t hese "cross c o r r e l a t i o n " e f f e c t s i s needed. This a s p e c t of t h e gus t encounter problem should be regarded, i n f a c t , as one of t h e top p r i o r i t y problems t h a t should be s tud ied n e x t .

APPLICATION OF MODELS I N AIRCRAFT STUDIES

It i s t h e consensus t h a t a l l f l i g h t and f ixed-based simu- l a t o r s t u d i e s of a i r c r a f t i nvo lv ing s t a b i l i t y and c o n t r o l and f l y i n g q u a l i t i e s e v a l u a t i o n should inc lude tu rbu lence as an i n p u t . This i n c l u s i o n , i n f a c t , i s cons idered e s s e n t i a l or a must. Thus, s imulated turbulence i n p u t s are t o be found i n nea r ly a l l f l i g h t or s imula to r s t u d i e s of a i r c r a f t . Th i s s e c t i o n l i s t s a few of t he s t u d i e s t ha t have been made.

Simulator or f l i g h t s t u d i e s .- References 234 and 239 through 252 a r e examples of f l i g h t and s imula to r s t u d i e s of a i r c r a f t which included tu rbu lence as a prime i n p u t . General ly , t he turbulence inpu t i s obta ined by p a s s i n g t h e s i g n a l from a white no i se genera tor through s imple e l e c t r i c a l f i l t e r s so that a des i r ed s p e c t r a l shape i s ob ta ined . Most of t h e e a r l i e r s t u d i e s were based on t h e h igh a l t i t u d e gus t models and t h e Dryden-type s p e c t r a . Inva r i ab ly , t h e i n v e s t i g a t o r s found t h a t t h e turbulence s e v e r i t y va lues and s c a l e of t u rbu lence as i n d i c a t e d i n these models were t o o l a r g e ; the main r e a c t i o n from the p i l o t s was that t h e tu rbu lence s i m u l a t i o n d i d n o t seem

usua l ly l e d t o a b e t t e r r e a c t i o n t o t h e tu rbu lence s i m u l a t i o n . It i s o f i n t e r e s t t o note t h a t t h e va lues a r r i v e d a t by a t r i a l and e r r o r process , IS of about 1 m / s i n s t e a d of 3 m / s and L of around 100 m i n s t e a d of 800 m, a r e i n conformity w i t h s e v e r a l low a l t i t u d e gus t models. A b e t t e r awareness of boundary l a y e r gus t models i s thus i n d i c a t e d . Another f a u l t i n many a p p l i c a t i o n s i s t h a t c r o s s - c o r r e l a t i o n e f f e c t s (non- uni formi ty of t h e gus t over t h e dimensions of t h e a i r c r a f t ) a r e no t included. It i s be ing r e a l i z e d , however, t h a t t h e s e e f f e c t s must be inc luded ,

r e a l i s t i c . " Downward adjustment of b o t h t h e IS and L values l l

20

One of the best t r ea tmen t s r e l a t i n g t o t h e development of a low- leve l tu rbulence model and w i t h r e s p e c t t o use i n a s imula tor s tudy of a p a r t i c u l a r a i rcraf t c o n f i g u r a t i o n i s r e f e r e n c e 234. The development cons iders (a) t h e average head- on and v e r t i c a l g u s t s on the wing, on the f u s e l a g e , and the t a i l , b t h e average s i d e g u s t s on the fuse l age and on the t a i l , and

I C 1 t h e spanwise shear of head-on and v e r t i c a l g u s t s a c r o s s t h e wing, The var ious f o r c e s and moments that a c t on the a i r c r a f t due t o g u s t s were s i m u l a t e d h r o u g h use of 8 independent gust gene ra to r s . Study of t h e behavior of t h e a i r p l a n e when f l y i n g a long a f l i g h t p a t h t h a t i s a t an angle t o t h e mean wind d i r e c t i o n w a s a l s o inc luded .

i t em of key concern i n s imula t ion s t u d i e s . Some f e e l tha t i n order t o achieve r e a l i s t i c r e s u l t s that t h i s p a t c h i n e s s i s one of t h e rnost important i t e m t o be included i n s imula t ion s t u d i e s . The terms nons ta t iona ry , non-Gaussian, o r i n t e r m i t t e n t c h a r a c t e r , a r e o f t e n used t o descr ibe the phenomenon. Reference 233 pre- s e n t s a s tudy of ways f o r developing a non-Gaussian gus t s i g n a l . Reference 229 p r e s e n t s some r e s u l t s comparing r e a c t i o n s of p i l o t s t o Gaussian and non-Gaussian i n p u t s .

r e f e rencescon ta ined t h e r e i n , d e a l with the modeling of the wake behind an a i r c r a f t c a r r i e r and w i t h s imu la t ion s t u d i e s of t h e f l i g h t of a i r c r a f t i n these wakes. Wake s t u d i e s have been r a t h e r thorough and from these s t u d i e s very s o p h i s t i c a t e d winds and turbulence models have been developed. The models are u s u a l l y given i n terms of t h r e e separable but superposable components: s t e a d y - s t a t e flow a n g u l a r i t y , ship-motion-induced d i s tu rbances , and random turbulence . Oddly , i t appears t ha t t h e models developed f o r c a r r i e r wakes a r e much b e t t e r and i n much g r e a t e r de ta i l than f o r the models of winds and tu rbulence ove r l a n d . Since the wake i s dependent on t h e conf igu ra t ion of t h e c a r r i e r , however, con t inu ing model developments a r e r equ i r ed .

The patchy na tu re of t he turbulence environment i s another

C a r r i e r ope ra t ion . References 253 through 259, and t h e

A n a l y t i c a l s t u d i e s . - Besides s imula to r s t u d i e s t h e r e a r e s t u d i e s which examine c e r t a i n o ther problems a s s o c i a t e d w i t h a i r c r a f t o p e r a t i o n i n winds and turbulence. s t u d i e s a r e mentioned he re . I n r e fe rences 229 and 260, t h e wind shear problem i s cons idered; t he approach t o l and ing of an a i r p l a n e a long a f l i g h t p a t h a t an angle t o t h e ground i s t r e a t e d , and r e s u l t s g iv ing t h e p r o b a b i l i t y of encounter ing a g iven wind shear w i t h i n a given a l t i t u d e increment a r e de r ived . The n a t u r e of t h e a n a l y s i s given i n r e fe rence 260 i s p ioneer ing , and much more work a long such l i n e s i s needed,

Reference 261 i s an i n t e r e s t i n g and d e t a i l e d t rea tment of t h e approach t o l and ing of a h e l i c o p t e r on the top of a t a l l b u i l d i n g . Very thorough measurements were made i n wind t u n n e l s t u d i e s t o e s t a b l i s h the wind p a t t e r n and turbulence that i s caused by t h e b u i l d i n g . An a n a l y t i c a l s tudy of t h e e f f e c t of wind p a t t e r n of a h e l i c o p t e r approaching the b u i l d i n g top was t h e n made.

Some of t h e s e

21

Reference 262 g ives an a n a l y t i c a l t rea tment of t h e s t a b i l i t y and response c h a r a c t e r i s t i c s of an STOL a i r c r a f t i n a gus t environment.

Wind tunne l p r o j e c t s . - It i s of i n t e r e s t t o note t h a t i n t h e United S t a t e s and Canada, many wind tunne l s have been con- ver ted t o s tudy wind flow over var ious o b s t a c l e s . Flows over c i t i e s , a c t u a l l y modeled on t h e wind tunne l f l o o r , 3x-e sf s p e c i a l i f i t e r z s t . Mucii effort has gone i n t o means f o r simu- l a t i n g t h e e a r t h ' s boundary l a y e r . The b a s i c problem i s t h e d i f f i c u l t y of s imultaneously s imula t ing t h e mean v e l o c i t y p r o f i l e and the turbulence c h a r a c t e r i s t i c s . References 263 through 266 a r e some r e p o r t s d e a l i n g with w i n d t u n n e l s h u l a t i o n . I n some u L U l ~ ~ ~ ~ b stuciies a r e be ing made of a n a i r p l a n e immersed i n a s imulated e a r t h ' s boundary l a y e r (both t h e wind p r o f i l e and tu rbu lence ) , i n a n a t tempt t o measure t h e f o r c e s and moments t h a t develop on t h e a i r p l a n e from the tu rbu lence . This p r o j e c t could be most s i g n i f i c a n t . type of wind s t u d i e s t h a t a r e a l s o made i n wind tunne l s .

+..-.- - 7 -

Reference 267 i s an example of t h e

SELECTED REFERENCES REPRESENTING OVERVIEW OF GROUND WINDS PROBLEMS

The long l i s t of r e f e r e n c e s given i n t h i s r e p o r t i s i n t h e n a t u r e of a shopping l i s t . A s tudy of a l l t h e r e p o r t s l i s t e d i s i n gene ra l not p r a c t i c a l . It w a s f e l t worthwhile, t h e r e f o r e , as an a i d t o the r eade r , t o s e l e c t a few r e f e r e n c e s which, i f read , would give a f a i r l y complete p i c t u r e of t h e va r ious f a c e t s of t h e su r face winds problem. This s e c t i o n i n d i c a t e s t h e s e r e fe rences .

General i n s i g h t t o a i r c r a f t problems. References 35 and 36

Measurement and a n a l y s i s of s u r f a c e winds and tu rbu lence . - Reference 223 i s a good example d e s c r i b i n g the measurements, t h e a n a l y s i s , and t h e r e s u l t s t h a t were obta ined i n a very ex tens ive t e s t program i n v e s t i g a t i n g s u r f a c e winds and g u s t s . References 152, 204 and 217 r e p r e s e n t good examples of r e p o r t s t h a t a r e t o be found wherein a t t e n t i o n i s focused mainly on t h e s tudy of var ious meteoro logica l parameters and l a w s which appear fundamental t o t h e a n a l y s i s of s u r f a c e winds and g u s t s .

Mathematical d e s c r i p t i o n s and models .- R e f e r a c e s 46, 234 and 235 p r e s e n t e x c e l l e n t d e s c r i p t i o n s of mathematical models t h a t a r e used t o c h a r a c t e r i z e suEface winds and g u s t s . Reference 225 develops e x t e n s i v e s p e c t r a l t u rbu lence models , while re ference 135 ana lyzes i n d e t a i l t h e d i f f e r e n c e s i n l o n g i t u d i n a l v e l o c i t y f l u e t u a t i o n s between two v e r t i c a l l y spaced po in t s . In t e rmi t t ency and t h e use of d i s c r e t e gusts models a re t reated i n r e f e r e n c e 226. References 114 and 229 a r e a l s o s i g n i f i c a n t wi th r e s p e c t t o model development. 22

F l i g h t and f ixed-based s imula tor s t u d i e s . - Random gus t s imula t ion and a n a l y s i s a r e t r e a t e d i n r e f e r e n c e 47. One of t h e b e s t and most ex tens ive model developments and s imula t ion e f f o r t s i s that covered i n r e f e r e n c e 234. Other s imula t ion s t u d i e s which inc lude turbulence as an i n p u t , i n var ious de- grees of de ta i l , a r e r ep resen ted by r e fe rences 240, 231, 242, 246, 250 and 251.

C a r r i e r approach.- References 253 and 254 r e p r e s e n t good examples i n d i c a t i n g t h e modeling and t rea tment of winds and g m t u u a i r - u approach t o c a r r i e r landings .

s e n t t r ea tmen t s of var ious o the r problems that a r e due t o s u r f a c e winds and g u s t s .

Other response s t u d i e s . - References 229, 260 and 261 r ep re -

THE ME'IZOROLOGIST AND THE AERONAUTICAL ENGIJXEER

I n gene ra l , t h e a e r o n a u t i c a l engineer o r s c i e n t i s t and t h e meteoro logis t have d i f f e r e n t p o i n t s of view towards the a n a l y s i s of s u r f a c e winds and gus t s . (Here, a e r o n a u t i c a l engineer r e f e r s mainly t o the s t r u c t u r a l dynamist, not n e c e s s a r i l y t o the f l u i d dynamist) of t h e s e d i f f e r e n c e s .

An a t tempt i s made i n t h i s s e c t i o n t o i d e n t i f y some

The a e r o n a u t i c a l engineer wants t h e models s p e l l e d out i n simple terms; f o r example, he wants t h e r m s gus t s e v e r i t y value, t h e i n t e g r a l s c a l e of tu rbulence , and the s p e c t r a l shape s p e c i f i e d . The meteoro logis t Speaks i n d i f f e r e n t terms. He i s concerned mainly w i t h energy budgets, and w i t h t h e sea rch f o r t h e b a s i c meteoro logica l parameters which cause d a t a t o condense t o a u n i v e r s a l form. H e i s accustomed t o such parameters as

he igh t mean wind v e l o c i t y f r i c t i o n v e l o c i t y (another way of des igna t ing equat ion (1) me an p o t en t i a l temp e r a t u r e f l u c t u a t i n g component of p o t e n t i a l temperature average p o t e n t i a l temperature f o r e n t i r e l a y e r von K&m& 1 s cons t an t -

W e a s c a l i n g temperature, - - u*

d i s s i p a t i o n r a t e

I n terms of t h e s e parameters t he meteoro logis t d e f i n e s c e r t a i n bas i c nondimensional terms :

f z - nz a dimensionless f requency U kz gT,

- _ - z - a dimensionless he ight - 2 L OU,

kzc % - - d a dimensionless d i s s i p a t i o n r a t e

- kz dU ' m - c z

kz dO

a dimensionless vel n c i t y g r a . d i ~ n t

a dimensionless temperature g r a d i e n t

When an a e r o n a u t i c a l engineer looks a t r e p o r t s by meteor- o l o g i s t s he i s a p t t o become confused. Often t h e , n o t a t i o n i s i n c o n s i s t e n t from one r e p o r t t o ano the r ; t he use of uT or u, f o r f r i c t i o n v e l o c i t y , f o r example. Even the b a s i c nondimensional parameters a r e de f ined i n a l t e r n a t i v e , bu t equiva- l e n t , forms. When t h e a e r o n a u t i c a l engineer s e e s the dimension-

i n t e g r a l s c a l e value of t h e tu rbu lence ; t o t h e me teo ro log i s t , L i s a b a s i c s t a b i l i t y parameter c a l l e d t h e Monin-Obukhov s t a b i l i t y s c a l e l e n g t h .

r e p r e s e n t a t i o n i n terms of @(LE) vs. C L , f i g u r e 5, where CT i s the r m s value of t u rbu lence i n t e n s i t y , w t h e

c i r c u l a r f requency, L t h e i n t e g r a l s c a l e , and V t h e speed of t he a i r c r a f t . The m e t e o r o l o g i s t ' s p r e s e n t a t i o n i s u s u a l l y as shown i n f i g u r e 6, f o r n e u t r a l s t a b i l i t y c o n d i t i o n s of t h e atmosphere. The f r i c t i o n v e l o c i t y u, i s used "as a more b a s i c parameter" i n p l a c e of au , 0 , or 0 i s used i n p l a c e of L , and t h e mean wind speed U i s used i n s t e a d of t h e a i r p l a n e speed V .

When t h e engineer t h i n k s f u r t h e r about u, he becomes f u r t h e r confused. He s e e s t h a t i t invo lves t h e covar iance value between u and w , y e t , because he i s accustomed t o t h i n k i n g i n terms of t he assumption of i s o t r o p y , he t h i n k s t h a t uw should be zero, not a b a s i c r e f e r e n c e va lue .

24

l e s s he igh t I; Z he may a t f i r s t t h i n k t h a t L r e p r e s e n t s an

The a e r o n a u t i c a l engineer u s u a l l y t h i n k s of s p e c t r a l

V ' w

t h e h e i g h t z w '

-

When the atmosphere i s uns t ab le , t h e m e t e ~ x o l o g i s t i n t r o - L x e S the Parameter @m , as shown i n f i g u r e 7, t o he lp c o l l a p s e t h e data; sometimes, i n s t ead , he in t roduces @ E . T-? others, and what i s the i r connect ion. Often the meteoro logis t in t roduces var ious frequency s c a l i n g parameters f o r t he a b s c i s s a

, where f, i s an e m p i r i c a l f a c t o r found t o s c a l e , such as - f m

make a chosen a n a l y t i c a l func t ion type f i t a given s e t of s p e c t r a l data.

The me teo ro log i s t s t u d i e s var ious c o r r e l a t i o n f u n c t i o n s such as the two-argument c o r r e l a t i o n f u n c t i o n shown i n f i g u r e 8; T i s t h e c o r r e l a t i o n time l a g , while 4 r e p r e s e n t s t h e s p a t i a l d i s t a n c e between two measuring p o i n t s which, i n t h i s case , f a l l e s s e n t i a l l y a long t h e d i r e c t i o n of the mean wind flow. The c o r r e l a t i o n contours obtained, resembling e l l i p s e s , a r e some- t imes r e f e r r e d t o as i s o p l e t h s . The alignment of the l i n e 4 = UT a long the main a x i s of these " e l l i p s e s " i s takeri as a measure of t h e v a l i d i t y of T a y l o r ' s hypothes is , o r t he f rozen gus t f i e l d concept.

I engineer wonders why i s used i n some i n s t a n c e s , @E i n

f

I n gene ra l , t h e c o r r e l a t i o n f u n c t i o n s s t u d i e d by the me teo ro log i s t a r e no t t h e f u n c t i o n s of concern t o the aero- n a u t i c a l engineer . A s an example, the engineer wants t o know t h e c o r r e l a t i o n between the v e r t i c a l gus t v e l o c i t i e s a t t w o d i f f e r e n t spanwise s t a t i o n s ; from these f u n c t i o n s he can deduce the r o l l i n g moment that i s generated on t h e a i r c r a f t by the g u s t s . Likewise, he wishes t o know whether t h e c o r r e l a t i o n s between u , v , and w a t d i f f e r e n t p o i n t s of t h e a i r p l a n e a r e s i g n i f i c a n t from a response p o i n t of view. But i f he does no t have these f u n c t i o n s he cannot eva lua te t h e i r importance. The fo l lowing example i s given t o i l l u s t r a t e how c r o s s - c o r r e l a t i o n e f f e c t s e n t e r i n t o a i r p l a n e response cons ide ra t ions , Consider an a i r c r a f t t o be a c t e d upon by three d i s t i n c t random i n p u t s w 1 Y w2 Y and v1 , as depic ted i n t h e fo l lowing sketch.

The value of any response q u a n t i t y , such as r o l l i n g motion, may then be w r i t t e n as

t t r r

- w - w

where hl i s the response due t o a u n i t impulse i n p u t for wi , and s i m i l a r l y f o r h, and E, . iiie Four i e r i n t e g r a l t ransform sf ti-lis equat ion i s

rnl

c I

F = F H 1 + F H 2 + F Y w1 w2

where H1 H2 , and G1 r e p r e s e n t t he frequency response func t ions for y f o r s i n u s o i d a l i n p u t s a t w1 w2 and v The spectrum f o r y fo l lows i n t u r n as

1 '

+ o v GlG1 1

It i s noted that the response depends not only on the s p e c t r a f o r w 1 Y w 2 and v1 , but on the c r o s s spectrum QWlw2 be - tween w and w2 and the c r o s s s p e c t r a and

between w and v1 and between w and v1 as w e l l . The c r o s s spec rum between the w1 and w components i s known t o be s i g n i f i c a n t t o the response. The c r o s s spectrum between w1

( i s o t r o p y ) ; i t i s not known, however, whether these c r o s s s p e c t r a can be ignored f o r an a i r c r a f t immersed i n s u r f a c e l a y e r tu rbulence . S t u d i e s t o e v a l u a t e t h e i r importances a r e t h e r e f o r e needed. I f t he component u had been in t roduced also, t he response s p e c t r a would appear even more complicated; c r o s s s p e c t r a between u and v and between u and w would appear i n a d d i t i o n , bu t aga in t h e i r importance would no t be known. Cer t a in c r o s s c o r r e l a t i o n s are known t o be of s i g n i f i - cance i n t h e formation of t h e boundary l a y e r - t h e vw c o r r e - l a t i o n appears important i n the Ekman l a y e r , f o r example - bu t t hese c ros s c o r r e l a t i o n s may not be s i g n i f i c a n t i n de te rmining a i r c r a f t response. It could be t h a t t h e u and w c o r r e l a t i o n 26

1

2 2 i

and v1 , normally has been taken as zero and v1 ' Or w?

i s s i g n i f i c a n t f o r response cons ide ra t ions , s i n c e the uw c o r r e l a t i o n i s s t r o n g i n the boundary l a y e r , bu t tha t o the r c o r r e l a t i o n s a r e of n e g l i g i b l e importance.

needed between the meteoro logis t and t h e a e r o n a u t i c a l engineer . Each must l e a r n the language of the o t h e r b e t t e r . o l o g i s t must become b e t t e r acquainted w i t h the needs of the engineer , and s t e p s must be taken t o ensure t h a t measurements taken a r e t h e type t h a t can be used i n t h e s tudy of a i r c r a f t des ign and o p e r a t i o n a l problems.

It may be s a i d i n genera l that a b e t t e r communication i s

The meteor-

SUGGESTED FORM OF MODELS

It i s not t h e i n t e n t of t h i s paper t o analyze var ious models or develop them f u r t h e r . It i s f e l t app ropr i a t e , however , t o s t a t e no t ions on the form t h a t newer models should t ake . Hopefully, t hese comments w i l l h e lp make c l e a r e r where a r e a s of weakness l i e , and where f u t u r e e f f o r t s should be d i r e c t e d i n t h e s tudy of su r face winds and g u s t s . A t t e n t i o n i s given mostly t o t h e turbulence models, s i n c e they appear t o be i n t h e l ess developed s t a t e . Some comments a r e o f f e red , how- eve r , t o t h e wind models by way of emphasizing comments made i n p rev ious s e c t i o n s of t h i s r e p o r t .

The wind models descr ibed i n r e f e r e n c e s 46 and 235 a r e considered good. A t t e n t i o n should not concen t r a t e o r be r e - s t r i c t e d t o t h e mean wind p r o f i l e s ; s p e c i f i c a l l y , more work i s needed t o develop t h e unusual and severe p r o f i l e s t h a t occur f r o m time t o t ime - those p r o f i l e s which have a "nonc la s s i ca l " behavior of wind speed with he ight . Involved a r e such a s p e c t s as d i u r n a l v a r i a t i o n s , s t a b i l i t y cons ide ra t ions , occurrence of a two-layer s t r u c t u r e , condi t ions w i t h low c e i l i n g s , gust f a c t o r v a r i a b i l i t y . Reference 114 i s a good r e p o r t t o read i n t h i s connec t ion . With r e s p e c t t o p r o f i l e s i n gene ra l , whether mean or odd i n shape, more work of t h e type i n d i c a t e d i n r e f e r e n c e 260 on wind shear i s needed. For c r o s s winds, a b e t t e r unders tanding i s needed of t h e i r v a r i a b i l i t y and, i n p a r t i c u l a r , what d i s c r e t e d is turbances a r e introduced i n t h e i r make-up by o b s t r u c t i o n s such as b u i l d i n g s , h i l l s , o r t r e e s .

The remainder of t h i s s e c t i o n touches upon su r face turbu- l e n c e . 268 and 269 should a l s o be s tudied , s i n c e the concepts s t u d i e d i n t h e s e r e f e r e n c e s a r e d i r e c t l y r e l a t e d t o t h e no t ions s e t f o r t h he re . Reference 23 cons iders t h e 3-dimensional gus t en- coun te r problem, and e v a l u a t e s t h e gus t components and, i n t u r n , t h e g u s t f o r c e s and moments t h a t a r e s i g n i f i c a n t from a response p o i n t of view f o r convent iona l a i r c r a f t . g iven i n t h i s r e p o r t i s needed f o r va r ious STOL conf igu ra t ions ,

27

I n r ead ing through these comments, r e f e r e n c e s 23, 24,

Work a long 'Le l i n e

s i n c e t h e mechanism f o r f o r c e and moment gene ra t ion due t o g u s t s may be d i f f e r e n t f o r t hese conf igu ra t ions than f o r convent iona l conf igu ra t ions . func t ions of t he type t h a t a r e needed i n response cons ide ra t ions . Reference 24 de r ives the gene ra l technique f o r handl ing m u l t i p l e random i n p u t s , which i s t h e c h a r a c t e r i s t i c s i t u a t i o n f o r a i r - c r a f t i n t he su r face l a y e r . Reference 269 demonstrates t he a p p l i c a t i o n of c r o s s s p e c t r a by cons ide r tng a sne-degree-of- freedom roll respcnse case.

Reference 268 develops c e r t a i n c r o s s - s p e c t r a l

It i s t h e b e l i e f of t he au tho r t h a t t h e major reasons f o r f a i l i n g t o achieve realism i n many s imula to r s t u d i e s are as fo l lows :

1) Use of excess ive gus t s e v e r i t y values ( t h e use of r m s values of around 9 f p s n e a r l y always l e d t o "un- r e a l i s t i c " response behavior ; the use of t h e more appropr i a t e values around 3 f p s gave a more r e a l i s t i c f e e l ) .

2 ) Use of excess ive i n t e g r a l s c a l e va lues ( t h e use of s c a l e values of around 2500 f t gave u n r e a l i s t i c r e s u l t s , as with the h igh s e v e r i t y va lues ; t h e use of s c a l e va lues of only s e v e r a l hundred f e e t , as i s more a p p r o p r i a t e , gave a much b e t t e r response i n t e r p r e t a t i o n ) .

t h e g u s t s ( f o r c e s and moments) were not used.

Many f e e l t h a t t h e ques t ion of i n t e r m i t t e n c y or n o n s t a t i o n a r i t y has a l o t t o do with achiev ing r ea l i sm. It i s f e l t , however, t h a t i f the t h r e e i tems l i s t e d had been handled more r e a l i s t i - tally, then n o n s t a t i o n a r i t y a s p e c t s may not be impor tan t .

With t h e s e comments i n mind, and wi th r e f e r e n c e s 46, 47, 223, 234 and 235 as a guide, t he fo l lowing turbulence model i s suggested. I f a n a l y t i c a l s t u d i e s a r e be ing made, use t h e von K&rmdn s p e c t r a l func t ions , equa t ions ( 4 ) , (5) and (6). If i n - f l i g h t s imula t ion or f ixed-based s i m u l a t o r s are be ing used, use t h e Dryden equa t ions , simply because s p e c t r a l shaping i s accomplished e a s i e r . For the tu rbu lence s e v e r i t y va lues use

3) I n p a r t i c u l a r , t he a p p r o p r i a t e f o r c i n g i n p u t s due t o

= 1.7 m/sec 5.8 f t / s e c OU

ov = 1.4 4.6

ow = 0.9 3

w i t h v a r i a t i o n s from one-half t o double t h e s e va lues . For i n t e g r a l s c a l e use

up t o 3GO meters a l t i t u d e ; use 250 m (800 f t ) a t h ighe r a l t i - tudes .

Force and moment s p e c t r a due t o t h e turbulence i n p u t s should be e s t a b l i s h e d i n accordance with the conf igu ra t ion . I n genera l , t he fo l lowing f o r c e and moment s p e c t r a w i l l dominate, f i g u r e 9.

1) V e r t i c a l f o r c e spectrum due t o w 2) P i t c h i n g moment spectrum due t o w 3) Yawing moment spectrum due t o v 4) Ro l l ing moment spectrum due t o spanwise

v a r i a t i o n i n w

For some conf igu ra t ions , t h e r o l l i n g moment due t o a spanwise v a r i a t i o n i n u may be s i g n i f i c a n t . On t h e basis of t hese i n p u t f o r c e and moment s p e c t r a , input gust gene ra to r s may be der ived f o r use i n s imula t ion . Each inpu t i s handled by an independent gust genera tor , with the shaping f i l t e r s be ing chosen such t h a t t h e output d u p l i c a t e s or approximates the s p e c t r a l na tu re of t he input be ing r ep resen ted .

It i s be l i eved t h a t i f t h i s model approach i s used, c l u s i o n of t h e n o n s t a t i o n a r i t y na tu re of t h e g u s t s may not be s i g n i f i c a n t . Nons ta t iona r i ty may be in t roduced , however, i f judged l ack ing , e i t h e r by simply switching the gene ra to r s on and of f i n random fash ion , or by randomly varying t h e ga in of t he gene ra to r s .

Some comments about a p o s s i b l e , or i n t e r i m , gene ra l i zed c o r r e l a t i o n d i s t a n c e for p o s s i b l e use i n d e r i v i n g a gene ra l gus t covar iance t enso r a r e given t o c lose out t h i s s e c t i o n . t r ea tmen t o r d e r i v a t i o n of t h i s i n t e r i m gene ra l i zed c o r r e l a t i o n d i s t a n c e i s not w i t h i n t h e scope of t h i s e f f o r t , bu t t h e ad- vancement of c e r t a i n not ions r e l a t i v e t o such a f u n c t i o n may be of i n t e r e s t . There a r e two p r i n c i p a l reasons o r mot iva t ions f o r d e r i v i n g a b a s i c c o r r e l a t i o n d i s t a n c e measure:

t h e i n -

The

1)

2)

To a l low t h e d e r i v a t i o n of a p p r o p r i a t e c r o s s - s p e c t r a l f u n c t i o n s (even though they may be approximations) .

To be a b l e t o t r e a t the case where the f l i g h t p a t h of t h e a i r c r a f t i s a t an angle t o t h e wind d i r e c t i o n .

With r e fe rence t o f i g u r e 10 , and without g iv ing t h e d e r i v a t i o n , o r t he l o g i c behind i t s meaning, t he fo l lowing i s sugges ted as a b a s i c c o r r e l a t i o n d i s t a n c e .

2 - x ) + y2 + 7 _1

which may be r e w r i t t e n

i n which

, ;;,Lcly, -h,.--- as i n d i c a t e d i n f i g u r e 10, U i s t h e wind speed, V t h e speed of t he a i r c r a f t r e l a t i v e t o the ground, 3 t h e ang le between the p o s i t i v e U and V d i r e c t i o n s ; U i s seen t o be t h e r e l a t i v e v e l o c i t y between the a i r c r a f t and ?he wind. Suppose t ha t i t i s d e s i r e d t o e s t a b l i s h t h e time c r o s s - c o r r e l a t i o n f u n c t i o n between, f o r example, the v e r t i c a l gus t v e l o c i t i e s a t p o i n t 2 and a t p o i n t 1, where x i s the sepa- r a t i o n d i s t ance i n t h e f l i g h t d i r e c t i o n and y i s the sepa- r a t i o n d i s t ance normal t o t h i s d i r e c t i o n . t h a t c r o s s - c o r r e l a t i o n f u n c t i o n and i n t u r n the c r o s s spectrum.

The fo l lowing touches upon the gene ra l way t o u t i l i z e On the assumption that i s o t r o p y i s p r e s e n t , i t can be shown that the c r o s s - c o r r e l a t i o n f u n c t i o n f o r w between p o i n t s 2 and 1 i s given by the p o i n t c o r r e l a t i o n f u n c t i o n g ( r )

/ g(r) = (1 - &)e terms of Bessel f u n c t i o n s , r e f e r e n c e 20. equat ion (12), i s s u b s t i t u t e d i n i n t e g r a l t ransform taken, t hen a cross-spectrum f u n c t i o n for w between the two p o i n t s w i l l r e s u l t . For the ua components, t h e turbulence components i n t h e d i r e c t i o n of t h e r e l a t i v e wind U a , i t may be shown, aga in on the assumption of i s o t r o p y , t h a t t h e c r o s s - c o r r e l a t i o n f u n c t i o n i s given by

It i s p o s t u l a t e d (12), can be used t o d e r i v e the r , given by equat ion

r .

g ( r ) func t ion by the Dryden formula t ion , f o r example, i s

The

- L ; f o r t he von K&m& form, g i s g iven i n

If r , given by g ( r ) , and t h e F o u r i e r

'('1 = f ( r > + y;l 2 r af

where f i s the p o i n t c o r r e l a t i o n f o r u , given, f o r example,

as f (r) = e f o r t h e Dryden model. The F o u r i e r i n t e g r a l t ransform of R , w i t h r given as by equa t ion (12), t hen g i v e s a cross-spectrum f u n c t i o n f o r t h e ua components. The c r o s s - s p e c t r a l func t ions genera ted i n t h i s way can be used t o e s - t a b l i s h the s p e c t r a l f u n c t i o n s f o r the g u s t f o r c i n g moments on t h e a i r c r a f t .

r - -

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It should be observed tha t equat ion (12) takes i n t o account convect ive e f f e c t s , s p a t i a l s epa ra t ion e f f e c t s , and t h e time- varying na tu re of tu rbulence . With r e f e r e n c e t o equat ion (ll), if y i s zero, then the use of r would lead t o two-function c o r r e l a t i o n f u n c t i o n s very similar t o that shown i n f i g u r e 8. If t h e a i r c r a f t i s d r i f t i n g w i t h the wind, 0 , ' a n d x = 0 , y = 0 , then r reduces simply t o the ?Ut "%?:my which i s a s s o c i a t e d wi th she time-varying proper ty of t h e turb6lence (a long t h e 4; = U t l i n e i n f i g u r e 8 ) . Values of '1 a r e not known, bu t an e s t ima te given he re i s 7 == 0.4 , When V becomes l a r g e r e l a t i v e t o the wind, the na tu re of r complies wi th the f rozen gus t f i e l d concept.

PROBABILITY CONSIDERATIONS I N MODELS

A p o i n t of i n t e r e s t noted during t h e course of t he survey w a s t h e f a c t t h a t t h e r e w a s v i r t u a l l y no rrention of p r o b a b i l i t y a s p e c t s of s u r f a c e wind and gus t encounter. T h i s f a c t i s con- s ide red s i g n i f i c a n t and m e r i t s s epa ra t e d i scuss ion . Mention w a s f r e q u e n t l y made of design wind and gust values , o r of t h e l a c k of knowledge of what t h e values should be, b u t d i scuss ion of t h e va lues i n terms of p r o b a b i l i t y cons ide ra t ions w a s no tab ly absent .

numbers i s probably due t o t h e f a c t t h a t most of t h e i n d i v i d u a l s con tac t ed r ep resen ted the ae ronau t i c s community. T r a d i t i o n a l l y , a i r c r a f t s t r u c t u r a l design people have worked i n terms of s p e c i f i e d des ign values , bu t r a r e l y do they dea l w i t h a s s o c i a t e d p r o b a b i l i t y cons ide ra t ions . A few examples may be given by way of i l l u s t r a t i o n . A s mentioned e a r l i e r i n t h i s r e p o r t , a i r c r a f t ca rgo doors are designed t o operate i n winds up t o 65 mph; t h e p r o b a b i l i t y of encounter ing such winds i s no t considered. For y e a r s , a i r c r a f t design f o r gust encounter has been based upon a s p e c i f i e d gust shape and a s p e c i f i e d maximum gus t v e l o c i t y ( u s u a l l y 50 f p s ) . The p r o b a b i l i t y of encounter ing t h i s des ign v e l o c i t y i s n o t known, nor considered. For commercial type a i r c r a f t , l and ing gears a r e designed and t e s t e d t o . w i t h s t a n d a l a n d i n g v e r t i c a l impact speed of 10 f p s ; f o r c a r r i e r opera t ion , a des ign v e r t i c a l v e l o c i t y has been 25 f p s , I n some i n s t a n c e s , des ign va lues t h a t have appeared s u i t a b l e f o r yea r s may become inadequate because newer conf igu ra t i cns are in t roduced or because increased performance i s demanded. Inadequacy i s judged on t h e basis t h a t t h e number of o p e r a t i o n a l f a i l u r e s have become "excessive;" t h e r e u s u a l l y i s no hard and f i r m r u l e , however, as t o what i s meant by "excessive." I f design values are found t o be inadequate , they a r e graded upward. An example case i s t h a t of c a r r i e r landing ope ra t ions . For c e r t a i n con- f i g u r a t i o n s , t h e design s inking speed of 25 f p s was found i n - adequate; a new design value of 31 f p s w a s t h e r e f o r e in t roduced . (Usual ly , a number of ope ra t iona l t e s t s and measurements a r e made and analyzed as a he lp i n e s t a b l i s h i n g how much of an i n - c r e a s e should be made.)

A primary reason for the l a c k of r e fe rence t o p r o b a b i l i t y

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The f a c t t h a t many a i r c r a f t des ign procedures a r e s t a t e d simply i n terms of s p e c i f i e d numbers does not n e c e s s a r i l y mean that no cons ide ra t ion w a s given t o t h e s t a t i s t i c s of t he des ign problem. Often a l a r g e body of s t a t i s t i c s on t h e problem are gathered. These data are analyzed and from t h e a n a l y s i s a de- s i g n value i s s e l e c t e d . The rea f t e r , t h e s t a t i s t i c s of the problem are s e t a s i d e . The p r o b a b i l i t y of encounter ing t h e design value i s not cons idered . I n some i n s t a n c e s , c e r t a i n of t h e s t a t i s t i c s a r e ussd f o r des ign purposes . I n t h e more r-ecent t r ea tmen t s on gus t encounter , f o r example, s p e c i f i c con- s i d e r a t i o n i s given t o load exceedance curves , which i n d i c a t e theanumber of t imes pe r second, on t h e average, t h a t given l o a d l e v e l s are reached. I n h e r e n t l y cont8ined I n Lhe curves i s the p ropor t ion of' timc tiubu12nce i s encountered on the average, and the o v e r a l l r m s value of t he turbulence s e v e r i t y .

It i s t o be noted t h a t some i n v e s t i g a t i o n s dea l s p e c i f i - c a l l y wi th p r o b a b i l i t y c o n s i d e r a t i o n s . References 21 and 260 a r e examples. Reference 21 con ta ins a fundamental d e r i v a t i o n which shows how t h e gus t l oad exceedance curves may be used t o e s t a b l i s h t h e p r o b a b i l i t y t h a t a given l o a d w i l l be encountered i n a s p e c i f i e d f l i g h t t ime. Reference 260 ana lyzes the proba- b i l i t y of encounter ing given wind shear values i n the l a n d i n g approach. There appears t o be no a p p l i c a t i o n of such ana lyses , however, f o r a i r c r a f t des ign purposes .

By c o n t r a s t , launch vehic le and space f l i g h t technology has developed wi th a s t r o n g a t t e n t i o n be ing given t o p r o b a b i l i t y a s p e c t s . Design phi losophy depends on the miss ion under con- s i d e r a t i o n , on the component of t he system tha t i s be ing s t u d i e d , and p a r t i c u l a r l y on t h e phase of t h e development cyc le that i s on hand. Sometimes, a c e r t a i n o v e r a l l veh ic l e performance c a p a b i l i t y i n terms of p r o b a b i l i t y may be mentioned as a guide l ine , b u t i t i s r e a l i z e d t h a t because of the many unknowns i n t h e veh ic l e c h a r a c t e r i s t i c s and des ign c r i t e r i a , i t i s no t r e a l i s t i c t o expect a des ign t o be developed t h a t w i l l p r e c i s e l y meet t he s p e c i f i e d performance c a p a b i l i t y . On the o t h e r hand, t h e r e a re s p e c i f i c ques t ions on p r o b a b i l i t y a s p e c t s t h a t a r e of primary concern i n the design, mission p lanning , and o p e r a t i o n of space v e h i c l e s . I n t h e d i scuss ion on s u r f a c e winds i n r e fe rence 46, f o r example, such q u e s t i o n s as the fo l lowing a r e brought out and d i scussed .

1) How probable i s i t tha t t h e peak s u r f a c e wind a t some s p e c i f i e d r e fe rence he igh t w i l l exceed (or no t exceed) a given magnitude i n some s p e c i f i e d time p e r i o d ?

2) Given a des ign wind p r o f i l e i n terms of peak wind speed versus height from 10 t o 150 mete r s , how probable i s i t t h a t t h e design wind p r o f i l e w i l l be exceeded i n some s p e c i f i e d time per iod?

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t

Much informat ion i s given i n r e f e r e n c e 46 on the f i r s t ques t ion a t s e l e c t e d geographic l o c a t i o n s . The second ques t ion , be ing more complex, i s t r e a t e d i n a l i m i t e d way. Reference 46 a l s o d i scusses and p r e s e n t s much informat ion on the concept of i nc lud ing exposure pe r iod p r o b a b i l i t i e s i n t h e design and ope ra t ion of space veh ic l e s . A s a n example, i f an ope ra t ion r e q u i r e s one hour t o complete, and i f the c r i t i c a l wind loads on t h e space veh ic l e can be defined i n terms of t he peak wind speed, then i t i s t h e p r o b a b i l i t y of occurrence of the peak speed du r ing a 1-hour p e r i o d that g ives a measure of the probable r i s k of t h e occurrence of s t r u c t u r a l f a i l u r e .

philosophy used f o r t he des ign of a i r c r a f t t o winds appears d i f f e r e n t than tha t used f o r space veh ic l e s . It would seem d e s i r a b l e t o b r i n g the t w o design communities t oge the r t o exchange viewpoints, and t o b r i n g ou t t h e m e r i t s of each i n d i v i d u a l design approach. S p e c i f i c cons ide ra t ion should be given t o t h e form t h a t design models f o r winds and g u s t s should take , p a r t i c u l a r l y wi th r e s p e c t t o t h e degree t h a t p r o b a b i l i t y a s p e c t s are included ,

The main purpose of this s e c t i o n i s t o p o i n t out that the

CONCLUSIONS AND RECOMMENDATIONS

Some of t h e main conclusions t h a t may be drawn from the survey are the fol lowing:

1, A s a n t i c i p a t e d , environmental su r f ace winds and g u s t s c r e a t e many des ign and ope ra t iona l problems f o r a l l a i r c r a f t . The l i s t i n g i n t h e s e c t i o n e n t i t l e d "Ci ted Problems" se rves as a convenient reminder t o t h e s e problems.

2. The problem of main ta in ing c o n t r o l of t h e a i r c r a f t , p a r t i c u l a r l y dur ing hover, t ake-of f , approach and l and ing , i s probably t h e most c r i t i c a l design and o p e r a t i o n a l problem t h a t i s due t o s u r f a c e winds and gus t s .

3. VTOL and STOL a i r c r a f t are most vu lnerable ; s t a b i l i t y and c o n t r o l and handl ing q u a l i t i e s , e s p e c i a l l y as aggravated by s u r f a c e winds and g u s t s , a r e the key f a c t o r s of whether a STOL system i s f e a s i b l e o r no t ; o p e r a t i o n a l problems w i l l i n - c r e a s e i n number i n t h e f u t u r e .

4. A s w i t h VSTOL conf igu ra t ions , wind shear and c ross - winds e f f e c t s must be s t u d i e d c a r e f u l l y f o r e x o t i c s i n g u l a r - type c o n f i g u r a t i o n s as the Space S h u t t l e and remotely p i l o t e d v e h i c l e s (RPV); automatic approach systems i n the presence of winds and g u s t s need much study.

5. R e l a t i v e l y l i t t l e i s known about t he unusual wind p r o f i l e s o r wind shear p a t t e r n s - t h e i r cause, t h e i r d e t e c t i o n , and t h e a b i l i t y t o cope w i t h them.

33

6. Likewise, unusual cross-wind s i t u a t i o n s , e s p e c i a l l y those having d i s c r e t e and severe d i s tu rbances as a r e s u l t of obs t ac l e i n t e r f e r e n c e , are l i t t l e understood.

7. B e t t e r s enso r s and cues or d i s p l a y s t o t h e p i l o t a r e needed; i t i s ve ry d i f f i c u l t f o r him to d e t e c t what t h e winds and gus t s a r e doing, and what h i s a i r c r a f t motions are.

8. A b e t t e r l i a i s o n between aeronaut ica l - design a n d ope ra t iona l people and me teo ro log i s t s i s needed.

9. There i s a need f o r s t a n d a r d i z a t i o n i n measurement of winds and g u s t s , e s p e c i a l l y i n t h e measurement and r e p o r t i n g of winds a t a i r p o r t s .

10 . The c r o s s - c o r r e l a t i o n measurements made i n s u r f a c e gus t s t u d i e s a r e u s u a l l y not a p p r o p r i a t e f o r a i r c r a f t response s t u d i e s ; t h e r e i s a c r i t i c a l need to measure the s p a t i a l c o r r e l a t i o n f u n c t i o n s t h a t are of concern to a i r p l a n e c o n t r o l , such as the spanwise v a r i a t i o n i n t h e u and w components. The s i g n i f i c a n c e of t h e c o r r e l a t i o n between t h e u , v , and w components must a l s o be eva lua ted .

11. C e r t a i n wind and turbulence models e x i s t , bu t on t h e whole they a r e not adequate; t he a b i l i t y to develop gus t i npu t moments i s a key i n g r e d i e n t missing; much e f f o r t should be expended to develop appropr i a t e models f o r use i n s imula t ion and design s t u d i e s .

1 2 . There i s a n u n c e r t a i n t y w i t h r e s p e c t t o how s p e c i f i c p r o b a b i l i t y cons ide ra t ions should be included i n des ign models f o r w i n d s and g u s t s . A i r c r a f t des ign procedures a r e based p r imar i ly on s t i p u l a t e d va lues of wind o r gus t s e v e r i t y ; by c o n t r a s t , space veh ic l e des ign o f t e n g ives e x p l i c i t cons ider - a t i o n t o the p r o b a b i l i t y of an event occur r ing i n a s p e c i f i e d t i m e .

13. Although c e r t a i n models e x i s t , only a r e l a t i v e l y few i n d i v i d u a l s are aware of them.

14. Gust i n p u t s are e s s e n t i a l i n s imula t ion s t u d i e s , f i x e d - based or i n - f l i g h t s imula t ion ; t echn iques f o r i n c l u d i n g g u s t s a r e i n need of much improvement.

15. It i s not known whether a w e l l developed model w i l l s e rve a l l a i r c r a f t c o n f i g u r a t i o n s , or whether models should be " t a i l o r e d " t o the type of c o n f i g u r a t i o n .

These 15 i tems se rve as some of t h e impor tan t conc lus ions t h a t can be made from t h e survey. The recommendation fo l lows t h a t these i tems need s tudy and development. P r i o r i t y i tems a r e : (1) measurement and a n a l y s i s of s p a t i a l c o r r e l a t i o n e f f e c t s p e r t i n e n t to a i r p l a n e ope ra t ion , ( 2 ) s tudy of unusua l and severe

34

wind shear and cross-wind condi t ions , and (3) development of b e t t e r models, w i t h b e t t e r disseminat ion.

The fo l lowing a d d i t i o n a l s p e c i f i c recommendations a r e made:

1. Improved a n a l y t i c a l means f o r s tudying the dynamic

2. Study t h e importance of the u , v , and w components

behavior of t h e atmosphere a t low a l t i t u d e a r e needed.

on d i f f e r e n t type conf igu ra t ions ( f o r example, u may be important t o some, b u t no t t o o t h e r s ) .

3. A n a l y t i c a l l y develop key c r o s s - s p e c t r a l f u n c t i o n s on the basis of in format ion known today and on p l a u s i b l e assumptions.

4. With ( 2 ) , develop the fo rce and moment s p e c t r a tha t a r e a p p l i c a b l e t o var ious conf igu ra t ions , f o r use i n s imula t ion and a n a l y t i c a l response s t u d i e s ; a t t h e same time, develop experimental procedures t o measure t h e s e f o r c e s and moments.

Develop procedures f o r i n v e s t i g a t i n g l and ing d i s - p e r s i o n s or o t h e r f a c t o r s of concern o p e r a t i o n a l l y , due t o wind shear and c r o s s winds.

5.

6. Study and develop b e t t e r models (poss ib ly a long t h e l i n e s advanced h e r e i n ) .

P repa ra t ion of a handbook on su r face winds and gus t s should be considered, which would serve t h e me teo ro log i s t and the a e r o n a u t i c a l engineer on an equal b a s i s , and he lp c l o s e the l a n s a g e gap between them.

on su r face winds and gus t s . ge the r the me teo ro log i s t and the a e r o n a u t i c a l design communi- t i e s , a l low f o r an exchange of not ions , show where developments s t a n d today, p o i n t ou t t h e need f o r b e t t e r models and, most impor tan t ly , would make a much l a r g e r number of i n d i v i d u a l s aware of t he su r face wind and gust problems. If success fu l , which seems c e r t a i n , schedul ing of such conferences on a regular basis, such as every th ree years , should be made.

7 .

8. F i n a l l y , i t seems h i g h l y d e s i r a b l e t o have a conference This conference would b r i n g t o -

35

APPENDIX

LIST OF ORGANIZATIONS AND INDIVIDUALS CONTACTED

Many groups a r e known t o e x i s t which could have provided an inpu t t o t h i s survey. ma jo r i ty of t h e s e groups due t o lack of t ime, and t h u s a sampling, hopefu l ly r e p r e s e n t a t i v e , w a s s e l e c t e d . z a t i o n s and some of t h e i n d i v i d u a l s t h a t were contac ted a r e i n d i c a t e d i n t h i s appendix.

A i r Force Cambridge Research Labora to r i e s Hanscom F i e l d , Mass.

Rene V. Cormier Donald D . Grantham Duane Haugen Norman Sissenwine Paul Tatt leman

It w a s not poss ib l e t o con tac t t h e

The organi-

1.

2. B a t t e l l e Memorial I n s t i t u t e (N .W. ) Richland, Washington

A . G. Dunbar G. E . E lde rk in T. W . Horst D. C . Powell J. V. Ramsdell

3. B e l l Aerospace Buffalo, N.Y.

Desmond E a r l Dav id Grupe Rob e r t Kaiser Nei l S u l l i v a n

F o r t Worth, Texas Meijer Drees Troy Gaffey Leo Kingston L. Rohrbough Dora S t r o t h e r

4. The Boeing Company S e a t t l e , Washington

James F u l l e r John Rogers Dwight R . Schaef fer Howard W . Smith Leroy Topp

37

1

5.

6.

7 .

8.

9.

10.

11.

12.

Colorado S t a t e Univers i ty F o r t C o l l i n s , Colorado

P ro f . J. E . Cermak

Corne l l Aeronaut ica l Laboratory, Inc . Buffa lo , N . Y .

Warren H a l l

Vic tor Lebacqz Roger Smith

Da:i.~id I(py

de Havi l land A i r c r a f t of Canada, L t d . TCrCEtc!, Czm2dZ

Richard Batch Jack P. Uffen

Fede ra l Av ia t ion Agency Washington, D . C .

Edmund Bromley, Jr. Arthur Hilsenrod Kenneth Kraus

General Dynamics Corp. F o r t Worth, Texas

R . L . Haller R . P. Peloubet

Ling-Tempco-Vought Dallas, Texas

B . J . Brock Lee Head Robert E . Ros t ine Mack S h i e l d s

Lockheed-California Company Burbank, C a l i f o r n i a

E d w a r d Ashburn Fred Hob l i t Maurice D. Lamoru John Lewolt Alfred P o t t h a r t Roger H . Shaar

Massachusetts I n s t i t u t e of Technology Cambridge, Mass.

Prof . Rene M i l l e r

13

14.

15 9

16.

17

18.

Nat iona l Aeronaut ics and Space Adminis t ra t ion Ames Research Center, Moffet t F i e l d , C a l i f .

S e t h Anderson Jack F r a n k l i n Ron G e r d e s Richard Kurkowski Maurry White

Jack R e e d e r Langley Research Center, Hampton, Va.

Manned Spacec ra f t Center , Houston, Texas Jake Kl ina r Bernie Marcantel Rudolph L. Saldana

Marshal l Space F l i g h t Center , A l a . George H. F i c h t l W i l l i a m W . Vaughan

Nat iona l Severe Storms Laboratory Norman, Oklahoma

Edwin Kessler Jean Lee

Pennsylvania S t a t e Univers i ty S t a t e Col lege, Pa.

S t a n H i l l a r d P ro f . J . L. Lumley P ro f . M. M. Sevick

Ryan Aeronaut ica l Company San Diego, C a l i f .

W i l l i a m Anderson W . Be r t Davis H. B. S ta rkey

Systems Technology, Inc. Hawthorne, C a l i f .

I r v i n g L. Askenas Duane T . McRuer

Un ive r s i ty of Toronto Toronto, Canada

Prof . B. E t k i n H . W. Teunissen

39

19. University of Wisconsin Madison, Wisconsin

P r o f . H. Lettau

20. Wright-Patterson A i r Force Base, Ohio Evard H. Flinn

40

REFERENCES

Group i n g of R e f e r e ne e s

p o s s i b l e

Ref 5 .

1-25

26-27

28-34

35-36

37-45 46-61 62-67

68-85

86-221

222-238

2 39- 2 52 253-260

261-263 264-268

269-270

I n a rough sense, the r e fe rences given h e r e i n a r e grouped

a i d t o the r eade r i n surveying the r e fe rence l i s t . i n var ious c a t e g o r i e s . This grouping i s i n d i c a t e d here as a

General gus t s t u d i e s , mainly f o r h igher a l t i t u d e s

Wind l o a d s on launch veh ic l e s

Wind l o a d s on s t r u c t u r e s

General no t ions on the a i r c r a f t o p e r a t i o n a l problems i n su r face winds and gus t s

Assorted s t u d i e s of w i n d and gus t problems

S p e c i f i c a t i o n s , c r i t e r i a , and g u i d e l i n e s

Books

Sensors and measurement techniques

Data a c q u i s i t i o n and a n a l y s i s , s t a t i s t i c a l i n fe rence , meteoro logica l a spec t s , bas i c r e sea rch of su r f ace winds and g u s t s

Mode 1 de ve 1 opment s

S imulat i o n s t u d i e s

Carrier wake s t u d i e s

A n a l y t i c a l re sponse s t u d i e s

Wind t u n n e l s t u d i e s

C r o s s - spe c t r a and r e sp onse

41

REFEiiENCE LIST

1.

2.

3 .

4.

5.

6 .

7 .

8.

9 .

10.

11.

42

Press , Harry; Meadows, May T . ; and Hadlock, Ivan: A Re- Evalua t ion of Data on Atmospheric Turbulence and Airplane Gust Loads f o r Appl ica t ion i n S p e c t r a l C a l c u l a t i o n s . NACA Rept. 1272, 1956.

Atmosphei-lc Turbulence and I t s Re la t ion t o A i r c r a f t . Proceedings of a Symposium, The Royal A i r c r a f t Es tab l i shment , Farnborough, England, 1 6 t h November 1961.

Meeting on A i r c r a f t Response t o Turbulence .- Compilation of p a p e r s p resented a t the NASA Larigley Research Center , September 24-25, 1968.

Jones, J . W . ; Mielke, R . H . ; Jones, G . W . ; e t a l . : Low A l t i t u d e Atmospheric Turbulence, LO-LOCAT Phase 111, Vol. I, Parts 1 and 2, Vol . 11, P a r t s 1 and 2, AFFDL-TR-70-10, November 1970.

Crooks, W . M . ; e t a l . : P r o j e c t H I C A T High A l t i t u d e C lea r A i r Turbulence Measurements and Meteorological C o r r e l a t i o n s . (AFFDL-TR-68-127, Vol. l), AF F l i g h t Dynamics Laboratory, Wright-Pat terson AFB, Ohio, 1968.

Ashburn, E . V . ; e t a l . : Development of High A l t i t u d e Clear A i r Turbulence Models. (AFFDL-TR-69-70), AF F l i g h t Dynamics Laboratory, Wright-Pat terson AFB, Ohio, 1969.

P r i t c h a r d , F .E . ; Easterbrook, C . C . ; and McVehil, G . : S p e c t r a l and Exceedance P r o b a b i l i t y Models of Atmospheric Turbulence f o r Use i n A i r c r a f t Design and Opera t ions . (AFFDL-TR-65-112), AF F l i g h t Dynamics Laboratory, Wright- Pa t t e r son AFB, Ohio, November 1965.

Saxton, D.W.: The Nature and Causes of Clear-Air Turbulence. A I A A Paper No. 66-966, p re sen ted a t t h e A I A A 3 r d Annual Meeting (Boston, Mass.), November 29-December 2, 1966.

0 'Hara, F . ; and Burnham, J. : The Atmospheric Environment and Aircraft--Now and the Fu tu re . Aeronau t i ca l J . , vo l . 72, June 1968, pp. 467-480.

Jones, J.G.: A Theory f o r Extreme Gust Loads on an A i r c r a f t based on the Representa t ion of t h e Atmosphere as a S e l f - S imi l a r I n t e r m i t t e n t Random Process . RAE Technica l Report 68030 1968

Jones, J.G.: A Unified D i s c r e t e Gust and Power Spectrum Treatment of Atmospheric Turbulence. RAeS/AIAA/CASI I n t e r n a t i o n a l Conference on Atmospheric Turbulence, London, May 1971.

1 2 . E tk in , B.: Theory of the F l i g h t Ai rp lanes i n I s o t r o p i c Turbulence; Review a n d Extension. Un ive r s i ty of Toronto Report N o . 72, February 1961.

13. E tk in , B. : Dynamics of F l i g h t : S t a b i l i t y and Cont ro l , Wiley, N.Y., 1959.

and Fur the r Research. J. Royal Aero. Soc:, v o l . 69, no. 649, 1965.

14. Zbrozek, J .K . : Atmospheric Gusts. P re sen t S t a t e of the A r t

15. Diederich, F rank l in W. : The Response of an Airplane t o Random Atmospheric Distrubances . NACA Report 1345, 1958.

16. Eichenbaum, F rede r i ck D.: A General Theory of A i r c r a f t Response t o Three-Dimensional Turbulence. J . A i r c r a f t , vol. 8, no. 5, May 1971, pp. 353-360.

17. P o r t e r , Richard F.; and Robinson, Al f red C. : A Proposed C r i t e r i o n f o r A i r c r a f t F l i g h t i n Turbulence. B a t t e l l e Columbus Labora to r i e s , Columbus, Ohio.

18. Hob l i t , F.M.; Paul , N.; Shel ton, J . D . ; and Ashford, F .E . : Development of a Power-Spectral Gust Design Procedure f o r C i v i l A i r c r a f t . FAA-ADS-53, January 1966.

19 . Dutton, John A:: Broadening Horizons i n P r e d i c t i o n of t he E f f e c t s of Atmospheric Turbulence on Aeronaut ica l Systems. A I A A Paper 68-1065, presented a t the A I A A Annual Meeting and Technical Display, Ph i l ade lph ia , Pa., Oct. 21-24, 1968; a l s o A I A A Se lec ted Repr in t s S e r i e s , vo l . X I I I .

20. Houbolt, John C . ; S t e i n e r , Roy; and P r a t t , Kermit G . : Dynamic Response of Ai rp lanes t o Atmospheric Turbulence Inc lud ing F l i g h t Data on Input and Response. NASA TR R-199, June 1964.

I 21. Houbolt, John C . : Gust Design Procedures Based on Power

I S p e c t r a l Techniques. AFFDL-TR-67-74, August 1967.

r 22. Houbolt, John C . : Design Manual f o r V e r t i c a l Gusts B a s e d on Power S p e c t r a l Techniques. AFFDL-TR-70-106, December

I 1970

I 23. Houbolt, John C . : On the Response of Ai rp lanes i n a 3- Dimensional Gust F i e l d . A.R.A.P. Report No. 161, Aero- n a u t i c a l Research Assoc ia tes of Pr ince ton , Inc . , Pr ince ton , N . J . , J u l y 1971.

Mul t ip l e Random I n p u t s . J ah r . 1957 der WGL, F r i e d r . Vieweg & Sohn (Braunschweig), pp. 296-305.

I 24. Houbolt , John C.: On the Response of S t r u c t u r e s Having

43

25.

26.

27.

28.

29

30.

31.

32.

33

34

35.

36.

37

Houbolt, John C . ; Atmospheric Turbulence. AIAA Paper No. 72-219, presented a t A I A A 1 0 t h Aerospace Sciences Meeting, San Diego, Ca l i f ., January 17-19, 1972.

Meeting on Ground Wind Load Problems i n R e l a t i o n t o Launch Vehicles . Compilation of papers p re sen ted a t the NASA Langley Research Center , June 7-8, 1966.

Prelaunch Ground Wind Loads. NASA Space Vehicle Design C r i t e r i a . NASA SP-8008, November 1965.

F i c h t l , George H . ; Kaufman, John W . ; and Vaughan, W i l l i a m W . : The C h a r a c t e r i s t i c s of Atmospheric Tir.~-hir?lernl~e 2 s F,elz.tzc! t o Wind Loads on T a l l S t r u c t u r e s . Nat iona l Bureau of S tandards , D e p t . o f Commerce Bui ld ing Science S e r i e s 30, 1970.

Anon.: Wind E f f e c t s on Bui ld ings and S t r u c t u r e s . Un ive r s i ty of Toronto Press , Toronto, 1968, 2 vo l s .

Davenport, A . G . : The Re la t ionsh ip of Wind S t r u c t u r e t o Wind Loading. Paper 2, Symp. 16, Proceeding of Conference on "Wind E f f e c t s on Bui ld ings and S t r u c t u r e s " at'NPL i n June 1963. HMSO, London, 1965.

Davenport, A . G . : The B u f f e t i n g of Large S u p e r f i c i a l S t r u c t u r e s by Atmospheric Turbulence. Repr in ted from t h e Annals of t he New York Academy of Sc iences , vo l . 116, a r t i c l e 2, June 1964, pp. 135-159.

Davenport, A.G. : Ra t iona le f o r Determining Design Wind V e l o c i t i e s . J . S t r u c t . D i v . , Proc. Amer. SOC. C i v i l Eng., May 1960.

Harris, R . I . : The Nature of t he Wind. P resen ted a t t h e Construct ion Indus t ry Research and Informat ion Assoc ia t ion Seminar on t h e Modern Design of Wind-Sensit ive S t r u c t u r e s a t London, England, June 18, 1970.

Hanna, S.R.: The Formation of Longi tudina l Sand Dunes by L a r g e H e l i c a l E d d i e s i n t h e Atmos he re . J . of Applied Meteorology, vol . 8, 1969, pp. 875-883.

F l inn , E.H.: Low A l t i t u d e Turbulence and V/STOL Gust Response. AFFDL/FGC-TM-70-3, September 1970.

Davis, H.M.; and F l i n n , E.H. : Research Requirements f o r V/STOL Gust Response and Low A l t i t u d e Turbulence Model. AFFDL-FDCC TM 66-11, October 1966.

McElreath, Capt. Kenneth W . , ed. ; Research and Development Programs of the F l i g h t Cont ro l D iv i s ion . AF F l i g h t Dynamics Laboratory (FGS) , Wright -Pa t te rson A i r Force Base, Ohio, prepared f o r t he SAE, January 1972.

44

I

i

,

I

i I 1

i

i

I

~

I

I

38

39

40.

41.

42.

43

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45.

46.

47

48.

49 9

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Some Comments on Reinges t ion - A m e s Wind Tunnel Tes t Resu l t s . XV-5A D a t u m - No. 4, 1 September 1963.

Snyder, C . Thomas: Analog Study of the Longi tudina l Response of a Swept-Wing Transport Ai rp lane t o Wind Shear and Sus ta ined Gusts During Landing Approach. NASA TN D-4477, 1968.

Cockayne, W i l l i a m ; and Rusnak, Walter: Study of Automatic F l a r e and Decrab Guidance and Control System f o r the Space S h u t t l e Vehicle . NASA CR-114436, J u l y 1971.

Crutcher , H.L.: P r o j e c t Crosswinds. Unpublished paper of t h e Nat iona l Cl imat ic Center , U.S. Dept. of Commerce, NOAA, Environmental Data Serv ice , Federa l Bui lding, Ashev i l l e , N . C . 28801.

Ryken, John M.: A Study of A i r Cushion Landing System f o r Space S h u t t l e Vehic les .

McElreath, Capt. K.W.: True Airspeed Sensor f o r V/STOL A i r c r a f t . AFFDL-TR-72-131, December 1972.

NASA CR 11803.

V/STOL Disp lays f o r Approach and Landing. AGARD Report No. 594, J u l y 1972.

Measurement and Report ing of Surface Wind and V e r t i c a l Wind Shear a t Aerodromes. Attachment t o S t a t e L e t t e r AN 10/11.9- 72/19, 5th A i r Navigation Conference, Montreal, 1967.

Danie ls , Glenn E. , ed. : T e r r e s t r i a l Environment (C l ima t i c ) Cr i t e r i a Guidel ines f o r use i n Space Vehicle Development, 1971 Revis ion. May 10, 1971.

NASA Technical Memorandum TM X-64589,

Chalk, C .R . ; Neal, T.P.; e t al . : Background Information and User Guide for MIL-F-8785B(ASG) , "Mi l i t a ry S p e c i f i c a t i o n - F l y i n g Q u a l i t i e s of P i l o t e d Airp lanes . " AFFDL-TR-69-72, August 1969.

F l y i n g Qual i t ies of P i l o t e d Airp lanes . M i l i t a r y Spec. No. MIL-F-8785B (ASG) , In t e r im Amendment-1 (USAF) , 31 March

F l y i n g Qua l i t i e s of P i l o t e d V/STOL A i r c r a f t . M i l i t a r y Spec. No. MIL-F-83300, 31 December 1970, used i n l i e u of

1971

MIL-H-8501A.

Airp lane S t r e n g t h and R ig id i ty , General S p e c i f i c a t i o n For. M i l i t a r y Spec. No. ~IL-A-008860A (USAF) , 31 March 1971, used i n l i e u of MIL-A-8860(ASG).

45

51. Airplane S t r eng th and R i g i d i t y , F l i g h t Loads. M i l i t a r y Spec. MIL-A-O08861A(USAF), 31 March 1971, used i n l i e u of MIL-A-8861 (ASG) . M i l i t a r y Spec. MIL-A-008865A (USAF) , 31 March 1971, used i n l i e u of MIL-A-8865(ASG).

Repeated Loads, and Fa t igue . M i l i t a r y Spec. MIL-A-008866A (USAF), 31 March 1971, used i n l i e u of MIL-A-8866(ASG).

54. Airplane S t r eng th and R i g i d i t y , V ib ra t ion . M i l i t a r y Spec. l * I I L i - L l - t U C J ~ C \ C l k H P 1 , -herlciment 1, i6 piovember 1971.

55. Airplane S t r e n g t h and R i g i d i t y , F l i g h t Loads. M i l i t a r y Spec. MIL-A-8861(ASG), 18 Mary 1960, supersedes i n p a r t t h e MIL-S-5700 s e r i e s specs and i n p a r t Spec. MIL-A-8629.

Airplane S t r e n g t h and R i g i d i t y , Miscel laneous Loads. M i l i t a r y Spec. MIL-A-8865(ASG), 18 May 1960, supersedes i n

ar t t h e MIL-S-5700 s e r i e s specs . and i n p a r t Spec. MIL-A- k 2 9 .

52. Airplane S t r e n g t h and R i g i d i t y , Miscel laneous Loads.

53. A i r p l a r ; ~ S t r eng th and R i g i d i t y , R e i i a b i i i t y Requirements,

R ~ T T A O O n n / r ~ n n n \

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Campbell, G.S.; and Standen, N.M.: P rogress Report I1 on Simula t ion of E a r t h l s Surface Winds by A r t i f i c i a l l y Thickened Wind Tunnel Boundary Layers. Nat iona l Research Council of Canada, NAE Report LTR-LA-37, J u l y 1969. Cermak, J .E. ; and Arya, S .P .S. : Problems of Atmospheric Shear Flows and Thei r Laboratory Simulat ion. Paper 12 , Aerodynamics of Atmospheric Shear Flows, AGARD Conference Proceedings No. 48, February 1970. Counihan, J.: An Improved Method of S imula t ing an Atmospheric Boundary Layer i n a Wind Tunnel. Atmospheric Environment, vol . 3, 1969, pp. 197-214. Templin, R.J . : I n t e r im Progress Note on S imula t ion of E a r t h ' s Surface Winds by A r t i f i c i a l l y Thickened Wind Tunnel Boundary Layers. Nat iona l Research Council of Canada, NAE Report LTR-LA-22, February 1969.

Durgin, h'rank H.; and ToLlg, PTr,: The E f f e c t of T w i s t Motion on the Dynamic W-ltimode Response of a Bui ld ing . P resen ted a t t h e Symposium on Flow-Induced S t r u c t u r a l V ib ra t ions , sponsored by IUTAM and IAHR a t Karlsruhe, Germany, August 14-16, 1972. Houbolt, John $.;,and Sen, A s i m : Based on von Karmanls S p e c t r a l Equat ion. March 1972.

Cross-Spec t ra l Funct ions NASA CR-2011,

Houbolt , John C . ; and Sen, A s i m : Single-Degree-of -Freedom R o l l Response due t o Two-Dimensional Vert ical Gusts. NASA CR-111966, J u l y 1971.

w

Y

CL;

x w w-4

#-I

\ I

65

I 0 0 0 N

I I

t z w J >

- n a 4:

(3

d

1333 - z

k 0

a, E-1

k a, > 0- 0

m c r ,

66

1

t

t

t

I

Cedar Hill data shoxving dispersion l imi t s

Log profile law: 3 = In E , zo = 0.1 ft 0

U 1

" 4

n I

W 1

v I

I300

!200

I100

1000

900

830

1 1 1 1 ~ .

0 2 4 6 8 IO 12 14 16 18 20 22 24 26 28 30 32 34 36 38 4 0

-

-

- -

-

-

u/u*

FIGURE 3. Average Values of Cedar H i l l Tower Wind P r o f i l e s f o r Combined Unstable-Neutral Lapse Rate (From Ref. 237)

Height

a

Wind Speed

F I G U R E 4. Examples of Nonclassical Wind P r o f i l e s

68

I

IO"

I 0-4 IO" I IO

LSZ

FIGURE 5. Gust Spectrum Function for w

I o2

61 a, ffi E 0 k

a, m cd u

5

d cd c 0 *rl

0

0 0

ai LO 10 100

FIGURE 7. Nondimensional w Spectra - Two Unstable Cases

Yawing mclment due t o v I /

P i t c h i n g moment J due tc w 7

R o l l i n g

\

The w field /

FIGURE 9. Predominant Force arid Moments that Act on an Airplane in Turbulence

73

Ua , a i r f l o w v e l o c i t y r e l a t i v e t o a i r c r a f t

/ I / V , a i r c r a f t ground v e l o c i t y

U , wind v e l o c i t y

(a ) The a i r f l o w v e c t o r Ua

'5; Y

( h ) R e l a t i v e l o c a t i o n of two p o i n t s

FIGURE 10. Geometry Involved i n C o n s i d e r i n g t h e C o r r e l a t i o n of Turbulence V e l o c i t i e s between Two P o i n t s f o r A i r c r a f t F l i g h t Path n o t Al igned with Wind D i r e c t i o n

NASA-Langley, 1913 74

~~ ~

'a

A i r c r a f t motion r e l a t i t o wind

ve

\

- 20

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