.

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(_

.TECHNICAL NOTE 3698-

PREUMINARYINVESTIGATIONOFSELF-EXCI’TED

OFSINGLEPLANINGSURFACES

ByEhno J.Mottard

La.ngley AeronauticalIa.ngleyField,

LaboratoryVa.

Washington

June1956

.

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TECHLIBRARYKAFB,NM

NATIONALADVISOKYCmMmTEEFORAERONAUTICS 11111101-------TEcBmcALNom3698

PRELIMINARY-TIGATION OFSELF-EXCHZD

I.11.lbhd/d

VIBRATIONS-i

OFSINGIEPLANINGSURFACES1

ByElmoJ.Mottard

suMMARY

A preliminaqinvestigationwasmadeofself-excitedvibrationsofsingleplaningsurfaces.A se~-excitedoscilk.tionrequiringfreedominrisebutnotintrimoccurredwithhighaspectratio’(orde~of10)ofthewettedportion.‘12hisvibrationcouldbe controlledmostsuccess-fullyby methods(suchastheaspectratio.

useofdeadrise)whichlimitedthewetted

mODUCTION

A tendencyforsingleflatplaningsurfacestovibratehasbeenobservedduringforcetests(ref.1). Themotionoccurredat SMUwettedlengthsandappearedtobe essentiallyan oscillationintrimorriseora combinationofthese.Thistypeofvibrationhasnotbeenencounteredon seaplanehu3d.s,perhapsbecausethesehullsgenerallydonothaveflatbottomsorbecausethehullstructuresareveryrigid.Thisvibration,nevertheless,issignificantforhydro-skis,whichmaybeflat-bottomednearthetrailingedgeandhavean inherentlylessrigidstructurethantheseaplanehull.TherehavebeeninstancesinwhichseverevibrationsofI@ro-skishaveoccurredandcaused,structuralme tothea@?l=e ad discomfortto”thepilot.

Becausethevibrationofplaningsurfaceshasbecomea practicalproblem,a prelhinaryinvestigationofthevibrationofsingleplaningsurfaceshasbeenmade.Varioustypesofplaningsurfacesweretestedona practicalhydro-skiconfiguration,theeffectsofvaryingsomeofthestructuralandgeometricpropertiesoftheconfigurationweresought,andtestsweremadeto determinethedegeesoffreedomrequisiteforthevibrationto occur.

‘supersedesrecentlydeclassifiedNACAResearchMemorandumL~~27byElmoJ.Mottard,1955.

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2

MODELANDAI’PAFWTUS

IJllcllTN3698

Thebasichydro-skimodelandtowingapparatusareshownhfigures1 and2. Theskiwasmadeofspruceintheshapeofa rectangularboardandwasattachedtoa nmsiveI-beamby meansofpivotssoastopermitfreedominbendhg. Themassofthetowinggear(exclmiveofthecounterweights)was2CX)pounds,whichwassufficientlylargeto eliminateverticaltranslationoftheskisupportatthevibratoryfrequenciesencountered.ThedistanceZ fromtherearpivottothetrailingedgeoftheskiwas17 inchesandthethichess t was0.94inch.ThemechanicalpropertiesoftheskiaregivenintableI. An accelerometer,usedmerelyasa vibrationdetector,wasfastenedtothehydro-skimidwaybetweenthepivots.

Inorderto.determinetheeffectsofvaryingsomeofthedesigncharacteristics,thebasichydro-ski(fig.3(a))wasmodifiedas follows:

Anglesofdeadriseof10°and20°(figs.3(b)and3(c))werepro-videdby fasteningblocksofthepropershapetothetrailingedge.Theblockswereshortandlightinweightsothatthemechanicalpropertiesofthehydro-skiwerenotappreciablyaltered.

Transversecircular-arccurvatureshavingradiiof9.14inchesand4.86inches(figs.3(d)and3(e))weresimilarlyprovided.Thecylin-dricalsurfacesweredesignedto circumscribetheprism.ticsurfacesoffigures3(b)and3(c).Thealterationdidnotappreciablyaffectthemechanicalpropertiesofthehydro-ski.

Thetrailingedgewasmadepointedinplanform.A single600point(fig.3(f)),twopoints(fig.3(d) des~ed to @ve thes~e variationwettedareawithdraftasthesinglepoint,w fivepointsof600(fig.3(h)) weretried.Themechanicalpropertieswerenotappreciablyaffectedby thesechanges.

Thethicknesst (fig.1)wasvariedfrom0.5inchto 1.5inches;theresultingvariationsinthemechanicalpropertiessreshownintableI(a).

of

Theoverhangz (fig.1)wasdecreasedfrom17inchesto 1 inchinfourstages;theeffectsonthemec~cal properties=e sho~ ~tableI(b).

In orderto determinetheeffectofeliminatingthechangeintrimoftheplaningsurface,thearrangementshow ~ fi~e 4 -S wed. meSMIJ-planingsurfacelocatedmidwaybetweenthefixedpivotsofthesimplebefiiseffectivelyconstrainedtomoveintheverticaldtrectionwhenthebeamvibrateswithsmallsmplitudeinthefirstmode. Two

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NACAm 3698

accelerometerswereusedinorderthefirstmodeandhighermodes.sprayfromhittingthebeam.

3

to facilitatediscriminationbetweenA sprayshieldwasprovidedtoprevent

TheLangleytmik no.1, inwhichthetestsweremade,isdescribedinreference2. Thewaterinthetankwas12feetdeepduringthetests.

PROCEDURE

Theloadandtrimwereheldconstantduringthetestrun. Becausethetrtiwassetandmeasuredat zeroload,thetrimofthetrai13ngedgeofthehydro-skiwhenunderloadwasdifferentfromthemeasuredtrim.Thetrimofthetrailingedgemaybe computed,ifdesired,by theuseoftableI. Themodelwasacceleratedata constantrateof1.5feetpersecondperseconduptoa maximumspeedof 80feetpersecond.Thespeedandaccelerometeroutputwererecorded,andthespeedatwhichvibrationstartedwasobtatiedfromtherecord,wherethestartoftheself-excitedvibrationwasevidencedby a suddenincreaseintheaccelerometeroutput(fig.5). As anaidindistinguishingmodesandvisualizingthevibration,motionpictureswere~de inwhi’chan illusionofslowingdowntheoscil-lationswasobtainedbysettingthe@era speed(inframespersecond)slightlyslowerthanthevibrationfrequency.A similsxtechniquewasusedwitha flashinglampfordirectobservation.

Inordertoprotectthetowinggearfromtheeffectsofsaltspray,shieldswereprovided.Checkrunsmadewithouttheseshieldsindicatedthatthedatawerenotappreciablyaffectedby theirpresence.

RESULTSANDDISCUSSION

GeneralDescription

Oscillationswereencounteredwhichwereevidentlyselfexcitedratherthanwaveexcited,sincetheydidnotdependonthepresenceofwavesorripplesonthewater.Thevibrationoccurredonlywhenthewettedlengthwassmallwithrespecttothebeam(athighaspectratio).Forexample,duringa typicaltestrunwithconstantloadandincreasingspeed,thehydro-skistartedtovibrateinthefirstmodeinbending(withlargestdisplacementatthetrailingedgeandmidwaybetweenthepivots)whentheaspectratioofthewettedportionreachedabout10.Thefrequencywasslightlyhigherthanthenaturalfrequencyinair. Asthespeedwasincreasedthefirstmodewassucceededby an intervalofirregularvibrationwhichwasfollowedby higherties. Thevibrationinmanyinstanceswasaccompaniedby a drumingsoundwhichwasapparently

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4 I?ACATN3698

causedby therapidsuccessionof impactsoftheplaningsurfacewiththewatersurface.At lowtrhs theoscillationsappearedgraduallyandwereof smalIlamplitude.At lightloadsandhightrims,themodelbecameairborneduringthetestrun.

A seriesofframesfroma motionpictureofthevibratinghydro-skiisshowninfigure6. Thecamerawaslocatedonthestarboardside,aboveandslightlyforwardofthetrailingedge.Theskiismovingtotherightandthetrailingedgeisslightlyabovethecenterofthepicture.Thewake,whichcanbeseentotheleftofthetrailingedge,iscomposedofapproximatelysquaredepressionswhicharesurroundedby spray.Thefre-quencyofvibrationwasSO cyclespersecondandthecameraspeedwasslightlylessthan50-framespersecond,theresultbeingthatthesequenceofphoto~aphsshowssuccessivestagesinthevibrationcycle.Thefirstphotographshowstheskienteringthewaterandphotographs2to 1.2showsubsequentstagesofthefomtion ofa depressioninthewater.Inphotograph13thehydro-skiisoutofthewater.Thelastthreephotographsshowthestartofthefomwtionofanotherdepression.

VibrationWithoutTrimMotion

By usingthearr&ngementoffigurek,a self-excitedvibrationwasobtainedwithoutmotionintrim.Thisvibrationoccurredunderthesameconditionsofload,trim,speed,andaspectratioandhadabotithesamefrequencyasthatexperiencedwiththeconfigurationoffigure1. 12vi-dentlythevibrationofthehydro-ski(fig.1)andthatofthearrangementshowninfigurek arebothexamplesofa vibrationphenomenonwhichoccurswithplaningsurfacesathighaspectratios(orderof10). Thisphenomenonisnotexplainableonthebasisof couplingbetweentrimandrisemotions,sinceitmayoccurwithoutfreedomintrim.

VibrationBoundaries

Figures7 to 12 showthespeed”atwhichthefirstmodeofvibrationstsrtedforthebasicskiandforthevariousmodificationsfora loadrangefrom5 to60pounds(withtheexceptionoftheO.S-inch-thickski,forwhichtheloadrangewasfrom5 to40pounds),speedsupto ~ feetpersecond,andtrims(measuredat zeroload)from3°to 25°. Theabsenceofdata.foranyconditionofload,speed,andtrimwithintheselimitsindicatesthatvibrationeitherdidnotoccuror startedsograduallythatthestartingpointwasnotdefinable.Thescarcityofdataattrimsbelow10°istheresultofa tendencyforthestartingpointtobeindefiniteat lowtrims.Theconditionsforwhichvibrationdidnotoccurareindicatedinthefigures.Thespeedatwhichvibrationstartedincreasedwithincreaseinloadand,ingeneral,decreasedwithincreaseintrim.

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NAC~TN3698 5

Theeffectsofloadandtrimonthevibrationboundaryofthebasicconfigurationareshowninfigure7. Inorderto facilitatecomparisonofthemodifications,thesecurvesforthebasichydro-skihavebeen.repeatedinallsubsequentfiguresas solidlines.

Theeffectsof changesintheshapeoftheplaningsurfacearepre-sentedinfigures8 to 10. Deadriseof10°causedthevibrationboundarytomovetohigherspeeds.(Seefig.8.) h evenmorefavorableeffect,however,wasa greatreductionintheseverityofthevibration.Increasingthedeadriseto 20°completelyeliminatedthevibrationfortherangeof speedandloadtested.Theeffectoftransversecurvature(fig.9)wasverysimilartothatofdeadrise. Thevibrationwiththe4.86-inchcurvaturewassofeeblethatitwaspracticallynegligible.Thevibrationspeedswithvarioustypesofpointedtrailingedgesareshowninfigure10. Thesinglepointandthetwopointscomplete~eliminatedthevibrationfortherangeofspeedandloadtested.Thevibrationboundsrywasnotsignificantlydifferentwiththefive-pointedtrailingedgethanwiththebasictrailingedge,buttheseverityofthevibrationwasmuchlesswiththepointsthanwithout.Becausetheseplaning-surfacechangeslhitedthewettedaspectratio,theirsuccessisadditionalevidencethatthevibrationonlyoccurswithhighwettedaspectratio.

TheeffectsofmodificationswhichalterthemechanicalpropetiiesoftheplaningsurfacearepresentedinfiguresI-1andI-2.FigureI-1showstheeffectofhydro-skithicknessonthevibrationboundaries.h general,thebehaviorofthe0.50-,0.75-,andO.$lk-inch-thickhydro-skiswassimi~, especiallyathighloadsandtrims.Thevibrationofthe1.25-inchhydro-skiusuallystartedsograduallythatthestsrtingpointcouldnotbe determined.Thespeedatwhichvibrationst=rtedforthe1.5Q~fichhydro-ski(6opercentthickerthanthebasicski)wasconsiderablyhigherthanfortheothers.Theeffectofreducingtheoverhangfrom17 inchesto 9 inches(fig.12)wasslight,butat 5 inchesoverhang(areductionof 70percent)thevibrationwaseliminatedfortherangeofspeedsandloadsinvestigated.

CONCLUDINGREM4RM

A preliminaryinvestigationofvibrationsofsingleplaningsm’faceshasindicatedthatself-excitedvibrationsoccurwitha highaspectratio(orderof10)ofthewettedportionandmayoccurwithouta changeintrimangle.Theoscillationscanbedecreasedin severityoreliminatedbythe

. useofplaningsurfaceswhichlimitthewettedaspectratio.Deadrise,transversecurvature,anda pointedtrail~ edgeare~ effecti~e~A 60-percentincreaseinthethicknessofthebydro-skicauseda definite

. increaseinthespeedatwhichvibrationfirstoccurred.A 70-percent

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6 NACATN3698

decreaseintheoverhangeliminatedthevibrationfortherangesof speedsandloadsinvestigated.

LangleyAeronauticalLaboratory,NationalAdvisoryCommitteeforAeronautics,

LangleyField,Vs.,September1, 1955.

REFERENCES

1.Shoemaker,JamesM.: TankTestsofFlatandV-E!dtomPlaningSur-faces.NACATN 509,1934.

2.Truscott,Starr:TheEnlargedN.A.C.A.TankjandSomeofItsWork.NACATM 918,1939.

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NACATN3698

TABLEI.-MECHANICALPROPERTIESOFHYDRO-SKZS

(a)Variousthiclmessesand17 inchesof overhang

Thiclmess,t,in.

0.50

a1.251.50

Firstmodenaturalfrequency,

Cps

23.231.137.453.673.0

Staticdeflection

Linear,in./lb

(a)

0.0326.0145.0083.0036.0018

Angular(trim),deg/lb

(b)

0.131.058.034.014.007

(b)VariousamountsofoverhangandO.~ inchthickqess

Overhang,1, Firstmodein. naturalfrequency,

Cps

C17 37.413 43.3

46.7; 47.51 47.8

Staticdeflection

Linear, -r (trim),in./lb deg/lb

0.0083.0048.0027.0012.0CQ2

0.034.027.018.010.002

abad appliedanddeflectionmeasuredatthetrailingedge,per-pendicularto thebottomofthehydro-ski.

bbad appliedatthetrailingedge,perpendiculartothebottomofthehyd.ro-ski.Angulardeflection(changeoftrim)measuredatthetrailingedge.

cBasicconfiguration(figs.1,2,and3(a)).

. .. . ..————.——r —. -——----- __ —.— — .— —— ——

8

— Counterweights

Directionofmotion

,x DashPot

I

L-J’ Towing point ,

Trimlink

Towinggate

6 X 6 I-be~ .

klydro-ski

tJ-

Staticwaterlevel

,2, J-C’-’.

Figure1.-Schematic

Trimangle. .

d.iagremofmodelandapparatus.Dimensionsinches.

arein

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JNACATN 3698 9

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Figure2.-Modelandapparatus.L+0488

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——, —.— .

=w”p-— Oerhr line

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ram piwt

6.’25i-

(8)WI. ca-flgmaticm.

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(b) A.i#.aCJJdand I,.

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I

(d)&B31J.aul”,’ature+

I

(e) Imgp au-mtmm.

(c) Au&e O&dead IJ.

Figore’3.-Plsning-surfaceconfigurations.DtinElorIsare in

E(f)

, ,

TSimple beam

~Center lineAccelerormtera

Directionofmotion

rd~

““V t \\’L

Spray shield

Figure k.- Arrangement of a~eratus withrise without mtion

LL Static water level

Planing surface

mcdel which permits freednm inh trim.

t

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F&u-e 5.-R@ of oscillograph record showing start of vibration.

NACATN 3698 13.

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1

2

3

4

5

6

8

L-90J89Figure6.- Consecutivephotographsofthevibratinghydro-ski.Thickness

ofhydro-ski,0.94inch;load,20pounds;speed,45feetpersecond;trim,20°.

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NACATN369814

9

10

xl

12

Figure6. - Cone

u

ti

s

I-6

L-9049oluded.

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,

..- ....0..-...s ~ 15

-—m-—s --A--— 20

-.-.~----lo -—b-–-es

60- p ,ti

id,1,/

40 -No vibration

20 -Vibration

o I I I I

had, lb

..................... 5 30

-—-— 10 --—--—40

------------ 20 —–-––-60

30 r

0 80 40 60 80 0 20 40

Sped, fp Speed,

Figure 7,- The effects of load ad trim on the B eed at which vibrationTstarts for the basic hydro-ski com?igure.tion figs. 1, ‘2,and 3(a)).

60 so

fpa

luv.xm 3698.

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Dead-rise 30angle,deg‘i~e

3(a)-G––– 1: 3(b)~o vibration20 3(C v : 20

a

NO vibrationof’

2

h-’ e?!

/& 10

‘h> ’50/ ~fl” Vibration

[

-lQVibration

No vibration ‘~,

+

Load,30 lb

o 20 40 60 80 0 20 40 60 BoSpeed,fpa Speed,fps

Figure8.-Theeffectof deadriseonvibrationboundaries.

60“ RadiusofCUrvacmeJFigureIn.

a 3(a)9.14 3(d)

~ 40---u- 4.86 3(e)

a’ No vibration:20 _

Trim,150

I Io 20 40 60 80

S~ed, fps

I Load,301b

I I I Io 20 40 60 80

Speed,fps

Figure9.-Theeffectoftransversecurvatureonvibrationboundaries.

60

.“4(

.--1

Configuration-vlbratlonFigure

— ~3(a}No Vf-

-+ ~s(h)

Vlbratlon

J I I ) 10 20 40 60 80

Speed,fps

30

14

i

I Vibrationi?20‘u No vlbra’qlon “

E-.-l -\k 10 ‘a

Load,30 lb

~o 20 80

Speed,fps

Figure10.-~e effectofpointingthetrailingedgeonvibrationbo-ies.

,

.

.

—— .———

u

.mm m 3698 17

Thickness,in.

60

r

Trim,3°

–-o–– “so –---A-——l.+-+-— .75 ---h---1.5cl— .94

@‘0‘ib;a’i”nqVlbrationo I I

60rTrim, 10°I I

:40– ~

m’ i:.20_

Y

.. Ll?ovibration,. a/ /’

,9’ ,“Vibrationpo [

60

‘,/,

Trim, 20° ,P

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~ 40 ‘Arl lbvlbrat.ion.o- </’:20

///“ VlbratIon

800 20 40 60Speed,fps

(a)

Figureil.- Theeffect

1Trim,5°

- VovibrationH/

8/ Vibration

/“ ,

-.Trim,15° 7#

Hovfbration

I

Iod. against

ofthickness

; ,L,,Trim,25° ?

P,

tiov.lbratlon P,/~: ~J,. ,/

. .,4{-!@

// i’ /dibratiOn-. .4..” k y

o 20 ).LO . 60 80‘Spe;d,fp-S

speed.

onvibrationboundaries.

-— —-.— —-—-

NACATN3698

‘Thickne3~,in.—–—— .50. 1.25—- —-...—-—-– .75 --—-— 1.50

.94

30–

(

\~/

(

i ,1,/

to2Q- ~i i is { :Vlbratlon No vlbratlon

HO vibration\, \l (vibration:-

& 10- \\ ‘\ “\

\Load,20lb Load,30lb

o I I [

30-

w2Q-

!

~’x./’/No vibration i “:Vlbpatlons

Fj \\g 10- ‘\

Load, 40lb1 I I

o 20 40 60 80Speed,~ps

r

I

\,/i

Novibration “ Vibration

\‘\

Load, 60 lbI I 1

0 20 4-0 - 60 8Qs~ed, fps

(b)!IYimagainstspeed.

FigureIl.-Concluded.

_. .—-

I , ,.

overhang, 2, in. !2

-—–-o—----

---------- U -----------

9I

No vibration

Vibration

TrlQ, 15°

d 13

NO vibration d

NO vibmtlon$

5

1

o !20 40 60 80 0 ’20 40 60 00

Speed, fpa Speed, fpa

Figwe 12. - The effect of overhang on vibration boundaries.

G

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