Some Aspects of the Design of Seagoing Aircraft Part 1

7/27/2019 Some Aspects of the Design of Seagoing Aircraft Part 1

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FLIGHT, JANUARY 16, 1931

ON IMPERIAL SER VICE : The S h o r t " C a l c u t t a " is f i t ted with three Br is tol "Jupiter" Engines( F L I G H T Plwto.)

SOME ASPECTS OF THE DESIGN OF SEA-GOINGAIRCRAFTBy A. GOUGE, B.Sc, A.F.R.Ae.S., General Manager of Short Brothers

Being a lecture delivered before the ltoy.il Aeron autical Society on January 8, 1931

R EADERS of F L I G H T wil l know that weh a v e c o n s t a n t l yadvocated an e x t e n s i v e and bold flying-boat policyfor Br i t i sh ai rcraf t con st ruc tors . T her e is not the^lightest doubt that the fu ture of ai rcraf t in the Br i t i sh^Empire lies far more wi th flying-boats t ha n wi th land a ircra ft .Mhc British Empire is essen t i a l l y an E m p i r e w h i c h has, forSmtration.s, been held together by the seawor t h i ness of b o t hK T ships and her men, and it therefore seems only logicalhat sheshould, in the fu t u re , bo h e l d t o g e t h e r b y ' t h e sea-worthiness of her a i r c raf t and her men. It is wel l knownthat wehave cons t an t l y dep reca t ed bo t h the a b o r t i v e andHe successful flights w hic h ha ve b een m ad e ov er w ideexpanses of w a t e r in l and ai rcraf t , s ince such could never be"tl ier than specta cul ar s tu nt s, and since sane and solid'evelopment of overseas f l ights must lie in the d e v e l o p m e n t" sea-going aircraft. On T h u r s d a y , J a n u a r y 8, Mr. A.delivered the fo l l owi ng excep t i ona l l y i n t e res t i ng paperthe Royal Aeronaut ical Society . Them a s s of i n t e r es t - f. information which Mr.G o u g e hasplaced before everyone,11 ins paper, will be w e l c o m e d . Mr. G o u g e has had"verynany years 'exp er ience wi th Sho r t Bro s, of"R o c h e s t e r , who everyone knows, have prod uce d some of the m o s t sea-c v e r - r - m S t a i r w o r t h y . wh i ch m eans m os t e ff ic ien t inv direction, flying-boats at presen t ex i s t en t , and w h o s eDf a T r 7 o n t h e cons t ruc t i on , opera t i on and m a i n t e n a n c ee-ini n S U l t a b l e f o r t e s t i n g o u t the charac t e r i s t i cs of b o t hipiane floats and f ly ing-boat hul l s must rank amongst theimnortant work which has been done by any onefron m fT m im t h i s c o u n t r y - So v a l u a b l e is the i n fo rma-it in f?,M r ,m Mr- G o u g ' s l ec t u re , t ha t be l ow we pub l i shPresident- u n t h e M a s t e r of Sempi l l , in the absence of theconnect R ' F a i r e y > w h o was a b r o a d in Bel g i um inlas b f J n i W , t h e r e c e n t o rder for F ai r ey a i r c r af t wh i chremark" , }' h e B s l 8 i a n G o v e r n m e n t , in his open i ng"*" th f"fi-v t h e a s s e m b l y t h a t M o n d a y , J a n u a r y 12,s m a l l m v e r s a r y of the Soci e t y , du r i ng wh i ch " t i mein? H n m S O m e 3 0 m e m b e r s , to over 4 ,000. He

0 , < ' a u ^ e d Mr. G o u g e as the Chief Designer of S h o r tlecture . _ c h e s t e r > whoproceeded t hen to give the followingThR'hich niari'e Ct l l a v e to t a l k ab ou t t h i s even i ng is one on' ^"y Previous pap ers hav e been read before th ism t a i T \ a a ' d ^ m u s t ask you not to e x p e c t m u c hen i j i a t e n a l from me. P r a c t i c a l l y the w h o l e of the*'-^ce aV H e , g a t h e r e d t o g e t h e r is the r esu l t of p r a c t i c a le tme m t h l S P o i n t o f v i e w J h o P e j t w i u p r o v e59

Dur i ng r ecen t year s , cons i derab l e deve l opmen t has beendone , and good p rog ress made wi t h bo t h the design andopera t i on of big flying-boats. Also, the Schnei der Cup r aceshave caused a c o n s i d e r a b l e a m o u n t of r esearch work to bed o n e on thed e v e l o p m e n t of t w i n - f l oa t seap l anes , par t i cu l ar l ywi t h r egard to t a k i n g oft the w a t e r at very h i gh speeds , andalso to the air r es i s t ance of the floats.I n the l as t year we have seen the a d v e n t of the Dorn i erDo. X, the largest flying-boat yet cons t ruc t ed , whose t o t a lf ly ing weight approximates to 50 t ons . T h i s , in itself, isan i mmense s t op fo rward , and all cred i t is due to Dr. Dorn i erfor his courage in concei v i ng , and his skil l in p r o d u c i n g , aboat cons i derab l y more t han t wi ce thewei gh t of any " h e a v i e r -t h a n - a i r " m a c h i n e p r e v i o u s ly a t t e m p t e d .W h i l e we h a v e n o t h i n g in Eng l and wh i ch is near l y equalt o the Do. X, in w e i g h t and size, we h a v e , h o w e v e r , d u r i n gt he l as t t h ree mon t hs , p roduced a successful boat whichhas flown at an a l l -up wei gh t of 40,000 lb. a p p r o x i m a t e l y .Recen t year s have a l so seen the success fu l deve l opmen tof the Bri t i sh f ly ing-boat for p a s s e n g e r and mai l - car ry i ngwork , and along these l ines we may e x p e c t , in the n e a rfuture, very rapid extensions, for it is i mposs i b l e to l ink up

( S D O DPLVNG BOAT - D I S P LA C E M E N T 3 6 2 0 0 LBS .

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2 2 26 30 54SP E E D IN KN OT S 3 6 4-2 4 6 3 0

TWIN FLOAT SEAPLANE DISPLA CEME NT 21,000LBS.

Z s o p o FK.IA

-.- . ".26 50 i4SPEED IN KNOTS

SO


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the outlying portions oftheBritish Empire without coveringin some parts ofthe journey considerable portions ofopensea.I propose now todescribe briefly themethod andprocedureadopted by myfirm when commencing anewdesign ofeither

a flying-boat or aseaplane.Assuming that all thepreliminary details have been settled,the first step is todetermine the suitability ofthe proposedhull orfloats forthe design under consideration. Amodelis, therefore, made of ahull orfloat asaccurate aspossibleto some convenient scale, usually th to thfull size,according tothesize ofthemachine and therange ofspeedsat which theresults arerequired. These models areusuallymade ofmahogany with apolished finish. Bymeansofweights andalight frame attached to it, themodel isbalancedabout atransverse axis, passing through thepoint corre-sponding to thecentre ofgravity of thefull-size aircraft,and isattached to asupporting apparatus on thetestingtank carriage.The model istowed bymeans of along towing rodwhichis arranged to beapproximately at thesame heightandinclination as thepropeller thrust axis. The other end ofthis rod isattached tosome form ofrecording apparatus,whereby, when themodel isbeing run, thepull required to

overcome thewater resistance can bemeasured.At anyparticular speed atwhich it isrequired torun themodel, dueallowance ismade for theequivalent lift obtainedfrom theplanes infull size, this being done by theapplicationof weights supported bylight wires over pulleys. It isusual inthepreliminary runs toassume this airlift propor-tional tothesquare ofthespeed. Iftheangle atwhich theboat runs departs much from theangle ofmaximum lift,further runs aremade correcting for this want ofair lift.A form ofparallel link apparatus attached to themodelenables theangle ofthe model, compared with some fixeddatum line, to bemeasured atany particular speed. Thedatum line usually used istheforebody keel line just forwardof themain step. The water resistance isassumed to bewholly duetowave formation, theresults being predictedfor full size from themodel records, on thebasis ofFroude'slaw ofsimilarity. Theabove is avery brief descriptionofthe apparatus used, and themethod ofcarrying out thevarious tests. Those who require further information onthis subject arereferred toR. & M. 655.

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SPEED INKNOTSFLYING BOAT DISPLACEMENT 36.2 00 LBS.

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22 26 3O 94 36 42SPEtO INKNOTS.

TWIN FLOAT SEAPLANE - DISPLACEMENT 21,000 LAS.

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TWIN'FLOAT SEAPLANE

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T A K EOF - 60KNOT

A 22 2 5O 34 VfiPEfJ Ifs KNOTSThe twomain objects ofmodel tests onatesting tankare(1) todetermine the resistance of afloat orhull duringthetake-off run; and (2) todetermine theangle atwhichthefloat orhull runs relative tothefixed datum line.The planes on thefull-size aircraft areattached atsomefixed angle relative to thedatum line, andconsequentlythe running angle of thehull orfloats has animportantbearing upon thelift obtained. Thus, bycarryingoutests at thetank, curves showing resistance andrunningangle at anyspeed may beplotted. It should benoted

here that it is notalways possible to setplanes on ahullat thebest angle fortaking off thewater, because otherconsiderations maydetermine that the planes should be setat afiner angle. Thus, if thechief consideration in thedesign of aboat orseaplane is topspeed, thewings willdefinitely be at afiner angle than ifthe chief considerationis cruising speed. This is one of thedifferences betweenmilitary andcivil aircraft. Military aircraft areusuallydesigned ontop speed, whereas civil aircraft are invariablydesigned to be asefficient aspossible at adefinite cruisingspeed.A typical resistance curve ofahull shows that theresistanceincreases steadily to amaximum value at aspeed approxi-mately 30per cent, ofthat atwhich themachine becomesairborne, andasspeed increases beyond this, theresistancedecreases more orless uniformly. SeeFigs. 1 and 1A.Fig. 1shows theresistance curve of anormal type flyinpboat at anall-up weight of36,200 lb. Fig.1A showstheresistance curve of atwin-float seaplane of anall-up weightof 21,0001b.Cases have arisen where acertain increase ofresistanceoccurs atabout 60 to 70 percent, oftake-off speed, buthis isusually duetosome characteristics ofthe hull line*and isseldom encountered inpresent-day design. See Fig -If it issuspected that onaparticular model theresistant'will increase athigh speeds, then asmall model isman1so that theresistance atspeeds approaching thetake-ofspeed may beinvestigated. 4These small models areused as anindication only,a1'1are notrelied upon, because thescale istoosmall foranig"degree ofaccuracy.The angle taken up byahull datum line increases gradual!)as speed increases, although very often aslight decrease i>noticed just prior tothe machine becoming airborne '^Fig. 3,which shows theangle curve corresponding totheresistance curve shown inFig.1 ,Floats angle back rather suddenly asthe speed atwhicnthe hump resistance occurs isreached, andthen usual!1 tnwforward slowly asspeed increases. Efficient floats can J*expected toangle forward about 1| to 2deg. from t.-maximum value, by thetime take-off speed isobai*See Fig.3A, which shows the angle curve correspondingthe resistance curve shown inFig. 1A.Load onWater/Resistance -As ameans ofrelative comparison between differen"typ._of hulls andfloats, acurve showing thevalue ofthe raload onwater/resistance atanyspeed, isplotted.

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JL1G:iT, JANUARY 16, 1931

: IMPROVING THEBREED : The Short " Singapore " Mark II has been cleaned up byplacing the four Rolls-f Royce " F " engines in tandem, and by abolishing thechine struts. Instead the inner part of the lower wingshas been thickened. ( FL I GHT Photo.)

This value is a form of eff iciency rat io , the a c tu a l lo a d on' water atany s p e e d b e in g thedif ference between theall-up weight ofthe a i r c r a f t and the lift from the p la n e s o b ta in e dat that speed.Usually, this ratio has a m in im u m v a lu e c o in c id in g w i ththe speed at w h ic h the " h u m p " r e s i s ta n c e o c c u r s , and athigher speeds thev a lu e in c r e a s e s a c c o r d in g to ther a t e ofdecrease of r e s i s ta n c e .Efficient present-day hulls have a m i n i m u m I. R r a t i o of5-0 to5-4, while for floats a v a lu e of 4-0 to 4-5 can beobtained. SeeF i g s . 4and 4 A .Fig. 4shows the L/R c u r v e c o r r e s p o n d in g tothe r e s i s ta n c ec u r v e g iv e n o n F ig . 1. Fig.4A isas im i la r c u r v e c o r r e s p o n d in gto theres is tance curve g iven onFig. 1A.Cross CurvesI 'or various reasons, it is r e q u i r e d to k n o w the c h a r a c -| teristies and b e h a v i o u r of h u l l s and floats as the take-off, . ina p p r o a c h e d , w h e n s u b je c t to m o m e n ts ; a p p l ie d by: loads on thet a i l p la n e . T e s t s can bec a r r ie d out, w h e r e b yat any c o n s ta n t s p e e d thea n g le and r e s i s t a n c e for a c o r r e -sponding applied moment are r e c o r d e d .i Usually, asthe hull orf loats are t r im m e d f o r w a r d s l ig h t ly

| from then a tu r a l r u n n in g p o s i t io n , a d e c r e a s e inr e s i s t a n c e is! obtained, notwith.standing the fact that less l i f t has t h e r e b ybeen obtained from the p la n e s .It will benoticed f rom thec u r v e s , h o w e v e r , th a t if t r i m m e dforward toom u c h , the r e s i s ta n c e in c r e a s e s , as is also thecase when them a c h i n e is t r i m m e d aft. Fig. 5 s h o w s themoment curv e at40k n o t s fora f iv ing-boat hull at a d is p la c e -ment of30,000 lb. ','& fi shows ac o r r e s p o n d in g c u r v e fora twin-f loat seaplane1/21,000 lb. d i s p l a c e m e n t .It should benoie-1. h e r e t h a t them o m e n t c u r v e s s h o w na o not allow forany m o m e n t due to thep r o p e l le r th r u s t ,a n asinboth cases theth r u s t l in e iss l ig h t ly a b o v e thec e n t r e0 gravity, ther u n n in g a n g le of thefu ll-s ize machine wil lslightly less than thea n g le in d ic a te d on thec u r v e s . InPtn cases them o m e n t did not excee d 15 ,000 f t . - lbs . T here considerable differen ce of o p in io n r e g a r d in g the e a s e w i thturf iL i ^ i n 8 ~ b o a t o r s e a p la n e s h o u ld bec o n t r o l la b le lo n g i -tak w h i l e on the w a t e r at s p e e d s a p p r o a c h i n g theis c + s P e e d . I amofthe o p in io n th a t allt h a t is r e q u i r e d

]{ o n t : ol sufficient to t r i m the m a c h i n e a b o u t t h r e e d e g r e e s .t mor_- th a n th i s is a l lo w e d th e r e is thep o s s ib i l i ty , if they hasn o r m a l Vs e c t i o n s , t h a t thehull or floats willa l v s e c t i o n s , t h a t theh u l l or floats willraach , d l r e c t i o n a l l y u n s t a b l e , w i t h the r e s u l t t h a t thestarbr,' -ri"^ h a v e the t e n d e n c y tos w in g q u ic k ly to p o r t or

A P p i i c a b l e t o F l o a t s O n l y^ :!in*l Stability (Static)varioi floats atr e s t u n d e r n o r m a l c o n d i t i o n s of lo a d in gn , l o m e n t s are applied , both fore and aft. and the" a m g a t t i t u d e t a k e n upis r e c o r d e d .orre

Jiy t h i s m e a n s a s ta b i l i ty c u r v e iso b ta in e d s h o w in g them o m e n ts w h ic h w i l l c a u s e thefloat to b e c o m e u n s ta b le f o r ea n d aft, andf r o m th is c u r v e thea p p r o x i m a t e l o n g i t u d i n a lm e t a c e n t r i c h e i g h t canbec a l c u l a t e d .E x p e r i e n c e w i t h v a r i o u s t y p e s of s e a p la n i s w h ic h h a v ep r o v e d s a t i s f a c to r y , has ledtothea s s u m p t i o n t h a t if the all-

u p w e ig h t ofm a c h i n e is IVlbs., t h e n for a r e a s o n a b le d e g r e eo f s ta b i l i ty :L o n g i t u d i n a l G.M. s h o u ld n o tb e le s s th a n1-77^" 'a n d them o m e n t toc a u s e in s ta b i l i ty aft should not bel e s s th a n

(1/10) W'l*F i g . 7shows such a m o m e n t c u r v e .

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FLIGHT, J A N U A R Y 16. 1931

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\Tftr tK TE5T5 OnTWIl i FLOWSMOMEfiT - ATTITUDE RESlSTAnCE' CURVESAT COHSTAhT SPEED 46 KriOTS

i I IDISPLACEMENT AT REST 21,000 \&~ 3 I60.00C

120.000

80.000

12.00

BOOTransverse Stability of Twin-Float SeaplanesTests are not usually c arried out at the tank to investigateth e transverse stability.The track of the floats is arrived at for any machine so thatthe calculated transverse G.M. is not less than :]J IVwhen Wis the all-up weight of the machine, and an additional marginof safety is allowed for in the case of high wing monoplanesIf twin-floats are to have the necessary degree of stability,together with reasonable seaworthiness", it is essential thatthey should have a reserve buoyancy of not less than 100 percent. For all commercial machines this figure should beexceeded.Single-Float SeaplanesSingle-float seaplanes stabilised with small wing tip floatshave not been used to any extent in this country, althoughthey are used in good numbers in the U.S.A. The y havedefinite advantages, in some respects, over the twin-float type,particularly with regard to longitudinal stability and effi-ciency on the water. Figs. 8 and 9 give a comparison betweentank tes ts of twin and single floats for the same machine, thetotal weight in each case being 1.650 lb. From these testsi t is apparent that the single-float ty pe is 60 lb., less resistancea t the hump, and the resistance falls off bette r after the hump.I t is very surprising that so little work has been done on thesingle-float type in this country.Before leaving the subject of tes ts on floats there are twoother tests which it is usual to make on any new design offloats. The first of these is to determine the effect of nosediving moments on the running angle of the floats at slowspeeds. All floats, so far as myexperience goes, are definitelyless stable longitudinally when taxying than when they areat rest. Fig. 10 gives the effect of"nose diving moments onthe running angle of a twin-float seaplane of 21,000 lbs.tot al weight. From this curve it is seen that a moment of42,500 ft.-lbs. subm erges the floats at just over 20 knots .Fig. 11 gives a similar curve for a single-float seaplane of5,500 lb. weight. Theother te st referred to above is the effectof moments on the floats when the machine is drifting back-wards.The foregoing is a brief description of the main resultsobtained from a model test, and theactua l results given are formodern boats and floats. The boats are of the two-step type

with the main step just aft of a vertical linethrough the centre of gravity, the exactposition being influenced by the height of200.000 t n e centre of gravity above the water huean d the position of the thrust line. The floatsare all of the long single-step type , the bowsbeing somewhat similar to the bows of aboat while the aft end runs in a sharp Vet.In addition to the standard tests pre-viously described, there are several othertests which it is usual to make, such asmeasurement of the height of the bowwaveand its position relative to the propellerdisc. This test is particularly important ona hull which may be overloaded by morethan 5 or 10 per cent. The position of thetail plane relative to the wave that leavesthe end of a float is also important, for caseshave arisen where this wave swamps thetail during landing.The longitudinal stability of the hull orfloat when running at high speed on thewater is also investigated. The period ofoscillation of the model does not representthe periods in full size, but it can, I thinkbe definitely stated that if the model runsstable on the testing tank the full size hulior float will also run stable.The Tank at RochesterBefore leaving the subject of tank tests itmay be of interest to describe some researchwork which has been u ndertaken in thepastyear on Messrs. Short Bros', testing tank.The object of this research work was toobtain some idea of the distribution of thepressures over the bottom of a hull or floatwhen moving through the water at highspeed. Considerable work has been done invarious parts of the world on the impactloads during take-off and landing ; theseimpact loads are very important and inter-esting, but it seems to me tha t if real pro-gress is to be made the first thing toinvestigate is the pressures under steadyconditions. After a considerable amount of work on amode!of a complete hull, the conclusion wasarrived at that we werestill working on a shape that was too complex for the resultsto be analysed with any degree of success. (The resultsoisome of this work were published in Aircraft Engineering forNovember 24,1927).

16 0. OOC.

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80.00062


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FLOATS INSTEAD OF HULL: The Shor t "Valet ta" mono-seap lane, Br is to lalso be fitted with land undercarriage. (FLIGHT Photo.) Jup i ter " engines, can

I The tests were, therefore, started once more, working thistime on a flat plate immersed in the water a constant depth,and running at a constant angle and a constant speed, theintention being to run at various angles and various speedsandfinally o divide the plate in two to form various angles ofVee and repeat the test through the various angles and speeds.This is, of course, a very extensive programme, but it ishoped on its completion to know a little more about pressureswhich occur on the forebody of either a hull or float understeady conditions.Fig. A shows the pressure measured on a flat plate, 18 in.of its length being immersed at an angle of 10 when run at aconstant speed of 15 ft. per second. The results are given inlb. per square inch, and in the form of lines of constant pres-sure. This simple case gives some idea howcomplex is the subject of pressures on thebottom of a hull or float.

Wing-Tip FloatsThe whole of the tests on hulls, which Ihave described previously, refer to hullsstabilised laterally with 'wing-floats. Therequisite size or volume of wing-float for anymachine to give a reasonable degree oftransverse stability has been the subject ofmuch discussion, which I do not propose tocontinue here.Fig- 12 gives the values of the displacementof the wing-float, multiplied by its distancefrom the centre of gravity, plotted against'he total weight of the machine for a familyof boats, of weights ranging from 1,750 lb. toThis family being generally similar, theangle of roll to submerge the floats remainspractically constant and equal to 6 to 6|.Ihe actual volume of the wing-floats in1S family is increasing slightly faster with than the main hull, though for exactlyf ilar machines the wing-float volume shouldW directly proportional to the total weightof the machine.I have drawn attention to this point

flri?"S,c ! t h a s b e e n st

ated that, finally, whents become large enough they willlaterally stable, using the main hull, this is contrary to my experience.T h e Effict of Non-Standard Conditionsn sake-off of a Flying-Boat . or

wSea.:,laneith :he rapidly increasing use of flying-EmD c o n n e c t up the outlying parts of thet>robl ' )articularly in the tropics, variousK lem have arisen, and I now wish toHensit - u t ' o n to the serious effect of air" at y U t h e take-off qualities of a flyingi or si-aplane.e :; ithematical investigation of this

problem is given in an appendix to this paper, and is anextension of an article published by myself in Aircraftfcngineering, for November 24, 1927.Although the underlying assumptions on which thismathematical investigation is based are not altogether exact,they are sufficiently close to the truth to give a very goodapproximation. The only reason for using these mathematicalexpressions instead of a step-by-step integration is that thelatter method is so tedious that it is almost impossible to drawany general conclusion therefrom.Calculations have been made relative to a particular machinewhose all-up weight, under normal conditions, is 22,500 lb.The full line in Fig. 13 gives the calculated time to take-offplotted against all-up weight in air of standard density of a


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Some Aspects of the Design of Seagoing Aircraft Part 1

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