NACA TM 426 Seaplane Floats and Hulls

7/27/2019 NACA TM 426 Seaplane Floats and Hulls

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NACA TM 426

TECHNICAL MEMORANDUMSNATIONAL ADVISORY COMMITTEE FOR AERONAUTICS

No. 426

SEAPLANE FLOATS A.ND HULLSBy H . H e r r m a n n

P A R T I

From " B e r i c h t e und A b h a n dl u ng e n d e r W i s s e n s c h a f t e nG e s e l l s c h a f t f u r L u f t f a h r t "D e c e m b e r , 1926

Was h ing tonAugus t , 1927


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12 84 24 12 132I t a l y 176 176Russia 30 30 120 180America 135 66 264 264 461

There a r e two methods f o r a government t o develop good seplanes : I n the f i r s t in s tance , by placing a~ order o r caJ.1


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ed f o r t h e same tak e-o ff speed, The same r e. by changing t h e take-off speed o s bo th the load

off speed simultan eously, which i s usu sal ly th e case. Then i t u d e of t h e v a r i a t i o n i s shown i n Figs . 3-5. Ther e f e r t o a p a i r of twin f l oa t s , Cond i tions a reing boat s.

The procedures outl ined above a re n ot s u f f i c i e n t f o r cornparing two p a i r s o f f l o a t s o r two f l yi ng boats . In t h i s con-

s a r e p l o t t e d as


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thet h eThe

-

t o t a l f l o a t capacity . A s far its seaworthiness i s concapaci ty i s u s u a l l y 1.8-2.2 times the displacement atwater r e s i s t a n c e of t tvin-f loats of 2 t o n s t o t

shown i n Fig. 1. The shape of the f l o a t i s shorn i n FiYJithout changing i t s submerged portion, a f l y i n g boat may

be provided with a cab in hu l l o r w i t h a rn i l i t 'vy hul l , The t o -


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measurement accuracy.

Water resistance dependsl a r g e l y o n flow underhull, which changesslowly with increas ingspeed.

There i s no acce le ra t io( 2 ) When there i s a c c e l e r a t i ot h e h u l l always travelsw i t h a flow diagsponding t o a l oVaria t ion of w a ttance i s with in range ofmeatjureraent accuracy( 3 )P a r t of water r e s i s t a n c e i s Owing t o f r i c t i o n , water re-due t o f r i c t i o n subject t o s i s t an ce measurements of

the Reynolds l a w . model are too high andmust be corrected,. A t low speeds the model i s(4)Controls a re i n e f f e c t i v e at


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D s p lacementMassSpeedAccelerat ion

L Z m xT t S 7

K k kg KVol vol m3 L3M

hm/s 7

V v

hB bA a m h K

L

K = k KVol=vol h3

~ = m7 2h

hv=v -ThB = b -2

A=ah K


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t o t a l weight. Consequently, t h e water re s i s ta nc e of modern boatsi s approximately t e n t i m e s t h e a i r res i s ta ncc . The seaplanet a k e s o f f when t he pr op el le r th rus t exceeds the combined waterand a i r r e s i s t ance .

A r e s u l t of t h e e l e va t e d p o s i t i o n of t h e p r o p e l l e r , withreference t o t h e cen te r o f g r a v i t y and e s p e c i a l l y t o the water-l i n e , i s nose -hea vine ss, which de pr es se s t h e bow. Ce rt ai n bowshapes produce suc t io n ef fe c t s , &de t o t h e i nc re as ed r e l a t i v ev e l o c i t y of t h e water f low. The result i s t h a t many seaplanesnose dovn while t a k i n g o f f , b ef or e t h e c r i t i c a l speed i s reached.This e f f e c t i s counterbalanced by ho ld ing th e e leva to r con t ro l


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Normally a seaplane takes o f f a nand waves, Very seldom, and only when t h e r e ar e w

t hus avo id ing the i r blows. When th e seapl ane a l i g h t s , th e con-d i t i o n s are reversed.t h e w a t e r u n t i i i t s speed decreases t o th e c r i t i c a l speed. Thenthe hull submerges and the seaplane soon comes t o r e s t .mass mul t ip l i ed by t h e r e t a r d a t i o n i s always equa3 t o t h e com-bi ne d water and a i r re s i s t ance .

The seaplane g l ides on the , surf ace . of

The

I n Fig. 9 , th e water res i s t anc e i s seen inc reas ing t o am a x i m u m value and th en de cr ea si ng again. Above is p l o t t e d t h ep r o p e l l e r t h r u s t , from which the a i r re s i s t ance ha s a l readybeen deduced. The tak e-of f time w i l l now be determined. Graph-i c a l means a re used , s ince th e water re s i s t an ce can h a rd l y bec a l c u l a t e d by analyticaJ. methods, A n i s o s c e l e s t r i a n g l e i s

the speed of 9.81 m / s (32.2 f t , / s e c . ) b e i n g i t s base and


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s u b j e c t thus be obtained.Summary o f Information Obtained


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ed and no suc t ion exe r ted on t h e r e a r p o r t i o n of t h e hu l l . Ifno s tep i s provided, a s t r o ng s u c t i o n e f f e c t i s c r e a t e d at t h es t e r n and t h e r e i s no, o r a very small, decresse o f r e s i s t a n c eaboze th e c r i t i c a l speed, Consequently, th e water r e s i s t a n c ecan be overcome by hul ls wi thout steps only when they are veryl i g h t l y loaded.water aornents acting on a s t ep le s s h u l l w i t h ordinary horizon-

I t i s f a r more d i f f i c u l t t o overcorne th e high-


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h i s r e s i s t a n c e a c t s at a c e r t a i n d i s t a nc e from t h e f i n e oust of th .e propel ler and develops a nose-heavy moment

stance and take-off t ime being increased correspondI n Fig. 16 t h e s t e p i s l o c a t e d far beh ind t h e c.g,, and

t h e seapla ne nose-heavy: one by w a t


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N . A , C , A , Technical Memorandum No, 626 16be de fl ec te d enough fur th er t o i n

A h u l l with a t o o e f f i c i e n t b o t t o m it he water before t h e take-off speed i s r el i f t i s not reached at t h a t moment, the seaplane fback on t h e water. The seaplane may alsojus t when, owing t o t h e p o s i t i o n of the s t e p , t h e e l e v a t o r i sa l re a dy f u l l y d e f l e c t e d and a ser i ous accide nt , such as side-sl ip pi ng , may th en resul t . By such le ap s considerable s t re ss esare exe rte d on t h e hul l . The speed at which they begin can bedetermined by tank te s t s . F o r well-designed seaplanes, they donot occur before 90$ of t h e take-off speed i s reached. Englishf l y i n g boats jumped even at 5 0 8 of t h e take-of f speed. TheEnglis h Felixstowe trFurytt (Fig. 25) w i t h f ive 250 HP. Rolls-Royce engines, w a s completely destroye d by such lea ps. I n t h i scase, tank t e s t s should have been made befo re t h e cons t ruc t ionof the seaplane and not a f t e r t h e crash. This tendency can o f -te n be avoided by a s l i g h t displacement of th e c.g. o r by anad di ti on al . moment sometimes f orw ard and sometimes back wa d. I fno improvement i s thus obta inea , the e f f i c i e n c y o f the bottommust be reduced by lowering the s tep o r s h i f t i n g i t forward, o r

by i t sof the c.g. means a reduct ion of t he e f f i c i e n t p a r t of t h e bot-


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waves axe more ea s i l y overcome by p u l l i n g t h e e l e v a t o r c o n t r o lback i n advance. To produce th e des i re d ef fe c t , the second s tmus%be some d i st a n c e behind th e f i r s t s tep. If i t i s too net h e f i r s t s t e p , no s a t i s f a c t o r y s t a b i l i z i n g e f f e c t i s produce

a r e s u b j e c t t o var ious va lua t ions , Therefore , th ere are s t i l lmany con trad ict ory opinions regarding th e rea r s tep ,

Condi t ions are d i f feren t with r e f e re n c e t o a t h i r d s t e

insu re a smooth separa


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N . A . C . A . Technical Meriorandum No. 426 20th e wings , t he h ul l and th e pro pel ler . The spray i s reduced bybending t he upper p a r t down, as i s done on t h e Linton-Hope h u l l s(Figs. 34, 36, 40, 42, 50 and 65), b y f i t t i n g a s t r i p beneaththe outer edge of th e ch ine , thus reducing t h e depth of immer-si on, and by in cr ea sin g t he angle of a t t ack of th e h u l l and byg iv ing a su i t ab le shape t o the bow. The cr oss s ec ti on should behollow and V-shaped w i t h a f l a t , wide ground p l a n and approxi-mately horizontal l a t e r a l and % o t t o m surfaces t o r ide the wa te r .The chines, o r the more o r l e s s horizontal bottom surfaces , mustbe gradua l ly r a i s ed t o w a r d t h e f ro nt . L i t t l e o r no reductiono f spray i s produced by longitudinal beams beneath t h e b o t to m.

The best shape of hul ls , and fl o a t s can be developed by tan kt e s t s o r by building a su f f i c i e n t number of models. The samer e s u l t s a r e ob ta in ed by bo th methods. The second method i s moreexpensive and considerably s l o w e r . Regardl ess of th e danger in-volved, t h i s method was worked out during the war at Felixstowe,E n g l a d , by Colonel J. C. P o r t e , o f f i c e r of t h e B g i t i s h n av ala i r s e r v i c e , who had no engineering training. The r e s u l t i n gs a c r i f i c e s o f human l i f e could have been avoided by ta nk t e s t s .These experiments were subsequen tly desc rib ed by Rennie i n anapologetic note.**Rennie, J. D. - Some Notes on t h e Design, Construction and Op-e r a t i o n of Flyin g Boats. '!The Jour nal of theRoyal. Aeronaut ica l Socie ty ," 1923, p. 123.


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load. Thus theo f developing hydroplaning e99 ciency; such quest io nsl a nd i ng , s ea wor th in es s, s t a b i l i t y , e t c. , were more o r 1l e c t e d u n t i l more powerful engi nes became av ai la bl e. The f i r shull . tes ted was a modified Curtiss l f&ericat l f l y i n g boat ( F i g17): weight, l i g h t , 3100 l b. ; f u l l y loaded, 4500 l b , ; horsepower, 160; l e n g t h of hull, 30 f t . ; single s t ep , p r o j e c t i n gf i n %osw,rd, ending at s t b p , which was undes thhe cog, Fore andaft angle between t h e unders ide of t h e t a i l and planing surfacof t h e s h i p was 10 degrees,

ould be h e l d up during th e accele


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t s obtained with t h e ffAmeSica!fh u l l s , P a r t i c u l a r s :


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s u l t . Fig. 25 shows profi le , plan, and body s ec t i ons i n d e ta i l .It was or ig ina l ly des igned f o r 24,000 l b . t o t a l we i g h t , and t obe f i t t e d w i t h th ree 600 HP. Rolls-Royce Condor en gi nes. A sthese engines d i d not become ava il abl e, f i v e Eagle VII I1s hadt o be used, which l e d t o a decided drop i n a ir performance.

From every poi nt of view, t h e boat was t h e best designtu rned out at Felixstowe. It was found t h a t the normal load

io us F-boats. Loading t e s t s were conti nued up t o


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i-ments i s t h a t t h e l i n e s md dimensions ( F i g .c e s s f u l f l y i n g b oa t h u l l f o r a given displacementevolved, I t now remai ns t o show how t h e var io us f e a tdes ign con t r ibu te toward th e f u l f i l l r r en t of t h e r el a i d down above, and t o ind ic at e where, i f at al l , t h ef r o m what might be deduced f r o m tank t e s t s .

The experience gained requi red severzl months ' TFrork,as th e sane r e s u l t s would have been obtained i n a f e v weeks by


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with i t ,Tlie l i n e s of th e N . 4 T i t a n i a and t h e N .4 Atalanta f ly ing-

boat h u l l s are shown i n Figs. 29-31.* Tank t e s t s were made on-l y a f t e r t he Titania was alr ead y under constru ct ion. I t wasf i r s t in tended to omi t the c e n t r a l p o r t i o n of t h e s t e p andonly th e l a t e r a l por t ions. Owing t o excess ive water res i s%ance ,t h e s t e p W a s subsequently extended over the whole width and even

e l a t e r a l p o r t i o n s were enla rged (F i g . 32) . When the step i ssmall, t h e r e s i s ta n c e i s t y p i c a l l y sirnilas t o t h e ca see i s no s t e p a t a;ll. I t does no t decrease su f f i c i e n t


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v/v s t a r t


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32003000280026002490

a 200c32 2000+>06 18GO;;f 16CO14001200loco8006004002000


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Fig.7

k0


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X.A.C.A.

.P

01

ABC

Figa. 12,13

0 LO 20 30 $OverloadFig.13 Ii>flucilce of ov,grload on take-off t i n e atd i f f e r e n t tcke-off Epceds.


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g.25 Lines of the Fe l ixs tone nFuryn. Better 1:have been obtained w i t h 3, a'nzrper V - b O t t O i 2 born.Inclined t o l eap before reaching taka-off speed, oving t o veryl a r g e and e f f i c i e n t b o t t o n . Trimed aft leaped :.iithinsuf f ic iex t lift, being subsequently crushed.


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NACA TM 426 Seaplane Floats and Hulls

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