7/27/2019 The Aircraft Engineer February 28, 1935

1/3

February 28, 1935

u>

Suppl em ent to

E^FD1 U 226a

No. 109 (V0 iU0m^ X ) 10th Year

F L I G H TENGINEERING SECTIONEd i t ed by C . M . P OU LSEN

February 28, 1935

THE R.A.E. TANK*Tests for Interference : Large M odels Found Practicable : Four-w ire Suspension

System : Accelerated Motion Tests : Dyna mically-similar ModelsB y L . P . C O O M B E S , B . S c , A . C . G . I .

- en g th .ft .6 5 01 , 0 5 01 ,9 8 0

W i d t h .ft .916.524

D e p t h .ft .4-58-31 2

Max . Sp eedof Carr iage.40 ft. /sec.6688

I N the introduction t o his pape r, Mr. Coo mbes recal ledthat seaplane tank tes t ing was an offshoot of ship tanktest ing, the technique of which was founded by Will iamFroude nearly 70 yea rs ago . A pa rt from th e fact th ata scale model was towed thr ou gh s t i ll w ater the re w as no wlit tle in common betw een th e tw o sys tem s of tes t , th e on eremaining link being Fr ou de 's law s, whi ch w ere still usedas a basis of corre lation bet we en mo del an d full sc ale.The dimensions of the ne w R .A .E . ta nk wer e fixed by asmall committee. The s ize of ta nk re com me nde d b y th ecommittee is com pare d, in th e fol lowing tab le, wit h tw oother tanks buil t abroa d a t ab ou t th e same t im e.

Tank.R.A.E., EnglandH.S.V.A., GermanyN.A.C.A., U.S.A. "Two immediate problems a wai ted cons iderat ion when thetank was finished. Th e first was the cal ibr atio n of th etank with reference to the size of model it was possible totest in it , and to find wh at cor rec tion s, if an y, wer e toe applied to the resul ts in orde r to pr edi ct t he perfo rma ncem the ful l-size mach ine. Th e second pro ble m w as th ewh'th f e x i s t i n g methods of tes t ing in order to decidep ether o r n o t a n e w technique of t es t ing should be devel -ped. A four-wire suspension of the m odel w as decide dP ,n T w o wires a t the cen t re grav i ty suppor ted the- oaei lateral ly, while fore-and-aft wires de term ine d the

that th IR P l t C h ' F r s i m P l i c i t Y ft c o u l d b e cons ideredof th tT C g ' w i r e s c o u n t e rb a l an c ed t h e ex c es s we i g h tlift anrT+K 1 ' - W h i l e t h e f o r e - a n d- af t wi res appl ied the wingthrust P l tc *ng moment due to wings , t a i lp lane andThfore an ?'**. W l t e S h a d c o u n t e r b a l a n c e w e i g h t s , w h i l e t h ethe air nr W IT eS W e r e a t t a c h e d t o a i r pis tons in which_ _ _ P res s u r e co ul d b e ac c u ra t e l y c o n t ro l led . Th e p i s t o n s>ummary 0f p a p e r r e a d b e f o r e t h e R A e S . o n F e b # 2 2 , 9 3 5

were air-lu brica ted and w orked with very li t t le frict ion.The damping in p i t ch due to cons t ra in t by the a i r p i s tonshad a s tabi l is ing act ion on the models , and i t was possibleto run even qui te uns tab le models wi thout the necess i tyof res t ra in ing the osc i l la t ions by ha nd . The drag balancewas a l s o p n eu m at i c .Wall Interference

When the balances had been ins ta l l ed , the f i rs t p roblemto be at tacked was that of finding the l imitat ions of thetank as regards s ize of model . W al l in ter ference wasmeasured fi rs t , the method employed being to run fi rs tone and then two s imilar models abreast , spaced half thewidth of the t ank apar t . F inal ly , th ree s imi lar modelswere spaced one- th i rd the width of the t an k apar t . Thesize of model used was 4.5ft . long, i .e. , the length w rashalf of the wid th of the t ank . The tes t s showed t ha tmode ls up to 1.5 t imes the width of the ta nk , i .e. , r3.5ft .long, could be tes ted without s ide-wall interference, prov ided that the water was as deep as the model was long .For twin floats spaced half their length apart the l imit ingleng th would be s l ight ly over 7ft . La ter tes ts on a twin-float model indicated that twin floats could be spaced quitec lose together wi thout mutual in terference, none beingfound w ith floats spaced less th an half their leng th a pa rt .

Tank tes ts on depth effects indicated that unless i t weretaxying at about the cri t ical veloci ty a flying-boat wouldnot experience any appreciable depth effects in taking offfrom shal low water.

S c a l e EffectHaving determined that i t was safe to t es t models upto 9ft . long, three models were made to the same l ines ,these being 3 , 4 .5 , and af t . long respec t ively , the corres p o n d i n g s c a le s b e i n g i / i 8 t h , i / i 2 t h , an d i / 6 t h fo r alarg e flying-boat. Th e sam e series of te sts were carrie dou t on each mod el . Th e resul ts showed tha t scale effectwas smal l a t low and h igh speeds and reached a maximum

7/27/2019 The Aircraft Engineer February 28, 1935

2/3

SUPPLEMENT TOFLIGHT2266

iTHE AIRCRAFT ENGINEER F E B R U A R Y 28 (935-

at the hump. As pa rt of the scale effect research, theeffect of rivets, lap joints, chine angles, etc., were represented to scale on the i / 6 t h scale model. A small increaseof resistance throug hou t the speed range was found, bu tthe effect was so small as to be negligible in practice, aresult which was in accord with calculations previouslycarried out and which had indicated that skin friction wasa very small proportion of the total drag.

In order to check the model results obtained in thetank, a slightly more than half full size model hull tothe same lines as those of the tank models had been constructed, and would be tested at Felixstowe on the seaplane fitted with a force-recording undercarriage capableof measuring the water forces on a large-scale float.One difference between tests in a tank and actual takeoff conditions appeared worth y of stu dy. This was theeffect of acceleration on th e forces. From full-scale test smade at Felixstowe it appeared that the " virtual-masseffect was small . From the point of view of th e Fa rn-borough tank it was desired to make a certain number oftests in accelerated motion, not only to clear up this pointbut also in order to develop a method of covering a rangeof speeds in one run. The drag in accelerated motion wasmeasured by the aid of a pendulum connected by a systemof levers to the drag balance. The length of this pe ndulumwas adjustable, so that the effect of acceleration on itwould be exactly equal and opposite to the effect on themass of the model. Thus, the drag balance measured wa terdrag only, taking no account of the large forces needed toaccelerate a heavy model. The tests indicat ed th atacceleration effects on drag and immersion were very smalland could safely be neglected.The lecturer pointed out that it was the practice at theR.A.E. to tow the model through the centre of gravity

and not thro ugh the th rust line. In an actua l flying-boatthe thrust moment was offset by slipstream effects, beingentirely balanced at the moment of take-off. They preferred to calculate thrust moments and apply them withthe air pitching moments.Improvement of Hull Design

The Farnborough tank had been in operation for twoyears , and th e basic research work described, toge therwith a considerable proportion of time spent on development work, had not left muc h time yet for progress. Itwas the opinion of the lecturer that improvement of design would only follow systematic tests of families ofmodels. One investigation was carried out which is ofgeneral inter est. This dealt with the use of stub s or spon-sons for the lateral stabilisation of hulls on the water.Experience with these stabilisers was very limited in thiscoun try. The investigation started with stubs of theDornier type, which were tested on two hulls of similardesign, one having rather flat vee sections followingDornier's design, and the other having more acute anglesin conformity with British practice. The test s showedthat there was no inherent reason why stubs should notbe successful on quite deep vee-bottom hulls.

It was found that the general characteristic of stubs wasthat they interfered markedly with the bow-wave systemat a speed of from 10 to 14 knots, and so introduced a waveinterference hu mp into th e resistance curve . The stubswere attached at various fore-and-aft positions, and theheight above or below the water-line, as well as the angleof att ack , was varied. Heig ht of stu b was found to bemore imp or tan t from a lateral stabil ity point of view. Ifthe sponsons were placed above the water-line the hull wasinitially uns tabl e. [This is interes ting in view of th e fairlyhigh placing of the stubs used on the latest large Latecoeref l y i n g - b o a t .ED . ] The effect of a stub below the water-line was to produce a sort of elongated resistance humpwhich actually seemed to reduce the normal hump resistance.Fro m the test s on fore-and-aft position of stubs it wasfound that the best setting was a compromise between

low resistance (which demanded a position well forwardand a setti ng above t he water -line at a large angle ofat tack) and latera l stab ili ty, which required a mediumposition and medium angle of set ting and a height somewhat below the water-li ne. A bette r compromise wa-obtained by sweeping back the stubs in plan form.The modifications tested in the tank were afterward-tried in the wind tunnel to find their effect on air performance , a s air dra g was one of the main objections to stubsThe ac tua l pos ition ing of the st ubs was found to have onlya minor effect on t op speed, and t he p enalty for fittingstubs instead of wing tip floats on this particular designwas about 2J m.p .h . Allowing for the fact th at it mightbe possible slightly to reduce the main wing area if thestubs gave a cert ain amo un t of lift a t high angles, it mightbe possible to design stubs with a lower drag and slightlyhigher speed. The greatest improv ement in air drag ofstubs was achieved by bet ter aerodynamic design and byfining down the trailing edge.

Of th e longitudinal ins tab ilit y kno wn as porpoising Mr,Coombes said this was still a problem for the designer,as up to now no hard-and-fast rules for its avoidance hadbeen laid down. At the R .A .E . the prob lem was attackedfrom two points of view : analytically and experimentally.It was found th at the assumptions which had to be madein the mathematical work were too crude, and that beforean y survey could be mad e the de rivatives would have tobe measured for actual hulls . Unt il tes ts could be madethe mathematical side had been side-tracked.Dynamic Models

Mr. Coombes concluded his paper with the followingremarks: " Another fact which emerged from th e theory was thatmodel experiments could no t be relied upon unless theair structure was represented. Dynamical ly similar modelscomplete with wings and tail plane of approximately thecorrect proportions are therefore used. The models aremade of balsa wood, waterproofed and strengthened asnecessary, a nd are of th e correct mass and moment ofiner tia. They are towed by a special fitting which allowsfreedom in rise and pi tch, b ut preven ts yawing or rolling.The technique of making and tes ting such models has beendeveloped so that an y i mpor tant design can be vetted inth e model stage , and, if necessary, al tera tions made andtried t o eradicate any tendenc y to instability. In thecourse of tes tin g these models it has been found that seaplanes have a cer tain range of angles within which theyare stable. If trimmed too far nose down or tail down,porpoising occurs. These angu lar limi ts vary with speed,but ar e well defined as a rule , and do not alter much withchange of e.g. position, loading or moment of inertia. Thisenormously simplifies the test ing, which has been boileddown to a relatively simple routine.

" The value of this dynamic model test is fully apparentwhen an unorthodox design is under consideration. As anext reme example I might quo te a recent proposal for a

o A E.A "dynamic " model of a flying boat, as used in the K.A-Tank. (Flight photograph).

7/27/2019 The Aircraft Engineer February 28, 1935

3/3

FEBRIARY 28, 1935- THE AIRCRAFT ENGINEER SU PPLEMEN T TOF L I G H T2 2 6 clarge tai l - less f ly ing boat having a Pterodactyl wing plustubs a combinat ion of tw o exp erim enta l features . InS,U c o u r g e of the tes ts we have discovered a few interes t ingfacts about porpois ing and i ts am el ior at ion , b ut n ot enou gh

warrant laying down rules for success . He re is ano the rcase where sys tem at ic c han ges as oppos ed to ad hoc t e s t sa r e desirable, and i t i s hoped to have t ime in the nearfuture to sta rt such a series . So far, we hav e ha d ve rylittle opportuni ty to com par e th e res ul ts pred icte d frommodels with full-scale be ha vio ur , b u t w her e this ha sbeen done the agre em ent bet we en m ode l an d full-scale isgood." Seaworthiness tes ts in a t rain of waves demand a s imilartechnique to porpois ing, and dy na m ic m odels are neces sary.A wave-maker has been ins ta l led in th e R. A .E . ta nk andexperiments in th is d i rect io n h av e been com me nce d."Another in teres t ing use of dynamic models was in anexperiment made in connect ion with a proposal for impacttests on hul ls . By pho tog rap hin g in s low motio n th e impact of a model in which v ario us a l tera t ion s w ere m ad e,e.g ., the ai r s t ructu re rem oved and l i f t appl ied by weig hts ,moment of inertia alt ere d, n o lift app lied , et c. , i t w aspossible to say wh at t yp e of tes t w as des irab le in o rde rto give approximately the r i gh t mo t ion a fter im pa ct .

Future Research"A few words should be said wi th regard to the workcontemplated. Several des ign ers ha ve expressed t he view

tha t the uns tab le yawing moments on cer t a in types o f hu l l sshou ld be inves t iga ted . A t the sam e t ime yaw ing mom entsa t l ow speeds a re requ i red fo r water rudder t es t s , and weare therefo re des ign ing a ba lance to measure yawingm o m e n t s . T h e s a m e a p p a r a t u s w il l m e a s u r e r ol li n gm o m e n t s m o r e s a t i s f a c t o r i l y t h a n t h e p r e s e n t c r u d ear rangement and fu r ther sys t emat i c t es t s on l a t e ra ls tabi l isers wi l l be made.Exper imen t s on changes in hu l l des ign in o rder to improve e ff ic iency hav e com me nced . Some of the changescon templa ted a re in the d i rec t ion o f unor thodox hu l l fo rms ,but the effect of al ter ing s tandard types wi l l also be t r ied .The s tud y o f po rpo i s ing i s go ing fo rward by b o th ana ly t i ca land exper ime n ta l me thods . The ana lys i s depends on theeva lua t ion o f the s t ab i l i t y der iva t ives , and th i s i s i n hand .The most profi table f ield for experiment is an invest igat ionon dynamic models in which the effect of sys temat icchanges in hul l des ign are correlated with their effects ons t a b i l i t y .

" In al l the foregoing discuss ion I have made no reference to deve lopment t es t s , t hough a l a rge p ropor t ion o fthe t an k t ime i s devo ted to thes e . Th e reasons a re obv ious ,as the resul ts are, in general , confident ial and appl icableon ly to the par t i cu la r des ign unde r t es t . The researchmethods descr ibed a re f requen t ly used u l t imate ly in rou t inetes ts , as in the case of porpois ing where a large proport ionof new des igns a re cons t ruc ted as dynamic model s in o rdert o a ss e ss t h e i r s t a b i l i t y . "

CONTROL SURFACE FLAPSfor TRIM and BALANCEGenerally referred to in Am erica as " Tabs," these Flaps are called " Trimmers"when used Instead of Adjustable Tailplanes, and as " Balances " when theirFunction is to Reduce Hinge M oments of Control Surfaces

A L T H O U G H t r i m m i n g t a i l p l a n e s a r e s t il l u s e d o nf-^ t h e m a j o r i t y of B r i t i s h a i r c r a f t , t h e r e i s at e nd e n cy i n t h e m o s t m o d e r n d e s i g n s t o a d o p tthe smal l t ra i l ing -edge f l ap w hich ha s bec om e po pu larwith Amer ican des ig ners o f rec en t yea rs . T he fo l lowingartic le will p ro bab ly be o f in t e r es t t o Br i t i s h des igne rs .I t is t a k e n , w i t h d u e a c k n o w l e d g m e n t , f r o m t h eJournal of the Aeronautical Sciences, w h i c h i s t h eAmer ican " oppos i t e n um be r " to ou r Journal of theRoyal Aeronautical Society. I t is p u b l i s h e d b y t h eI ns ti tu te of t h e A e r o n a u t i c a l S c i e n c e s , I n c . w h i c hc or re sp on ds t o o u r R . A e . S . a n d n o w a p p e a r s s i x t i m e sper annum. Th e au th or o f the a r t i c l e i s Mr . A . E .

r d - of t h e C u r t i s s A e r o p l a n e a n d M o t o r C o . , I n c .l l t 7 t l l e Pas t few years ( savs Mr . Lombard) thePpncat ion of smal l f laps to the movable control surfacesaerop lanes has become ex tens ive . T ha t these f laps g ivequick, Hght and hig hly effect ive co ntr ol i s show n b y th ed i J n t h e C u r t i s s - W r i g h t " C o n d o r " a 9 0 d e g.ono r n \ 0 f t h e e l e v a to r f l ap con t ro l whee l changes theon ? h !nf' t n m o f t h e a e r o p l a n e a s m u c h a s 2 8 t u r n sOn S t a b I h s e r a d i u s t m e n t c o n t r o l .n t i L a e i P , e s o f m o r e t h a n one eng ine the con t ro l f l ap5 t h e rudder has proved to be ofwhen flying

fuselage, an d th e ru dd er wi th i t s contr ol flap has sufficientp o w e r t o h o l d t h e a e r o p l a n e o n a s t r a i g h t c o u r s e w i t h o n eeng ine ou t o f commiss ion .When used to g ive con t ro l i n l i eu o f an ad jus t ab les t ab i l i s e r o r ver t i ca l f in the con t ro l f l ap may be ca l l ed a" t r i m m e r , " a n d w h e n u s e d t o r e d u c e t h e h in g e m o m e n t sof a con t ro l su r face the t e rm " ba lanc e " i s appr op r i a t e .In the fo l lowing no tes the theore t i ca l fo rmulae fo r theeffect of these f laps are presented as wel l as the resul ts ofw i n d t u n n e l t e s t s o n s e v e r a l t y p e s o f t r i m m e r s a n db a l a n c e s .Nomenclature

a, = s tabi l iser angle, see Fig . 1 .S = e l eva to r ang le , s ee F ig . 1.i = con trol f lap angle, see Fig . 1 .c t e lev a to r cho rd a f t o f h inge , wh ich inc ludes chord o fins et flap.* See Fig . 1.c f f lap chord aft of h ing e.* See Fig . 1 .c, = o ver al l cho rd of tai l . * See Fig . 1 .S , = e l ev a to r a r ea a f t o f h inge inc lud ing a rea of in se tflap.*S f = flap area aft of h inge.*

ilot g r e a t a s s i s t a n c e t o t h eesponsibl f m g W a t h n e en& ne d e a d - a n d i s I a r g e l yan be d C- r * ^ e ^ a c * t n a t m o d e r n t w i n e n g i n e a e r o p l a n e sor e v a , ? n e d s a t i s f a c t o r i l v w i t h a s in g l e v e r t i c a l t a i l .929-1Tcoo hA the o r i & i n a ] C u r t i s s " C o n d o r , " b u i l t i n'reams fr' d t w o f in s a n d rudders , l oca ted in the s l ip -0 be mai T - t w o P r o P e l l e r s to ena b le s t r a ig h t f li gh tu i lt with - ed n n e e n S i n e - T h e new " Condor " i sn a s ing le ver t i ca l t a i l mo un te d r ig id lv on th e

See Eqs . (1 an d 2 ) .See Eqs. (1 and 2) .E, = c,/c t.p t , y c . A, = p a r a m e t e r s e l e v a t o r .y / , A/ = par am eters con t ro l f lap.q = d y n a m i c p r e s s u r e ( = pV-lz).C L = Uit/qS.CHf ( E l e v a t o r h i n g e m o m e n t ) jqc fS f.

* In th e case of the tests with th e external flaps, see Fig. 8, the elevator chord andarea, c,. and S, do not include the chord and area of the flap. Also, in this casect an d S f represent the tota l chord and area of the flap rathe r than just th e chord andarea aft of the hinge.