Gessow: Helicopter Research Problems

7/30/2019 Gessow: Helicopter Research Problems

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By Alfred Gessow

L r ~ l g l e y Aeronaut ical Laboratory

T h i s paper i s r e s t r i c t e d t o a pre sen ta t ion o f t he more impor tan tproblems as so ci at ed with th e development of 'the m s t s u c c e s s f u l t y p e o fr o t a t i n g - w i n g a i r c r a f t - t h e h e l i c o p t e r - and t o a n i n d i c a t i o n o f t h ep r e s e n t status of res earc h of th ese problems.

The he l i cop te r as t ho u gh t o f a t p r e s e n t i s an a i r c r a f t i n w hichl i f t , p r op u ls i on , and c o n t r o l a r e a l l p ro v id e d by one o r more p r o p e l l e r -l i k e r o t o r s t u r n i n g ab o ut a n a pp ro xi m at el y v e r t i c a l axis . The funda-me nta l advan tage of such a n ar-ement i s t h a t t h e means f o r o b t a i n i n gand c o n t r o l l i n g f l i g h t i s s e p a r a t e d fr om t h e t r a ~ n s l a t i o n a l p ee d s of t h ef u s e l a g e . In s p i t e of t h e many advantages a ffo rded by t h i s f ea t -m e ,n ot ab ly t h a t o f v e r t i c a l f l i g h t , i t w a s only dur ing the l a s t decade thathe l i cop t e r s hav ing sa t i s f ac to ry pe rformance and h a n d l i n g q u a l i t i e s h a v eb e e n b u i l t and flown. T h e i r s u c c es s c a n b e a t t r i b u t e d t o im provedsower pl an ts , an incr eas ed knowledge of g en er al aerodynamics as w e l l a sthe aerodynamics of rota t ing-w ing f l i g h t , and th e backlog of exp er iencega thered from the hundreds o f unsuccess fu l he l i co p te r bu i lde r s s ince th etime of D a Vinc i .

The present-day hel icopteri s s t i l l

i n an ea r l y s t ag e o f development.I t s per fo rmance , hand ling ch ar ac te r i s t i c s , s a fe t y, and r e l i a b i l i t y, h o w -ever, though s t i l l poor when judged by modern airplane standards, area l r e a d y a c c e p t a b l e f o r a number of important applications where i t ss p e c i a l c a p a b i l i t i e s are a t a premium. Prospec t s f o r fu r t he r improvementare good and a w ide f i e l d of a p p l i c a t i o n , b o th mi l i t a ry and commercial,i s assured .

DISCUSSION

The ge ne ra l h e l ic o p te r r e s e q h f i e l d i s , f o r t h e p r e s e n t d i s c u ss i o n ,

d i vi d ed i n t o f o u r b ro ad c l a s s i f i c a t i o n s : performance, vi br at io n andf l u t t e r , s t r e s s e s , and s t a b i l i t y and c o n t r o l . A d e s c r i p t i o n of t h ep r o b l e m e nc o un te re d i n ea ch o f t h e s e f i e l d s i s given, and l i n e s o ff u t u r e r e s e ar c h a r e p o i n t ed o u t .

Performance

The problem of determining th e aerodynamic c ha ra ct e ri st ic s of al i f t i n g ro t o r f o r purposes of des ign o r per fo rmance es t im a t ion i s com-pl ic at ed by the la rg e number of va r ia ble s involved. Consequent ly, t heapproach could no t be wholly em piri cal, and some t h e o r e t i c a l frame workwas r e q u i r e d t o c o r r e l a t e e x pe r im e n ta l d a t a . The performance problem has


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been a t t ac ked the re f o re by t r y i ng t o deve lop a method of ca l cu la t ing th ec h a r a c t e r i s t i c s of t h e r o t o r f rom t h e c h a r a c t e r i s t i c s of t h e b la de a i r f o i lse ct io ns . The method i s similar t o t h a t o f c a l c u l a t i ng p r op e l l e r c ha ra c -t e r i s t i c s by bl a de e le me nt o r s t r i p t h e or y b u t i s much more complicatedbecause of t he f lap pin g motion of th e hinged blad es and because the t r ans -la t ional (edgewise) component of veloci ty i n forw ard f l i g h t , p a t a l s o beaccounted for.

I n o r d e r t o m&e t h e pr ob le m c a pa b le of p r a c t i c a l ~ o l u t i o n , c e r t a i nassunp t ions and s imp l i f i ca t io ns had t o be incorpora ted i n t h e t h eo ry i nadd i t ion t o the pr imary one of us ing two-d imensional a i r f o i l cha r ac te r-i s t i c s i n summing up th e f o rce s a c t i ng on the b l a d es o f t h e r o t o r. Ap r i n c i p a l a s s u m p t i o n s p e c i f i e d t h a t t h e r o t o r i n d u c e d v e l o c i t y c o u l d b eca lc ul at ed by t he momentum the ory and could be conside red t o be uniforma c r o s s t h e r o t o r d i s k . (S ee r e f e r e n c e 1. T h e r e s u l t i n g c a l c u l a t i o n e ,

which were extremely lengthy and comglicated,were

s impl i f ied andcondensed in to des ign char t s tha t g ive a good ineight as t o t h e e f f e c t sof changes i n r o t o r d e si g n p a ra m e te r s. ( see re fe rence 2 - S u f f i c i e n tcomparisons of the theory with exper imental data have been obtained fromf l i g h t and f u l l - s c a l e t u nn e l t e s t s t o prove t h e v a l i d i t y of theory.(See references 3 t o 9 f o r an e xp er im e nt al v e r i f i c a t i o n o f t h e t h e o ryi n v ar i ou s f l i g h t c o nd i ti o ns . )

The accuracy of the theory i s i l l u s t r a t e d by f i p 1, which showsthe good agreement between th e cal cu lat ed m d rmsasured ch ar ac te r i s t ic s ,a s ob ta ined in f l i g h t , o f a t e s t r o t o r i n term s of a plot of powera g a i n s t v e l o c i t y. It might be ment ioned th a t model t e s t s in g e n e ra l a r eno t s a t i s fa c t o r y fo r he l i cop te r-pe r fo rmance work because o f the e f fe c t s

of s ca le on th e aerodynamic ch ar ac te r i s t ic s of th e blade e lements.

The t ransi t ion region between hover ing and about 30 m i l e s p e r h o u rshown i n t h e f i g u r e r e p re s e n ts a speed range i n which acc ura te d a t ac o ul d n o t be o b ta i ne d i n f l i g h t b ec au se o f i n s t a b i l i t y and c o n t r o ld i f f i c u l t i e s o r i n fu l l - sca le wind tunne l s because of t h e l a r g e l yunknown in te r f e ren ce cor r ec t i ons a t low a i r speed s . F u l l - s c a l e d a t awere obtaine d i n t h i s region, however, by me- of a r e l a t i v e l y newr e s e a r c h t o o l - t h e h e l i c o p t e r t e s t t ow er . It w a s f o u n d t h a t t h e t e s t -tower r es u l t s checked c lo se l y wi th th eo re t i ca l ca lcuLa t ions over mostof t h e t r a a s i t i o n r e gi o n.

As a r e s u l t of t he exper ience gained through the use of %he theoryand i t s e x p er i m en t al v e r i f i c a t i o n , s e v e r a l f a c t o r s were f oun d t o i n f l u e n c econs ide rab ly the per fo rmance cha ra c t e r i s t i c s o f ro to r s . One such fa c t o rwas th e importance of smooth, nondeformable bl ad e sur fac es i n reduci ngthe power requi red by th e ro to r i n a l l f l i g h t c on di ti on s. (See r e f e r -ences 9 and 10. ) The importance of wel l -bui l t b lades ar ises f rom thef a c t t h a t i n c r u i s i n g and hi gh -s pe ed f l i g h t b l ad e p r o f i l e drag accountsf o r from o n e- ha lf t o t w o -t hi rd s of t h e t o t a l r o t o r l o s s e s .

Rotor theory and experiment have also shown t ha t rotor performancei s dependent t o an qp re c ia b le ex ten t on the amount of t w i s t and taper


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b u i l t i n t o t h e b l a de s of t h e r o t o r. S tud ies ( re fe rence ll) have indicated,f o r excunple, th a t the r o t o r induced losses , which a r e the pena l ty th a tmt

b e p a i d f o r t h e t h r u s t produced by the r ot or , comprise approximately75 p e r c en t o f t h e t o t a l power l o s s e s i n t h e h o ve r in g c o n d i ti o n and t h a tthese los ses can be reduced t o th e ex te n t o f inc reas ing t he hovering payload by approximately 20 percen t i f the b lades were des igned wi th amoderate amount of taper and twist, instead of being untapered anduntwisted .

Theory and experiment have als o pointe d out th e values of designv a r i a b l e s t h a t would r e s u l t i n maximum performance. An example of t h i si s i l l u s t r a t e d i n f i g u r e 2, which shows t h e i q o r t a n c e of low r o t o rsp ee ds and h i gh b la d e l i f t c o e f f i c i e n t s f o r h ov er in g and v e r t i c a l - f l i g h tperformance. The to p curve shows t h a t a reduc t ion o f t i p speed from600 f e e t pe r second t o 400 f e e t pe r second would reduce the power

r e q u i r e d t o h o ve r a t f ixed th rus t by approx imate ly 25 percen t . The lowercurve shows t h a t a t a f ix ed power and t h r u s t , t h e same re d u c t i o n i n t i papeed resu l t s in a s u b s t a n t i a l i n c re a s e i n t h e v e r t i c a l r a t e o f c lim b,namely from 200 f e e t pe r minute t o approximately 1150 f e e t pe r minute.The ques t ion migh t na tu ra l ly a r i s e as t o what c o n s t i t u t e s a l ow er l i m i tt o t h e t i p s peed and why the he l i co p te r cou ldn ' t a lways opera te a t th a tl i m it i n g c on di ti on . The m w e r l i e s i n the f a c t t h a t l ow t i p s pe ed s a r every undesirable a t high speeds and that a good hel icopter design muste i th e r compromise between t he two cond itions o r must d e l i b e r a t e l y f a v o rone a t the expense of the other. (See reference 12. )

The choice of th e proper t i p speed and othe r design parameters foxe f f i c i e n t hig h- sp ee d f l i g h t must b e i n v e s t i g a t e d as p a r t o f t h e g e ne r a lproblem of rotor -blad e s ta l l in g . This problem has received and i sg e t t i n g a gre a t de a l o f a t t en t ion , fnasmuch as i t considerably reducest h e e f f i c i e n c y o f a h e l i c o p t e r f l y i n g a t h i g h s pe ed s and i s t h e d e c i s i v ef a c t o r in l fm i t ing the top speed of p resent -day he l i co p te r s .

B la de s t a l l i n g r e s u l t s from t h e f a c t tha t a s t h e l i f t i n g r o to rmoves forward, the advancing bla des encounter progr ess ive ly high erve l oc i t i e s , whereas the re t r ea t i ng b lades encounte r p rogress ive ly lowerv e l o c i t i e s . Thus, i n o rder to main ta in approximately equa l l i f t on bo ths i d e s o f t h e r o t o r s o a s t o pr e ve n t t h e h e l i c o p t e r fr om r o l l i n g o ve r,t h e l ow - ve lo c it y r e t r e a t i n g b l a de m st opera te a t higher angles ofa t t ack than the h igh-ve loc i ty advanc ing b lade . I t fol lows t h a t a s t h eh e l i c o p t e r i n c r e a s e s i t s forward &peed, th e angles of a t t a ck of ther e t r e a t i n g b l a de w i l l i n cr e a se p r o p o r t io n a l l y u n t i l a t some value offorward speed the ang les o f a t t ac k of the re t r ea t i ng b lade w i l l reacht h e s t a l l . As s t i l l higher speeds are reached, t h e s t a l l i n g becomesprogressively more severe and spreads t o a l a r g e r p a r t o f t h e r o t o r d i s ku n t i l t he s e ve r e v i b r a t i o n s and the l os s o f c on t ro l b rought about by t h es t a l l pr ev en ts t h e h e l i c o p t e r from f l y i n g f a s t e r .

The effect of forward speed a.nd r o t o r t i p s peed on s t a l l i n g i si l l u s t r a t e d i n f i gu r e 3. The c i r c l e s repr esen t p lan v iews of the ro to rd i s k , t h e d i r e c t i o n of f l i g h t and d i r e c t i o n of r o t a t i o n b e in g a s R ~ O W T ~ .


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The dark region a t the cen ter repres ents the swept ar ea of the hub andblade s and the shaded crescents represent the regione where thed i r ec t ion o f f low ove r the r e t r e a t fng b l ades i s reversed. F or t h i s h e l i -copter, a t 40 miles pe r hour and r o t o r rpm, s t a l l i s beginning tooccur nea r t he t i p o f t he r e t r e a t in g b l ade; When th e q e e d i s increasedt o 70 miles per hour, s t i l l keeping the same rotor rpm, t h e s t a l l e d a r e ahas Increase d consid erably= The redu ctio n in s t a l le d area brought aboutby incre the ro to r speed to 225 rpsn i s shown on the bot tom c i rc le .A t the s onmrd speed, the high er rot at io na l speed reduces thed i ff e r en t i a l i n speed between the advancing and r e t r e a t in g b l ades and socuts down the stal led area.

A c r i t e r i o n has been developed f o r pred ic t in g the Um it ing q e e ddue to s ta l lFng. (See reference 13. It lm.e been found that theopera t ional l imi t can be cons idered as reached when t he ca l cu la t ed ang leof a t tack a t t he t i p o f t he r e t r e a t in g b l ade exceeded the s t a l l i n g ang leof" th e blade a i r f o i l by approximately 4. O n e u se of t h i s c r i t e r i o n i si l l u ~ t r a t e d n f i g u r e 4 which shows the variation of t h e m-lnlrmnn allowablem t o r speed, as s e t by b lade stall-, with f0maa-d speed. The m L d m mm$or weed was calc ulat ed f o r each value of folVElrd speed by s et t i n g a16 t i p of a t t a c k a t t he r e t r ea t ing b3ade as t h e o p e r at i o na l l l m i t .Thus, f o r a given fo& wee d, a hel ic opte r cannot be opera ted i n thehatched por t ion of th e p lo t but rrmsf, i ac reaee i t s r o to r t i p q e e d u n t i lt h e t i p a ng le of a t t a c k Is 1@ o r less ( t h a t is , it must be operate d t othe r ight of the curve . )

Although a helicopter can be f luwn unti l the 4 t i p - a n g l e l i m i t i sexceeded, th e pro fi le-d rag lo ss due t o s t a l l - beg ins as t i p s t a l l in g

a e t s in .It

ha8 been found tha t the prof i le d r a g approximately doublesby the t ime the l im i t in g top speed i s reached. (See ref ere nc e 14 . )The ef fe c t s of s t a l l i n g on r o t o r p r o f i l e d r a g can be seen in f i g u r e 5,in which the profile-drag power absorbed by two sets of blades arep lo t t ed aga ins t weed . The dashed l i n es i n the f igu re r epresen t t hecalc ulat ed power with no allowance fo r blade s ta l l in g, whereae the so li dl i n e s i nc lu d e l o s se s due t o s t a l l i n g and thw r ep resen t t he ac tua lp ~ o f l e power absorbed. Note th a t s t a l l in g los ses are Lar@;e incomparison t o th e profi l e-dr ag pa rer absorbed by th e un sta l le d bladesand tha t , therefore , the top speed of the he l ic opte r i s also reducedbecause of th e addit ion al s t a l l power.

Once th e e ff e c ts of blade st al lF ng were understood, means f o r

a l l ev ia t i ng o r de lay ing these e f f ec t s were inves t iga t ed . A s a t i s f a c t o r yw ay t o d e la y t h e s t a l l was t o t w i s t t h e r o t o r b la d es s o t h a t t h e t i psec t ions wo rhd a t lower angles of a t ta ck than they would i f the b ladeswere untwisted. (The effects of blade twist were investigated i n f l i g h tend the r e su l t s a r e r epor ted i n r e fe rence 17.) The effectiveness ofblade twis t i n reducing the de t r imenta l e ff ec ts of s ta l l in g can be seeni n f i g u r e 5 . The fig ur e show t h a t an incre ase of about 10 percent inth e l im i t ing speed of the t e s t he l ico pter appears poss ib le wi th the useof -8O of blade t w i s t . Alternat ive ly, twist re du ce s t h e s t a l l i n g p r o f i l e -drag losses by approximately 40 percen t of the pr ofi le- dra g power absorbed


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by the r o t o m in th e uns t a l l e d con di t ion once s t a l l in g had deve loped onboth ro to rs . The use o f b lade twist i s desirable inasmuch as , a t t h e

very least,it

appears t o have no de t r im enta l e ff ec t on ro to r pe rfomnancefn, a.uy o t h e r f l i g h t c o n d i t i o n .

Another and s a n e w h a t o b v i o : ~ means f o r m in im iz in g t h e e f f e c t s o fb l a d e s t a l l i n g I s by increasing t h e b la d e -s e c ti o n s t a l l i n g a n g le o fa t t a c k . The b e n e f i t s t o b e h ad by s o d o in g, in terms of an inc rease i npermissable load a t a f i x e d t i p sp eed, i s shown in t h e l e f t p a r t off i g u r e 6. It can be seen From the f i gu re t ha t th e permissab le h e l i c op te rlo ad c ould be i n c r e y d by a f a c t o r of 3 i f t h e s e c t i o n s t a l l e couldbe increased from 12 t o 20'. The suc ces sfu l ap pl i ca t io n of var io us high-l i f t dev ices t h a t w o u l d s u b s t a n t i a l l y i n c r e a s e t h e s e c t i o n s t a l l anglew i th o u t p r o h i b i t i v e drag i n c r e a s e s 16 th e high -velo ci ty low-angle-of-a t t a c k r e g i on s o f t h e d i s k w f l l pr ov e a f e r t i l e f i e l d f o r f u t ur e h e l i -

c o p t e r r e s e a r c h .

J u s t asr b l a d e s t a l l i n g p r e s e n t s a lo we r l i m i t t o t h e a l l ow a b ler o t o r t i p s peed, a n o th e r l i m i t e x i s t s t h a t p re v e n ts o p e r a ti o n a t extremelyhigh t i p s pe ed s. That l i m i t i s c o q r e s s i b i l i t y e f f e c t s on t h e hi gh -v e l oc i ty t i p s e c t i on s of the advancing blade . F o r a given s ta l l ing angle ,a higher sec t ion c r i t i c a l Mach number w i l l permi t opera t ion a t l a r g e rgross weights because it p er mi ts t h e w e o f U g h e r t i p 8 pe ed sa It can beseen f rom the r igh t p a r t of f igure 6 t h a t l a rg e i n c r e a s e s in pay load canb e r e a l i z e d by i n c r e a s h g t h e c r i t i c a l Mach number o f a i r f o i l s e c t i o n sused in t h e b l a d e s .

Although most r o t o r b l a d es a t the present time are composed ofconvent ional w i n g s e c t i o n s , a t t e n t i o n i s befng gfven t o th e developmentof a i r f o i l s e c t fo n s de si gn ed e s p e c i a l l y f o r r o t o r s as dist-ishedf w m wings o r p r o p e l le r s . In a d d i t i o n t o a h ig h s t a l l angle and a highc r i t i c a l Mach nmiber, th e de s i ra b le ae rodynamic c ha ra c t e r i s t i c s of a . i r-f o i l s e c ti on s s u i ta b le f o r us e as r o t o r - b l a d e s e c t i o n s are: ( 1 ) n e a r l yzero pitching moment, (2 ) low drag throughout the range of lowmoderate l i f t s , an d (3 ) moderate drag a t h ig h l i f t s .

Most of the NACA l o w - d r a g a i r f o i l s tha t have been developed havetoo high a pitching-moment co ef f ic ien t t o warran t con s idera t ion f o r usew i t h c u r r e n t h e l i c o p t e r d e s i g n s . (High pitching-moment c o ef fi ci e n tsl e a d t o u n d e s i ra b l e p e r i o d i c s t i c k f o r c e s and t o v ib r a t io ns b rought aboutby per iodic blade twis t . ) Althorn t h i s o b j e c t i o n i s removed with thelow-drag symmetrical sections t h e s e s e c t i o n s a r e n o t a p p l i c a b l e b e ca us e

4,ha lf of th e low-drag 'bucket, o r, i n o t h e r w ords, h a l f o f t h e l i m i t e drange of l i f t c o e f f i c i e n t s in which the important drag r e d u c t i o n s a r eachieved, i s below zero l i f t ; whereas th e f a s t e r moving por t io ns o f th eh e l i c o p t e r b l a d e are nearly always operat ing a t p o s i t i v e l i f t coefficient^.

In ord er t o pla ce th e low-drag "bucket" in a us ef ul ra.nge of l i f tc o e f f i c i e n t s a n d s t i l l r e t a i n zero o r a lmost zero moment co ef f ic ien t , anumber of sp ec ia l a i r f o i l s have been der ived. (See refere nce 16. ) Oneof these, th e NACA 8-H-12, shows the most promise. A comparison of the


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NACA 8-H-12 sec t io n with the conven tional RACA 23012 a i r f o i l i s given inf i g u r e 7, which shows a r educ t ion in d r ag o ve r most o f t h e l i f t c o e f f i -cient range conibined with an e a r l i e r

s t a l l .Calcu l a ti ons o f t he pe r fo r-mance of ro to rs inco rpor at ing the new sec t io n have ind ica ted th e

supe r io r i t y of t h e sp ec i a l s ec t i o n ove r t he conven tiona l s ec t i ons . Full-sca l e t e s t s of p r ac t i ca l - cons t ruc t i on b l ades i nco rpo ra ti ng t he NACA 8-H-12se c t io n a re needed , however, t o de termine the t ru e worth of t he a i r f o i lw d e r a c t u a l o p e r a t in g c o n di ti on s .

Vibra t ion and F l u t t e r

It i s commonly accepted t h a t where Large, ro $a tin g masses a r einvolved, v ib ra t i ons o f some k ind a r e l i ke ly t o appear - and the h e l i -cop t e r i s no exception. In f a c t , t he de s igne r s of most of t h e e a r l i e r

tries of he l ico pte rs had as m h i f f i c u l t y in r educ ing t he v ib ra t i ont o a c ce p ta b le l e v e l s as they had i n obta ining adequate performance. Agood de al of th e tro ubl e w a s caused by poorly b u il t , unbalanced bladesand w a s l a rg e ly e l imina ted wi th more ac cura te des igns and an increasedhowledg e of blade balancin g and trac ki ng procedure. A second sourceof t h e v i b r a t i o n d i f f i c u l t i e s e n co un te re d were i n h er e n t i n t h e h e l i c o p t e ri t s e l f and cou ld o nly be av oided when th e phenomenon th a t c aused it wasthoroughly analyzed and understood. A n example of such a phenomenon i sa s e l f - exc i t ed mechan ica l v ib r a t i on known as "ground resonance, " whichhas been r e spons ib l e f o r t he de s t ruc t i o n o f s eve ra l au tog iro s cwdh e l i c o p t e r s .

Es se nt ia l ly , "ground resonance " i s a s e lf -exci ted mechanicalv i b r a t i o n t h a t i nv ol ve s a coupling between t he motion of t he r o to r bladesa b o u t t h e i r drag hinges and th e motion of the h e l ic op ter as a whole oni t s lan din g gea r. When th e fre qu en cie s of th e two motions approach eachother, a v io le n t shaking of the a i r c r a f t occurs which, i f undamped, wouldr e s u l t in i t s complete de st ru ct io n. Th is phenomenon w a s t h e o r e t i c a l l yinves t i ga t ed and a theory was developed which su gge sted means f o ravoiding "ground resonan ce." (see ref er enc es 17 t o 19.) Ln o r d e r t omaZre t h e th eo ry ea sy t o use, it was put i n th e form of s imple cha rtswhich predicted the range of rotor speeds i n which t he i n s t ab i l i t yoccurred and th e amount of dasrping nec essa ry t o avo id dangerous f r e -quencies.

Another example of a v ibr a t io n problem pecul ia r t o he l ico pte rs w a sencountered i n th e operat io n of two-bladed ro to rs . The phenomenon wasca l l ed b l ade 'heaving " from the appearance of the wavy path traced byt h e b l ad e t i p s and w a s found t o be an aerodynamic i n s t a b i l i t y o r ty peo f f l u t t e r . The problem w a s i nves t i ga t ed t he o re t i ca l l y ( r e f e r ence 20)~ m d ls o by means of model te s t s . The genera l re su l t o f th e s tudy wast h a t a s ee -saw r o t o r w i th a coning angle i s more uns tab le than an a i r-plane wing having corresponding parameters. The add i t i o na l uns t ab i l i z inge f f e c t i s as soc i a t ed w ith t he d i f f e r ence i n moments o f i n e r t i a i n f l app ingand i31 r o t a t i o n . In f a c t , i t w a s found tha t wi th cer ta in combina t ions ofconing angles and b lade des ign par me te rs , f l u t t e r could occur even when


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th e chordwise cen te r of mass of th e blades w a s we ll ahead of t he 23-p er ce nk - ch or d p o i n t P ro po se d r e m d i e s f o r t h e f l u t t e r i n v e s t i g a t e dincluded decreasing the coning angle of th e b lades , des ign ing the b ladess o tha t t h e i r mass ten ds t o be confined t o t h e p h e o f r ot a t io n ,h c r e a s i n g t h e c on tr ol -s ys te m s t i f f n e s s and fo rward pos i t ion o f the cen te rof mass, and adding mechanical damping t o t he ro t o r system.

The he l i cop te r is s u b je c te d t o a t h i r d t y p e o f v i b r a t i o n t h a t c a x m o tbe e l im in ate d i n m c h as i t i s a f o r c e d v i b r a t i o n i n h e r e n t in t h e a e r o -m c s f t h e r o t o r i t s e l f . T h is t yp e of v i b r a t i o n i s encountered, f o re q l e , w it h two-bladed h e l ic o pt er s i n t h e t r a n s i t i o n r e g i o n be tw eenhovering arnd fo rward f l i g h t where in cyc l i c va r i a t io ns o f induced andp r o f i l e drag g iv e r i s e t o h o ri z o nt a l hub v i b r at i on s o r, f o r e x q l e , whenb l a d e s t a l l i n g i s encountered in high-speed f l i g h t . (See re fe rence 21. )Although inheren t v ib ra t ion s o f th ese typ es casno t be e l imina ted , theycaa be i s o l a t ed by s u i t a b l y shock mount ing the ro to r system, and bywb.g i r r e v e r s i b l e c o n t r o l s t h a t c an no t trm sm 3-t v i b r a t o r y f o r c e s t o t h ep i l o t 's c o n t r o l s . A gr ea t de a l o f work remains t o be acconrplished i nr ed u ci n g t h e o v e r -a l ll v i b r a t i o n l e v e l o f t h e h e l i c o p t e r s o t h a t it ca nbe f lown f o r long pe r iods o f t b i t ho u t u n n e c e s s a ri l y a dd in g t o p i l o tf a t i g u e .

S t r e s s e s

Although the achievement of maximum he li c op te r performance a.ndr e l i a b i l i t y c a l l s f o r a thorough knowledge of the stresses imposed ont h e r o t o r a nd fu s e la g e of t h e h e l i c o p t e r i n a l l s teady and acce le ra ted

f l i g h t c o n d i t io m , t h e g e n e r a l f i e l d of h e l i c o p t e r s t ress a n a l y s i s h asbeen c om i de r ed secondary t o t h ~ erodynamic problems. L i t e r a t u r e o nh e l f c o p t e r st ress analysis does ex i s t , bu t , in t h e main, c o m e n t i o n a lznethods have been applied in analyziw t h e f u s e l a g e a nd r o t o r b la d e s .Blade analys es, f o r example, have been made by pr op el le r s t r i p methodsal though an add i t iona l compl ica t ion tha t has been t aken in to accoun ti s th e spanwise bending of th e blades , which tends t o change the d ir ec t i onof t h e c e n t r i f u g a l l o ad i n g o n t h e b l a d e s . (See references 22 to 26 Tori n f o m a t i o n on b l ad e s t r e s s a n a l y s i s . ) A s ye t , however, ac tu a l s t r e ssvalues , and the var i ous ass lxtqptions regarding blade loa ding th a t ar eFncorpora ted i n these methods, have no t been d i re c t ly ve r i f i ed by r e l i a b l efu l l -s ca le t e s t measurements. Aside f rom a d i r e c t c h e c k o n t h e a c t u a ls t r e s s e s , t h e s i g n i f i c a n c e o f t h e s e c a l c u l a t i o n s would b e g r e a t l y

s t rengthened i f exper imental da ta were obtained on the induced f low i nf o m d f l i g h t , s o t h a t t h e aero-c l o a d i n g c a n b e more a c c u r a t e l yc a l c u l a t e d . (Tlae induced flow h h o v e r i n g h a s b e e n d i r e c t l y v e r i f i e dby B r i t i s h f l i g h t t e s t s . )

I n conn ecti on w it h induced-flaw measurements, i t might be mentionedt h a t t h e o v e r - a l l m ag ni tu de and g e n e r a l d i s t r i b u t i o n of t h e i nd uc edveloci ty have been ver i f ied by rotor-blade-motion and performance t e s t smade i n f l i g h t . The in du ce d v e l o c i t y a c t u a l l y a p p ea r s t o v a ry n o n l i n e a r l yi n magni tude acr oss t he di sk, however, and would the re fo re be e q e c t e d t o


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inf luence cons iderably local s t ress va lues a long the b lade . The problemof dete induced veloci t ies i s amenab le to theo re t i ca l so lu t ion jand although same work ha^ been done along these l ines ( r e f e r a c e 27), agood deal &ill remains t o be done before ro tor-blade st re ss es can bepredicted with confidence

S t a b i l i t y and Control

The in fom at ion th a t h a s been ax;cunnilated on the stability asdcontrol of helicopters during the past years has been rather L-lmited.In t h e i r d e s i r e t o e s t a b l i s h th e p r a c t i c a b i l i t y of tb h e l l c o p t e r as af Q b g machine, des igne rs have con cen trate d on in~roving he performancean d mducing the v ibra t ions of the he l icopter, while accepting mmginals t a b i l i t y and c o n t r o l c h a r a c t e r i s t i c s . AEJ a r e s u l t , t h e h e l i c o p t e r in

i t s present stage of development i s d i f f e r e n taJld

more d i f f i c u l t t o f l ythan m s t f ixed -wiq a i rp l anes . Ln response t o t he inc rea sin g demandsplaced upon the he lic op ter by the armed ser vice s and by commercialoyerato m, however, th e improvement of the s t a b i l i t y asd c o n t r o lc h m a c t e r i s t i c s o f t h e h e l i c o p t e r and of i t s f l y i n g and - l ingq u a l i t i e s i s perhaps th e most important he lic op te r resea rch problem a tthe present t i m e .

A nuni$er of th eo re ti ca l papers have been wri t te n on the sub ject ofh e l i c o p t e r s t a b i l i t y and con trol . (See reference s 28 t o 32.) Althought h e t h eo r ie s pre se nte d i n t h e ~ e apers are somewhat di ff er en t and so w -times contradictorg, it i s gene ra l ly agreed th a t (1 ) i f t he he l i cop te ri s dis turbed while hover ing, asd i f th e cont r o l s t ic k remaizlEI f ixed,

t h e h e l i c o p t e r w i l l describe an osc i l l a t ion abou t i t s or ig inal hover ingpos i t i on , eLnd (2) the ampli tude of the os c iU at io n w i l l increase withtime. According t o def in i t io n , the he l ic opte r i s thus dynamicallyunstable i n hovering . Cal cula t ion s indi ca te th a t the per iod of theo sc il la ti o n of a two-place, 2700-pound he lic op te r i s of th e orde r of10 secon&s and that the rate of divergence i s s m a l l . Limited f l ightdata , obtahed in t h i s country (refere nce 33) and in England, haveroughly checked the calculat ione asd have ind ica t ed tha t t he Fns t ab i l i t yof the hover- os ci l l at io n i s no t a p rob lem t o the p i l o t .

The helicopter does have some handling characteristics in hoveringt h a t me f requen t ly objec tionable , esp ecia l ly t o the novice p i lo t . Oneof the handling problems th a t the t ra ine e m a t overcome wi th the smal lers i zed he l i cop te r a r i s e s f rom the h igh con t ro l s ens i t i v i ty o f t he he l i -copter i n m l l or, i n o ther words, the h igh ra te of r o l l pe r inch ofst ick displacement. Th is s e ns i t i v i ty f r equen t ly l eads t o ove r-con tro l li ng ,which may r e s u l t in a s h o rt -p e ri od p i lo t -i n du c ed l a t e r a l o s c i l l a t i o n .Con trol se n s it iv it y becomes le sa of a problem with l ar ge machines becausef o r a g iven s t i ck d isplacement th e ro l l in g veloc i ty obta ined w i l l varyinverse ly as the diameter. Prequently, undesirable st ic k-f or ce grad ientsa r e a d d i t i o n a l f a c t o r s t h a t add t o the co nt ro l problem of th e unexperi -enced heUcopter p i lo t .


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Another cont ro l d i ff icu l ty that might be mentioned has been encoun-te red in th e partial-power vert ical- des cen t region between approxi-

mately 500and PfSOO

fee& pe r minute.In

this vert ical-descent raage,the s ib ra ti o n of th e he lic op ter Becomes q ui te pronounced. Rather violent ,random yawing motions the n occur with some r o l l ; th e r a t e ofdescent apparent ly increases rap id ly j the ro tor ro ta t iona l speed var iesn o ti ce ab ly j q d more often than not the hel icopter eventual ly pi tchesnose down a.nd recovers by gaining speed, desp ite ap pli cat ion of conaid-erable rearward control. There i s rrmch t o be le arn ed abou t t h i8 regimeof ope ration, bu t prel9mlnaz-y in di ca tio ns are that the fundamentd causeof the phenomenon i s an unsteady, mixed flow of a i r through th e ro t or*ZmeguLar f law i n th i s in termediate f l i g h t condi t ion might log ic beerpected i m c h as air i s blown downward through the r o t o r i nhovering, whereas in completely power-off descen t an, upward flow of a i r

place. Although p i l o ts have exgerienced no di ff ic ul ty i n vering

the maneuver a t any st ag e de sir ed , th e phenomenon could be erousi f it occurred a t very l a w a l t i t u d e s .

The helicopter has c e r t a i n u n d es ir ab le s t a b i l i t y and co nt ro l eh;arac-t e r i s t i c s i n forward f l ight as wel l as i n hovering a.nd in v e r t i c a ldescents. The major complaint reported by pilots i s t h a t they f ind itqui te d i ff ic u l t t o hold s teady condi tions in forward f l ight bemuse ofa stro ng tendency of th e machine t o diverge in p i t ch . Inves t i ga t i on hasshown tha t t h i s tendency r e s ~ + l t s rom the f ac t t ha t t he he l i cop t e r 191general i s m at ab le with angle of at tac k. There ase two logicalsou rces fo r t h i s I n s t a b i l i t y . The f i r s t sou rce i s t h e usual unstablefuselage, and the second results f r o m the flapp ing of the roto r. Whena f lapping ro tor LEI subjec ted t o an angle-o f-attack chmge i n forwardfl ig h t, th e respaat- change i n blade f lapping w i l l , be such as t o herfncrease the ro tor angle change.

Theoret ical izalculat ions indicate t ha t t h e i n s t a b i l i t y of t b o t o rand fuselage with angle of att ack , i f not overcome by a ~ t a b i l i z i n gm e a m such as a t a i l su r f ace , r e su l t s in an unstable dynamic oscillation.F l i gh t t e s t r e s u l t s of s t i ck - f i xed o sc i l l a t i ons , r epo r ted in r e f e r e m e 34,qu ali t at i ve ly checked the calcu lat io ns. An example of an o s c i l l a t f o nobta ined a t 40 miles per hour i s shown i n f ig ur e 8. The osc i l la t ion w a si n i t i a t e d by a momentary a f t motion of th e s ti c k . The peri od of themotion i s about 1 4 seconds, which i s long enough so that the pi lot doesnot have trou ble con tro llin g the osc il la ti on . The motion doubles Inamplitude i n about 1 cycle. Resu lts obtained a t higher speeds, however,have indicated that the motion following a disturbance i s a divergence,ra-bher tha n an osc i l l a t i on . Ae you can w el l imagine, a di verg ent raotiont h a t could be brought about by a sudden gu st i s a dangerous maneuver i fc o ~ c t i v e c t i on i s not immediately inftiated.

An exanple of such a maneuver obta ined a t 65 miles per hour frs shownin f igure 9 . Again the h el ic op te r was distu rbed by an i n t e n t i o n a l s t i c kmotion, a f t e r which the s t i c k was h el d f i x ed a t t h e trim pos i t i on . Thehelicopter nosed up mildly and then nosed down. It was s t i l l nosing downa t an increasing rate, as the accele rat io n curve indicates, about 4 econds


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a f t e r t he 1 g axis w a ~ rossed, and recov ery had t o be made by c o n tr o la p p l i c a t i o n . In fa ct , considerable d if f i c u l t y was encountered i nrecovering from the maneuver because the ac cel era t io n cont inued t o bu i ldup 2 s eco nd s a f t e r t h e c y c l i c c o n t r o l s t i c k was a t i t s forward stop. Thep i l o t had t o r ed uc e t h e t o t a l p i t c h and had t o r o l l the machine as i n a

-oTer before s teady f l ight could be reached.

Ln general , it was found tha t though the he l icop te r i s unstable overthe ent i re speed range, i t s i n s t a b i l i t y i s l e a s t in t h e 40 t o 63 milesper hour region. A t h i gh e r q e e d s , t h e p i l o t h a s p r og r es s iv e ly l e s stim e t o i n i t i a t e r ec ov er y f rom a d i ~ t u r b a n c e sd t h e w h i m becomesr a p i d l y m re uns tab le .

It should be unde 'mtood tha t the undes i rab le s tab i l i ty and c o n t r o lc h a r a c t e r i s t i c s j u s t d i sc u s se d do n o t p r o h i b i t t h e p re se n t- da y h e l i c o p t e rfrom be ing a usefu l to o l f o r spec ia l ized purposes. Qar ioue meass f o re li m in a ti n g t h e se c h a r a c t e r i s t i c s are under coneiderat ion in o r d e r t ou t i l i z e all the po te n t ia l i t i ' e s o f the he l icop te r, bu t the cho ice andap pli ca t io n of thes e s olu t io ns depend upon cont inued rese arch anddevelopment.

Future Research Deeds

AD a t t e q t has been made here in t o acquaint t he read er with thepresen t s ta tw of he l icop te r research . It may the re fore be appropr ia tet o conclude with a sta%ement on fu tu re rese am h needs.

R eq uire men ts f o r s a t i s f a c t o r y f l y i n g q u a l i t i e s of h e l i c o p t e r s s ho ul dbe es tab l i shed , similar t o t h os e a l r e a d y s e t up f o r t h e a ir p l a n e , asdm an s f o r meet ing thes e requirements should be inv est ig ated . In p a r t i c u -Par, methods should be found to give the he l ic op ter s t ic k- f ix ed and s t i ck -f r e e s t a b i l i t y i n ho verin g and i n f orw ard f l i g h t . Wi th th i s in mind,automatic-f l ight devices should be i n v e s t i g a t e d j and t h e e f f e c t i v e n e s sand appl ica t ion of aerodymmlc servocontrols and other conltrol mrmgements,including power controls, should be s tudied. Also. , theoret ical andexperimental s tudies are needed t o e q l a l n and c o r re c t t h & c o n t ro l d i f f i -cu l t i es encounte red by p i lo t s in the t r an s i t i on reg ion be tween hover ingmd c r u i s i n g f l i g h t a;nd when descending vertically a t pmtial-powercondi t ions .

The t r en d toward la rge-d iameter load-car ry ing he l ic op te rs c a l l s f o ra mom extensive knowledge of rotor-blade aerodynamic loading and blades t r e s s e s . Induced veloci-ty an d s t r e s s measurements should, the refo re,be made and thoroughly analyzed. The w e of more than one l i f t i n g ro to ron t h e l a rg e l o a d -c a rr y in g h e l i c o p t e r s c a l l s f o r a thorough invest igat iono f t h e aerodynam ic c h a r a c t e r i s t i c s of' t h e v ar io u s d t i r o t o r ar ra ng em en tst h a t a r e being proposed. In pa rt ic ul ar , induced flow stu die s should bemade fo r the various co nfigurat io ns th a t are now bein g used. Such st u d ie swould be u s e f u l f o r s t a b i l i t y work and, al so , f o r performance inasmuch asinduced power requirem ents appea r t o be th e primary unknown in computingthe performance c ha rac te r i s t i cs o f mul t i ro tor conf igura t ion%.


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The app l ica t i on of j e t p ropuls ion t o he l icop te rs has long beenconsidered as a d e s i r a b l e me- f o r i n cr ea si ng th e s b n ~ l i c i t y nd t h eload

carcying a b i l i t y of t h e h e li c o p te r. S e v e r a l h e l i c o p t e r s u tl i z

n gt h e j e t p r h c i p l e h ave a lr e ad y been b u i l t and flown. A g r e a t d e a l ofresearch, however, i s s t i l l needed t o e st ab lf sh th e aerodynamic req uir e-

4 ments of je t -d r ive n he l ic op ter s and t o produce an e f f i c ie n t j e t s y s t m .The use of j e t s al so br ing s a3out additian &l problems Involving bladedesign, vibrat ion, asd s t a b i l i t y c h a r a c t e r b t i c s t h a t should beas t ic ipa ted and so lved

It i s hoped that an early and s u c c e s s f u l s o l u t i o n of t h e s e p r o b l e mwill make the helicopter a t r u l y dependable asd i n d i s p e n s a b l e a i r c r a f t .


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1. Bailey, F. J ., J r. : A Sim plifie d The ore tica l Method of Determiningt h e C h a r a c t e r i s t i c s o f a L i f t i n g R oto r i n Forwud F l igh t . NACARep. No. 716, 1941.

2. Bailey, F. J ., J r. , and Gustafson, F. B. : C h a r t s f o r E s t im a ti o n o ft h e C h a r a c t e r i s t i c s o f a Helicopter Rotor in Forward FHght. I -Pr of i le Dra g-Li f tRa t io f o r Untwisted Rec tangular Blades. IlACAACR Do. ~ 4 r n 7 ,194-4-

3. Gwtafson, F. B I : Flight Tests of the Sikomlgy BNS-1 ( ~ r m y R - 4 ~ )Hel icop te r. I - Experimental Data on Level-FUght Performancewith Origlnal Rotor Blades. NACA ME3 no. L s 1 0 , 1945

4. Gwtafson, Fb B e , and Gessow, Alfred: Fl i gh t Tests of th e SikorskyIDTS-1 (Army Y R - 4 ~ ) elicopter. I1 - Hovering and Ver t ica l -F l igh tPerformance with the Original and an Alternate Set of Main-RotorBlades, Including a Comparison with Hovering Performance Theory.XACA MR No. L5DOga, 1945.

5. Gessow, Alfred, and Myers, G-arry.C., J r. : F l i g h t Te s t s o f a H e l i -copter in Autorotation, Including a Compwiaon with Theory. NACATN No. 1267, 1947.

6 . Dingeldein, Richard C . 4 chaef er , Rapmnd F . FuLl-Scale Lnvesti -ga%ion of th e Aerodymmic Ch ar ac ter is tic s of a Typ ical Single-RotorHelicopter i n Forward Fl i gh t . NACA TI? No. 1289,

1947.7 . Myers, Garry C., Jr. : Fl ig ht Measurements of Ze lic op te r Blade Motion

with a Co~l~parison etween Theoretical and Experimental Results.rnmA TN NO . 1266, 1947

8. Migotsky, Eugene: Fu ll-S cal e In ve st ig at io n of th e Blade Motionof the W-2 Helicopter Rotor. NACA TN Xoi 1521, 1948.

9. Gustaf son, F. B . , nd Gessow, Alfred: Psalysis of Flight-PerformanceMeasurements on a Twisted, Plywood-Covered H el ic op te r Rotor inVarious Flight Conditions. NACA Tm No. ' 1595, 1948.

10 . Gustsfson, F. B.: Ef fe ct on Helico pter Performasce of M odificationsi n P r o f i l e - D r a g Ch ara cte r is t ics of Rotor -Blade A ir fo i l Sect ions.NACA ACR NO L4H05, 19440

11. &s%ow, Alfred: E ff ec t of Rotor-Blade Twist asd Ph-Form Taper onEelicopter Hovering Performance 3ACA Tm No. 1342, 1948.

12. Gustafsoa, F. B., and Gessow, Alfre d: E f fe c t of Roto r-Tip Speed onEelicopter Hovering Performance asd Maximum Forward Speed. NACAARB No. L6~3.6, 946.


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13. Gustafson, F. B., and Myers, G. C. , J r. : St a l l i ng of Hel icopterBlades. BXA !L% Roo 1083, 1946.

3-4. Gustafaon, P o B., and Gessai, Alfred: Effect of Blade Stal-llfng mthe Eff iciency of a Helicopter Rotor a~ M e m d i n FlfghL. N X ATfa no. 1250, 1947.

15- Gessow, Alfred: Fl ig ht Inv es tig at io n of Ef fe ct s of Rotor-Blade T w i s t%onHelicopter Performance in th e High-Speed and Ve r t i c a l -Autorotative-Descent Conditions. RACA TI? Bo. 1666, 1948.

l6. Stivers , Louis S. , J r. , asd Rice, Fred J . , J r . : AerodynamicCharac te r i s t ics o f Four NACA Ai rfo i l Sec t ions Des igned fo r G l i -copter Rotor Blades. M A B Ro. LTg02, 1946.

17. Coleman, RobertPo:

Theory of Self-Er ci ted Mechanical O s c i U t i a n sof Hinged Rotor B l a h s . W A AN? o. 3G29, 1943.

18. Peingold, Arnold Me : Theory of Mechaelcal Oscillations of Rotorswit h Two Hinged Blades . W A ARR No. 3113, 1 9 3

1 9 . Coleman, Robert P., and FeingoLd, Ax-n~Ld M.: Theory of G r a dVibrations of a Two-Blade Helicopter Rotor on AnisotropfcFlexible SupporDe . RACA 'I3 Ro. u& , 1947.

20. Coleman, Robert P. , sd Stempin, C a r l W. : A Prel iminary Theoret icalStudy of Aemdynajmic In s t a b i l i t y of a Two-Blade E e l i c q t e r R otor.XAcA RM Ro . ~ 6 ~ 2 3 ,946

21. Seibel, Chmles: Periodic derodynamfc Forces on Rotors Fn FomaxdF l f & t o JOW* B e ~ o - %I., O ~ . U , no. 4, Oct. 1944, pp* 339-3420

22 Owen, J . B e B e : The Str es si ng of Gyroplane Blades i n Steady ~ F ll gh t.R. & M. no. 1875, B r i t i s h A.R.C., 1939.

23. Duberg, John E., and Luecker, Arthur R.: Comparisons of Methods ofComputing Bending Moments i n H el ic op te r R otor Blades in the Planeof Flapping. NACA ARR No. L5E23, 1945.

24. F k , Ale~a31der H.: The Bending of Rotor Blades. Jo ur . Aero. S c i . ,vole 14, IIO. 1, J ~ s - 947; pp. 42-50.

25. Yu a n , Shao Wen: Beanding of Rotor Blade in the Plane of Rotation.Jour. em. ~ c i . , o l . l h , no. 5 , Mag 1947, pp. 285-293-

26. Horvay, Gabriel: S tr e ss Analysis of Rotor Blades. Jour. &roe Sc i . ,v o l e 14, no. 6, June 1947, .DO 315-336.

27. Coleman, Robert P., Feingold, Arnold M., and Stenpin, C a r l W.:Evaluation of the Induced-Velocity Field of as. Ideal ized HelicopterRotor. nACA ARR No. L5El0, 1945.


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: Contribu tions t o th e Dgnamic S t a b i l i t y of Rotary-Wingraft with Articulated Blades. P& I - G en er al P r i n c i ~ l e s .

l a t i o n No. F-TS-690-RE, Air Mate riel Command, July 29;1946.

: The Dynamic S t a b i l i t y of He&icoptere with Ar ti cu la te dRotors - Second P a rt ia l Report. Trans la t ion no. F-TS -1002-RE,Air Mate ri el command, Sept . 9, 1946.

30 . Bohenemser, K.: S t a b i l i t y in Hovering of'the Helicopter with CentralRotor Location. Tra nsla tion Ao. F-TS-~~T-HE, ir M ateriel Commasd,m * 1, 1946-

31. Hohenemer, K O : Long itudinal-Stabil i ty of th e Helicopter in ForwardP l igh t . Translat ion no. F-TS -688 -HE, Air M aterie l Commasd,aug. 2, 1946.

3 2 , Donovan, A. F., and Goland, M.: The Response of Helicopters withArtic ulate d Rotors t o Cyclic Blade Pi tc h Control. Jour. &ro.Sci . V O ~ . U, no. 4, ~ c t . 944, pp. 387-398.

33. Gustafson, F. B., and Reeder, J. P.: He li cop t er S t ab i l i t y . NACAm no. L71104, 1948. .

34* Rseaer, John P., asd Gustafsan, 3'. B . Notes on the Fly- Q u a l i t i e sof Helicopters. Paper presented a t th e Fourth Annual Forum of th eh r i c m He li cop te r Soc ie ty (Ph il ade lph ia , Perxla. ) , Apri l 22 -24,1948


15/19

ROTOR SH-AFT POWER,

H P

vCALCULATED

0 20 40 6 0 8 0

TRANSLATIONAL VELOCITY, M PH

Figure .l. - Comparison of r oto r cha ract eris ics as calculate& and measu red' in flight.

MAIN-ROTORSHAFT, HP

RATE OFCLIMB, FPM

ROTOR TIP SPEED, FPS

Figure 2.- Effect of rot or tip speed on hovering and ver tica l flight performanceof sam ple helicopter.


16/19

OF ROTATION

Figure 3.

FORVVARDSPEED, MP

V = 40 V. 7 0R P M = 2 1 0 R P M = 210

DIRECTIONOF FLIGHT

V = 70

R P M = 22 5

Effec t of fo rwa rd speed and r o to r t ip speed on ro torcb ladestall.

2 0 0

160

120

8 0

4 0

0I

3 0 0 3 8 0 46 0 5 4 0 620MINIMUM ROTOR TIP SPEED, FPS

Figure 4.- Variat ion of minimum r o to r t i p speed , as s e t by ro to r-b lade s t a l l ,wi th fo rward speed .


17/19

ROTOR PROFILE-DRAG POWER, HP

1207

BLADE

WITHOUT BLADE STALLINGA

TRANSLATIONAL VELOCITY, MPH

Figure 5.- Effects of rotor-blade st al l and blade twist on rotor profile-dragpower.

E F F E C T O F S TA L L A N G L E E F F E C T O F C R I T IC A L M A C H N O.

MACH NO. AT ADVANCING TIP = -75 S T A L L A NG LE = 12

12 14 16 18 20 -7 ' .8 .9 I-0

ROTOR - B L A D E - S E C T I O N R O T O R - B L A D E - S E C T I O NS T A L L A N G L E L I M I T I N G M A C H N U M B E R

Figure 6. - Effect of r otor-blade -sect ion stal l angle and limiting Machnumber on the permissible load carried by a sample helicopter,


18/19

SECTION D R A GCOEFFICIENT, c d

.028iI

.024NACA 23012

\AG A 8-H-12

7 --C-- REYNOLDS NO.\---- - 2 . 6 ~ lo6- .0 x 1o6

SECTION LIFT COEFFICIEMT, c,

Figure 7.- Comparison of the profile-drag characteristics of the NACA 23012and NACA 8-H-12 a ir fo il sect ion s.

PITCH ATTITUDEl o

DEG-10

A

ACCELERATION1.2

9

.8

I I

10 2 0

SECONDS

Figure 8. - Time history of a he licopter oscillation obtained in flight at40 miles per hour.


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,-STICK POSITION0

IN.Fw 'D

5

ACCELERATION

SECONDS

Figure 9.- Time history of the divergent motion of a helicopter obtained

in flight at 65 miles p er hour.

Documents

Gessow: Helicopter Research Problems