Document
PNEUMATIC BOOT FOR HELICOPTER ROTOR D E I C I N G Bernard J. Blaha and Peggy L. Evanich L e w i s Research Center SUMMARY Although t h e y have many d e s i r a b l e c h a r a c t e r i s t i c s , pneumatic d e i c e r boots Modern have received l i t t l e c o n s i d e r a t i o n f o r a p p l i c a t i o n t o h e l i c o p t e r s .
polyurethane pneumatic d e i c e r boots are l i g h t i n weight, low i n power consump- t i o n , e a s y t o c o n t r o l , and c a p a b l e of f i e l d r e p a i r . The L e w i s Research Center, i n c o o p e r a t i o n w i t h t h e B. F. Goodrich Company, has t e s t e d pneumatic d e i c e r boots f o r h e l i c o p t e r r o t o r blades. The te.sts were conducted i n t h e L e w i s 6- by 9-ft I c i n g Research Tunnel on a s t a t i o n a r y s e c t i o n of a UH-1H h e l i c o p t e r main- r o t o r blade. The b o o t s were e f f e c t i v e i n removing ice and i n reducing aero- dynamic drag due t o i c e . R e s u l t s of t h e s e tests are p r e s e n t e d i n t h i s paper.
Because of t h e s e promising r e s u l t s a program w a s begun a t t h e NASA Ames Research Center t o t e s t boots on f u l l - s c a l e , r o t a t i n g UH-1H r o t o r blades.
I N TROD UCT I O N To d a t e , t h e r e a r e no U.S.-manufactured h e l i c o p t e r s c e r t i f i e d t o f l y i n t o f o r e c a s t e d i c i n g c o n d i t i o n s . None are expected t o be c e r t i f i e d f o r a t l e a s t 2 years. However, much work i s i n p r o g r e s s t o develop both c e r t i f i c a t i o n c r i t e r i a (which c u r r e n t l y are n o t d e f i n e d e x c l u s i v e l y f o r r o t o r c r a f t ) and de- i c i n g systems f o r r o t o r s ( r e f . l). The r o t o r d e i c i n g systems being developed The pneumatic boot concept f o r employ t h e e l e c t r o t h e r m a l concept ( r e f . 1).
r o t o r b l a d e i c e p r o t e c t i o n was analyzed i n 1973 by t h e Lockheed-California Although t h e s t r o n g advantages of low weight, Company and r e j e c t e d ( r e f . 2).
low power, blade leading-edge p r o t e c t i o n , and simple c o n t r o l s were pointed o u t i n t h i s study, Lockheed l i s t e d s e v e r a l reasons f o r q u e s t i o n i n g t h e pneumatic boot concept. These reasons included materials problems, p o s s i b l e a d v e r s e aerodynamic e f f e c t s , and b a s i c i c i n g questions. O f t h e reasons l i s t e d , t h e most damaging c e n t e r e d on t h e materials technology of t h e day. A primary q u e s t i o n w a s whether t h e pneumatic boot could w i t h s t a n d t h e s e v e r e dynamic environment of t h e h e l i c o p t e r r o t o r blade. A s p e c i f i c concern was t h a t t h e b o o t s might be damaged or completely t o r n o f f by t h e high c e n t r i f u g a l f o r c e s .
Furthermore t h e r a i n a b r a s i o n r e s i s t a n c e of neoprene w a s unacceptable. Also t h e r e were p o s s i b l e a d v e r s e aerodynamic e f f e c t s of t h e i n f l a t e d t u b e s on t h e small-chord, t h i n a i r f o i l s of a r o t o r . These problems were s u f f i c i e n t t o e l i m i n a t e t h e boot from f u r t h e r c o n s i d e r a t i o n .
As a r e s u l t of t h e s e e a r l y s t u d i e s t h e B. F. Goodrich Company has f u r t h e r i n v e s t i g a t e d m a t e r i a l s and t e c h n i q u e s of pneumatic boot manufacture and h a s conducted l i m i t e d t e s t i n g . They claim t h a t a polyurethane e l a s t o m e r i c mate- r i a l , r a t h e r t h a n t h e c u r r e n t l y used neoprene, c a n be compounded t o e x h i b i t many s u p e r i o r p r o p e r t i e s , such as a b r a s i o n ( r a i n and sand) r e s i s t a n c e (3 t o 5 t i m e s g r e a t e r t h a n t h a t of neoprene), f i e l d r e p a i r a b i l i t y , g r e a t e r compatibil- i t y t o e s t e r o i l s , h i g h e r s t r e n g t h and f a t i g u e r e s i s t a n c e , and minimal d i s t o r - t i o n under high c e n t r i f u g a l f o r c e s .
A schematic diagram of t h e pneumatic system a p p l i e d t o a UH-1H h e l i c o p t e r is'shown i n f i g u r e 1 ( r e f . 1 ) . According t o r e f e r e n c e 1 t h i s system would approximately 13.6 kg (30 lb) (43 percent of the electrothermal system weight), would apply t o e x i s t i n g r o t o r b l a d e s , and would c o s t much less t h a n t h e pro- posed e l e c t r o t h e r m a l system.
I n a n i n i t i a l a t t e m p t t o e v a l u a t e t h e d e i c i n g c a p a b i l i t y and aerodynamic performance of b o o t s f o r r o t o r b l a d e s , tests were conducted i n 1979 i n t h e NASA Lewis 6- by 9-ft Icing Research Tunnel (IRT) on a 1.83-m (6-ft) span, full-scale segment of a stationary UH-1H rotor blade. In these tests three boot geometries were evaluated. These boots comprised both spanwise and chordwise tubes. Since the model blade was stationary during a run, neither the rotating nor vibrating l o a d s of a r e a l r o t o r were simulated.
Also t h e h i g h r o t o r t i p speeds could n o t be simulated s i n c e t h e maximum t u n n e l a i r speed was 134 m/sec (i.e., M o ,., 0.4). Angle of a t t a c k was v a r i e d from 0 ' t o 16" ( s t a l l ) without i c e and from 0 ' t o 10' w i t h i c e . The lower a i r speeds and t h e absence of r o t o r dynamic tests probably made t h i s a c o n s e r v a t i v e t e s t of t h e b o o t ' s l o a d s i n t h e s e e f f e c t i v e n e s s a s a d e i c e r because both dynamic l o a d s and h i g h e r a i r speeds should a i d i n removing t h e i c e . Some tests were made t o roughly s i m u l a t e t h e c y c l i c motion of a r o t o r b l a d e by i c i n g t h e model a t one a n g l e of a t t a c k and d e i c i n g t h e model a t another. With t h e b e s t boot c o n f i g u r a t i o n a s e r i e s of model d r a g measurements were made w i t h a t r a n s l a t i n g wake-survey probe. The t e s t r e s u l t s are i n c l u d e d h e r e i n along w i t h a d e s c r i p t i o n of a N A S A Ames-Lewis program p l a n t o t e s t t h e pneumatic boot concept w i t h f u l l - s c a l e , r o t a t i n g UH-1H blades.
SYMBOLS
s e c t i o n drag c o e f f i c i e n t , 2 j k (1 - k) d z
' d C wing chord, 0.533 m (1.75 f t )
d r o p l e t median volume s i z e , urn
Dmed H l o c a l s t a g n a t i o n p r e s s u r e f ree-stream s t a g n a t i o n p r e s s u r e HO LWC l i q u i d water c o n t e n t , g/m free-stream Mach number M O f r e e - s t re am s t a t i c p r e s s u r e PO free-stream s t a g n a t i o n temperature, OC T O
x, p o s i t i o n c o o r d i n a t e s , m
V v e l o c i t y , mlsec free-stream v e l o c i t y , mlsec vO a s e c t i o n a n g l e of a t t a c k a t t u n n e l c e n t e r l i n e , deg TEST APPARATUS AND PROCEDURE The t e s t model was made from a 1.83-m ( 6 - f t ) span segment of a f u l l - s c a l e UH-1H r o t o r blade and was mounted v e r t i c a l l y i n t h e test s e c t i o n of t h e L e w i s 6- by 9-ft I c i n g Research Tunnel ( f i g . 2). S i n c e t h e model was c u t from a n a c t u a l r o t o r blade, it included a uniform t w i s t of approximately 0.5" p e r f o o t o r about 3" from f l o o r t o c e i l i n g . The r o t o r b l a d e on a UH-1H h e l i c o p t e r i s 14.63 m ( 4 8 f t ) i n d i a m e t e r and i n c o r p o r a t e s a constant-chord (0.533 m The model w a s mounted on t h e t u n n e l (1.76 f t ) ) NACA 0012 a i r f o i l s e c t i o n .
f l o o r - p l a t e , and t h e a n g l e of a t t a c k could be v a r i e d from n e a r zero t o s t a l l .
The pneumatic b o o t s were a p p l i e d o v e r t h e e x t e r n a l s u r f a c e of t h e l e a d i n g edge, and t h e supply a i r l i n e was routed i n s i d e t h e model and through t h e tun- n e l f l o o r p l a t e . The c o n t r o l system f o r t h e boot t e s t was t h e same as t h a t shown i n f i g u r e 1. For t h e wind t u n n e l t e s t s t h e t u r b i n e bleed a i r was re- placed by regulated t u n n e l s e r v i c e a i r . This system i s a l s o t h e same a s t h e The system was designed around a one c u r r e n t l y used on fixed-wing a i r c r a f t .
two-position v a l v e ( e j e c t o r flow c o n t r o l valve) t h a t used a v e n t u r i o r i f i c e t o Upon a c t i v a t i o n t h i s provide vacuum t o t h e boot when i t was n o t a c t i v a t e d .
valve c l o s e d and h i g h e r p r e s s u r e a i r ( 1 0 . 5 ~ 1 0 ~ t o 21x103 kg/m2 (15 t o 30 p s i g ) ) w a s provided t o r a p i d l y i n f l a t e t h e b o o t . This system can b e operated e i t h e r manually o r a u t o m a t i c a l l y w i t h a programed p u l s e sequence and timing.
A t r a n s l a t i n g wake-survey probe was used t o h e l p e v a l u a t e t h e d e i c i n g per- formance of t h e boot c o n f i g u r a t i o n s . The probe, as shown i n f i g u r e 3, con- s i s t e d of a s i n g l e s t a g n a t i o n p r e s s u r e t u b e t h a t c o u l d be r e t r a c t e d down behind a wind screen. When t h e a i r f o i l was exposed t o t h e t u n n e l i c i n g cloud, t h e probe w a s r e t r a c t e d behind t h e windscreen. Then a f t e r t h e cloud w a s t u r n e d o f f , t h e probe w a s i n s e r t e d i n t o t h e a i r stream and t h e wake survey was made.
This probe, which w a s l o c a t e d about one chord downstream of t h e a i r f o i l a t mid- span, w a s i n s t a l l e d as shown i n f i g u r e 4 t o y i e l d t h e v e l o c i t y decrement r a t i o V I V O i n t h e a i r f o i l wake. By t r a n s l a t i n g l a t e r a l l y through t h e wake a p l o t of V I V O as a f u n c t i o n of p o s i t i o n X w a s obtained. I n t e g r a t i o n of t h e wake d e f e c t gave a measurement of a i r f o i l s e c t i o n d r a g c o e f f i c i e n t .
Sketches of t h e pneumatic boot d e s i g n s t e s t e d are shown i n f i g u r e s 5 and 6. I n t h e i n i t i a l p a r t of t h e test program t h r e e c a n d i d a t e d e s i g n s were screened i n terms of t h e i r d e i c i n g c a p a b i l i t y . These boots w e r e designed t o u s e a combination of both chordwise and spanwise tubes. R e s u l t s from tests performed i n t h e 1950's ( r e f . 3) on pneumatic boots f o r f i x e d wings suggested t h a t t h e aerodynamic e f f e c t of i n f l a t i n g t h e t u b e s was less w i t h chordwise t u b e s t h a n w i t h spanwise c o n f i g u r a t i o n s .
Because t h e r o t o r a i r f o i l s e c t i o n was both s h o r t e r and t h i n n e r t h a n f i x e d wings, it was f e l t t h a t t h e boot f o r a h e l i c o p t e r r o t o r should i n c o r p o r a t e p r i m a r i l y chordwise tubes. However, w i t h t h e small leading-edge r a d i u s of a r o t o r blade, i t was e v i d e n t t h a t chordwise t u b e s would crimp o v e r i n t h e leading-edge r e g i o n and n o t provide t h e d e f l e c - t i o n necessary t o f r a c t u r e and remove t h e ice. As a r e s u l t it w a s necessary t o a l s o i n c o r p o r a t e i n t o t h e boot d e s i g n a spanwise t u b e ( o r t u b e s ) a t t h e l e a d i n g edge. Two boot geometries were t e s t e d i n i t i a l l y : a small-diameter-tube con- f i g u r a t i o n (similar t o f i g . 5, but w i t h a s i n g l e spanwise t u b e ) and a l a r g e r - diameter-tube c o n f i g u r a t i o n ( f i g . 6 ) . These boot c o n f i g u r a t i o n s i n c o r p o r a t e d t u b e s i z e s t h a t were i n t h e same range (1.27 t o 3.18 cm diam) as t h o s e c u r r e n t - l y used on l a r g e r chord, fixed-wing a i r c r a f t . For t h e smaller-chord r o t o r air- f o i l s it would be d e s i r a b l e t o u s e smaller d i a m e t e r t u b e s t o minimize t h e aero- dynamic e f f e c t , e s p e c i a l l y upon a c c i d e n t a l or m u l t i p l e i n f l a t i o n . However, g e t t i n g t h e d e f l e c t i o n s r e q u i r e d t o break t h e i c e w i t h smaller t u b e s would re- q u i r e h i g h e r i n f l a t i o n p r e s s u r e s t h a n a v a i l a b l e on e x i s t i n g r o t o r c r a f t . Conse- q u e n t l y t h e t u b e s i z e s used i n t h i s program were from 5 t o 7 t i m e s l a r g e r , r e l a t i v e t o t h e chord l e n g t h , t h a n t h o s e c u r r e n t l y used f o r f i x e d wings. The b o o t s were designed t o provide coverage of about 20 p e r c e n t of t h e chord on t h e upper s u r f a c e and 30 p e r c e n t on t h e lower. When choosing t h e amount of chord- w i s e boot coverage, b o t h t h e l i m i t s of impingement of water-droplet t r a j e c t o r - i e s and runback should be taken i n t o account.
During t h e i n i t i a l d e i c i n g t e s t s t h e two c o n f i g u r a t i o n s w i t h a s i n g l e span- w i s e t u b e on t h e l e a d i n g edge were i n e f f e c t i v e i n removing t h e i c e . Therefore, a s d i s c u s s e d i n t h e s e c t i o n RESULTS AND DlSCUSSION, t h e boot d e s i g n was modi- f i e d by s p l i t t i n g t h e s i n g l e spanwise t u b e i n t o two t u b e s ( f i g . 5 ) . Once it w a s determined t h a t t h e boot w i t h two spanwise t u b e s on t h e l e a d i n g edge w a s e f f e c t i v e i n d e i c i n g t h e blade, t h e t r a n s l a t i n g probe w a s i n s t a l l e d . Measure- ments of a i r f o i l s e c t i o n d r a g n e a r t h e model c e n t e r l i n e were made o v e r a range i n a n g l e of a t t a c k from 0" t o s t a l l (-16" without i c e , and -9.4" w i t h i c e ) .
Data were o b t a i n e d , both w i t h and without t h e boot i n s t a l l e d , a t t u n n e l speeds These speeds are lower t h a n t h o s e n e a r of 67 and 112 m/sec (150 and 250 mph).
t h e outboard s e c t i o n s of a r o t o r b l a d e ; t h e r e f o r e c o m p r e s s i b i l i t y e f f e c t s and aerodynamic h e a t i n g e f f e c t s were n o t simulated. Data were i n i t i a l l y t a k e n a t t h e lower speed, without i c e , t o check o u t t h e probe and t o check t h e s t a l l c h a r a c t e r i s t i c s of t h e r o t o r blade. S t a l l w a s d e t e m i n e d by applying t u f t s t o t h e s u c t i o n s u r f a c e of t h e b l a d e and observing where t h e flow began t o r e v e r s e d i r e c t i o n or become u n s t a b l e . When it w a s determined t h a t t h e probe could w i t h s t a n d t h e t u r b u l e n c e generated by t h e model, d a t a were t a k e n a t r t h e h i g h e r speed w i t h i c e .
Data were o b t a i n e d a t v a r i o u s i c i n g c o n d i t i o n s and a t v a r i o u s angles of By s e l e c t i n g t u n n e l temperature b o t h g l a z e (-6.1" C) and r i m e a t t a c k .
(-14.4" C) i c e c o n d i t i o n s were i n v e s t i g a t e d . I n a l l c a s e s t h e i c i n g cloud Dmed of 20 urn c o n d i t i o n s were k e p t c o n s t a n t a t a volume median d r o p l e t s i z e and a l i q u i d water c o n t e n t (LWc) of 1 g/m3. Most i c i n g and d e i c i n g sequences were done a t c o n s t a n t a n g l e s of a t t a c k , b u t f o r some c o n d i t i o n s t h e model would The a n g l e s of a t t a c k be i c e d a t one a n g l e of a t t a c k and d e i c e d a t another.
were kept w i t h i n t h e range of t h o s e t y p i c a l l y expected on a r o t o r b l a d e , namely, between 0" and 8'. For example t h e model would be i c e d a t 1.4' and d e i c e d a t or vice versa; the model was also iced at 5 . 4 O and deiced at 9 . 4 O . These 5.4", v a r i a t i o n s were a n a t t e m p t t o s i m u l a t e , i n a very slow way, t h e c y c l i c p i t c h v a r i a t i o n s of a real r o t o r blade.
I n each i c i n g t e s t sequence about 1 cm of ice w a s a c c r e t e d on t h e b l a d e b e f o r e d e i c i n g was attempted. One c e n t i m e t e r of i c e w a s chosen as a good test c o n d i t i o n f o r two reasons. F i r s t , f o r t h e pneumatic boot t o work, a c e r t a i n amount of i c e h a s t o be p r e s e n t . I f t o o l i t t l e i c e i s p r e s e n t , t h e ice w i l l be f r a c t u r e d i n t o small p i e c e s , b u t t h e i n t e r f a c i a l bonds w i l l not be broken and consequently t h e i c e w i l l n o t be removed. Second, from unpublished f l i g h t d a t a from r e c e n t r o t o r c r a f t i c i n g tests behind t h e HISS (U.S. Army h e l i c o p t e r i c i n g spray system) t a n k e r and a t t h e O t t a w a spray r i g , it w a s e v i d e n t t h a t , when ice a c c r e t i o n s exceeded approximately 1 cm on t h e r o t o r , t o r q u e r o s e g r e a t l y .
RESULTS AND DISCUSSION x A s w a s noted i n t h e previous s e c t i o n t h e pneumatic boot c o n f i g u r a t i o n s t h a t had a s i n g l e spanwise tube a t t h e leading edge proved t o be inadequate f o r t h e s e d e i c i n g tests of a s t a t i o n a r y r o t o r blade. I n f l a t i o n of t h e b o o t s a t v a r i o u s c o n d i t i o n s of t u n n e l speed, temperature, and model a n g l e of a t t a c k r e s u l t e d i n t h e i c e being s e v e r e l y f r a c t u r e d , b u t t h e i c e cap would n o t l e a v e e i t h e r 'the upper o r lower s u r f a c e s . A f t e r each of t h e s e tests t h e i c e adhe- s i o n w a s found t o be so s i g n i f i c a n t l y reduced t h a t t h e i c e could be e a s i l y removed by wiping t h e s u r f a c e of t h e model. However, t h e aerodynamic f o r c e s would n o t remove t h e i c e . S i m i l a r r e s u l t s were observed w i t h both tube s i z e s . It was decided t h e r e f o r e t o change t h e b a s i c boot design by s p l i t t i n g t h e s i n g l e spanwise t u b e on t h e l e a d i n g edge i n t o two t u b e s ( f i g . 5 ) . With t h i s new d e s i g n t h e aerodynamic f o r c e s were e f f e c t i v e i n removing t h e i c e on t h e s u c t i o n s u r f a c e . Therefore t h i s new boot c o n f i g u r a t i o n was used through- out t h e remainder of t h e t e s t i n g with t h e wake-survey probe. It must be noted, however, t h a t t h e s e i n i t i a l t e s t s , s i n c e t h e r e was no blade r o t a t i o n w i t h t h e corresponding c e n t r i f u g a l a c c e l e r a t i o n and b l a d e v i b r a t i o n , c a n be considered as p r e l i m i n a r y and probably c o n s e r v a t i v e . It i s p o s s i b l e t h a t even t h e single-spanwise-tube c o n f i g u r a t i o n would work i n a r e a l r o t o r environment.
A i r f o i l Drag The model s e c t i o n drag c o e f f i c i e n t d a t a without ice a r e p r e s e n t e d i n f i g u r e 7 as a f u n c t i o n of s e c t i o n a n g l e of a t t a c k . I n t h i s f i g u r e d a t a are p r e s e n t e d f o r t h e c l e a n model w i t h o u t t h e boot and f o r t h e model w i t h t h e boot, both d e f l a t e d and i n f l a t e d . Also shown i n f i g u r e 7 a r e published d a t a ( r e f . 4) f o r a NACA 0012 a i r f o i l s e c t i o n , both smooth and w i t h standard rough- ness. These d a t a provide a means t o e v a l u a t e and v a l i d a t e t h e measurements made w i t h t h e wake-survey probe. F i g u r e 7 a l s o i n c l u d e s t h e r e s u l t s of t h e flow s e p a r a t i o n s t u d i e s , made by observing t u f t s , which show t h e e f f e c t of t h e pneumatic boot on t h e a i r f o i l s t a l l c h a r a c t e r i s t i c s .
4 29 The d a t a i n f i g u r e 7 are f o r a t u n n e l speed of 67 m/sec (150 mph). A s noted i n t h e p r e v i o u s s e c t i o n , d a t a w e r e o b t a i n e d a t two t u n n e l speeds, 67 and Since b o t h speeds were w e l l below t h e r e g i o n 112 m/sec (150 and 250 mph).
where compressible f l o w e f f e c t s become important (i.e., Mo 2 0.41, t h e d r a g
c o e f f i c i e n t s were e s s e n t i a l l y t h e same f o r t h e two t e s t speeds. F i g u r e 7 shows t h a t t h e clean-model d a t a agreed very w e l l w i t h t h e s m o o t h - a i r f o i l r e f e r e n c e d a t a , thereby v a l i d a t i n g t h e probe r e s u l t s .
The d a t a w i t h t h e boot i n s t a l l e d , but u n i n f l a t e d , i n d i c a t e a drag p e n a l t y t h a t decreased w i t h a n g l e of a t t a c k .
T h i s p e n a l t y was about 20 p e r c e n t a t low a n g l e s and decreased t o z e r o a t h i g h e r angles. However, t h i s p e n a l t y could probably be reduced t o zero i f t h e boot were recessed f l u s h w i t h t h e s u r f a c e of t h e w i n g . I n any case t h e p e n a l t i e s were less t h a n t h e d i f f e r e n c e between t h e smooth and standard-roughness r e f e r - ence a i r f o i l d r a g d a t a .
The drag a s s o c i a t e d w i t h t h e i n f l a t i o n of t h e boot was q u i t e l a r g e , w i t h drag i n c r e a s e s ranging from about 50 p e r c e n t a t t h e low a n g l e s of a t t a c k t o n e a r l y 300 p e r c e n t a t h i g h e r a n g l e s of a t t a c k . S i m i l a r r e s u l t s were observed i n t h e s t a l l a n g l e d a t a . With t h e boot d e f l a t e d , t h e s t a l l a n g l e of a t t a c k was about 16', n e a r l y t h e same as t h a t of t h e smooth r e f e r e n c e a i r f o i l . When t h e boot was i n f l a t e d , however, t h e s t a l l a n g l e w a s reduced t o about 9.4'. This r e s u l t , although s e v e r e , may s t i l l be a c c e p t a b l e s i n c e t h e r o t o r b l a d e c y c l i c p i t c h e x c u r s i o n s r e s u l t i n a n g l e s of a t t a c k t h a t a r e t y p i c a l l y less t h a n 8 ' .
Consequently a c c i d e n t a l boot i n f l a t i o n should n o t c a u s e b l a d e stall.
F i g u r e 8 p r e s e n t s p l o t s of drag c o e f f i c i e n t as a f u n c t i o n of a n g l e o f (1) d a t a repeated from f i g u r e 7 f o r t h e i n f l a t e d boot a t t a c k f o r two c a s e s : without any i c e p r e s e n t ; and ( 2 ) t h e envelope of t h e drag d a t a t a k e n when t h e test s e c t i o n had about 1 cm of i c e on i t s l e a d i n g edge. (Data from both r i m e and g l a z e i c e c o n d i t i o n s are included w i t h i n t h i s envelope.) A s noted e a r l i e r , h e l i c o p t e r p i l o t s and test e n g i n e e r s have t o l d u s i n informal c o n v e r s a t i o n s t h a t h e l i c o p t e r s l i k e t h e UH-1H c a n t o l e r a t e about 1 cm of i c e on t h e main r o t o r s without s e v e r e consequences, such as i n o r d i n a t e torque r i s e caused by i c e d r a g o r e x c e s s i v e shaking and v i b r a t i o n due t o unsymmetrical ice shedding on t h e main r o t o r s .
F i g u r e 8 shows t h a t w i t h 1 cm of i c e on t h e l e a d i n g edge t h e flow s e p a r a t e s when t h e a n g l e of a t t a c k exceeds about 6'. Therefore w e should expect t h a t w i t h 1 cm of i c e t h e a i r f o i l performance w i l l d e t e r i o r a t e d r a s t i c a l l y f o r a n g l e s of a t t a c k g r e a t e r t h a n 6'. On t h e o t h e r hand, f i g u r e 8 shows t h a t w i t h t h e boot i n f l a t e d and no ice, t h e a i r f l o w s e p a r a t e d a t about 9.5' and t h e d r a g c o e f f i c i e n t was about t h e same o r lower t h a n it was w i t h 1 cm of i c e . W e s i n c e t h e h e l i c o p t e r c a n f l y w i t h 1 c m of i c e , t h e t h e r e f o r e conclude t h a t , i n f l a t i o n o f t h e boot w i t h no i c e should n o t produce s e v e r e or c a t a s t r o p h i c re s u l t s.
Deicing Performance The pneumatic boot d e i c i n g performance and c h a r a c t e r i s t i c s are e v a l u a t e d i n f i g u r e s 9 t o 19. I n f i g u r e s 9 t o 13, comparing t h e d r a g measured b e f o r e and a f t e r a c t u a t i n g t h e boot y i e l d s a d i r e c t i n d i c a t i o n of t h e boot d e i c i n g p e r f o r - mance. F i g u r e s 1 4 t o 19 are a series of photographs of t h e boot f o r several d e i c i n g sequences. F i g u r e s 9 t o 13 show d a t a for t h e two t y p e s of icing- d e i c i n g sequences. I n f i g u r e s 9 and 10 t h e model w a s i c e d and deiced a t t h e same a n g l e of a t t a c k . I n f i g u r e s 11 t o 13 t h e model w a s i c e d a t one a n g l e and d e i c e d a t another. I n e a c h case t h e f i r s t a n g l e l i s t e d i s t h e a n g l e a t which t h e model w a s i c e d , and t h e second is t h e d e i c i n g a n g l e . Data are p r e s e n t e d f o r two t u n n e l temperatures, namely -6.1' C (21' F) and -14.4' C (6' F). These temperatures gave r e p r e s e n t a t i v e g l a z e and r i m e ice c o n d i t i o n s , r e s p e c t i v e l y .
For e a c h temperature shown, ice a c c r e t i o n r e s u l t e d i n a s i g n i f i c a n t i n c r e a s e i n drag c o e f f i c i e n t . However, t h e i n c r e a s e s i n d r a g c o e f f i c i e n t were g e n e r a l l y less at t h e c o l d e r temperature t h a n a t t h e warmer temperature. T h i s r e s u l t i s c o n s i s t e n t w i t h t h e f a c t t h a t r i m e i c e shapes are smoother t h a n g l a z e i c e shapes. A s shown i n f i g u r e s 9 t o 1 3 a c t i v a t i n g t h e pneumatic boot a t e i t h e r temperature r e s u l t e d i n a s i g n i f i c a n t d e c r e a s e i n t h e s e p e n a l t i e s . The resi- d u a l d r a g was due t o t h e r e s i d u a l i c e l e f t on t h e model ( b o t h on t h e boot and I n each c a s e shown, t h e d a t a r e p r e s e n t one c y c l e of boot behind t h e boot).
i n f l a t i o n ; however, a d d i t i o n a l c y c l i n g of t h e boot seemed t o have l i t t l e addi- t i o n a l e f f e c t on removing t h e r e s i d u a l ice. The r e s i d u a l d r a g s shown are t h e r e f o r e a d i r e c t measurement of t h e boot performance, and as shown i n f i g - u r e 9 t h e boot w a s q u i t e e f f e c t i v e e s p e c i a l l y a t t h e warmer temperatures. The boot tended t o be less e f f e c t i v e a t t h e c o l d e r temperatures, but i n each c a s e t h e boot r e s u l t e d i n a r e d u c t i o n i n d r a g t h a t could be t h e d i f f e r e n c e between a r o t o r c r a f t completing i t s m i s s i o n o r g e t t i n g i n t o s e r i o u s d i f f i c u l t y . For ex- ample, i n f i g u r e 13 (for To = -14.4' C) even though t h e r e s i d u a l drag a t t h e c o l d temperatures w a s 55 p e r c e n t , a c t i v a t i n g t h e boot a t 9.4' a n g l e of a t t a c k r e s u l t e d i n t h e flow o v e r t h e b l a d e r e v e r t i n g from a s e p a r a t e d t o an a t t a c h e d c o n d i t i o n . Comparing t h e two t y p e s of icing-deicing sequences t e s t e d d i d not show any d e f i n i t e trend. It i s inconclusive whether t h e slow v a r i a t i o n i n c y c l i c p i t c h used h e r e c o u l d i n any way be r e p r e s e n t a t i v e of t h e real r o t o r motion.
T y p i c a l icing-deicing sequences are d e p i c t e d i n f i g u r e s 1 4 t o 1 9 f o r b o t h t h e upper ( s u c t i o n s i d e ) and lower ( p r e s s u r e s i d e ) s u r f a c e s . F i g u r e s 14 t o 1 7 show t h e g l a z e i c i n g c o n d i t i o n ( i . e . , a t warmer temperatures) a t two d i f f e r e n t a n g l e s of a t t a c k .
A s shown i n t h e s e photographs t h e i c i n g l i m i t s along t h e chord, s i n c e both of t h e s e t e s t p o i n t s were a t p o s i t i v e a n g l e s of a t t a c k , were g r e a t e r o n t h e lower s u r f a c e t h a n on t h e upper s u r f a c e . Also t h e i c i n g l i m i t on t h e lower s u r f a c e i n c r e a s e d w i t h i n c r e a s i n g a n g l e of a t t a c k . F i g u r e s 18 and 19 show a r i m e i c e c o n d i t i o n .
Compared w i t h t h e previous two photographic sequences, t h e i c e a t t h i s lower temperature was much w h i t e r and g r a i n i e r and was n o t as peaked or double-horn shaped a t t h e l e a d i n g edge. I n e a c h of t h e d e i c i n g c a s e s shown, t h e boot was f a i r l y e f f e c t i v e i n removing i c e on t h e upper s u r f a c e b u t n o t as e f f e c t i v e i n removing ice on t h e lower s u r f a c e . Also t h e e f f e c t i v e n e s s of removing i c e from t h e lower s u r f a c e w a s less a t t h e lower tem- p e r a t u r e s . Comparing t h e s e r e s u l t s w i t h t h e d r a g r e s u l t s g i v e n p r e v i o u s l y in- d i c a t e d t h a t most of t h e observed d r a g rise r e s u l t e d from t h e i c e on t h e upper s u r f a c e . T h i s w a s e s p e c i a l l y e v i d e n t f o r t h e r i m e i c e c a s e ( f i g s . 18 and 191, where very l i t t l e of t h e lower ice w a s removed b u t , as shown i n f i g u r e 13 ( f o r To = -14.4' C ) , t h e d r a g w a s reduced from a l a r g e v a l u e w i t h s e p a r a t e d flow t o a lower v a l u e w i t h a t t a c h e d flow. Again, as shown i n f i g u r e s 14 t o 19, t h e r e s i d u a l i c e w a s g r e a t e r f o r t h e r i m e i c e c a s e and r e s u l t e d i n h i g h e r r e s i d u a l drags. I n a l l t h e sequences shown t h e r e s i d u a l i c e w a s w e l l f r a c t u r e d , and t h e r e f o r e t h e boot performance should be b e t t e r i f t h e c e n t r i f u g a l and v i b r a t o r y f o r c e s o n a real r o t o r were p r e s e n t .
N A S A Ames-lewis Rotor Program The n e x t s t e p i n t h i s program i s t o see how pneumatic d e i c e r b o o t s perform NASA Ames h a s begun a program t o test b o o t s on on f u l l - s c a l e r o t a t i n g blades.
a UH-1H h e l i c o p t e r . A series of nonicing tests w i l l f i r s t be performed -
i n c l u d i n g tie-down, hover, and f u l l - f l i g h t e v a l u a t i o n . I f t h e s e n o n i c i n g t e s t s are s u c c e s s f u l , i c i n g tests should follow.
CONCLUDING REMARKS These i n i t i a l tests of a pneumatic d e i c e r boot on a h e l i c o p t e r r o t o r b l a d e y i e l d e d some answers t o several of t h e b a s i c aerodynamic q u e s t i o n s posed by t h e Lockheed-California Company i n 1973. S i n c e t h e b l a d e was f i x e d i n t h e t u n n e l ( n o n r o t a t i n g ) and s i n c e t h e t u n n e l speeds were l i m i t e d t o Mach numbers less t h a n 0.4, t h e s e t e s t s could n o t s i m u l a t e t h e mechanical, r o t a t i o n a l , c y c l i c p i t c h , v i b r a t i o n a l , and high-tip-speed environment of an o p e r a t i o n a l r o t o r .
However, some important r e s u l t s were observed. F i r s t , t h e d r a g p e n a l t i e s of u n i n f l a t e d boots were small as compared w i t h d r a g p e n a l t i e s caused by 1 cm of ice. These p e n a l t i e s were no worse t h a n experienced w i t h t o d a y ' s blade foreign-object-damage s h i e l d s and would probably be e l i m i n a t e d i f t h e b o o t s were recessed f l u s h on new blade designs. Second, although t h e aerodynamic of i n f l a t i n g t h e boot without i c e was s i z a b l e , f o r most a n g l e s of a t t a c k e f f e c t t h e p e n a l t i e s were no worse t h a n t h o s e a l r e a d y accepted on fixed-wing a i r - c r a f t . Even t h e r e l a t i v e l y l a r g e r t u b e d i a m e t e r s on t h e small-chord a i r f o i l d i d n o t lower t h e s t a l l a n g l e i n t o t h e normal r e g i o n of r o t o r operation. A t t h e same t i m e t h e s e p e n a l t i e s proved t o be s i g n i f i c a n t l y less t h a n t h o s e ob- served w i t h 1-cm a c c r e t i o n s of ice. T h i r d , t h e pneumatic boot proved t o be an e f f e c t i v e d e i c e r even a t low temperatures (-14.4" C) and i n a probably very c o n s e r v a t i v e t e s t environment. It must be noted, however, t h a t some of t h e s e r e s u l t s could be d i f f e r e n t on a real r o t o r , e s p e c i a l l y t h e aerodynamic e f f e c t s a t t h e h i g h e r t i p speeds, but i n t h a t c a s e t h e d e i c i n g performance would prob- a b l y be more e f f e c t i v e .
Because t h e pneumatic boot e f f e c t i v e l y reduced t h e i c i n g d r a g p e n a l t y with- o u t c a u s i n g any o t h e r s e r i o u s aerodynamic p e n a l t i e s , NASA Ames h a s begun a pro- gram t h a t i n c l u d e s f u l l - s c a l e f l i g h t t e s t i n g of t h e pneumatic boot on helicop- t e r r o t o r s . I f t h e boot material w i t h s t a n d s t h e s e v e r e r o t o r environment i n f l i g h t and i f no f u r t h e r s i g n i f i c a n t aerodynamic p e n a l t i e s arise, perhaps t h e pneumatic boot c a n be developed i n t o a l i g h t w e i g h t , low-cost, low-power, and e a s i l y maintained d e i c e r system f o r r o t o r a p p l i c a t i o n s .
REF E RE NCES 1. A i r c r a f t Icing. NASA CP-2086, 1979. (Also FAA-RD-78-109.)
2. Werner, J. B.: I c e P r o t e c t i o n I n v e s t i g a t i o n f o r Advanced Rotafy-Wing A i r - c r a f t . LR-25327-10, Lockheed-California Co., 1973. (USAAMRDL-TR-73-38, AD-771182.)
E f f e c t of Pneumatic D e i c e r s and Ice Formations on Aero- 3. .Bowden, Dean T. : dynamic C h a r a c t e r i s t i c s of a n A i r f o i l . NACA TN-3564, 1956.
4. Abbott, I r a H.; and von Doenhoff, A l b e r t E.: Theory of Wing Sections.
Dover P u b l i c a t i o n s , Inc., 1959.
PNEUMATIC DEICER EJECTOR FLOW CONTROL V REGULATOR-TIMER RELIEF VALVE CHECK VALVE TURBINE BLEED LOW-VACUUM WARNING SWITCH Figure 1.- Main rotor application pneumatic deicer.
Figure 2 . - Pneumatic boot on rotor model installed in 6 X 9 ft. NASA Lewis Icing Research Tunnel.
Figure 3.- Pneumatic boot on rotor model and wake survey probe in NASA Lewis Icing Research Tunnel.
r TRANSLATING PROBE TEST AIRFOIL 7, TERAL POSITION POT TO CONTROL ROOM
MODULE FUNCTION: io - (Ho:;)uz -
X Figure 4.- Translating probe instrumentation.
Figure 5.- Typical cross section of i n s t a l l e d deicer ( i n f l a t e d ) , small tube.
,- 2.54 cm TUBE - CHORDWISE (TYP) r 3.18 cm TUBE- 3. 18 cm TAPERE // SPANWISE
EDGE (TYP) - ~
0.2 cm THICKNESS (TY / .....
4 LEADING EDGE Figure 6.- Typical cross section of i n s t a l l e d deicer ( i n f l a t e d ) , large tube.
---
SMOOTH AIRFOIL (REF. 4)
---
STD. NACA ROUGH (REF. 4) -0- CLEAN MODEL
+ BOOT DEFLATED
* BOOT INFLATED
SECTION ,r BOOT INFLATED 8’ r STD. NACA ROUGH (REF. 4)
. 01 ,/‘ ,/’ , r B O O T DEFLATE0
,-SMOOTH AIRFOIL (REF. 4) 0 4 a 12 16 SECTION ANGLE OF ATTACK, a, deg Figure 7.- Helicopter rotor model section drag.
NACA 0012 a i r f o i l (no ice) ; V = 67 m/sec.
BOOT INFLATED W I T H I c m I C E 0 4 a 12 16 SECTION ANGLE OF ATTACK, deg Figure 8.- Helicopter rotor model section drag.
NACA 0012 a i r f o i l ; VO = 1 1 2 m/sec.
TO = -6.1' C CLEAN WITH I C E .c-.c.
DEICED .01 SECTION DRAG COEFF, cd
.01 :"i 0
a Figure 9.- Section drag of helicopter rotor model with pneumatic boot. Ice-deice sequence; a = 1.4O/1.4O; V g = 1 1 2 m/sec.
I To = -6.1' C
a CLEAN
.@I WITH ICE DEICED .01 SECTION DRAG COEFF. 0 ' d
.01 :"I- O
a Figure 10.- Section drag of helicopter rotor model w i t h pneumatic boot. Ice-deice sequence; 01 = 5.4O/5.4O; V o = 1 1 2 m/sec.
CLEAN To = -6.1' C wim I C E - 0 5 1 . .
a DEICED
SECTION DRAG COEFF, 16% ' d I .01 a Figure 11.- Section drag of helicopter r o t o r model with pneumatic boot. Ice-deice sequence; 01 = 1 . 4 O / 5 . 4 O ; v0 = 1 1 2 m/sec.
Tn = -6.1' C CLEAN wim I C E DEICED .02 SECTION DRAG COEFF, O L cd *04 To = - 1 4 4 ' C
:~~~ . 01
Figure 1 2 . - Section drag of helicopter r o t o r model with pneumatic boot. Ice-deice sequence; 01 = 5.4O/1.4O; VO = 1 1 2 m / s e c .
?
CLEAN To = -6.1' C WITH ICE DEICED ? SEPARATION 'd
~:i . 01
9.40 9.40 Figure 13.- Section drag of helicopter rotor model with pneumatic boot. Ice-deice sequence; a = 5.4O/9.4O; VO = 1 1 2 m/sec.
Upper surface; Figure 14.- Typical ice-deice sequence.
a = 1.4O/5.4O; To = -6.lOC; V o = 1 1 2 m/sec.
4 4 0 Figure 15.- Typical ice-deice sequence.
Lower s u r f a c e ; a = 1.4O/5.4O; TO = -6.lOC; VO = 1 1 2 m/sec.
Figure 16.- Typical ice-deice sequence. Upper s u r f a c e ; a = 5.4O/5.4O; To = -6.lOC; V o = 1 1 2 m/sec.
4 41 ~1~~ ICE DEtCFO Lower surface; Figure 17.- Typical ice-deice sequence.
a = 5.40/5.40; TO = -6.1OC; V o = 1 1 2 m/sec.
DEICED HIM ICE Figure 18.- Typical ice-deice sequence.
Upper surface; 01 = 5.4O/9.4O; TO = -14.4Oc; Vo = 1 1 2 m/sec.
4 4 2 WITH 1CE F i g u r e 19.- Typical ice-deice sequence. Lower s u r f a c e ; a = 5.40/9.4O; TO = -14.4OC; VO = 112 m / s e c .