The RSRA, an experimental helicopter, is equipped with an active isolation system that allows the transmission to move relative to the fuselage. The purpose of the motion compensator is to prevent these motions from introducing unwanted signals to the main rotor control. A motion compensator concept was developed that has six-degree-of-freedom capability. The mechanism was implemented on RSRA and its performance verified by ground and flight tests

Huber, R.

Jeffrey, P.

English

NASA Technical Reports Server

DESIGN AND DEVELOPMENT OF A MOTION COMPENSATOR 
FOR THE RSRA M A I N  ROTOR CONTROL 
P. Je f fe ry * ,  R. Huber* 
ABSTRACT 
The RSRA, an exper imental  h e l i c o p t e r  developed by S i  korsky A i r c r a f t  f o r  
NASA and t h e  U. S. Army, i s  equipped w i t h  an a c t i v e  i s o l a t i o n  system t h a t  a l lows 
the  t ransmiss ion  t o  move r e l a t i v e  t o  the  fuselage. The purpose o f  t h e  mot ion 
compensator i s  t o  p revent  these motions from i n t r o d u c i n g  unwanted s igna ls  t o  t h e  
main r o t o r  c o n t r o l .  
Review o f  mot ion compensator concepts i n d i c a t e s  t h a t  most f u n c t i o n  o n l y  f o r  
l i m i t e d  motion. A new concept was developed t h a t  has six-degree-of-freedom 
c a p a b i l i t y .  The mechanism was implemented on RSRA and i t s  performance v e r i f i e d  
by ground and f l i g h t  t e s t s .  
INTRODUCTION 
The Rotor Systems Research A i r c r a f t  (RSRA) i s  an exper imental  h e l i c o p t e r  
in tended f o r  f l i g h t  research on advanced r o t o r  systems, designed and developed 
by Sikorsky A i r c r a f t  under the  j o i n t  sponsorship o f  NASA and the  U. S. Army. Two 
a i r c r a f t  have been b u i l t .  A t  p resent  one a i r c r a f t  i s  i n  compound con f igu ra t i on ,  
equipped w i t h  a v a r i a b l e  inc idence wing, a moving s t a b i l a t o r  and a u x i l i a r y  
t h r u s t  f a n  j e t  engines (F igure 1). Th is  ve rs ion  w i l l  be used f o r  high-speed 
research on h i g h l y  loaded o r  p a r t i a l l y  unloaded r o t o r s .  The o the r  a i r c r a f t  i s  
con f igured  as a pure h e l i c o p t e r  (F igure 2) and w i l l  be used f o r  a d d i t i o n a l  
research on a v a r i e t y  o f  r o t o r s .  Both a i r c r a f t  a r e  p r e s e n t l y  f l y i n g  w i th  a 
base l ine  r o t o r  system v i r t u a l l y  i d e n t i c a l  t o  the  Sikorsky S61 i n c l u d i n g  b i f i l a r  
v i b r a t i o n  absorbers, and t h e  v i b r a t i o n s  t ransmi t ted  t o  the  fuselage are  small i n  
a l l  f l i g h t  regimes. However, i t  i s  a n t i c i p a t e d  t h a t  some o f  t h e  r o t o r  systems 
t h a t  may be mounted on t h e  a i r c r a f t  i n  the  f u t u r e  w i l l  cause v i b r a t i o n s  t h a t  
cannot be a l l e v i a t e d  by b i f i l a r  absorbers alone. Thus the re  are  p rov i s ions  f o r  
an i s o l a t i o n  system t h a t  w i l l  a l l o w  t h e  r o t o r  and i t s  t ransmiss ion t o  move on 
spr ings r e l a t i v e  t o  the  fuselage, so p reven t ing  t h e  t r a n s m i t t a l  o f  r o t o r  induced 
v i b r a t i o n s  t o  the  fuselage. 
The RSRA r o t o r  c o n t r o l  i s  convent ional .  C y c l i c  and c o l l e c t i v e  blade p i t c h  
are c o n t r o l l e d  by pushrods at tached t o  a swashplate whose v e r t i c a l  p o s i t i o n  and 
tilt about l o n g i t u d i n a l  and l a t e r a l  axes are  determined by t h e  extens ion o f  
th ree  h y d r a u l i c  servo ac tua tors  mounted on t h e  t ransmiss ion  base. The s igna l  t o  
each servo ac tua to r  i s  a mechanical displacement t ransmi t ted  from t h e  c o c k p i t  by 
a s e t  o f  rods and cranks. When the  i s o l a t i o n  system i s  opera t ive ,  t he  
t ransmiss ion base i s  sub jec t  t o  t r a n s i e n t  and p e r i o d i c  motions r e l a t i v e  t o  t h e  
fuselage. Wi th no compensating dev ice these motions would i n t roduce  unwanted 
s igna ls  t o  the  servos and so cause c o l l e c t i v e  and c y c l i c  blade p i t c h  changes. 
t he re  a re  obvious Apart  f rom undesi rab le changes t o  the  f l i g h t  p a t h  
p o s s i b i l i t i e s  f o r  c o n t r o l  coupled i n s t a b i l i t i e s .  
* S i  korsky A i r c r a f t  D i v i s i o n ,  Un i ted  Technologies Corporat  
15 
on, S t r a t f o r d ,  Conn. 
https://ntrs.nasa.gov/search.jsp?R=19790014370 2020-03-21T23:46:48+00:00Z
TEE ACTIVE ISOLATION SYSTEM 
Various con f igu ra t i ons  o f  a c t i v e  i s o l a t i o n  were under cons ide ra t i on  i n  t h e  
e a r l y  days o f  RSRA design. The purpose o f  t h e  RSRA i s  t o  serve as a f l y i n g  t e s t  
bed f o r  advanced r o t o r  systems w i t h  d i f f e r e n t  dynamic c h a r a c t e r i s t i c s  rang ing  
from two-bladed t e e t e r i n g  r o t o r s  t o  h ingeless,  bear ing less,  and f u l l y  
a r t i c u l a t e d  designs. For some a p p l i c a t i o n s  i t  was known t h a t  ext remely s o f t  
t ransmiss ion supports would be requ i red  f o r  acceptable i s o l a t i o n ,  w h i l e  t h e  
t o t a l  excurs ion o f  t h e  t ransmiss ion under steady loads must be r e s t r i c t e d  
because o f  t he  l i m i t a t i o n s  o f  i n t e r f a c i n g  systems - i n  p a r t i c u l a r  engines, t a i l  
r o t o r  d r i v e ,  and c o n t r o l s .  The s o f t  r e s t r a i n t s  were thus prov ided by ' a c t i v e  
i s o l a t o r s  ' c o n s i s t i n g  o f  servo-nul 1 ed hydro-pneumatic ac tua to rs  w i t h  
displacemetit feedback t o  recenter  t h e  t ransmiss ion under steady f l i g h t  loads 
wh i l e  a l l o w i n g  o s c i l l a t o r y  motion a t  t h e  c r i t i c a l  v i b r a t i o n  frequencies. Other 
r e s t r a i n t s  cons is ted  o f  very  s t i f f  p i v o t e d  l i n k s  con ta in ing  l oad  c e l l s .  The 
number o f  independent s o f t  r e s t r a i n t s  determines t h e  number o f  degrees o f  
freedom o f  t h e  t ransmiss ion  r e l a t i v e  t o  the  fuselage. 
Dur ing t h e  course o f  development, a design evolved c o n s i s t i n g  o f  f o u r  
a c t i v e  i s o l a t o r s  (two independent) i n  t h e  h o r i z o n t a l  plane, a to rque r e s t r a i n t  
l inkage i n  t h e  h o r i z o n t a l  plane, and f o u r  l oad  c e l l  l i n k s  w i t h  t h e i r  axes 
focused t o  a p o i n t  low down on the  r o t o r  a x i s  (F igure 3). The a c t i v e  i s o l a t i o n  
system has been descr ibed by Kuczynski and Madden (Ref. 1). Th is  system has two 
degrees o f  freedom - o r  th ree ,  when f a i l u r e  modes, e.g., f r a c t u r e  o f  t h e  a n t i -  
torque 1 i nkage, are considered. 
COMPENSATOR REQUIREMENTS 
The p r e l i m i n a r y  design o f  t he  RSRA c o n t r o l  system took  p lace  w h i l e  t h e  
i s o l a t i o n  system was undergoing many i t e r a t i o n s .  Many combinations and con- 
f i g u r a t i o n s  o f  s o f t  r e s t r a i n t s  and r i g i d  l i n k s  were inves t iga ted .  The dec i s ion  
was made t h a t  t h e  main r o t o r  c o n t r o l  would i nc lude  compensating l inkages  w i t h  a 
f u l l  six-degree-of-freedom c a p a b i l i t y ,  Thus t h e  i s o l a t i o n  system design cou ld  
proceed w i t h o u t  any cons ide ra t i on  o f  r e s t r a i n t  imposed by c o n t r o l  system 
requirements. Also, t he re  would be no i n h i b i t i o n  o f  development and 
r e c o n f i g u r a t i o n  o f  t h e  i s o l a t i o n  system a t  any f u t u r e  time. 
The requirement f o r  a c o n t r o l  compensating l i nkage  may be s imply  described. 
The i n p u t  c o n t r o l  s igna l  i s  generated by t h e  p i l o t  commanding a displacement 
between h i s  s t i c k  and t h e  fuselage. The mechanical c o n t r o l  system l i nkage  
t ransmi ts  t h i s  s igna l  t o  one o f  t he  hyd rau l i c  servo ac tua tors  t h a t  p o s i t i o n  t h e  
swashplate and are  mounted on t h e  t ransmiss ion  base. I f  an i s o l a t i o n  system i s  
incorporated,  t he  t ransmiss ion base can move r e l a t i v e  t o  t h e  fuselage. Thus a 
mechanical displacement r e l a t i v e  t o  one body must be f a i t h f u l l y  copied by a 
displacement r e l a t i v e  t o  a second body, w h i l e  remaining unaf fec ted  by r e l a t i v e  
motion between t h e  bodies. I n  t h e  most general case, t he  motions c o n s i s t  o f  
t r a n s l a t i o n s  a1 ong th ree  mutual l y  perpend icu la r  axes and th ree  r o t a t i o n s  about 
these axes. 
16 
COMPENSATOR CONCEPTS 
I f  t h e  number o f  degrees o f  freedom a re  l i m i t e d ,  then simple s o l u t i o n s  a re  
poss ib le .  For  example, cons ider  two bodies w i t h  r e l a t i v e  t r a n s l a t i o n a l  freedom 
along one a x i s  (F igure 4). A l i nkage  c o n s i s t i n g  o f  a crank p i v o t e d  a t  each body 
and a l ong  r o d  perpend icu la r  t o  t h e  a x i s  i s  an e f f e c t i v e  approximate 
compensator, s ince smal l  motions produce l i t t l e  d is turbance t o  the  s igna l  i n  t h e  
rod. This  s imple idea was t h e  sub jec t  o f  a pa ten t  i n  1971 (Ref. 2). I f  t h e  
r e s t r a i n t  between the  two bodies i s  p rov ided by p a r a l l e l  p i v o t e d  bars and t h e  
c o n t r o l  r o d  i s  made p a r a l l e l  t o  the  bar  and equal i n  length ,  then a p e r f e c t  
compensator r e s u l t s .  
I f  t h e  bodies have t r a n s l a t i o n a l  freedom along a l l  t h ree  axes b u t  a re  
r e s t r a i n e d  r o t a t i o n a l l y  about a t  l e a s t  one a x i s ,  then a s o l u t i o n  i s  poss ib le  
c o n s i s t i n g  o f  a torque tube, u n i v e r s a l l y  connected t o  the  c o n t r o l  l i nkage  a t  
each end and having a te lescop ic  f e a t u r e  w i t h  a sp l i ned  o r  sc i sso rs  l i n k  
connection. The c o n t r o l  s igna l  cons i s t s  o f  r o t a t i o n  of t he  torque tube. Th is  
device,  sub’ect  o f  a pa ten t  by Durno & Dean (Ref. 3) ,  was used i n  t h e  Sikorsky 
XH-59A ABCJ’” (Advancing B1 ade Concept) research he1 i c o p t e r ,  which had 
p rov i s ions  f o r  a pass ive i s o l a t i o n  system w h i l e  be ing r e s t r a i n e d  i n  r o l l  by 
torque l i n k s  (F igure 5). 
A s imple apparent s o l u t i o n  t o  t h e  problem o f  t r a n s m i t t i n g  a displacement 
s igna l  between t w o  bodies w i t h  complete freedom o f  r e l a t i v e  mot ion i s  t o  use a 
f l e x i b l e  push/pul l  cable ope ra t i ng  w i t h i n  a f l e x i b l e  condui t .  I n  f a c t ,  t he  bes t  
o f  these devices e x h i b i t s  more f r i c t i o n  and hys te res i s  than can be t o l e r a t e d  i n  
a pr imary f l i g h t  c o n t r o l .  
COMPENSATOR DEVELOPMENT 
Development o f  t he  RSRA mot ion compensator s t a r t e d  w i t h  cons idera t ion  o f  
t he  problem o f  t r a n s m i t t i n g  a mechanical c o n t r o l  s igna l  between two bodies 
having r e l a t i v e  mot ion i n  a plane, i . e . ,  t r a n s l a t i o n  a long two axes and a 
r o t a t i o n  about a t h i r d  a x i s  perpend icu la r  t o  these two. Thus a s o l u t i o n  
cons is t s  o f  a summing l i nkage  con ta in ing  two s igna l  paths i n  p a r a l l e l  p lanes 
(F igure 6). One pa th  t ransmi t s  the  c o n t r o l  s i g n a l ,  t he  o the r  a c a n c e l l i n g  
s igna l  t h a t  removes the  e f f e c t  o f  t h e  r e l a t i v e  displacement between t h e  two 
bodies. I f  body 2 i s  subjected t o  displacements dg, dp, and r o t a t i o n  8, w h i l e  a 
c o n t r o l  i n p u t  c i  i s  made the t o t a l  displacement o f  t he  c o n t r o l  ou tpu t  p o i n t  i s  
d, + c i .  Hence the  ou tpu t  s igna l  cg = c i  and compensation i s  achieved. This  
re1 a t i  onship and the  independence o f  t he  c o n t r o l  ou tpu t  displacement on d, and 
8, i s  t r u e  a t  the  c o n t r o l  s igna l  mid-point  and very  nea r l y  t r u e  f o r  small 
displacements from the  mid-point .  
I f  t h e  mechanism i s  implemented w i t h  s e l f - a l i g n i n g  b a l l  t ype  connect ions,  
then t h i s  device prov ides compensation f o r  t he  case where the  two bodies a l so  
have t r a n s l a t i o n a l  freedom along the  z a x i s  and r o t a t i o n a l  freedom about the  x 
a x i s  (F igure 7). The c o n t r o l  ou tpu t  i s  independent o f  t he  displacement d, o r  
r o t a t i o n  8% and i s  very  nea r l y  unaf fec ted  by a combination o f  these two motions. 
Rota t ion  about the  y a x i s  i s  another mat ter .  I f  t h e  d is tance between the  s igna l  
pa th  planes i s  h, then a r o t a t i o n  8, w i l l  produce a f a l s e  ou tpu t  s igna l  co = h 
s i n  Ely. It i s  ev ident  t h a t  i n  order  t o  achieve a f u l l  six-degree-of-freedom 
compensator, i t  i s  s u f f i c i e n t  t o  l e t  h = 0. 
17 
The i m p l i c a t i o n  of h = 0 i s  t h a t  t h e  two s igna l  paths occupy t h e  same plane. 
The f i r s t  development o f  t h i s  i dea  i s  shown i n  F igure  8. The summing beams a re  
separated and connected by rods, arid one beam has become shor te r  than t h e  o the r  
t o  accGmmodate a be l l c rank .  Implementat ion o f  t h e  i dea  t o  a mechanism designeci 
t o  f i t  i n t o  the  space a v a i l a b l e  around t h e  t ransmiss ion  base a f t e r  t he  a c t i v e  
i s o l a t o r s  had been loca ted  l e d  t o  even more compl icat ions (F igures 9, l o ) .  Two 
l inkages  a re  i n s t a l l e d  a t  t h e  r i g h t  s ide  o f  t he  t ransmiss ion and one a t  t h e  
l e f t .  Local p e c u l i a r i t i e s  of space and al ignment imposed an i n d i v i d u a l  approach 
a t  each l o c a t i o n  and t h e r e  are  no common p a r t s  anywhere i n  t h e  l inkages.  A lso a 
number o f  compromises had t o  be made away from the  i d e a l  geometry, each 
i n t r o d u c i n g  another smal l  e r r o r .  A t  t h i s  t ime i t  was decided t o  make a new 
s t a r t .  
The nex t  development reve r ted  t o  the  bas ic  s i m p l i c i t y  o f  t he  o r i g i n a l  
concept (F igure  11). I n  order  t o  b r i n g  t h e  s igna l  paths i n t o  one p lane,  t h e  
summing l i nkage  took  t h e  form o f  an open frame beam, w i t h  t h e  o the r  beam 
centered i n s i d e  (F igure 12). Likewise, t h e  two rods take  the form o f  concen t r i c  
t u b u l a r  members. It was n o t  poss ib le  t o  a t t a c h  t h e  ou te r  ( cance l l i ng  s igna l )  
rod  t o  t h e  t ransmiss ion base a t  t he  same p o i n t  t h a t  t he  i nne r  ( c o n t r o l  s i g n a l )  
r o d  at taches t o  i t s  ou tpu t  crank (F igure 13). Th is  in t roduces an e r r o r ,  b u t  i t  
i s  extremely smal l  i f  the  rods are  l ong  compared t o  the  attachment o f f s e t .  F o r  
t h i s  reason, the  rods were a l i g n e d  h o r i z o n t a l l y  and a b e l l c r a n k  was naeded a t  
the  output .  The mot ion of t he  i nne r  rod  i s  l i m i t e d  by be ing  stepped down 2 : l  a t  
t h e  i n p u t  end and stepped up 1:2 a t  t h e  ou tpu t  be l l c rank .  
IMPLEMENTATION OF THE COMPENSATOR ON THE RSRA 
Implementat ion o f  t he  mot ion compensators on t h e  RSRA fo l l owed  very  c l o s e l y  
A compromise fo rced by t h e  l i m i t a t i o n s  o f  space was the  l i n e s  o f  t h i s  concept. 
t h e ' i n v e r s i o n  o f  t he  summing l i nkage  geometry. 
The th ree  l inkages  a re  s i m i l a r  i n  c o n f i g u r a t i o n  and many common p a r t s  a re  
used (F igure  14). A number o f  innovat ions were incorpora ted  i n  the  d e t a i l  
design of t he  summing l inkages  t o  make these u n i t s  compact and prevent  the  
p o s s i b i l i t y  o f  fas teners  s lackening o f f  and jamming t h e  c o n t r o l s  (F igure  15). 
True compensation i s  achieved w i th  the  i d e a l  l i nkage  and f o r  smal l  c o n t r o l  
s igna l  d isplacements about neu t ra l .  The compromises t h a t  had t o  be accepted 
in t roduced small e r ro rs .  A lso e f f e c t i v e  compensation was requ i red  over t h e  
whole range o f  c o n t r o l  displacements. A c r i t e r i o n  was s e t  t h a t  t he  e r r o r  i n  
output  should n o t  exceed 2% f u l l  s t roke  a t  any c o n t r o l  p o s i t i o n  w h i l e  the  
t ransmiss ion base was sub jec t  t o  motions i n  any d i r e c t i o n  up t o  the  maximum 
s t roke  o f  t h e  a c t i v e  i s o l a t o r  (+9 mm) taken i n  the  wors t  combinat ion o f  poss ib le  
con f igu ra t i ons .  A geometric mct ion ana lys i s  was performed by computer program 
and performance w i t h i n  s p e c i f i c a t i o n  was determined. 
18 
The compensating 1 inkage described was ins ta l  led on both RSRA a i r c r a f t  
(Figures 16, 17). Only the second a i r c r a f t  has been configured w i t h  the act ive 
isolat ion system operative,  and t e s t s  of the compensator performance were 
conducted on this a i r c r a f t .  The t e s t  consisted of s e t t i ng  a control i n p u t  w i t h  
a fixed s t i c k  i n  the  cockpit and driving the transmission base t o  extremes of 
travel on the isolat ion system, while observing motion a t  the control output 
poin ts .  The  resu l t s  of these t e s t s  confirmed the predictions of analysis.  
CONCLUSIONS 
The RSRA has made many f l i g h t s  w i t h  the  i so la t ion  system action and records 
indicate t h a t  the motion compensators have functioned t o  completely eliminate 
any measurable spurious control signals.  
REFERENCES 
1. Kuczynski , W. A. , and Madden, J . ,  The RSRA Active Isolation/Rotor Balance 
System, Paper No. 18, Four th  European Rotorcraft and Powered L i f t  Aircraf t  
Forum , Stresa , I t a l y  , September 1978. 
2. Dueweke, J. E. , U.S. Patent 3,563,499, Mechanism t o  Transfer Engine Torque 
and Control Motion Across He1 icopter Rotor Vibration Isolator .  
3. Durno, R. A. , and Dean, J. C . ,  U. S. Patent 3,908,399, Primary Flight ' 
Control W i t h  Isolated Gear Box. 
19 
Figure 1 RSRA Compound Configuration 
Figure 2 RSRA Helo Configuration 
20 
Figure 3 RSRA Transmission Active Isolat ion System 
B O D Y  2 
( D I S P L A C E M E N T  
Figure 4 Compensator Concept - 
L i n ea r 1.10 t i  on 
S E R V O  A C T U A T O R  0 u T p ” T - g :  , ,  
F U S E L A G E  /U 
“’TRA‘N SM IS s I o N 
- S Y S T E M  
M I X E R  
I N P U T  
Figure 5 Controls Motion Compensator 
on Sikorsky ABC 
2 1  
t d ”  DISPLACEMENTS DISPLACEMENTS d e x  
B O D Y  2 
I N P U T  S I G N A L  C ;  
B O D Y  2 
Y 
INPUT S I G N A L  C i  
Figure 6 Compensator Concept - 
P1 ane Motions 
Figure 7 Compensator Concept - 
General Motions 
0 RELATIVE TO 
TRANSMISSION 
SERVO ACTUATOR 
NSMlSSlON BASE 
PUT 
NAL 
Ci  
Figure  8 Compensator Concept Applied t o  RSRA 
22 
Fiqure 9 Preliminary Design for RSRA Motion Compensator - Right Side 
Figure 10 Preliminary Design for RSRA Motion Compensator - Left Side 
23 
TRANSMISSION BASE 
OUTER ROD 
I 
INNER ROD 
M 
Figure  11 Revised Concept f o r  RSRA Motion Compensator 
INNER ROD 
/ OUTER ROD 
0 
TRANSMISSION 
Figure 12  Revised Concept for  RSRA Figure  13 Revised Concept f o r  RSRA 
Motion Compensator - I n p u t  
and Summing Linkage Output Crank 
Motion Compensator - 
24 
Figure 14 Final Design - RSRA 
Mot i on Compensator 
Figure 16 Motion Compensator 
Installation on RSRA 
25 
Figure 15 Final Design - RSRA Motion 
Compensator - Detail 
Figure 17 The RSRA Main Rotor and 
Transmission, Isolation 
System, and Flight Contro 1s 


Design and development of a motion compensator for the RSRA main rotor control

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