The variable twist wing concept was used to investigate the relative effects of lift and turbulence distribution on the rolled up vortex wake. Several methods of reducing the vortex strength behind an aircraft were identified. These involve the redistribution of lift spanwise on the wing and drag distribution along the wing. Initial attempts to use the variable twist wing velocity data to validate the WAKE computer code have shown a strong correlation, although the vorticity levels were not exactly matched

Holbrook, G. T.

Morris, D. J.

English

NASA Technical Reports Server

BASIC RESEARCH IN WAKE VORTEX ALLEVIATION 
USING A VARIABLE TWIST WING 
By Dana J. Morris and G. Thomas Holbrook 
NASA Langley Research Center 
INTRODUCTION 
Aircraft trailing vortices are one of the principal factors affecting air- 
craft acceptance and departure rates at airports. 
posed by the vortex would allow reduction of the present spacing requirements. 
Such reductions would allow full utilization of advances in automatically aided 
landing systems while maintaining or improving safety within the terminal area. 
For several years, NASA has been conducting an in-house and contractual research 
effort involving theoretical and experimental studies of various wake vortex 
minimization techniques (refs. 1 and 2). This work was done in conjunction 
with the Federal Aviation Administration's investigation of various sensing 
devices for detecting the presence of vortices within the terminal area. 
Minimization of the hazard 
This work has identified several methods of reducing the vortex strength 
behind an aircraft. These involve the redistribution of 1 ift (vorticity) span- 
wise on the wing and drag (turbulence) distribution along the wing. 
continued effort involves experiments and theoretical analysis aimed at improv- 
ing the understanding of the physics of vortex dissipation. This report sum- 
marizes one area of NASA's basic research in wake vortex alleviation and con- 
tains the highlights of model tests using a variable twist wing to investigate 
various wing span-load and drag distributions. 
NASA's 
b 
- 
C 
c1 
cD 
cL 
C1 
cP 
SYMBOLS 
All force and moment data are referenced to the wind axes. 
reference wing span, m 
wing mean aerodynamic chord, m 
sectional lift coefficient 
drag coefficient, Drag force 
qms 
Lift force lift coefficient, 
qms 
trailing-wing rolling-moment coefficient, 
pressure coefficient, P - Pm 
Rolling moment 
qmSb 
409 
https://ntrs.nasa.gov/search.jsp?R=19810010532 2020-03-21T09:03:38+00:00Z
P 
9 
S 
U 
V 
VTW 
V 
W 
% 
Y 
Z 
na 
rl 
s t a t i c  pressure, Pa 
dynamic pressure, Pa 
reference w i n g  area, m 
component of velocity parallel t o  the x-axis, m/sec 
2 
resultant velocity magnitude, m/sec 
refers  t o  Variable Twist Wing 
component of velocity parallel to  the y-axis, m/sec 
component of velocity parallel to the z-axis, m/sec 
longitudinal axis referenced to  Variable Twist Wing centerline,  
positive a f t ,  m 
la teral  axis referenced to  Variable Twist Wing centerline,  positive 
o u t  r igh t  wing, m 
vertical  axis referenced to  Variable Twist Wing quarter-chord l i ne ,  
positive above wing ,  m 
wing-segment twist angle re la t ive  to  w i n g  center panel, wing leading 
edge u p  i s  posit ive,  deg 
vor t ic i ty ,  counterclockwise flow i s  positive, per sec 
Subscripts: 
TW refers  to  t r a i l i ng  wing 
VTW refers  to  Variable Twist Wing 
cn refers  to free-stream conditions 
TEST FACILITIES 
The t e s t s  were conducted in the Langley V/STOL Tunnel and ,  under contract ,  
in the Hydronautics S h i p  Model Basin. 
V/STOL Wind Tunnel 
The t e s t  section of the V/STOL tunnel has a height of 4.42 m y  a w i d t h  of 
and 6.63 m y  and a length of 14.24 m. 
maintained a t  2 .2  m above the t e s t  section f loor  
t r a i l i ng  edge) during t e s t  runs. 
The Variable Tw 
Angle o f  at tack 
st-Wing was blade mounted 
floor-to-wing center pane 
was determined from an 
41 0 
accelerometer mounted i n  t h e  fuse lage ( r e f .  3) .  
balance measured l i f t ,  drag, and pitching-moment data.  
sampling of t he  t h r e e  wake v e l o c i t y  components o r  r o l l i n g  moment on a t r a i l i n g  
wing i n  t h e  wake o f  t h e  V a r i a b l e  T w i s t  Wing. 
anywhere from 1.2 m t o  13.7 m downstream o f  t h e  wing. The approp r ia te  sensor 
was mounted t o  a motor -d r iven  t r a v e r s e  mechanism on t h e  survey r i g  t o  a l l o w  
moving t h e  sensor bo th  l a t e r a l l y  and v e r t i c a l l y .  
nism o u t p u t  t h e  l a t e r a l  and v e r t i c a l  p o s i t i o n  o f  t h e  sensor du r ing  t e s t  runs. 
A six-component s t ra in -gage 
A survey r i g  (see f i g .  1 )  was u t i l i z e d  i n  these t e s t s  f o r  crossplane 
The survey r i g  cou ld  be p o s i t i o n e d  
D i g i t a l  encoders on t h e  mecha- 
Hydronaut ics Ship Model Basin 
The Hydronaut ics Ship Model Basin i s  a water  tank  125 m long and 7.3 m 
Two independent ly  powered c a r r i a g e  systems wide, w i t h  a water depth o f  3.8 m. 
propel  t h e  V a r i a b l e  T w i s t  Wing and t r a i l i n g  wing down t h e  tank (see f i g .  2 ) .  
The Var iab le  T w i s t  Wing was loca ted  0.56 span below the  water  l i n e  and a t tached 
t o  t h e  l ead  c a r r i a g e  by a s t r u t  mounted t o  a t i l t  tab le .  
v ided f o r  ang le -o f -a t tack  adjustment ( r e f .  3 ) .  
cen ter  body measured l i f t ,  drag, and pitching-moment data.  
i n g  a 0.46-m v e r t i c a l  survey o f  t h e  wake d u r i n g  runs a t  a scanning r a t e  o f  
0.04 m/sec. The l a t e r a l  p o s i t i o n  o f  t h e  t r a i l i n g  wing i s  changed manual ly 
between runs. 
us ing  t h e  t ime d i f f e r e n t i a l  f o r  t h e  two ca r r i ages  t o  pass a p o i n t  ha l f -way down 
the  tow tank and t h e  measured speed o f  the  ca r r i ages .  
The ti lt t a b l e  pro- 
Balances i n t e r n a l  t o  t h e  model 
The t r a i l i n g - w i n g  c a r r i a g e  has a motor-dr iven v e r t i c a l - s c a n  system a l l ow-  
The separa t ion  d i s tance  between the  two models was determined 
MODELS 
Var iab le  Tw is t  Wing 
The V a r i a b l e  T w i s t  Wing (VTW), shown i n  f i g u r e  3, i s  a unique research 
model capable o f  genera t ing  a des i red  span l o a d i n g  by t w i s t i n g  spanwise wing 
segments t o  the  proper  l o c a l  ang le  o f  a t tack .  I n  t h i s  manner, t h e  e f f e c t  o f  
h i g h l y  v a r i e d  span load ings  on the  r o l l e d - u p  wake can be i nves t i ga ted .  The 
e f fec t  o f  tu rbu lence on t h e  r o l l e d - u p  wake can be determined f rom t e s t s  made 
w i t h  va r ious  tu rbu lence i n j e c t i o n  devices a t tached t o  t h e  VTW model. 
The a s p e c t - r a t i o  7 metal  wing has a span o f  2.489 m. As shown i n  f i g -  
u r e  4, the  f i x e d  0.35-m cen te r  span i s  bounded on each end by 36 independent ly 
movable sec t ions ,  each about 0.03 m wide. 
t i o n s ,  each w i t h  29 pressure po r t s ,  were e l e c t r o n i c a l l y  scanned and recorded i n  
about one- tenth o f  a second t o  o b t a i n  pressure c o e f f i c i e n t  data, Cp, d u r i n g  
t e s t s  i n  t h e  V/STOL tunne l .  
The 20 inst rumented spanwise loca-  
41 1 
Trailing Wing 
Rolling moment on a smaller t r a i l i ng  wing has been used as a means of e s t i -  
mating the hazard posed by a vortex wake system. The aspect-ratio 5.35 t r a i l i ng  
wing used for  these t e s t s  has a span 13 percent of the VTW span. A photograph 
and dimensions of the unswept trailing-wing model installed on the V/STOL survey 
rig are  presented i n  f igure 5. In each t e s t  f a c i l i t y ,  the model was mounted on 
a ro l l  balance and attached to  a traverse mechanism capable of positioning the 
model both l a t e ra l ly  and ver t ical ly  i n  the VTW wake. The model and i t s  ro l l -  
balance system were used to  measure the roll ing moment caused by the vortex 
flow downstream of the VTW model. 
METHOD OF TESTING AND ANALYSIS 
The t e s t s  were a l l  made a t  a Variable Twist Wing 
number of about one million, based on w i n g  chord. 
t ions tested i s  shown i n  table I .  
data taken a t  the different  downstream locations. 
C L  
Figure 6 shows schematically the types of 
of 0.6 and a Reynolds 
A matrix of the configura- 
L i f t  distributions f o r  each configuration tested i n  the V/STOL tunnel were 
calculated from the measured C p  d a t a .  During the t e s t s ,  an on-line computer 
program ut i l ized abou t  half the pressure port data t o  produce rough plots of 
spanwise loading. This enabled "f ine tuning" of the VTW twist dis t r ibut ion to  
match the desired loading. 
Force and moment data ( l i f t ,  drag, and pitching-moment) on the Variable 
Twist Wing were taken throughout an angle-of-attack range in the V/STOL tunnel. 
Generally, only the force and moment data necessary t o  assure testing a t  
C L ~ ~ ~  = 0.6 were taken i n  the water towing tank. 
Measured trailing-wing rolling-moment coefficients given in this report 
The V/STOL technique fo r  measuring rol l ing moment i s  t o  position 
represent an averaged CITW for  V/STOL d a t a  and a peak CITW for the water- 
tank data. 
the probe and take 10 data p o i n t s  per second over a 5-second period. 
d a t a  points are averaged and used as the C 1  for  t h a t  y,z location. Data 
are  taken a t  a suf f ic ien t  number of y,z positions t o  insure location of the 
position of the maximum 
technique fo r  measuring roll ing moment uses the probe to  traverse the wake 
ver t ical ly  fo r  a single la teral  position. 
ent probe la te ra l  positions t o  insure locating the y,z p o s i t i o n  fo r  the 
peak '1TW. The Hydronautics measured CITW t h u s  represents the maximum 
instantaneous value of rol l ing moment obtained i n  the y,z 
wake. 
wing semispan. 
These 
TW 
value obtained by th i s  method. The Hydronautics c1 TW 
Sufficient runs are  made a t  d i f fe r -  
crossplane of the 
On several VTW configurations, multiple vortices were shed from each 
Only the maximum strength vortex i s  noted in this report. 
Three-component wake velocity measurements were made using a three- 
component hot-film probe, mounted i n  the V/STOL survey r ig .  Hot-film voltages 
41 2 
and y,z potentiometer outputs from the traverse mechanisms were signal con- 
ditioned and recorded on magnetic tape . .  This tape was digit ized to  a matrix 
of y, z ,  u ,  v ,  and w data values over a 1.524 m x 1.524 m y  0.0254-m mesh,cross- 
p lane  g r i d .  Vorticity contours were computed from the crossplane veloci t ies  a t  
each of the downstream data s ta t ions.  
plots a t  the downstream sampling planes shows the development of the wake. 
Additionally, the data from the half-span downstream station were used as  
in i t i a l  conditions fo r  the two-dimensional , time marching, viscous wake simu- 
lation computer code ( W A K E ) .  (See re f .  4 . )  
Comparison of velocity and vor t ic i ty  
RESULTS 
Data were taken a t  the discrete  locations shown i n  table  I and a l l  the data 
curves shown are  fa i red t h r o u g h  these data points. Among the t e s t s  ( table  I )  
were three relat ively simple VTW loadings, each of which resulted in a rolled-up 
wake w i t h  one s ignif icant  strength semispan vortex. 
(1 ,  2 ,  and 4 )  and t he i r  measured ClTW values a re  compared i n  f igure 7. The 
comparative values of C 1  
loading generating the maximum roll ing moment and the simulated triangular (4 )  
loading creating the minimum. The re la t ive  relationship of ClTW data for 
configurations 1 and 4 also agrees between t e s t  f a c i l i t i e s .  Thus, u p  to  a 
30-percent reduction i n  trailing-wing roll ing moment can be achieved i n  a single 
semispan vortex wake by span-load a1 teration on the generating wing .  Turbulence 
differences are  considered t o  be insignificant since the C D  
cal for the three‘configurations a t  a CL of 0.6. 
80-percent flapped wing  and resu l t s  i n  a downstream semispan wake composed of 
an inboard, o r  f lap ,  vortex and an outboard, o r  wingtip,vortex. For this con- 
figuration, the f lap  vortex i s  highly dominant - in the V/STOL tunnel, the wake 
velocity data indicate vort ic i ty  levels for the f lap vortex a re  a t  l eas t  
30 percent greater t h a n  those for  the wingtip vortex a t  3 spans downstream and, 
in the water tank, ClTW for  the f lap vortex was measured a s  60 percent greater 
t h a n  the wingtip vortex. 
(configuration 7s) resu l t s  in a f a r  downstream semispan wake w i t h  only one 
vortex and ,  as shown in figure 8, a significantly lower 
t o  note that  the l i f t  distribution for  configuration 7s varies greatly from that  
of configuration 7 ,  as does the turbulence distribution (evidenced by a 
230-percent increase i n  C D  for  configuration 7s a t  a C L  of 0 .6) .  Therefore, 
the reduction i n  measured 
lence dis t r ibut ions,  the increased turbulence level , or a combination of these 
factors.  
These three configurations 
are as expected, w i t h  the simulated rectangular ( 2 )  TW 
i s  nearly identi- 
T h e  l i f t  d is t r ibut ion for  configuration 7 i s  similar t o  tha t  of an 
Addition of a spoiler centered a t  0.61(bvTw/2) 
I t  i s  important ‘lTW. 
CiTW may resu l t  from the modified l i f t  and t u r b u -  
In an e f fo r t  t o  separate the e f fec t  of  turbulence from tha t  of span load, 
the VTW was adjusted (configuration 9) to  match the span load of configu- 
ration 7 w i t h  the spoiler (configuration 7s). 
7, 7S, and 9. 
Figure 10 compares configurations 
I t  i s  apparent tha t  configuration 9 does not achieve the complete 
41 3 
reduction between configurations 7 and 7s. In f a c t ,  the V/STOL tunnel 
f o r  configuration 9 
TW 
c1 TW data, for x/bVTW < 5.5, show a large increase i n  
as opposed t o  configuration 7.  However, a t  an X/bVTw of 35, the water-tank 
data indicate an 11-percent reduction in ClTW from configuration-7 values as 
opposed t o  an overall 32-percent reduction between configurations 7 and 75. 
T h u s ,  i t  seems tha t  one-third of the measured ClTW reduction between configu- 
rations 7 and 7s may be accounted for  by span-load a l te ra t ion  - the remainder 
occurring due t o  turbulence dis t r ibut ion and level changes. 
f igure 11. 
i n t o  two vortices by five-and-one-half spans downstream. The measured values 
a t  the half-span s ta t ion were used t o  i n i t i a l i z e  the WAKE code t o  predict the 
development downstream. The vor t ic i ty  contours predicted by WAKE a t  five-and- 
one-half spans downstream a re  shown i n  figure 1 2  for  comparison w i t h  the 
measured values shown i n  f igure 11 ( c ) .  While the predicted vor t ic i ty  levels  
are  higher than those measured, the locations and shapes of the contours are  
approximately the same. 
The nondimensional vor t ic i ty  contours for  configuration 7s are  shown in 
The f o u r  vortices present one-half span behind the wing  have merged 
CONCLUDING REMARKS 
The Variable Twist Wing concept has been used t o  investigate the re la t ive  
e f fec ts  of  l i f t  and turbulence dis t r ibut ions on the rolled-up vortex wake. 
extensive data gathered will a s s i s t  i n  understanding the development and decay 
of the wake. Also, i n i t i a l  attempts to  use the Variable Twist Wing velocity 
data t o  validate the  WAKE computer code have shown a s t rong  correlation, 
a1 though  the vor t ic i ty  levels were n o t  exactly matched. Further data analysis 
and verification of the computer code i s  proceeding. 
The 
41 4 
REFERENCES 
1 .  Wake Vortex Minimization. NASA SP-409, 1976. 
2. Proceedings of the A i r c r a f t  Wake Vor t i ces  Conference.  March 15-17, 1977. 
3. S t i c k l e ,  Joseph W.; and Kel ly ,  Mark W . :  Ground-Based F a c i l i t i e s  f o r  
Wake Vortex Minimization. 
Report  No. FAA-RD-77-68. 
Evalua t ing  Vortex Minimization Concepts. 
NASA SP-409, 1977, pp.  129-155. 
4. B i l a n i n ,  Alan E . ;  Hirsh, Joe l  E . ;  Teske, Milton E . ;  and Hecht, Arthur M . :  
Atmospheric-Wake Vortex I n t e r a c t i o n s .  ARAP Report  No. 331, 1978. 
41 5 
Q 
V 
O 
S 
m c ,  
S 
W W  
V E  
L O  
O E  
LL 
v) u 
c, 
3 
m 
S 
O 
% I 
-1 
0 
I- 
v, 
\ > 
.r 
e 
S 
0 .- 
\ \ \ \ \  
\ \ \ \ \  \ \  
x r n r n  
m m x  x 
l - - N m b e r n U J h h h .  h h co co co m m m  
41 6 
Figure 1.- Survey r i g  i n  Langley V/STOL Tunnel (with 
t ra i l ing-wing  model a t tached  to  t r a v e r s e  mechanism. 
VTbI carriage trailing-wing carriage 
~ _ _ _  
~- -~ -____ 
water line 
blade mount 
trailing-wing model 
tank bottom 
/ / / /  / / / / /  / / / / / / / / /  / /  / /  / / / / / / / / / / /  / /  / / 7 7 - 7 / /  / / /  / / / / /  / / / /  //////////////// 
Figure 2.- Diagram of Hydronautics Ship  Model Basin. 
41 7 
Figure 3.- Variable Twist Wing model, blade mounted in 
Langley V/STOl Tunnel test section. 
Dots indicate semispan locations where C data 
taken - upper and lower wing surfaces 
have a total  o f  29 pressbre ports a t  
each location. 
P 
0 n e e e e e e  * . e . e e e * . e . a e  
Figure 4.- Variable Twist Wing (VTW) model. 
41 8 
Figure 5.- Photograph and dimensions of unswept trailing-wing 
model on traverse mechanism. Model has NACA 0012 airfoil 
section. 
Forces. momciits . and 
Figure 6.- Types of data taken with Variable Twist Wing model. 
41 9 
Figure 7.- Wing twist, wing loading, and trailing-wing 
rolling-moment comparisons for VTW configurations 1, 
2, and 4. 
.08 
''Ti4 .06- 
.04- 
- 
- - - -__  -... - _  --_ 
- -  --- --. 
--_ 
.02 -10- 
A a -5- configuration 7s 
~ from water tank 
420 
0 
0 
0 10 20 30 40 
.08 
1.0' K c o n f i g u r a t i o n  7s 
__-- - - -  "TW .06. 
C 1  .04 
0.2.  -02 
0 
. 
--_ - -  --- 
--- --_ - 
. from V/STOL tunnel 
A a -5 
A a configuration 8.5 , P ' l i ,  
O 
. configuration 7s 
Configuration 9 { , \ I
- 5 1  
o t - -  
A a -lo 
.10 
.08- 
C 1 y ~ ~  .06- 
-5 configuration 7 
0 
.04- 
-10- 
A a  
-5. configuration 75 .a2 
0 
8 1 
0.2 0.4 0 . 6  0 8 l.o 
Y/(byTy/Z) 
Figure 9.- Comparison of different wing twist 
distributions used to simulate lift distri- 
bustion attained with configuration 75. 
- 
-__ -- -  
- - - _ _ _  - _  - _  - _  - - - _ _ _  
._ - 
from water tank - 
'd 0.2 014 0.6 018 I'.O 4 ; 2 3 4- 5 i 
Y/ (byT@ 1 X h T W  
Figure 10.- Wing twist, wing loading, and trailing-wing 
rolling-moment comparisons for VTW configurations 7, 
7S, and 9. 
421 
z 
bVTW’2 
nbVTW 
0.0 0.3 0.6 0.9 1.2 1.5 
Y w 
(a) x / h  = 0.5. 
0.6 
0.4 
0.2 
z 
bVTW/2 
-0.2 
-0.4 
-0.6 
0.3 0.6 0.9 1.2 1.5 0.0 
Y 
bVTW/2 
(b) x / b v ~  = 3.0. 
Figure 11.- Vorticity contours measured behind configuration 7s. 
422 
VTW 
2v-J 
(c) x/bm = 5.5. 
Figure 1 1  .- Concluded. 
0.5 
-0.: 
-0.f 
- 0.2 
* 0  
I 1 1 1 
0 0.3 0.6 0.9 1.2 1.5 
y /  (bvT1,//2 
Figure 12.- Predicted vorticity contours at 5.5 spans 
downstream of configuration 7s. 
423 



Basic research in wake vortex alleviation using a variable twist wing

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