The effectiveness of apex fences on a 60-deg delta wing at low speeds was experimentally investigated. Resembling highly swept spoilers in appearance, the fences are designed to fold out of the wing apex region upper surface near the leading edges, where they generate a powerful vortex pair. The intense suction of the fence vortices augments lift in the apex region, the resulting positive pitching moment being utilized to trim trailing edge flaps for lift augmentation during approach and landing at relatively low angles of attack. The fences reduce the apex lift at high angles of attack, leading to a desirable nose-down moment. The above projected functions of the apex fence device were validated and quantified through low speed tunnel tests, comprising upper surface pressure surveys on a semispan model and balance measurements on a geometrically similar fully span wing/body configuration. Fence parameters such as area, shape, hinge position and deflection angle were investigated. Typical results are presented indicating the apex fence potential in controlling the longitudinal characteristics of a tail-less delta

Frassinelli, M. C.

Hoffler, K. D.

Rao, D. M.

English

NASA Technical Reports Server

BASIC STUDIES OM D E L T A  WING FLOW MODIFICATIONS 
BY MEANS OF APEX FENCES 
Keith D. Hoffler and Dhanvada M .  Rao 
Vigyan Research Associates, I n c a  
Hampton, V i  rgi ni a 
Mark C. Frassinelli  
Air Force Wright Aeronautical Labs 
Wri ght-Patterson Ai r Force Base, Ohi o 
SUMMARY 
The effectiveness of 'apex fences' on a 60-deg delta wing a t  low speeds has 
been experimentally investigated. Resembling highly swept spoilers in appearance, 
the fencesaredesigned t o  fold out of the wing apex region upper surface near the 
leading edges, where they generate a powerful vortex pair. The intense suction of 
the fence vortices augments l i f t  in the apex region, the resulting positive pitching 
moment being ut i l ized t o  trim trailing-edge flaps fo r  l i f t  augmentation during 
approach and landing a t  re lat ively low angles of attack. The fences reduce the apex 
l i f t  a t  high angles of attack, leading to  a desirable nose-down moment. 
The above projected functions of the apex fence device were validated and quanti- 
. f ied through low-speed tunnel t e s t s ,  comprising upper surface pressure surveys on a 
semi-span model and balance measurements on a geometrically simi 1 a r  fu l l  -span wing/ 
body configuration. Fence parameters such as area, shape, hinge position and , 
deflection angle were investigated. Typical resul ts  are  presented indicating the 
apex fence potential in control 1 ing the longitudinal character is t ics  of a t a i l  - less  
delta.  
SYMBOLS 
AVERAGE CPU - Span-averaged CPU a t  local s ta t ion 
C L - Lif t  coefficient,  based on total  wing area 
CM - Pitching moment, based on total  wing area and mean aero- 
dynamic chord 
CPU - Upper surface pressure coefficient 
C~ - Root chord (inches) 
L / D  - Lift-to-drag r a t io  
a( A L P H A )  
- Chordwi se distance measured from apex (inches) 
- Spanwise distance from root nondimensisnalined by the 
90ca9 semi-span 
- Angle of attack (degrees) 
https://ntrs.nasa.gov/search.jsp?R=19860017727 2020-03-20T14:13:46+00:00Z
ACL% - (GL, Fence on - C L ,  fence off)/(CL, fence o f f )  x EOO 
'TE, ELEVATOR - I r a i  l i ng-edge F l  ap deft ection, i nhoard on1 y (degrees) 
- Fence deflection (degrees) 
INTRODUCTION 
The aerodynamics of pitch control and longitudinal trim of highly swept f ighter  
configurations have received considerable attention in recent years. Close-coupled 
canards are currently popular because of the i r  abil i ty to  generate powerful pitching 
moments and low trim drag. However, the canard downwash reduces wing efficiency, 
and a t  high angles of attack canards tend to  lose pi tch-down capability. The adverse 
interaction between canard and wing vortices in s idesl ip  also leads to  non-lineari- 
tes  and rol l  ins tab i l i ty  a t  high alpha ( re f .  1 ) .  Unloading the canard above a 
c r i t i ca l  angle of attack i s  made d i f f i cu l t  by the strong upwash induced locally by 
the forebody and wing. 
A different approach towards pitch control of highly swept wings, v iz , ,  t o  
modulate vortex 1 i f t  in the apex region, was explored in the apex Tlap concept ( r e f .  
2) .  The appeal of t h i s  concept was the ab i l i t y  to  undeflect the apex, for  cruise 
f l i gh t  conditions, and deflect downward for  recovery from high alpha. Tests showed 
however that  l i ke  the canard the up-deflected apex f lap  also generated strong down- 
wash over the wing, and suffered a severe 1 i f t  loss in the neighborhood of the 
transverse hinge-1 ine. The wing-alone model tested in reference 2 a1 so could not 
represent the fuse1 age interference which i s  1 i kely t o  degrade apex flap effective- 
ness. These considerations led the second author to  propose an al ternate  method 
of apex 1 i f t  control , viz. 9 the apex fence. 
Resembling highly swept spoilers,  the apex fences are  hinged to the wing upper 
surface along the leading edges ( f ig .  1 ) .  When folded out ver t ical ly  a t  low angles 
of attack, the fences generate an intense vortex pair whose suction augments l i f t  in 
the apex region, resulting in a nose-up moment. Conversely, a t  high angles of attack 
the fence vortices are  greatly weakened and also raised higher above the apex; the 
combined ef fec t  i s  t o  reduce apex l i f t  in comparison with the basic wing, thus 
generating a desirable nose-down moment. The apex fences will not be subject t o  
fuselage interference and they also avoid the adverse transverse corner of the apex 
f lap hinge. A noteworthy advantage of apex fences i s  tha t  they can be shaped and 
oriented for  most e f f i c i en t  vortex-generation capabi 1 i ty  quite independently of the 
wing pl anform. 
Exploratory small-scale wind tunnel investigations of the apex fence concept 
applied to a 74 and 65 deg delta wing have been reported in reference 3. Upper 
surface pressure surveys supplemented with oi l  flow and helium bubble visualization 
confirmed the existence of strong and stable vortices produced by apex fences. These 
promising early resu l t s  encouraged a more comprehensive study of the concept applied 
to a 60-deg delta wing, th i s  sweep angle being more in keeping with the current 
f ighter  design studies,  This investigation was undertaken primari 1y to  va1 i date and 
quantify the hypothesized aerodynamics effects of apex fences in controlling the 
longitudinal character is t ics  of a t a i l - l e s s  delta through the angle-ofpallack range. 
MODELS AND TEST DETAILS 
Pressure Model 
Major dimensions of the generic semi-span 60 deg delta wing body model are 
shown in figure 2. This model incorporated four spanwise rows of pressure Laps, the 
f i r s t  row being well inside the apex region occupied by the fence. The model was 
mounted on a boundary layer bypass plate seven inches above the tunnel floor.  Six 
fence shapes were tested on th i s  model, only two shapes being presented herein ( f ig .  
3 ) .  The t e s t  was conducted in the North Carolina State University Merrill Subsonic 
Wind Tunnel a t  a mean-aerodynamic-chord Reynold's number of 0.67 m i  11 ion, and angles 
of attack ranging from zero to  30 deg. 
Force Model 
Major dimensions of the force model are  shown in figure 4. This model was 
geometrically similar to  the pressure model and was f i t t e d  with four t r a i l  ing-edge 
flaps.  Only the inboard f lap  segments were deflected during the present t e s t s .  A 
total  of eleven fence shapes were investigated, some a t  different  mounting positions 
on the wing and some in asymmetric arrangement. Eight of the fences, a l l  in 
symmetric configuration and mounted along the leading edge, are  discussed herein. 
The fence shapes and the i r  respective areas a re  presented in the figures with the 
resul ts .  Unless otherwise noted the fence deflection i s  90 deg (i .e., perpendicular 
to  the wing plane). The t e s t  was conducted i n  the Air Force Ins t i tu te  of Technology 
5-Foot subsonic wind tunnel a t  a meak-aerodynamic-chord Reynolds number of 1.11 
million. The sting was mounted in two al ternate  positions, giving a low (-6 t o  30 
deg) and a high (20 t o  45 deg) angle-of-attack range. 
RESULTS AND DISCUSSION 
Pressure Results 
Typical spanwise distributions resulting from vertical  apex fences placed a t  
the leading edge of the delta wing will be examined a t  a constant angle of attack of 
10 deg (representative of the 'low-alpha' range). The Gothic (18.7 percent area) 
fence (f ig .  5)  resu l t s  in broadening of the vortex suction footprint a t  the f i r s t  
two pressure s ta t ions ( A  and B ) ,  and a significant increase of the span-averaged 
local -CPU above the basic wing value with the load center shifted inboard. A t  the 
downstream stat ions ( C  and D )  the spanwise distribution i s  similarly a1 tered but the 
average -CPU i s  somewhat reduced. The Delta (11.7 percent area) fence ( f ig .  6) 
produces more accentuated suction peaks while the vortex footprints in th i s  case are 
not as broad as with the Gothic fence. Nevertheless, the resulting -CPU average i s  
practically equal with both fence configurations. A t  the a f t  s ta t ion,  the pressure 
f ie lds  due to  the Gothic and Delta fences are almost identical.  
The longitudinal variation of -CPU AVERAGE presented i n  figure 7 clearly shows 
the augmented apex suction due to  both fences a t  alpha = 10 deg, Just the opposite 
effect  is  evident a t  alpha = 30 deg (representing the high-alpha case),  when the 
apex suction is  reduced below the basic wing value, Accordingly, a nose-up moment 
increment a t  low alpha and a nose-down effect  a t  high alpha are t o  be expected due 
to  fence deployment, as pootul ated, This trend was encountered i n  varying degrees 
with a1 9 the fence configurations tested. 
Oi 1 - F1 ow Study 
Typical oil-flow photographs of the basic wing and the wing with the delta fenee 
on a t  ALPHA = 9.5 deg on the force model are presented in Figure 8. In th i s  
comparison, the o i l  streaks in the apex region are longer and more highly curved in a 
spanwise direction, indicating a significantly stronger vortex with the fence on. An 
inboard s h i f t  of the fence vortex i s  evident downstream and a separate leading-edge 
vortex appears, as observed in the foregoing pressure resul ts .  
Balance Resul t s  
The Gothic and Delta fences studied on the semi-span pressure model were 
i n i t i a l l y  tested on the balance model. The l i f t  and pitching moment characteristics 
are compared with the basic model i n  f igure 9. The 1 i f t  increment due to  fences i n  
the low-alpha range i s  evident, as i s  the nose-up pitching moment anticipated from 
the foregoing pressure resul ts .  Between the two fence shapes compared, the Gothic 
generates higher pitching moment increments; however, since t h i s  fence was a1 so nearly 
60 percent larger in area than the De1 La, i t  was decided t o  study the area effect  in 
some detail  on these two fence shapes. 
The original Gothic fence area was reduced se r i a l ly  in two steps: the height 
was reduced a t  constant length, and then the length was shortened. The resul t  of 
height reduction ( f ig .  10) shows vir tual ly  no change in 1 i f t  characteristics and a 
relat ively small reduction in moment; length reduction resu l t s  in a vis ible  drop in 
1 i f t  and a more pronounced reduction in the pitching moment. 
The Delta fence was cut in length in two successive steps. The resul ts  ( f ig .  11) 
show a roughly proportional drop in 1 i f t  as well as pitching moment in the low-alpha 
range, the moment increments narrowing towards higher angles of attack. 
To obtain a broader picture of the effect  of fence area, the l i f t  increments 
with various fence configurations a t  a constant angle of attack of 12 deg, with and 
without trailing-edge f lap  deflection for  trim, are compared with the basic model (or 
fence-off case) in figure 12.  Included in th i s  comparison i s  a Double-Gothic fence 
shape, i n  which the rear half was tapered down to  zero width. Most of the fences 
increased the untrimmed 1 i f t ,  with the exception of the smallest fences in each shape 
family which showed a l i f t  loss a t  t h i s  angle of attack. However, a l l  fences 
irrespective of s ize and shape produced marked increases in the trimmed 1 i f t  due t o  
down-deflected t r a i l  ing-edge f laps (as  indicated by the blackened portion of the 
bars). Generally, reduction of fence area also reduced the trimmed 1 i f t  increment. 
In an attempt to  separate out the fence shape and area effects  on the trimmed 
l i f t  capability, the incremental l i f t  a t  ALPHA 1 2  deg i s  plotted versus fence area 
ra t io  for  the three shape families in figure 13. An almost l inear  increase of 
trimmed l i f t  coefficient with fence area i s  evident, an outstanding exception being 
the large Double-Gothic fenee. Note tha t  the smaller Double-Gothic fence was no t  
geometrically similar, having a convex a f t  taper in contrast t o  a concave taper of 
the larger Double Gothic. While the present data are quite inadequate t o  draw 
conclusions regarding the Double-Gothic fenee, the i r  potential as an area-efficient 
fence shape i s  worthy s f  further investigation. 
As already mentioned, the vortex load on the apex fences produces a drag 
component. While drag increment in combination with l i f t  augmentation i s  a desired 
feature during approach and Sanding, i t  i s  of in te res t  t o  examine the aerodynamic 
efficiency of apex fences as a trimming device, This may be done by comparing the 
L / D  a t  a constant l i f t  coefficient with and without the fences (see Table 1). The 
corresponding trailing-edge f lap  deflections for  trim and angle of attack are also 
given. Because the basic delta wing requires an up-deflected trailing-edge f lap t o  
trim with a positive s t a t i c  margin, the angle of attack must be increased to  obtain 
the same 1 i f t  coefficient.  In contrast, fence deployment allows a down deflection 
of t r a i l  ing-edge f lap f o r  trim and therefore the angle of attack can be reduced for  
the same approach speed. For example, the Gothic fence provided a nearly 6 deg 
reduction in angle of attack from ALPHA = 18 deg o f t h e  basic delta.  The consequent 
wing drag reduction compensates for  the fence drag to  a large extent, as indicated 
by the relat ively small decrease in L/D.  
The foregoing resul ts  pertained to  ver t ical ly  deployed apex fence, i .e. B F  = 
90 deg; in practice, the hinged fences may be actuated t o  a smaller or a larger 
angle. The effect  of varying fence deflection on e i ther  side of 90 deg i s  presented 
in figure 14 fo r  the case of the large Double-Gothic fence. The resu l t s  indicate 
tha t  the fence angle controls the pitching moment in an almost l inear  fashion. 
In some t e s t s  the high-alpha range was explored to  observe the apex fenceeffect 
on pitching moment. A typical resu l t  i s  shown in figure 15 using Gothic fences, 
where a reversal of the longitudinal moment i s  evident a t  high angles of attack. 
Thus the apex fence can be viewed as a natural alpha-limiting device. 
CONCLUSIONS 
Exploratory low-speed wind tunnel investigations were conducted to  evaluate the 
effects  of apex fences on a 60 deg delta winglbody configuration. An i n i t i a l  t e s t  
program surveyed upper surface pressures on a semi-span model including the apex 
region between the fences, fo l l  owed by bal ance measurements on a geometrical ly  
similar full-span model with t r a i l  ing-edge flaps.  The scope of the investigation 
covered varying fence shape, area and defl ection angles. 
The apex fences produced opposite effects  over the wing apex region in the low- 
alpha and high-alpha regimes. A t  low angles of attack fence vortices augmented the 
suction level over the apex, whereas a t  high angles of attack the apex suction was 
reduced from the basic wing case. Balance data showed corresponding l i f t  increase 
together with a nose-up pitching moment a t  low alpha, and l i f t  loss with a nose-down 
moment a t  high alpha. 
In combination with down-defl ected t r a i l  ing-edge f laps,  fences in the low a1 pha 
range produced marked increases in the trimmed 1 i f t  capabi 1 i ty of the configuration. 
The trimmed l i f t  increment was essentially proportional to  fence/wing area r a t io  in 
case of Gothic and Delta fences. An exception was the Double-Gothic fence of 8.8 
percent area, which indicated an area efficiency almost twice as high as the others. 
Varying fence deflection angle (on ei ther  side of the nominal 90 deg position) 
was found to  control the pitching moment in an almost-9 inear fashion, showing the 
apex fence t o  be a promising pitch control and trimming device. The effectiveness 
of asymmetri c fence depl  oynsnl i n  1 ateral and di rectisnal control i s  current1 y being 
eval uated. 
REFERENCES 
1. Wedekind, G . :  Tail Versus Canard Configuration, An Aer~dynamic Comparison with 
Regard t o  the  Su i t ab i l i t y  f o r  Future Tactical Combat Aircraf t .  ICAS Proc. 
1982, pp. 247-254. 
2. Rao, D. M. and Buter, T. A. : Experimental and Computational Studies of a Delta 
Wing Apex Flap. AIAA Paper No. 83-1815, July 1983. 
3. Wahls, R. A . ,  Vess, R. J .  and Moskovitz, C.  A. : An Experimental Investigation 
of Apex Fence Flaps on Delta Wings. AIAA Paper No. 85-4055, October 1985. 
ACKNOWLEDGMENTS 
This research was supported by the  Air Force Flight  Dynamics Laboratory, 
Wright-Patterson AFB, under contract  No. FY1456-85-00032. Considerable assistance 
was received from L t .  Mike S tuar t  and Capt. Chris Smith, graduate students of the  
Air Force In s t i t u t e  of Technology (AFIT) , which i s  great ly  appreciated. The authors 
a l so  extend t h e i r  gra t i tude t o  AFIT f o r  the  use of a model, the  5 - f t  Tunnel Facil i t y  
and associated personnel. 
Table 1 
FENCE TYPE 
FENCE OFF 
GOTHIC FENCE 
DELTA FENCE 
TRIMMED CL = 0.82 
AREA RATIO 
FENCE WING %E,ELEvAToR - L/D ALPHA 
APEX FLAP (RAo, BUTER, 1983) APEX FENCE 
INCREASED L I F T  
HYPOTHESIZED AT LOW ALPHA 
VORTEX PATTERNS 
I N  APEX REGION : 
WITH LEFT FENCE DOWN, 
RIGHT FENCE UP 
AT HIGH ALPHA 
F i  g. 1. Apex fence concept. 
PRESSURE STAT1 ONS 
4 
X/C, = 0 . 8 7  
CR = 22 i n ,  
F i g .  2 ,  60-deg d e l t a  semi-span p ressure  model. 
F ig .  3. Typ i ca l  fence shapes 
DELTA FENCE 
t e s t e d  on semi -span d e l t a  model . 
ALL DIMENSIONS 
F i g ,  4. 60-deg d e l t a  full-span f o r c e  model, 
0-FENCE OFF 
A-FENCE ON 
(b - AVERAGE VALUE 
I I\ PRESSURE STATIONS 
Y-LOC Y-LOC 
F i g .  5. Spanwise upper sur face  pressure d i s t r i b u t i o n s  a t  10 deg alpha 
w i t h  go th i c  fence. 
Y-LOC Y-LOC 
0-FENCE OFF 
A-FENCE ON 
A @-AVERAGE VALUE 
I\ PRESSURE STATIONS 
F i g ,  6,  Spanwise upper s u r f a c e  p ressure  d i s t r i b u t i o n s  a t  10 deg  a lpha  
w i t h  go th ic  fence. 
0'5 
0 IlAPEX 
CHORD STATION 
*FENCE OFF 
F ig .  7. Longi t u d i  na l  d i s t r i b u t i o n s  o f  span-averaged upper su r f ace  
pressure c o e f f i c i e n t s .  
F i g ,  8 ,  Typical o i l - f l o w  patterns on f o r c e  model, showing fence e f f e c t  
a t  95 deg a1 pha, 
AREA RATIO 
FENCE / WING 
11.7 % 
*FENCE OFF 
ALPHA 
F ig .  9. L i f t  and p i t c h i n g  moment e f f e c t s  o f  l onges t  d e l t a  and 
go th i c  fences. 
AREA RATIO 
FENCE / WING 
9.1 % 
@--FENCE OFF 
ALPHA 
F i g ,  10, L i f t  and p i t c h i n g  moment e f f e z t s  o f  g o t h i c  fence v a r i a t i o n s ,  
AREA RATIO 
FENCE / WING 
3'3 % 
@-- FENCE OFF 
ALPHA 
F ig .  11. L i f t  and p i t c h i n g  moment e f f e c t s  o f  d e l t a  fence v a r i a t i o n s .  
% AREA RATIO 
FENCE / WING 
I 7 '  1 DOUBLE 
I 6,0 DELTA I 
FENCE OFF, 
TRIMMED 
FENCE ON, 
UNTRI MEED 
FENCE ON, 
TRIMMED 
Fig ,  12, Trimmed and untrimmed l i f t  increments due t o  var ious  fences a t  
12 deg a l p h a ,  
60 
AC, % / 
40 
2 0 
0 
AREA RAT IO ,  F E N C E  / W I  \G 
Fig .  13. Fence area e f f e c t  on trimmed l i f t  increment a t  12 deg alpha. 
SFEKE - 
97 .  5' 
AREA FENCE / W I N G  = 8.84 % 
----_-_-_____ 
- 75,0°  
FENCE OFF 
ALPHA 
F i g ,  14 ,  L i f t  and p i t c h i n g  moment e f f e c t s  o f  fence d e f l e c t i o n ,  
@.-- 
~ F E N C E  OFF
25 30 35 40 45 50 
ALPHA 
F ig .  15. Fence e f f e c t  on p i t c h i n g  moment a t  h i g h  alphas. 


Basic studies on delta wing flow modifications by means of apex fences

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