Experiments on a jet-flap delta wing in ground effect

THE COLLEGE OF AERONAUTICS

C R A N F I E L D

E X P E R I M E N T S ON A J E T - F L A P D E L T A WING

IN GROUND E F F E C T

T H E C O L L E G E O F A E R O N A U T I C S C R A N F I E L D E x p e r i m e n t s on a J e t - F l a p D e l t a Wing in Ground Effect b y -A. J . A l e x a n d e r , M . S c , P h . D . , A . F . R . A e . S . SUMMARY

T e s t s have b e e n m a d e on a 70 c r o p p e d d e l t a wing with t r a i l i n g edge slot blowing both with and without a fixed ground b o a r d . J e t deflections of 0 and 30 w e r e u s e d . T h e i n c i d e n c e r a n g e w a s - 1 < a < 20 and h e i g h t / r o o t chord r a t i o s v a r i e d between 0 . 1 2 and 0 , 2 2 , the l o w e s t height c o r r e s p o n d i n g to touch-down at 15 i n c i d e n c e .

Lift i n c r e a s e d s t e a d i l y a s the ground w a s a p p r o a c h e d due t o the " b a u l k i n g " o f the airflow b e t w e e n t h e m o d e l and the g r o u n d . T h e effect of the t r a i l i n g edge j e t w a s s m a l l e x c e p t when the jet impinged on the ground p l a t e , c a u s i n g s o m e r e d u c t i o n in lift at the h i g h e s t i n c i d e n c e . D r a g w a s i n c r e a s e d at a given i n c i d e n c e in ground effect, the i n c r e a s e b e i n g a p p r o x i m a t e l y equal to the i n c r e a s e in induced d r a g due to the

I n c r e a s e d p r e s s u r e lift. In g e n e r a l , both the a e r o d y n a m i c c e n t r e and c e n t r e of p r e s s u r e moved r e a r w a r d s a s the ground w a s a p p r o a c h e d but moved f o r w a r d v e r y quickly when the jet impinged on the ground b o a r d .

Page

Summary

List of Symbols

1. Introduction 1

2. Model and range of t e s t s 1

3 . Discussion of results 2

4. Conclusions 5

5. References 6

Figures

c o c h q

s

m . 3 C ^ L ^ L p ^ L ^ G ^ D C m

n

Root chord = 3 . 3 3 ft. a e r o d y n a m i c m e a n c h o r d = 2 . 6 0 ft.

d i s t a n c e b e t w e e n ground b o a r d and pivot point ( 0 , 6 8 c ) m a i n s t r e a m d y n a m i c head wing a r e a = 3 . 6 0 s q . ft. m e a s u r e d r a t e of m a s s flow s l u g s / s e c . final j e t v e l o c i t y a s s u m i n g i s e n t r o p i c expansion t o f r e e s t r e a m p r e s s u r e . blowing m o m e n t u m coefficient = — ^ q . b lift coefficient = —rr-q . S

p r e s s u r e lift coefficient due t o jet flap effect lift i n c r e m e n t due to ground effect

d r a g coefficient = — ^

q . b

. , . , ^„. . , pitching m o m e n t about 0 . 6 8 Co pitching m o m e n t coefficient = *^ '^—r—=

t r a i l i n g edge flap deflection

Suffices b indicates value obtained with T . E . blowing o indicates value obtained without T . E . blowing

1. Introduction

In the last decade a considerable r e s e a r c h effect has been directed towards exploring the aerodynamic characteristics of slender wings. One aspect which seems to have been neglected, judging by the almost complete lack of published work, is the effect of ground proximity on these slender shapes. Another effect which has not been studied is the effect of jet exhaust when a powerful battery of jet engines is mounted at the r e a r of the aircraft.

Comprehensive experimental test programmes on the effect of ground proximity^ ' and the effect of jet configurations^^' a r e currently in p r o g r e s s at the Royal Aircraft Establishment, but in order to obtain quickly some information on both ground and jet-induced effects a s e r i e s of t e s t s was made on a 70 cropped delta wing, both with and without full span trailing edge slot blowing. It was not possible to simulate a battery of round jets as in conven-tional aircraft and the t e s t s are representative of a jet-flap delta wing, but it is possible to draw some tentative conclusions for more conventional slender wing aircraft.

2. Model and range of tests

The model is a 70 cropped delta wing having an aspect ratio of 0. 73, with the tip chord equal to one third of the root chord. It is of rhombic cross section and has a 0. 040 in. slot round the periphery which was sealed in these t e s t s except for the trailing edge slot. Interchangeable b r a s s edges enabled tests to be made with the trailing edge jet undeflected relative to the model and deflected downwards 30 . Excluding a small region at each end of the slot the variation in jet velocity was about - 5% but owing to the construction of the model it was difficult to improve on this figure. The blowing momentum coefficient, C ( ™ J . .1) was calculated in the usual way, i . e . m. was measured directly using orifice plates and V. was calculated, assuming isentropic flow from measured slot p r e s s u r e s to atmospheric p r e s s u r e .

The tests were made in the College of Aeronautics 8 ft x 6 ft low speed wind tunnel with the model mounted on a Warden type six-component balance, The ground was represented by a large wooden plate eight feet square and two inches thick, stiffened by " L " shaped steel supports to ensure flatness. The plate had an elliptic leading edge and a chamfered trailing edge and was set at zero incidence relative to the tunnel s t r e a m . It projected forward about two feet ahead of the wing apex and could be positioned vertically by means of screw jacks.

Balance measurements were made in all the tests of lift, drag, and pitching moment for z e r o sideslip only. Corrections for p r e s s u r e constraints on the balance are similar to those of Ref. 3. Conventional corrections for ground effect tests are small ' ^ ' and none have been applied. Tests were made with the jet undeflected and deflected downwards 30 over the range of incidence

a = - 1° to + 20°. C' values ranged between 0 and 0.2 approximately, thus covering the range for conventional supersonic transport type aircraft although due to p r e s s u r e limitations in the model it was not possible to cover

the full jet-flap range. Ground clearances with the undeflected jet were / c = 0.122, 0.162, and «>, and with the 30° downward deflected jet / c = 0.140, 0.158, and 0.220, a n d » , where h is the height of the pivot point (0.68 c ) above the ground plate.

3. Discussion of Results 3 . 1 . Lift

The variation of lift with incidence for the basic model configuration, i . e . no blowing or ground, is shown in Fig. 2. The non-linear slope is typical of slender wings and it is interesting to note that the deflection of a full span trailing edge flap ( ^L - 0.040)- gives a lift increment A C L of 0.15 which is unchanged throughout the incidence range.

The effect of mounting the model close to a fixed ground board without blowing is shown in Fig. 3, where lift increments due to ground proximity, A C L Q , are plotted against incidence for rj = 0 and 30 . The lowest values of ^/ c^ tested (0.122 for n - o and 0.140 for r; = 30 ) correspond to an 8 ft. trailing edge clearance on an aircraft with a 200 ft. root chord at 15° incidence and represent a practical minimum, i . e . , touchdown conditions. At zero incidence with no flap, the venturi effect between model and ground board actual gives r i s e to a small reduction in lift but as incidence is increased the lift r i s e s rapidly due to the "baulking" of the flow under the model, reaching a maximum C L of 1.07 at

a = 20°, an increase of almost 50% over the no ground case.

With the full span trailing edge flap deflected downwards 30°, the lift increments due to ground proximity increase at low incidence but fall below the undeflected flap case at higher incidence. As no p r e s s u r e plotting was possible on this model, the cause of this loss of lift due to flap deflection in ground effect could not be determined but is probably caused by e a r l i e r separation on the wing upper surface due to larger adverse p r e s s u r e gradients at the r e a r of the wing.

1 o

In F i g s . 4a - 4c the variation of C L with Cp 2 for ri = 0 is shown for

1^0 ~ » . 0.162, and 0.122. The variation is linear in the range tested and

is similar to the trend observed on a low aspect ratio straight wing f^' and a 60° Delta wing ^^^. The fact that ^ is constant in most cases up to C^ values of about one is a reasonable justification for extrapolating the present results to higher values of C^ if necessary. Even with an undeflected jet sheet there is some increase in p r e s s u r e lift due to the ability of the jet sheet to support a p r e s s u r e difference with the wing at incidence.

Figs. 5a - 5d give similar results for rj = 30^and '^0= " , 0.220, 0.158 and 0.140. Again the variation of C L with C^^is linear above a certain critical C|U value, say C^ul below which the jet has no appreciable effect. C . is about 0. 004 for the no-ground case for all the incidences tested. The critical value of C needed to suppress flow separation over the flap for the no-ground case was about 0. 030 and was roughly independent of incidence. This value is rather high for a flap deflection of only 30 but is due to the fact that the blowing slot is located at the trailing edge well behind the separation line.

C ^

With rj = 30 , a = O , and no g r o u n d , the slope j = 0 . 6 0 and

C 2

the c o r r e s p o n d i n g value in Ref. 5. on an a s p e c t r a t i o 2 . 7 5 i s 1.40. D e s p i t e the d i f f e r e n c e in wing s h a p e and s e c t i o n , the change in slope can s t i l l be l a r g e l y

A^ A2 a c c o u n t e d for with a s i m p l e a s p e c t r a t i o c o r r e c t i o n -r — -*- -r — even for t h e s e v e r y s m a l l a s p e c t r a t i o s .

A b r e a k d o w n of the o v e r a l l lift into i t s v a r i o u s component p a r t s i s given in F i g s . 6 a , and 6b, for rj = 0 and 30 r e s p e c t i v e l y at a = 15 . T h e l o w e r b o u n d a r y at C = 0 . 4 9 i s t h e lift obtained with n e i t h e r blowing, flap, n o r

L-j

g r o u n d . T o t h i s i s added the lift due to flap deflection, w h e r e a p p r o p r i a t e , the d i r e c t jet lift c o m p o n e n t , and the jet induced p r e s s u r e lift to give the lift with blowing but without g r o u n d . T h e upper b o u n d a r y i s the lift with blowing, flap dl >o) and g r o u n d .

U s i n g F i g s . 6 a , and 6b, a t e n t a t i v e m e t h o d of e s t i m a t i n g ground effect on low a s p e c t r a t i o j e t - f l a p w i n g s , if the unblown t e s t r e s u l t s a r e a v a i l a b l e , h a s b e e n d e v e l o p e d .

A e r o d y n a m i c lift without ground o r blowing i s known, and a l s o the lift i n c r e m e n t due to ground which i s a s s u m e d independent of C . T h e jet r e a c t i o n lift can be c a l c u l a t e d at the a p p r o p r i a t e C value and the only unknown i s the jet induced p r e s s u r e lift. T h i s cannot be c a l c u l a t e d with any a c c u r a c y from Spence" s 2 D r e s u l t ' ' ' modified by M a s k e l l and S p e n c e ' s t h r e e d i m e n s i o n a l jet flap c o r r e c t i o n ^ " ' s i n c e the A . R . ' s c o n s i d e r e d h e r e a r e too low. H o w e v e r ,

C

f r o m Spence* s t h e o r y we m a y obtain the r a t i o —:= ~ : °-—-^—^— which •' -^ C no blowing o r flap

L

i s a l m o s t independent of a s p e c t r a t i o for the s m a l l v a l u e s of C c o n s i d e r e d h e r e , although the a p p r o p r i a t e c o r r e c t i o n could be applied for l a r g e C v a l u e s .

T h e t h e o r e t i c a l " value for p r e s s u r e lift coefficient due to flap and blowing is now t a k e n t o be :

-. „ -I L with blow & flap

L I C no blowing o r flap L no blowing o r flap, e x p e r i m e n t a l .

J

t h e o r e t i c a l

T h e s e ' { h e o r e t i c a l " v a l u e s a r e plotted in F i g . 7 and c o m p a r e d with e x p e r i m e n t a l v a l u e s of A C , _L

P

i" {''^B'\'''^"'" ")-J

T h e t h e o r y m i g h t be e x p e c t e d to o v e r e s t i m a t e the lift s o m e w h a t s i n c e it i s l i n e a r , although good a g r e e m e n t with e x p e r i m e n t w a s obtained' for a s p e c t r a t i o s a s low a s 2. 7 5 .

It i s t h e n a s s u m e d that t o t a l lift in ground effect i s m a d e up of the following c o m p o n e n t s .

C A C C AC L no flap, blow, o r ground + L _ , , + 1^ sin a + L ^

The first two t e r m s are known from tests with an unblown model, the third t e r m depends on two t e r m s which can be postulated, and the fourth term can be

calculated from Spence's blown flap theory. For a = 15 and C = 0 . 2 , this method predicted the lift to within 5% of the measured lift. ^

It is not known whether predictions of total lift could be made with confidence outside the range of the present t e s t s , but it is suggested as an empirical method

until more information, both theoretical and experimental, is available.

3. 2.Drag

Comparison of the measured and theoretical thrust is made in Fig. 8, both with and without a main s t r e a m flow out of ground effect. In still air the theoretical C exceeds the measured thrust by about 2%, and with wind on by about 6%, botn values being comparable to those obtained elsewhere ^^'.

Drag is plotted against lift for rj = 0. in F i g s . 9a - 9c. Drag is reduced at a given lift as the ground is approached due to the smaller induced drag, but at a given incidence, which is of more practical interest, there is an increase in drag of about a .AC , where AC does not include the direct jet lift

^G ^G

component. Similar trends are observed for ri = 30 (Figs lOa - lOd) except for large values of C & a, where the jet impinges on the ground plate.

3. 3 Pitching Moments

The pitching moments presented here a r e as measured about the pitching axis at 0.68c and thus have a positive slope, the aerodynamic centre being well forward of this point.

For n = 0 (Figs. 11a - lie) the variation of C with C, is linear. At zero m L

incidence, the venturi effect created close to the ground giving the small negative lift referred to in S 3.1 also gives r i s e to a small nose up pitching moment. At a given lift, with C constant, the pitching moment is reduced on approaching the ground, but at a given (moderate) incidence the change is very small. Since the pivot point (0.68 ) is close to the centre of a r e a for this model, it suggests that the ' baulked' flow under the model produces a roughly constant p r e s s u r e r i s e over the whole of the lower surface.

With ri = 30 (Figs. 12a - 12d) the pitching moment at a given lift with C constant is also reduced on approaching the ground, but at constant (moderate) incidence

ground proximity i n c r e a s e s the pitching moment. This tendency agrees with the trend observed in § 3 . 1 , where the cause of lower values of AC with flap

^G

deflected at moderate incidence was said to be due to increased flow separation on the upper surface as the ground was approached.

The movement of the aerodynamic centre and the centre of p r e s s u r e with height

and blowing is shown in Fig. 13 for a = 15° and r? =0°.and 30 . These values are typical for all moderate incidences. With rj " 0 there is a steady rearward movement of both aerodynamic centre and centre of p r e s s u r e as the ground is approached. This is in line with an increase in p r e s s u r e on the undersurface acting near to the centre of the area, and indicates an increase in static stability as the ground is approached.

Wich the flap deflected 30 , the aerodynamic centre is only slightly forward of the undeflected flap position and without blowing, hardly moves as the ground is approached. With blowing, as soon as the jet impinges on the ground, the aerodynamic centre moves forward very rapidly and the model becomes unstable. The centre of p r e s s u r e is also virtually fixed v>'ithout blowing, and moves forward only a small amount with blowing. With the flap deflected it is likely that flow separation on the upper surface counteracts to some extent the effect of increased p r e s s u r e on the lower surface.

4. Conclusions

These tests were made in order to obtain some idea of the effect of ground proximity on a delta wing both with and without a jet flow. Blowing was from a full span trailing edge slot and the results are strictly applicable only to a jet-flap delta configuration, but tentative conclusions are drawn of the probable effects of round jets on conditions close to the ground.

At zero incidence there was a small reduction in lift; the p r e s s u r e on the wing lower surface was reduced slightly by the venturi type flow between the wing and the ground. At most positive incidences, however, there was a substantial increase in lift at small ground heights. The maximum increase in lift without blowing and zero flap deflection at the highest incidence (20 ) and the smallest clearance corresponding to touch-down conditions! was AC = 0.35, an increase

^G

of almost 50% over the free-air value. With the flap deflected 30 downwards and C = 0 , the lift at low incidences was greater than with the flap undeflected, but for high incidences close to the ground there was a small adverse effect which was attributed to e a r l i e r flow separation on the upper surface caused by the increased adverse p r e s s u r e gradient at the r e a r . The effect of the presence of the jet close to the ground was small except when impingement occurred, resulting in some loss of lift .

The effect of ground proximity on drag at a given incidence was to increase the drag by an amount equal to the induced drag increment based on p r e s s u r e lift increment due to ground. Without flap, at moderate incidences, both the a e r o -dynamic centre and the centre of p r e s s u r e moved backwards as the ground was approached and this movement was not changed by the presence of the jet. With flap deflected corresponding movements were very small until the jet impinged on the ground plate when both centres moved forward.

In view of the almost total absence of information on this subject, a very tentative method of predicting lift in ground effect with a jet-flap from unblown results is suggested, and over the range of tests covered here would seem to agree with experim.ent to about - 5% accuracy.

Although it was not possible to test a round jet configuration, the results obtained with an undeflected j e t - f ap suggest that, in this case, jet-induced effects will be negligible and only the increase in lift due to the vertical jet component need be considered.

5. References

1. Kirby, D. Private communication.

2. Moss, G. F . Private communication.

3. Alexander, A. J. Experimental Investigation on a Cropped Delta Wing with Edge Blowing.

College of Aeronautics Report 162, 1963. 4. Brown, W. S. Wind tunnel corrections on ground effect.

A . R . C . R. & M. 1865, 1938.

5. Williams, J . & Alexander, A. J .

Some exploratory three-dimensional jet-flap experiments. Aero Quarterly Vol. VIII, February 1957.

6. Williams, J . & Alexander, A. J.

Some exploratory jet-flap tests on a 60 delta wing. A . R . C . R. & M. 3138, 1961.

7. Spence, D.A. The lift on a thin aerofoil with a blown flap. R . A . E , T . N . Aero 2450, 1956.

8. Maskell, E . C. & Spence, D.A.

A Theory of the jet-flap in three dimensions. R . A . E , Report Aero 2612, 1958. 9. Williams, J, , Butler, S . F . G . , & Wood. M.N. The Aerodynamics of J e t - F l a p s . A . R . C . R. & M. 3304, 1962.

0 1 S '

/

V

}

V

0 — = 0 0 ^ 0 C

/

/ c / • l = 0

/

/

/ !

/

/ 0 1 ^ • « • - • ' ' ' // 1 ' ' / L-^o lói/' 1 """.^^

y

0 SYMBOL 1 n 30°

FIG. 3. LIFT INCREMENT DUE TO GROUND EFFECT Cp'O.

FIG. 2. VARIATION OF LIFT WITH INCIDENCE.

a b o «=5° o J , 0 3 « • 0 ' F I G . 4 b . VARIATION OF L J L S O 162. IFT WITH C „ ''2. rj • 0 °

FIG. 4 c . VARIATION OF L I F T WITH Cn" l| « O ,

— « 0 1 2 2 .

FIG. 5b. VARIATION OF LIFT WITH C ^ / 2 . , , 3 o ° F I G . S d . VARIATION OF L I F T WITH C^ '^ i) = 3 0 ° : 0 2 2 0 . _{i ^ = 0 M O .}

- - —

s s s ^

s ^

OROUND EFFECT i - x » »

3

RES6URE LIFT — 7 — r - ' / / JET REACTION AERODVNAMIC LIFT V° f.--" o Ol o-a i. o*3 o 4 o 5

FIG. 6a.VARIATION OF LIFT INCREMENTS DUE TO BLOWING AND GROUND EFFECT. « B I S " , I I . O " .

O 6 ^ ^ L , , 0 — H ' l S _{c ^ - «}

——^—r^"""

o ' " ' _ _ — o - _ — — 0 - : - — — 0 » o -" ^ « • 3 0 * . — 0 -• -•o

°—°"—

" ~ ~ THEORETICAL F I G . 7 . C O M P A R I S O N OF T H E O R E T I C A L A N D EXPERIMENTAL J E T - I N D U C E D PRESSURE LIFT. K ' l S l - ^ s o o . ( S o - ' ^ o è ) / / " / " »»0 J?-»oo / ^ / f /> / /

7°

SYMBOL —t. - • VOB O l O O Ulut •y ' ^ U O - ' ^ D B Cn o 9 « l 0 » 3 5

FIG .a COMPARISON OF MEASURED AND THEORETICAL THRUST.

FIG. 6b. VARIATION OF LIFT INCREMENTS DUE TO BLOWING AND GROUND EFFECT. K « I S ° i | . 3 0 ?

FIG. 9a. VARIATION OF DRAG WITH LIFT. r|« o", -=-x oo

7 t g

FIG. 9b. VARIATION OF DRAG WITH LIFT. i | « 0 ° , 7 - » O 162.

o 40

"^"-^^^^ï^^

o 60 O 80

•^L

_L^

* p 014 0 0 3 2 0 072 O i l ? 0 162 02C6 0 4 0 0 60 C,

FIG. Ilo. VARIATION OF PITCHING MOMENT ABOUT 0-68 CQ WITH LIFT.

, = 0°, 4=00.

FIG. l i b . VARIATION OF PITCHING MOMENT ABOUT 0-66 Cj, WITH LIFT. i ) = 0 ' ' , 7- = 0 162

0 3 0

FIG.lie. VARIATION OF PITCHING MOMENT ABOUT O-68 Cj, WITH LIFT. i l x O " , - ! ^ = O I 2 2 .

FIG. 12b. VARIATION OF PITCHING MOMENT ABOUT O M c^ WITH LIFT.

1, « 3 0 % ^ = O 2 2 0 .

F I G . l 2 d . VARIATION OF PITCHING MOMENT ABOUT 0-66 Cj, WITH LIFT. ii^SCf. ^ = 0\A0.

FIG. 13. MOVEMENT OF CENTRE OF PRESSURE AND AERODYNAMIC CENTRE WITH HEIGHT « - I S "