AIRFOIL DESIGN CONCEPT THAT INCREASES LIFT, REDUCES DRAG AND IMPROVES STALL

Demeter G . Fertis* University of Akron Akron, OH 44325

Abstract

The research in t h i s paper consists of an experimental study regarding a new a i r fo i l de- sign concept that (a) improves s t a l l character- i s t i c s a t virtually a l l operational airspeeds, (b) increases functional l i f t coefficients over a broader range of operational angles of attack, (c) increases functional l i f t t o drag rat ios over a broad range of operational angles of attack, and (d) that is suitable for use with a i rc ra f t of the fixed- wing type and also the rotary wing type. The author has combined his efforts with L.L. Smith and a U.S. Patent, en- t i t l e d "Airfoil", Patent Number 4,606,519, was obtained on August 9, 1986. Patents in other countries such as Greece, France, England, I ta ly Germany, Japan, Brazil, Israel and Canada have been obtained or the application for a patent i s submitted. The results obtained by using the new a i r fo i l design concept have been compared with those obtained by a conventional NACA 23012 a i r fo i l .

The New Airfoil Design Conce~t

The main purpose with any a i r fo i l design i s f i r s t t o produce an a i r fo i l design that has a greater amount of l i f t without detrimentally in- creasing drag, and second to enable the a i r fo i l t o function a t greater angles of attack without s ta l l ing . If we couple these desired parameters with a wide range of airspeeds that the a i r fo i l may be acted upon, a multitude of a i r fo i l de- signs can be obtained, each having i t s own aero- dynamic characteristics t o perform a t specific f l igh t conditions. quite often the present wing deslgns do not work exactly the way we want them to , and desi n modifications, or controls, have been introduce % t o improve performance.

In the past, movable s lo t s and/or flaps on the leading and t ra i l ing edges of the wing have been introduced in order t o improve the overall aerodynamic characteristics of the airplane wing. However, because of the increase in dra , the extension of such s lo t s and/or flaps i s onfy advisable a t relatively low airspeeds and there- fore they are unable t o improve the l i f t and s t a l l characteristics of the wing a t high cruising airspeeds as well as other aerodynamic characteristics. Dispite the extensive work conducted in t h i s f ie ld , no a i r fo i l as yet has been developed which provides improved s t a l l characteristics a t virtually a l l operational airspeeds simultaneously providing improved l i f t drag, and l i f t t o drag rat ios .

Recently the writer of th i s paper, in col- laborat ion w i t h L . Smith, has originated an a i r fo i l design that has (a) improved s t a l l characteristics a t virtually a l l operational airspeeds, (b) increased functional l i f t coef- f ic ien ts over a broader range of operational angles of attack, (c) increased functional l i f t t o drag rat ios over a broad range of operation- a l angles of attack and (d) that i s suitable for use with a i rc ra f t of the fix-wing type and also the rotary wing type.

The basic idea in th i s new a i r fo i l design concept i s the introduction of a premeditated stepwise aerodynamic excitation on the upper surface of an existing a i r fo i l section as shown in Fig. ( I b). The existing a i r fo i l section i s shown in Fig. (1 a ) . The purpose of t h i s aero- dynamic excitation i s t o improve the aerodynam- i c characteristics of the existing a i r fo i l i n terms of l i f t , drag, l i f t t o drag rat io , and s t a l l , and by also retaining the basic function- a l characteristics of the existing shape of the a i r fo i l . If desired, however, a completely new a i r fo i l section incorporating one or more aero- dynamic stepwise excitations can be designed.

I t should be pointed out, however, that the aerodynamic characteristics of th i s new de- sign concept depend upon the location of the aerodynamic stepwise excitation along the chord len th of the a i r fo i l , i t s depth, upon whether surgace (A) in Fig. (1 b) i s cambered or f l a t , and upon how f a r it extends towards the t r a i l - ing edge of the a i r fo i l . Several configurations have been studied experimentally by the writer using a wind tunnel, appropriate instrumentation and carefully prepared a i r fo i l models. A 7-foot (2.134111) airplane model was also constructed and several advanced f l i h t patterns have been in- vestigated and compare! w i t h those of the ori- ginal a i r fo i l by using remote control devices.

Professor, Civil Engineering Department

Because of tunnel speed limitations and avai labi l i ty of funds, tlie studies and t e s t s made by tlie writer were limited to low subsonic speeds rangin between 40 f t sec (12.2mJsec) and 260 f t / s ec $ 79.25m/sec), w i licli i s approximately 25 percent of the sonic speed. The trend i n resul ts however, indicates that the improvements get better or remain constant with higher sub- sonic speeds. No t e s t s are made a t the present time using sonic or supersonic speeds but the idea i s expected t o apply for such speeds by using proper optimization for the various para- meters associated w i t h t h i s new a i r fo i l design concept. Such studies are expected t o be per- formed in the near future, depending of course upon the avai labi l i ty of a high speed tunnel and appropriate funding.

I t should be pointed out however, that the main purpose of t h i s research work was t o inves- t igate the advantages and possible disadvantages of t h i s a i r f o i l design concept, and a f a i r l y large number of cases have been examined and tested in terms of l i f t , drag, l i f t t o dra ra- t i o , and pichin moment. As a part of t h i s study, several afvanced model f l i ht patterns have been performed by using a foot (2.13411) airplane model and remote control devices. The airplane model incorporated the new a i r fo i l design concept and also wings that u t i l i z e the NACA 23012 a i r f o i l design. The model f l igh t re- su l t s of both cases have been compared in terms of f l igh t performance and certain special aero- dynamic characteristics.

Ex~erimental Investigations and Results

The experimental studies and investigations have been performed by using a NACA 23012 air- f o i l that incorporates the new a i r fo i l design concept shown i n Fig. (1 b ) . Since the aerody- namic characteristics of the new a i r fo i l con- cept i n terms of l i f t , drag, l i f t t o dra ra- t i o , and piching moment, as pointed out ear 'i i e r , depend upon the location of the premeditated stepwise excitation along the chord length of the a i r f o i l , upon how f a r Surface (A) i n Fig. (1 b extends towards the t ra i l ing edge, upon w 1 ether Surface (A) i s cambered or f l a t , and on the depth of the stepwise excitation, the fol- lowing cases have been tested and studied:

1. Three different locations of the premedita- ted stepwise aerodynamic excitation, namely, 40 X , 50 X , and 60 % chord distance from leading edge, were investigated.

2. The depth of the stepwise excitation varied from 0 X t o 50 X of the thickness of the a i r f o i l a t the location of the aerodynamic step, w i t h the 0 % depth representing the unchanged 23012 NACA a i r f o i l .

3. Both cases of f l a t and cambered Surface (A) in Fig. (1 b) were examined.

4. The extent of Surface A) i n Fig. (1 b) was taken t o ran e from 0 4 chord len t l t o a l l the wa t o t ee t r a i l i ng edge, w i t k the 0 % chord ! ength case representing the original unchanged 23012 NACA a i r f o i l .

5. Each t e s t case was subjected to f ree a i r - stream velocities ranging from 40 f t / sec

(12.2 m sec) t o 260 f t /sec (79.25 mlsec), and ang i es of attack ranging between 2 de-

grees and 38 degrees.

6. Flow separation studies for both the ori- inal 23012 NACA a i r f o i l and the new air-

f o i l design concept for angles of attack from zero degrees to 30 degrees,

7. Flight performance was investigated by using a 7- foot (2.134111) airplane model and remote control devices. Advanced flight patterns as well as gliding capabili t ies were examined and compared for both tlie NACA 23012 a i r f o i l and the new a i r fo i l design concept.

A small portion of these results together with the associated improvements and comparisons re- garding the new a i r fo i l i s presented below.

Figure (2) , fo r example, shows that the de- sign configuration with an aerodynamic stepwise excitation a t 50 % chord and 33 % depth retains about the same maximum l i f t as the basic a i r - f o i l , but it does not s t a l l up to an angle of attack of 38 degrees, which i s the maximum value used in the experiment. The design conf igura- t ion with the 50 % chord, 50 % depth and a s l ight ly cambered second upper surface shows large gains of l i f t a t virtually a l l angles of attack and also improved s t a l l conditions com- pared t o the basic unchanged 23012 NACA a i r fo i l . Since the depth of the stepwise excitation can be made adjustable during f l igh t by simple me- chanical means, the shaded area i n Fig. (2) re- presents the improvement i n l i f t and s t a l l for both cases combined.

1 . 5 4 NACA 23012 . - 0

f t improvement a reas

- B a s i c A i r f o i l --- 50% Chord, 33% Depth -. - 50% Chord, 50% Depth

V = 2 0 0 f t I s e c R,, = 470,000

10 20 30 I

40 ANGLE OF ATTACK o( (DEGREES)

FIGURE 2

Figures (3) and (4) show the improvements i n s t a l l with increasing airspeeds up t o 240 f t / s ec (73.15 m/sec) . Lar e improvements i n l i f t , as compared t o the %asic 23012 NACA a i r f o i l , are also associated w i t h these two tes t cases. It i s important t o note that for the airspeeds shown i n Figs. (3) and (4) , the premeditated stepwise aerodynamic excitation prevents s ta l l ing of the a i r f o i l fo r angles of attack up t o 30 or 38 degrees. No t e s t s have been performed for angles of attack larger than 38 degrees.

240 f t l s e c

P 2 0 0 f t / s e c

NACA 3012

10 20 30 4b ANGLE OF ATTACKa (DEGREES)

FIGURE 3

- NACA 23012

ANGLE OF ATTACK CX (DEGREES)

FIGURE 4

- D L I D Improvement V=BOft/sec

":

50'; C?:rd 25% Chord

NACA 23012

2 I o 1 to 26 36 46

ANGLE OF ATTACK ff (DEGREE:

F I E R E 5

KFY - Basic A i r f o i l

-.-SO% Chord, 50% Depth

... . .. 50% Chord, 33% Depth

'" t --- 50% Chord. 19% Depth

L I D Improvement l=150 f t / sec

BASIC AIRFOIL

Y- - 2 I

o 1 I 10 2 0 30 4b

ANGLE OF ATTACKCl (DEGREES) p* 'hnrd W d F:;URE 6

>0%.19'%.33%.50% >

NACA 23012

The resul ts shown i n Figs. (5) and (6) i l - lustrate l i f t t o drag ra t io comparisons between the new a i r fo i l design concept and the basic un- chan ed 23012 NACA a i r fo i l for freestream air- speeis of 60 f t / sec (18.29 m/sec) and 150 f t / sec 45.72 m/sec) . Note the large gains i n l i f t t o

bTag rat ios associated w i t h the new a i r fo i l de- sign concept, particularly a t the operating angles of attack presently used in airplane f l igh ts .

The research work has also included boun- dary layer studies and flow separation research for both the basic a i r fo i l and the new design concept i n order t o understand better the per- f ormance improvements stated above. These studies show that the basic a i r fo i l s t a l l s a t an angle of attack of 20 degrees. The flow separ- ation s t a r t s very close t o the leading edge and the boundary layer becomes very wide as we pro-

towards the t ra i l ing edge. On the ohter k h e new a i r fo i l desi n concept, for the cases tested, prevents fyow separation a t 20 degrees angle of attack. The boundary layer becomes somewhat wider a t an angle of attack of 25 degrees w i t h very mild flow separation. The flow separation becomes more noticible a t the angle of attack of 30 degrees, but the boundary layer thickness i s much smaller compared t o the one obtained for the basic a i r fo i l a t the angle of attack of 20 de rees. That i s , the new air- i f o i l design concept emonstrates a great abi l i ty in controlling flow separation and boundary lay- e r thickness, which resul ts in either very mild s ta l l ing or no s ta l l ing a t much higher angles of attack.

Conclusions and Pmctical Benefits

From the experimental results regarding the new a i r fo i l design concept i t ma,y be concluded that t h i s a i r fo i l design produces (a) improved s t a l l characteristics a t virtually a l l opera- t ional airspeeds, (b) increases functional 1 if t coefficients over a broader range of operational an les of attack, and (c) increases functional ! li t t o drag ra t io over a broad range of opera- t ional angles of attack. The premeditated step- wise aerodynamic excitation proves t o be capable of controlling flow separation, and thus pre- vents s ta l l ing a t angles of attack much higher than those of conventional a i r fo i l s .

Some of the advantages and benefits that can be obtained by using a i rc ra f t that incorpor- ates th i s new a i r fo i l design concept are as follows:

1. Better maneuverability and control of the airplane.

2. By permitting the use of hi her an les of attack, the aircraf t w i l l o%tain t i e de- sign al t i tude much faster .

3. Fuel saving, and thus fighter airplanes can have longer combat missions.

4. Shorter runways and a i r f ie lds can be used since higher l i f t and higher angles of attack can be util ized.

5. Improvement of environmental noise problems in congested areas large cities) can be d realized since the esired altitude can be obtained much faster and at shorter radius.

6. With improved stall characteristics, the aircraft becomes safer, .particularly with new and inexperienced pilots.

7. Aircraft carriers can utilize shorter land- ing and take-off air strips by incorporating the new design.

8 . Simpler airfoil design may be utilized with possible reduction or elimination of several present control devices.

9. Simplicity in design leads to reduction in manufacturing costs.

10. The use of higher lift in conjunction with higher angles of attack, can drastically re- duce landing and take off speeds.

11. Better maneuverability and control at lower speeds can be obtained.

12. New as well as existing aircraft can utilize this new wing concept .-

Large military transports can land at lower speeds and in smaller fields with greater control and stabilit , also allowing larger loads to be carried gy the aircraft. The key advantages of the new wing design are safety, economy and manueuverability. All are very important for both military and commercial aircraft. It also increases lift and reduces drag.

References

Abbott, I.H. of Wing Sect York, 1959.

Fertis, D.G. Structures", York, 1973.

and Von Doenhoff , A.E. ,"Theory ions", Dover Pub., Inc.,New

, "Dynamics and Vibration of John Wiley and Sons, Inc. ,New

Fert is, D. G. , "Dynamics and Vibration of Structures", Revised Ed.,Robert E. Krieger Pub. Co . , Malabar, FL, 1984.

Fert is, D . G . , "Dynamic Response of Nonuni- form Rotor Blades", J. of Aircraft, Vol. 14, No. 5, Am. Inst. of Aeronautics & Astronau- tics, May 1977.

Houghton, E.L. and Brock, A.E., "Aerodynam- ics for Engineering Students" Edward Arnold Pub. Ltd. , London, 1970.

Rae, W.H. Jr. and Pope, A., "Low-Speed Wind Tunnel Testing", John Wiley and Sons Co. , Inc, New York, 1984.