Michael Papadakis*, Roy Y. Myose', Ismael Heron?, and Bonnie L. Johnson3 Department of Aerospace Engineering

Wichita State University Wichita, KS 67260

straet

Results from a low speed wind tunnel test are presented for a two-dimensional GA(W)-2 airfoil with 25% chord single slotted flap and 1% chord Gurney flaps. The experimental investigation was conducted at a Reynolds number of 2.2 million and a Mach number of 0.13. Flap deflections of 0 (nested), 10, 20 and 30 degrees were considered. For each slotted flap deflection experimental data were obtained for the baseline configuration (no Gurney flap) and for three Gurney flap configurations. The Gurney configurations included a Gurney flap attached to the slotted flap only, a Gurney flap attached to the main element only and Gurney flaps attached to both elements. In all cases the Gurney flaps were located at the trailing edge of the lower surface of each element and were set perpendicular to the local chord. Experimental results were obtained for angles of attack ranging from -8 deg. to +16 deg. and include aerodynamic forces and moments, surface pressure distributions, and wake velocity profiles at two locations downstream of the slotted flap trailing edge.

The experimental results indicate that the use of 1% Gurney flaps increased the maximum lift coefficient of the baseline configuration for all deflections of the slotted flap. However, in most cases the lift to drag ratio was decreased. The addition of a Gurney flap in the cove region of the main element had an adverse effect on the pressure distribution of the flap.

Gurney flaps are small flat plate flaps, typically less than 2% of the wing chord, which are usually attached to the pressure side of the wing near the trailing edge and perpendicular to the chord line.

*Associate Professor, Member AIAA 'Assistant Professor, Senior Member AIAA ?Research Assistant, Student Member AIAA $Aerodynamic Lab Director, Senior Member AIAA Copyright 0 1996 by the Authors. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission.

The Gurney flap was first used by Daniel Gurney to improve the performance of rear wings attached to Indianapolis type racing cars (Ref. 1). It was found that the use of these flaps increased the down- force needed for lateral traction.

Experimental investigations (Refs. 2-4) conducted with various single element airfoils have shown that in general Gurney flaps increase the lift coefficient relative to the clean airfoil for a given angle of attack. The influence of Gurney flaps on the drag coefficient however, appears to be configuration dependent. In some cases small drag increases have been observed while in other cases small drag reductions have been obtained with the use of these simple flap devices.

Experimental investigations on the effect of Gurney flaps on multi-element airfoils have also been conducted (Refs. 5-7). In Ref. 5 the application of Gurney flaps to a two-element airfoil was investigated experimentally. The Gurney flap was attached near the trailing edge of the main element. This investigation showed that the tab increased the loading on the main element while separation on the flap was delayed. Optimum performance was obtained with a 0.5 % Gurney flap. In general, the use of the Gurney flap increased the maximum lift and the lift to drag ratio of the two-element airfoil.

The details of the physical mechanisms responsible for the benefits obtained from Gurney flaps are not clear. However, a number of theories currently exist that attempt to explain these flow mechanisms. Simply put Gurney flaps increase the effective camber of the airfoil near the trailing edge (Ref. 8). More elaborate explanations relate the increased lift obtained with Gurney flaps to the Kutta condition. It is hypothesized based on experimental evidence that the Gurney flap causes a downward turning of the flow as the flow leaves the trailing edge of the airfoil. This flow turning causes an increase in the downward momentum of the fluid near the trailing edge. This increase in momentum

487

assists the fluid over the suction side of the airfoil to overcome the adverse pressure gradient encountered near the trailing edge. The net effect of the Gurney is an increase in lift and a smaller separation region on the suction side of the airfoil (Ref. 8).

In this study an experimental investigation was conducted to determine the influence of 1% chord Gurney flaps on the performance of a GA(W)-2 airfoil with a 25% chord slotted flap. Force, moment and pressure coefficients as well as wake velocity profiles were obtained with three Gurney flap configurations as well as with the baseline (clean) configuration for four deflections of the slotted flap and for a range of angles of attack. In the following sections the experimental method, the test configurations, experimental force and moment data as well as selected experimental pressure distributions and wake velocity profiles are presented.

The experimental investigation was conducted in the Wichita State University (WSU) Beech memorial 7 ft x 10 ft low speed wind tunnel. This is a single-return closed circuit facility with a maximum speed of 160 mph. For this investigation two-dimensional wall inserts were used to support the 3 ft span two-dimensional test model. The experimental set up is shown in Fig. 1 below. The dynamic pressure for all tests was 35 Ib/ft2. This corresponds to a freestream velocity of 120 mph (1 76 ft/sec) and a Reynolds number of 2.2 million based on the airfoil chord of 24 inches.

Fig. 1 GA(W)-2 airfoil installation in WSU wind tunnel facility (looking upstream).

Fig. 2 Close up of rake probe used to measure wake velocity profiles (looking upstream).

The GA(W)-2 airfoil section 24 inch chord and 36 inch span was mounted in the two-dimensional inserts. Transition strips 0.1 inch wide made of #80 carborundum grit were attached at 5% chord on the upper surface and at 5% chord on the lower surface of the main element. Transition on the flap was not forced.

Lift, drag and pitching moment were measured with the tunnel pyramid-type external balance. Pressure distributions were obtained with a Pressure Systems Inc (PSI) 8400 Industrial System processor. The PSI system was capable of measuring pressure in the range of k2.5 psid with an accuracy of 0.001 psi. The test model had 52 pressure ports on the main element and 18 pressure ports on the slotted flap. These pressure ports were distributed along the model midspan.

Wake velocity profiles were measured with a 14 inch high total pressure rake. This rake had a total of 37 total pressure ports and three static ports as shown in Fig. 2. Wake velocity profiles were taken at two locations 1/4 and 1/2 airfoil chords downstream of the slotted flap trailing edge. Force, moment, pressure and wake data were obtained for angles of attack ranging from -8 to 16 degrees. Angle of attack was set by rotating the airfoil about its 50% chord point using motor driven plates located on the two-dimensional wall inserts.

488

Experimental data were obtained for the airfoil with the flap in the nested position and for three flap deflections namely 10,20 and 30 degrees.

20 30

With the flap nested the configurations tested included : 1. The airfoil with no Gurney attached. This is referred to as the single element baseline configuration - Clean. 2. The airfoil with 1% (0.25 inch) Gurney attached to the lower surface of the airfoil at the trailing edge - Gurney.

0.943 I -0.03 0.938 I -0.025

a. Clean airfoil configuration (24 inch chord)

L- Gurney Flap

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 b. Airfoil with 1% chord Gurney flap.

Fig. 3 Nested flap configurations

For each deflection of the 25% chord slotted flap four test configurations were investigated as follows:

1. Baseline configuration - Clean 2. 1% chord (0.25 inch) Gurney attached to the trailing edge of the main element on the lower surface - Main Gurney 3. 1% chord (0.25 inch) Gurney attached to the trailing edge of the slotted flap on the lower surface - Flap Gurney 4. Two 1% chord (0.25 inch) Gurneys attached to the trailing edge of the main element and to the trailing edge of the flap - Flap + Main Gurney

Flap pivot locations for each flap deflection are given in Table 1. The origin of the x and z axes is at the airfoil leading edge, x is positive to the right and z is positive upwards.

Table 1 : Slotted flap pivot locations.

a. Clean

b. Main Gurney

c. Flap Gurney

l - . ~ . - i . ~ ~ ~ - . . - , ~ ~ ~ . . . , ~, ~~~1 ~~r ~1~ , , ~ ~1~~

d. Flap + Main Gurney

; i 4 ,

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20

Fig. 4 Slotted flap configurations

The total number of test geometries investigated in this study was 14 [ 3 flap deflections x ( 3 Gurney configurations + Clean configuration ) + 2 configuration with the flap nested]. Geometry details are provided in Ref. 9.

Experimental force and moment coefficients for all the configurations tested as well as surface pressure distributions and wake velocity profiles for the 20 deg. flap case are presented in this section. Flow conditions include Mach number of 0.13, Reynolds number of 2.2 million based on airfoil chord (24 inches) and angles of attack of -8 to 16 degrees.

ornent Coefficients All force and moment data have been corrected

using the wind tunnel corrections described in Ref. 10. Pitching moment coefficients have been computed about the 50% chord location.

489

Flap Nested Lift, drag and pitching moment coefficients for

the baseline and Gurney flap configurations with the slotted flap in the nested position are shown in Figs. 5 , 6 and 7.

0.10 -

-

0.08 -

cd 0.06 -

0.04 -

0.02 -

0.00

The effect of the Gurney flap is to increase the lift coefficient for all angles of attack. This increase in lift is accompanied by an increase in drag for angles of attack below stall, and by a more negative pitching moment for all angles of attack. Another effect of the Gurney is to shift the lift curve to the left by approximately 2-3 deg., indicating that for a given lift coefficient the angle of attack is reduced. In addition, the lift curve slope is marginally increased.

4 Clean, Run 55

+ Gurney, Run56

( 1 1 ( 1 1 1 1 I l l

The performance of the two configurations at maximum lift are compared in Table 2. The percentage values given in Table 2 are with respect to the "Clean" configuration.

0.60 -

0.40 -

0.20 -

Cm

0.00 -

-0.20 - -

Table 2: Comparison of Baseline and Gurney configurations at maximum C1 (flm nested). CAW-2 - flap nested

I I Gurney 2.1 I) Clean 1.7

3.00 -

2.00 -

1.00 -

0.00 -

CI

GA(V9-2 - flap nested

-+- Clean, Run55

+ Gurney, Run56

-1.00 1 1 1 1 1 1 1 1 I ,

-10.00 -5.00 0.00 5.00 10.00 15.00 20.00 Alpha

Fig. 5 Experimental lift curves for GA(W)-2 airfoil with 25% slotted flap and 1% Gurney flap; M=O.13, Re=2.2 million, flap nested.

0.12

GA(V9-2 - flap nested

I

0

Fig. 6 Experimental drag coefficient versus lift coefficient for GA(W)-2 airfoil with 25% slotted flap and 1% Gurney flap; M=0.13, Re=2.2 million, flap nested.

+ Clean, Run55

+ Gurney, Run56

-0.40 I I I ,

-1.00 0.00 1.00 2.00 3.00 CI

Fig. 7 Experimental pitching moment coefficient about 50% chord point versus lift coefficient for GA(W)-2 airfoil with 25% slotted flap and 1% Gurney flap; M=0.13, Re=2.2 million, flap nested.

In Fig. 8 the aerodynamic efficiency of the two configurations is compared in terms of the lift to drag ratio. For lift coefficients below 1.5 the clean configuration provides higher l i f t to drag ratios than the airfoil with the Gurney flap. Furthermore, at maximum lift the Gurney flap has a lower C K d than the clean airfoil. This is also evident from Figs. 5 and 6 which indicate that near stall the Gurney flap increases the lift but it also generates a large amount of drag.

490

200.00

-

- 150.00 -

103.00 -

50.00 -

0.03 -

-50.00

Fig. 8 Experimental lift to drag ratio versus lift coefficient for GA(W)-2 airfoil with 25% slotted flap and 1% Gurney flap; M=0.13, Re=2.2 million, flap nested.

-l G4pVp2-flapnested + Clean

- + G u r n e y

I l l , I , , , 1 1 1 1 1 1 I I

-1.00 0.00 1 .00 2.00 3.00

Slotted Flap Deflection: 10 deg. With the slotted flap deflected 10 degrees

trailing edge down the experimental lift, drag and pitching moment data for all four test configurations (Clean, Main Gurney, Flap Gurney and Flap+Main Gurney) are presented in Figs. 9-1 1. The lift to drag ratio versus lift coefficient for all configurations tested are compared in Fig. 12.

-

-

+ Clean, Run54

+ Main Gurney, Run 51

GA(W)-2 - 10 deg. flap

CI

--++ Flap + Main Gurney,

10.00 15.00 Alpha 5.00

-10.00 -5.00 0.00

Fig. 9 Experimental lift curves for GA(W)-2 airfoil with 25% slotted flap and 1% Gurney flaps; M=0.13, Re=2.2 million, 10 deg. flap deflection.

0.30 - GA(W)-2 - 10 deg. flap I + Clean, Run54

0.25 _ _ . I - -. - .

cd

0.30

+ Clean, Run54

+-- Main Gurney, Run 51

FlapGumey, Run53

+ Flap + Main Gurney,

0.25

0.20

0.15

0.10

0.05

0.00

0.00 1 .OO 2.00 3. 00 4.00 CI

Fig. 10 Experimental drag coefficient versus l i f t coefficient for GA(W)-2 airfoil with 25% slotted flap and 1% Gurney flaps; M=0.13, Re=2.2 million, 10 deg. flap deflection.

--+ MainGumey, Run51

-+- FlapGumey, Run53

-&- Flap + Main h e y , Run 52 -0.40 , 1 1 1 , 1 , 1 1 1 1 1 1 1

CI Fig. 11 Experimental pitching moment coefficient about 50% chord point versus lift coefficient for GA(W)-2 airfoil with 25% slotted flap and 1% Gurney flaps; M=0.13, Re=2.2 million, 10 deg. flap deflection.

A positive increment of approximately 0.1 in lift coefficient is obtained with the Gurney attached to the trailing edge of the main element. At maximum C1 coefficient however, the "Clean" and "Main Gurney" configurations produce the same amount of lift. The corresponding increments in C1 for the "Flap Gurney"

491

and "Flap+Main Gurney" configurations with respect to the "Clean" configuration are 0.4 and 0.5 respectively. At maximum CI these configurations provide a lift increment of approximately 0.3 with respect to the baseline airfoil. The largest increment in CI is obtained with Gurney flaps attached to both elements.

The Gurney flaps cause the lift curve to shift 2 to 4 degrees to the left depending on configuration. This shift suggests an increase in effective camber and is similar to that observed by other investigators.

For a given CI coefficient all Gurney flap configurations have higher drag than the baseline airfoil. For C1 coefficients less than 2.65 the largest Cd for a given CI is produced by the "Flap+Main Gurney" configuration. For lift coefficients greater than 2.65 the "Flap Gurney" and the "Flap+Main Gurney" generate less drag than the other two configurations.

The nose down pitching moment curves of the "Clean" and "Main Gurney" configurations are very similar. The largest negative pitching moment coefficients are produced by the "Flap Gurney" and "Flap+Main Gurney" configurations.

The lift to drag ratio curves are shown in Fig. 12. The complete curves are given in Fig. 12a while in Fig. 12b the results near maximum C1 are presented for each configuration. For lift coefficients less than 1.8 the highest lift to drag ratio is obtained with the clean airfoil followed by the "Main Gurney", the "Flap Gurney" and the "Flap+Main Gurney". For lift coefficients in the range 2.65-3 (Le., near maximum lift) the results indicate that for the most part the "Flap Gurney'' performs better than the "Flap+Main Gurney". Note that the other two configurations stall at a lift coefficient of approximately 2.7.

The performance characteristics of all four configurations at maximum CI are provided in Table 3. All percentage values in this table have been determined with respect to the "Clean" configuration.

Table 3: Comparison of the four test configurations at maximum CI (Slotted flap deflection: 10 deg.)

+ Flap Gurney

-&- Flap+Main Gurney

003 1.03 203 303 4

CI a. CI/Cd curves for full CI range

+ Clean

-+ MainGurney 60.03

20.03 3 1.80 2.03 2.20 2.40 2.60 2.80 3.03 3

CI b. CI/Cd curves near maximum CI

Fig. 12 Experimental lift to drag ratio versus

a

'0

lift coefficient for GA(W)-2 airfoil with 25% slotted flap and 1% Gurney flaps; M=0.13, Rez2.2 million, 10 deg. flap deflection.

Slotted Flnp Deflection: 20 deg. For this deflection of the slotted flap the use of

Gurney flaps increased the lift coefficients at all angles of attack and reduced the stall angle of attack with respect to the baseline (clean) configuration as shown in Fig. 13. The largest increment in lift coefficient was obtained with the "Flap + Main Gurney" and was in the range of 0.3-0.65. The "Flap Gurney" configuration produced C1 increments in the range of 0.3 to 0.48. The smallest increment in C1 was obtained with the "Main Gurney" and was in the range 0.06 to 0.28. The

492

maximum lift coefficient obtained with the "Main Gurney" was the same as that of the clean airfoil. The maximum lift coefficient obtained with the other two Gurney configurations was approximately 0.36 higher than the "Clean" configuration.

0.60

0.40

0.20

Cm 0.00

-0.20

-0.40

Drag coefficient plotted against lift coefficient are depicted in Fig. 14. For C1 coefficients less than approximately 2.5 the use of Gurneys increased the drag with respect to the baseline configuration. However, at C1 coefficients greater than 3.1 the drag produced by the "Flap Gurney" and the "Flap+Main Gurney" is less compared to the "Clean" and "Main Gurney" configurations.

- GA(W)-2 - 20deg. flap

- + Clean, Run66 - - + MainGumey, Run67

- FlapGumey. Run69

- --&- Flap t Main Gurney. Run 68

-

-

I ' I 1 1 I 1 ' ' I ' 1 ' 1 I I I I I

4.00

3.00

ci 2.00

1.00

(nose down) pitching moments than the baseline configuration. From the three Gurney configurations used the "Flap Gurney" and "Flap+Main Gurney" produced the largest negative increment in Cm. The "Main Gurney" configuration produced pitching moments similar to that of the clean configuration.

- GA(W)-2 - 20 deg. flap

-

-

- -++ Main Gurney, Run 67

+ Flap Gurney, Run 69

The lift to drag ratios for this case are presented in Fig. 16. For lift coefficients below 2.4 the trend is similar to the 10 deg. case with the highest Cl/Cd for a

Fig. 13 Experimental lift curves for GA(W)-2 airfoil given C1 produced by the "Clean" configuration with 25% slotted flap and 1% Gurney flaps; M=0.13, followed by the "Main Gurney", the "Flap Gurney" and Re=2.2 million, 20 deg. flap deflection. the "Flap+Main Gurney". For lift coefficients between

2.5 and 3.0 the lift to drag ratio obtained with the "Main Gurney" is higher than the other three configurations. At lift coefficients greater than 3 the "Flap Gurney'' and "Flap+Main Gurney" outperformed the "Clean" and "Main Gurney" configurations in terms of Cl/Cd.

4 Flap + Main Gurney, Run 68

Alpha (dGy

0 . 0 0 , , , , 1 1 1 1

-10.00 -5.00 0.00 10.00 15.00

0.20

+ Clean, Run 66

-+- Main Gurney, Run 67

+J-- Flap Gurney, Run 69

-y+- Flap + Main Gurney, Run 68 The performance of the clean and the three Gurney configurations at maximum lift coefficient is summarized in Table 4. The percentage changes are with respect to the clean configuration.

Cd 0.10

Table 4: Comparison of the four test configurations at 0.00

0.00 1.00 2.00 3.00 4.00 CI

Fig. 14 Experimental drag coefficient versus lift coefficient for GA(W)-2 airfoil with 25% slotted flap and 1% Gurney flaps; M=O.13, Re=2.2 million, 20 deg. flap deflection.

Pitching moment coefficients are depicted in Fig. 15. The use of Gurney flaps produced more negative

493

3 3

i m m

am-

4003 -

G 4 0 - 2 - 20 deg. flap

0 . 0 , , , , I , , , , I , , , I

0.m 1.m 2.03 3.m 4.m CI

a. C K d curves for full CI range

-+- Flap Gurney, Run 46

-&- Flap + Main Gurney, Run 45

0.00 -10.00 i -5.m 0.00 10.00

70 MI

6om

5om

3 3

4003

a m .

2003.

15.00

G 4 0 - 2 - 20deg. flap

+ Clean

+ MainGurney

-++ FlapGurney

--&- FlapMain Gurney

0.30

- - -

0.20 -

cd

0.10 -

0.00

2m 2.40 2.60 2.80 3.03 3.m 3

b. C K d curves near maximum C1 CI

GA(W)-2 - 30 deg. flap

+-- Clean, Run39

+ Main Gurney, Run 50

-++ FlapGtuney, Run46

-++- Flap + Main Gurney, Run 4

, , , , , , , , I I , , , , , , 0.00 1 .00 2.00 3.00 4.00

Fig. 16 Experimental lift to drag ratio versus lift coefficient for GA(W)-2 airfoil with 25% slotted flap and 1% Gurney flaps; M=0.13, Re=2.2 million, 20 deg. flap deflection.

Slotted Flap Deflection: 30 deg. The force and moment coefficients for this test

case are depicted in Figs. 17-19. The lift curves corresponding to the "Clean" and "Main Gurney" configurations are very similar. The "Flap Gurney" and "Flap+Main Gurney" configurations produced a notable increase in lift coefficient which for positive angles of attack was approximately 0.3 higher than that of the "Clean" configuration. For these two Gurney cases the CI curves are shifted to the left by 2 to 3 degrees and the stall angle is decreased. The drag coefficient of the

baseline airfoil and the "Main Gurney" are similar for the most part of the CI range. The "Flap Gurney" and the "Flap+Main Gurney" produced significantly higher drag coefficients than the other two configurations. The most negative pitching moment coefficient about the midchord point was generated by the "Flap Gurney" while the "Flap+Main Gurney", "Clean" and "Main Gurney" produced progressively less negative pitching moments. It is noteworthy that the "Main Gurney" has a more positive pitching moment than the baseline configuration.

4.00 GA(W)-2 - 30 deg. flap

494

cm

- GA(W)-2 - 30 deg. flap

+ Clean, Run39 0.40

0.20 -

0.00 -

-0.20 -

1-4- -% 7!+

Lift to drag ratios for the 30 deg. flap deflection are presented in Fig. 20. For positive angles of attack below stall, corresponding to lift coefficients of 2 to 3.2, the "Flap Gurney" and the "Flap+Main Gurney" produced considerably lower Cl/Cd for a given C1 coefficient than the baseline configuration. The "Main Gurney" configuration produced marginally higher lift to drag ratios at moderate angles of attack (2.6 < C1 <3) than the clean airfoil.

-0.40

{ G4O-2 - 30 deg. flap

1 1 , , 1 1 1 , 1 , 1 , 1 1 1 1 1 1

B 5

-t-

-Et- napamey + flaptMainWmey

0.00

0.00 1 .00 2.00 3.00 4.00 CI

a. Cl/Cd curves for full C1 range

40.00 ,

0.00

2.40 2.60 2.80 3.00 3.20 3.40 3 CI

b. Cl/Cd curves near maximum C1

Fig. 20 Experimental lift to drag ratio versus lift coefficient for GA(W)-2 airfoil with 25% slotted flap and 1% Gurney flap; M=0.13, Re=2.2 million, 30 deg. flap deflection.

The performance of all four configurations at maximum lift coefficient is summarized in Table 5 below.

Table 5: Comparison of the four test configurations at maximum C1 (Slotted flap deflection: 30 deg.)

Additional Comments The lift to drag ratios for all baseline airfoil

configurations (i.e., slotted flaps of 0, 10, 20 and 30 deg. with no Gurney flaps) are shown in Fig. 21. It is clear from this figure that in most cases increasing the camber reduces the lift to drag ratio for a given C1 coefficient. The performance of these four slotted flap configurations at maximum lift coefficient is summarized in Table 6. The percent change in maximum CI and in Cl/Cd at maximum C1 are with respect to the nested flap configuration (single element airfoil). For the GA(W)-2 airfoil the 10 deg. flap deflection provides the highest lift to drag ratio at maximum lift. As the deflection of the slotted flap is increased to 20 and 30 degrees the lift to drag ratio is considerably reduced.

495

2ooW

180.0

160.W

140.W

12O.W

1 0 . W

? 80.W a

60.0

40.00

20.M)

0 . 0

-2o.W

4 . W

Comparison of Clean Configurations

+ flapnested

++ 10deg.flap

+ 20deg.flap

050 OM) 050 1 0 150 2 W 250 3 W 3 a

Fig. 21 Experimental lift to drag ratio versus lift coefficient for baseline GA(W)-2 airfoil with 25% slotted flap ; M=0.13, Re=2.2 million; 0, 10, 20 and 30 deg. flap deflections.

Table 6: Comparison of the four "Clean" airfoil configurations at maximum CI (Slotted flap deflections: 0, 10,20 and 30 deg.)

-15.m

-12.50

-1o.m -0- WnCuTRy,Fhn67

-7.50

CP -5.m

-2.50

0.M

2.50

0.m 0.20 0.40 0.60 0.80 1.03 0.80 l.m x/C

Fig. 22 Cp comparison for the GA(W)-2 airfoil with 25% slotted flap;a = 12 deg., M=0.13, Re=2.2 million, 20 deg. flap deflection; Clean, Main Gurney, Flap Gurney, Flap+Main Gurney.

In Fig. 22 the pressure distributions for all four configurations are compared for an angle of attack of 12 degrees. The "Flap Gurney" configuration produced higher suction pressure coefficients on the flap than the other three configurations. The use of a Gurney on the main element appears to reduce the suction coefficient on the flap leading edge for the 20 deg. flap deflection case. On the main element higher suction coefficients were produced with the "Flap Gurney" and "Flap+Main Gurney" configurations.

rofiles - 20 deg. flap Wake velocity profiles for an angle of attack of

12 deg. and at 114 chord downstream of the flap trailing edge are presented in Fig. 23 for all four configurations tested. In all cases the slotted flap was deflected 20 deg. trailing edge down. In Fig. 23 the horizontal axis is the wake velocity normalized with the local freestream velocity. The vertical axis is the vertical location in inches with respect to a reference point which corresponds to the zero inch location. This reference point was set 2.75 inches behind the flap trailing edge and 13 inches below the airfoil chord and corresponds to the bottom total pressure probe of the wake rake (see Fig. 2).

Flap defeidion 20 deg.

Angle of Attack = 12 deg.

- - - -

7.50 - z (inch) 1

- -

5.00 - -+ - Clean, Run

+ MainGumey, Run67

+ FlapGumey, Run69

+ Flap+MainGumey, Run68

2.50

0.00

0.00 0.20 0.40 0.60 0.80 1.00 1.20 V / Wreestream

Fig. 23 Experimental wake velocity profiles for GA(W)- 2 airfoil with 25% slotted flap; a = 12 deg., M=0.13, Re=2.2 million, 20 deg. flap deflection; Clean, Main Gurney, Flap Gurney, Flap+Main Gurney.

From Fig. 23 it is observed that the "Flap+Main Gurney" and "Flap Gurney" configurations produced a larger momentum deficit than the "Clean" and "Main

496

Gurney" configurations. The configurations with a Gurney attached to the flap operate at a higher C1 coefficient and for the 12 deg. case these configurations are near stall conditions. Note that the use of a Gurney on the main element modifies the slot flow between the airfoil and the flap and causes the wakes from the two elements to merge sooner.

The application of 1% airfoil chord Gurney flaps to a GA(W)-2 airfoil with 25% chord slotted flap was studied experimentally. Flow conditions included Mach Number of 0.13 and a Reynolds number of 2.2 million. Slotted flap deflection of 0, 10, 20 and 30 degrees were considered. For each non zero deflection of the slotted flap three Gurney flap configurations were investigated. These included a Gurney flap attached to the trailing edge of the main element, the trailing edge of the flap and to the trailing edges of both elements. Based on the experimental results presented the following conclusions are made:

1.

2.

3.

4.

For the single element case (flap nested) the use of a Gurney flap at the trailing edge of the airfoil produced a 0.2 to 0.4 increase in the lift coefficient over the range of angles of attack considered in this investigation. The lift to drag ratio hdwever was decreased with respect to the baseline configuration for the most part of the C1 range.

For the two element configurations the Gurney flaps exhibited similar performance for all deflections of the slotted flap. In all cases the maximum increment of lift coefficient was provided by the "Flap Gurney" and "Flap + Main Gurney" configurations. When the Gurney Flap was attached to the main element only (Main Gurney) the increase in C1 was significantly smaller than the other two Gurney arrangements and in some cases the maximum lift coefficient was reduced compared to the "Clean" airfoil.

The addition of Gurney flaps to the two element configuration increased the drag coefficient and reduced the lift to drag ratio. The nose down pitching moment coefficient about the 50% chord point of the airfoil was also substantially increased.

Limited pressure and wake data presented in this paper indicate that the addition of a 1% Gurney flap at the trailing edge of the main element has an adverse effect on the performance of the flap. In

general the Gurney modifies the slot flow and reduces the suction peak on the flap.

owle ts

The authors would like to acknowledge the Beech Wind Tunnel staff for their assistance with the test article installation and also for all facets of tunnel operation. Thanks are also due to Santiago Matallana and Laura Myers for their assistance in processing the experimental data.

eferences

1. Liebeck, R.H., "Design of Subsonic Airfoils for High Lift," AIAA Journal of Aircraft, Vol. 15, No. 9, 1978,

2. Neuhart, D.H. and Pendergraft, O.C., "A Water Tunnel Study of Gurney Flaps," NASA TM 407 1, 1988. 3. Kentfield, J.A.C, and Clavelle, E.J., "The Flow Physics of Gurney Flaps, Devices for Improving Turbine Blade Performance," Wind Engineering, Vol. 17, No. 1,

4. Storm, B. L., and Jang, C. S., "Lift Enhancement of an Airfoil Using a Gurney Flap and Vortex Generators," Journal of Aircraft, Vol. 31, No. 3, 1994, pp. 542-547. 5. Ross, J. C., Storms, B. L. and Carrannanto, P. C., "Lift-Enhancing Tabs on Multi-Element Airfoils," AIAA Journal of Aircraft, Vol. 32, No. 3, 1995, pp. 649-655. 6. Katz, J. and Largman, R., "Effect of 90 Degree F] on the Aerodynamics of a Two-Element Airfoil," Journal of Fluids Engineering, Vol. 11 1, March 1989, pp. 93-94. 7. Storms, B. L. and Ross, J. C., "Experimental Study of Lift-Enhancing Tabs on a Two-Element Airfoil," AIAA Journal of Aircraft, Vol. 32, No. 5, September-October

8. Jang, C.S., Ross, J.C., Cummings, R.M., "Computational Evaluation of an Airfoil with a Gurney Flap," AIAA Paper 92-2708-CP, June 1992. 9. Rogers, E.J., Wentz, W.H., and Ostowari, C., "Experimental Studies of Pressure Distributions and Flow Fields on the GA(W)-2 Airfoil Incorporating a Flap and Spoiler," AR 79-4, Wichita State University, July 1979. 10. Johnson, B.L., Leigh, J.E., and Moore, K.A.,"Three- Dimensional Force Data Acquisition and Boundary Corrections for the Walter H. Beech Memorial 7 x 10 Foot Low Speed Wind Tunnel," Wichita State University Report AR 93-2, June 1993. 11. Myose, R., Heron, I., and Papadakis, M. "The Effect of Gurney Flaps on a NACA 001 1 Airfoil," AIAA Paper 96-0056, January 1996.

pp. 547-561.

1993, pp. 24-34.

1995, pp. 1072-1078.

4 9 7