25TH INTERNATIONAL CONGRESS OF THE AERONAUTICAL SCIENCES

EXPERIMENTAL AND NUMERICAL STUDY OF A TWO- ELEMENT WING WITH GURNEY

F.M. Catalano PhD.( [email protected] ) *, G. L. Brand * * Aerodynamic Laboratory EESC-USP Brazil

Keywords: Gurney flap, two element wing

Abstract brought attention back for the use of Gurney A quasi two-dimensional experimental and flap on multi elements wings. Numerous wind numerical investigation was performed to determine the tunnel tests on Gurney flaps have been effect of a Gurney flap on a two-element wing. A Gurney conducted in both single and multi-element flap is a flat plate of the order of 1 to 4% of the of 2-D and 3-D wings. Giguerre et al [2] in length, oriented perpendicular to the chord line presents an extensive list of these works. The and located on the airfoil windward side at the . The effects of various tabs were studied at a first work concerning Gurney flaps was carried constant Reynolds number on a two-element airfoil with a out by Liebeck [1] who found that is slotted flap. The experiments were conducted in a low increased with the attachment of a Gurney flap speed wind tunnel for the wing with Gurney flap sizes of at the trailing edge of airfoils. Liebeck also 1.0%, 2%, and 4.0% of the airfoil chord, located at the found that drag increases but for Gurney flaps flap trailing edge and also at the main element trailing edge. The flowfield around the airfoil was numerically with a height below 2% of the chord a slight predicted using CFX, with the non-linear RANS decrease in drag can occur. For Gurney flap turbulence model. Computational results were compared heights beyond 2% drag penalty may be with the experimental results. The results from both prohibitive even if overall L/D increases. A numerical and experimental work show that the Gurney parametric study of the application of Gurney flaps can increase the airfoil with only a slight increase in drag coefficient. Although these effects flaps on a single and multi element three- with a Gurney flap positioned at the main element trailing dimensional wing was carried out by Moyse and edge are highly dependent on the gap and overlap Heron [3]. Moyse and Heron found that the adjustments for maximum performance, Use of a Gurney Gurney flap increased lift for the majority of the flap increases the airfoil lift coefficient decreasing the configurations tested with a drag penalty required to obtain a given lift coefficient. The numerical solutions show details of the flow structure dependent of the Gurney flap height. They also at the trailing edge and provide a possible explanation for founded that placing a Gurney flap at the the increased aerodynamic performance. trailing edge of the main element; in general there is no significant improvement in the wing performance mainly due to the change in the 1 Introduction gap of the slotted flap. On the other hand, Ross There have been quite a number of works et all [4] demonstrated experimentally and on the Gurney flap since the race driver Dan computationally that placing a Gurney flap on Gurney used this flap on the inverted wing of both flap and near the trailing edge of the main his car, increasing down force, in the 60’s. element can achieve an increase in lift of 12% Despite this fact, the use of Gurney flaps still and 40% on the lift to drag ratio. The range of almost restricted to race cars with little use in Gurney flap heights tested was from 0.125 to design mainly due to the increase in 1.25% of the airfoil reference chord. Ross et al drag that may occur especially in cruise [4] positioned the Gurney flap at 0.5% chord conditions. However, the demands for a high lift upstream of the trailing edge of the main system that could be at same time highly element rather than right at the trailing edge. efficient and with a minimum complexity, has This allowed the Gurney flap to be retracted

1 F.M. CATALANO, G.L. BRANDT

when the high lift system is stowed. Intensive 2 Experimental set-up computation analysis was performed by Jang et The experiment was conducted in the Wind al [5] and Carrannanto [6] in order to investigate tunnel of the Aerodynamic Laboratory of Sao the effect of the Gurney flap on an airfoil by Carlos Engineering School, University of Sao using an incompressible Navier-Stokes solver Paulo. The Wind tunnel is closed circuit and with the one-equation turbulence model of closed working section, with dimensions of Baldwin and Barth. The numerical data was 1.75m width, 1.30m high and 4m length. compared with experimental data available in Turbulence level and maximum velocity are order to validate the computational flow 0.25% and 50m/s respectively. The wing model visualization in the Gurney flap region. The has two elements with a flap and main element general conclusion on the effect of the Gurney and the experiment is shown in Fig. 1 and Fig 2. flap on the local flow is the production of a pair The flap and main element are made of of counter rotating vortices downstream of the fiberglass with steel spars fixed to circular end trailing edge. These vortices act like an airfoil plates to simulate a two-dimensional wing. The extension, increasing the effective chord and main dimensions of the wing are in Table 1. camber. In this sense, it is expected that the Using end plates does not assure two- Gurney flap at the main element trailing edge dimensional flow, especially in high lift wings region will change the flow inclination in order such as this model. However, at the center of the to alleviate flap adverse pressure gradient and wing, where chordwise pressure measurements thus delaying separation. Delaying flap are performed the three-dimensional flow separation is good news for the high lift system induced by the secondary vortices at the end in the climb configuration but is not necessarilly plate is minimal. Also, the comparative analysis welcome for landing when drag is also of this work assumes that any secondary vortex necessary. Unless the Gurney flap is of the effect at the center of the wing will be present in retracted type as suggested by Ross et al [4] the both configurations: with and without the use of this device will not be directly applicable Gurney flap installed at the trailing edges. The in aircraft design before a trade-off analysis. An flap incidence angle can be changed but within alternative to solve this problem is to use a a small range due to the subsequent change in different device at the main element trailing the wing/flap gap and overlap. A total of 90 edge that would delay flap separation in the chordwise pressure taps were used to measure climb configuration but would be less effective pressure coefficient distribution on both at high flap angles such as in landing surfaces of the wing main element and flap. The configuration. Lemes and Catalano [7] proposed pressure coefficient distributions were measured a serrated trailing edge that promotes mixing by two mechanical D48 scanivalves fitted with between the higher-pressure flow from the ± 1.0-psia Setra transducer. The wing was lower surface with the flow on the upper surface positioned in a vertical position attached to the to produce vortices. These vortices feed high aerodynamic balance below the tunnel floor. momentum and low turbulent kinetic energy The aerodynamic balance has only two flow into the flap boundary layer delaying components so that drag and side force (lift) separation. A serrated main element in were measured for a range of incidence angle of conjunction with a flap with Gurney flap could o o –4 to 20 . The two-component balance used is create a high lift system that would bring of the strain gage type and has a measurement benefits to both take-off/climb and accuracy of ± 0.7% for maximum loading. approach/landing. In this work, application of the Gurney flap Therefore, accuracy for Lift and Drag are ±1.0N in a quasi two-dimensional two element wing is and ±0.19N respectively. Incidence angle was analysed experimentally and numerically in measured with an accuracy of ±0.1 deg. order to produce more information of its effect on the wing aerodynamic characteristics. 2 Experimental and Numerical Study of a Two-Element Wing with Gurney Flap

working section flow. Also, the computational domain length was the shortest for the least computational cost. The non-structured mesh of the computational domain and model are shown in Fig. 3. Details of the wing and end-plate non-structured mesh can be seen in Fig. 4. One of the main objectives of the numerical work was to analyze the region of the Gurney flap and the effect that this element imposes on the whole model. In the above mentioned region, Fig. 1 Experimental set-up flow separation is likely to occur thus, it was decided to apply the k-ε turbulence model which is a non-linear Reynolds Averaged Navier-Stokes model (RANS). The main advantage of this formulation is that it simulates with more accuracy the phenomena and has less computational cost compared with LES (Large Eddy Simulation) and DNS (Direct Numerical Simulation).

Fig. 2 Wing model lay-out.

Table 1 Model dimensions Main Flap Flap 0º 8º Elem incidence ent Refer 0.399 0.396 Gap 8,80 10,1 2 2 ence m m (mm) 5 Fig. 3 Computational domain and model area Span 1.00 1.00 m Overlap 12,0 13,0 m (mm) 0 0 Chord 0.186 0.170 m m

2.2 Numerical Set-up The computer program used was the Ansys CFX with its auxiliary software for grid generation the ICEM CFD 5.0, and CFX- Fig 4 Unstructured grid for the two element Pre/Pos 5.7 for pre-processing and pos- wing with Gurney flap and end-plate. processing. The computational domain adopted has exactly the wind tunnel working section dimensions: 1.3m x 1.75m with a length 3 Results and Discussion sufficiently long in order to not be affected by Experimental results: Previous results [1,2 and the presence of the wing model and simulate the 3] suggest that Gurney flap with heights of less 3 F.M. CATALANO, G.L. BRANDT

Alpha = 8o, flap at 0o than 5% of airfoil chord may produce less drag -4 penalty. Therefore, in this work only the Gurney -3.5 flaps of heights of 1%, 2% and 4% of reference -3 chord were used in the tests. Because the model -2.5 wing was not fitted with a mechanism for -2 P changing flap gap and overlap, the Gurney flap C -1.5 used for the main element trailing edge was the -1 -0.5 1% chord height. The results for pressure 0 coefficient are presented always in comparison 0.5 between the three Gurney flaps and the wing 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 without Gurney flap. Figures 6 to 9 presents the X/C Clean 1% Gurney 2% Gurney 4% Gurney results for incidence angles of: 0o, 8o, 16o and o 18 with the Gurney flap installed at the trailing o Fig. 7 Pressure coefficient for alpha=8 and edge of the flap set at a flap angle of zero o flap at 0 . degrees. For this wing the flap at zero degree means that the flap is in the design position, see -9 Fig 5 for reference. For all the experimental -8 results it is clear that the Gurney flap increases -7 effective camber as can be seen in Figs 6 to 10, -6 in which the suction peak becomes bigger as -5

P -4 Gurney flap height increases. Also it is clear C that at the bottom surface near the flap trailing -3 edge the pressure increases with the Gurney -2 flaps. These two effects combined can result in -1 an increase in lift of up to 10%. 0 Main 1 Flap at “zero” Element Deg position 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Clean 1% Gurney 2% Gurney 4% Gurney X/C Reference o Flap at 12o Line Fig. 8 Pressure coefficient for alpha=16 and flap at 0o. -6 LS Fig. 5 Wing reference angles. -5 LS= laminar separation T T = transition o o Alpha = 0 , Flap at 0 -4 R = reatachment -2.5 R S = Turbulent separation

-2 -3 P -1.5 C -2 S -1 P

C -1 -0.5

0 0

0.5 1 1 0 0.2 0.4 0.6 0.8 X/C 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 X/C 1 Clean 1% Gurney 2% Gurney 4% Gurney Clean 1% Gurney 2% Gurney 4% Gurney o o Figure 9 Pressure coefficient for alpha=20 Fig. 6 Pressure Coeficient for alpha=0 and and flap at 0o. flap at 0o

4 Experimental and Numerical Study of a Two-Element Wing with Gurney Flap

It can be seen from Fig. 7 that the turbulent -5 separation point has been slightly delayed for the 4% Gurney flap installed. This effect could -4 be related with the higher suction peak at the flap imposed by the Gurney flap. From Fig. 9 is -3 P C clear that even with the flap fully separated the -2 pressure increase at the bottom surface near the flap trailing edge still to occur, but it does not -1 affect the main element flow. Fig. 10 shows the curve of CL x α from integration of pressure 0 distribution. Despite the fact there are a small number of points, it can be seen the Gurney flap 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 X/C 1 effect on CL. Clean 1% Gurney 2% Gurney 4% Gurney

3.5 Fig. 11 Pressure coefficient for alpha=8o and o 3 flap at 8 .

-11 2.5 -10 -9 -8 2 -7

L -6 C

P -5 1.5 C -4 -3 1 -2 -1 0 0.5 1

0 0.2 0.4 0.6 0.8 X/C 1 0 Clean 1% Gurney 2% Gurney 4% Gurney -5 0 5 alpha 10 15 20 Fig. 12 Pressure coefficient for alpha=16o and clean 1% Gurney 2% Gurney 4% Gurney flap at 8o.

Fig. 10 Lift coefficient from pressure -3 distribution integration, flap at incidence -2.5 zero. -2 The Figs 11 and 12 show the pressure -1.5 o o coefficient distribution for alpha 8 and 16 with SClean o -1 S 2% P S 1% S 4%

the flap at 8 of incidence. Similar effect of the C Gurney flaps still occurring, although with less -0.5 intensity at the flap. For alpha=16o the Gurney 0 flap effect delayed turbulent separation at the 0.5 flap as it can be seen in more details in Fig. 13. 1

A combination of two effects: the increase of 1.5 velocity through the slot and the high pressure 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 X/C 1 at the bottom rear end due the Gurney flap may Clean 1% Gurney 2% Gurney 4% Gurney explain the decrease of the adverse pressure gradient at the flap upper surface. Fig. 13 Pressure distribution at the flap for alpha=16o and flap at 8o.

5 F.M. CATALANO, G.L. BRANDT

3.5 -3 -2.5 3 -2 -1.5

P -1 2.5 C -0.5

2 0 L

C 0.5

1.5 1

0 0.2 0.4 0.6 0.8 X/C 1 1 GFmain2% flap crean Clean Fig. 15 Pressure distribution for alpha=8o, 0.5 flap at zero without Gurney flap. 2% Gurney flap at the main element trailing 0 edge. -5 0 5 alpha 10 15 20 -5 clean 1% Gurney 2% Gurney 4% Gurney -4 Fig. 14 Lift coefficient for flap at 8o. -3

P

The Gurney flap effect is less effective for the C -2 flap at high angles due to the turbulent separation at the flap is more intense as it can be -1 seen in Fig.14. Figs. 15 and 16 show the pressure distribution for the case with a 2% 0 Gurney flap at the main element trailing edge for the flap set at zero. Fig.14 shows that the 1 rise in pressure at the main element trailing edge 0 0.2 0.4 0.6 0.8 1 is in accordance with previous results [3]. X/C Gf main2% main clean 5% Gurney Clean However, the presence of the Gurney flap at the slot changed the gap between flap and main Fig 16 Pressure distribution for alpha=8o, element affecting the flap performance. These flap at 8o. 2% Gurney flap at the main results show that the gap and overlap should be element trailing edge. changed in order to establish the best performance for the flap. Fig 16 shows the gap . change due to the presence of the Gurney flap. Different positions of positioning the Gurney flap inside the main element and flap slot probably as suggested in [4] could bring better results, although still the need to optimize the gap and overlap.

Fig. 17 Gap change due to the Gurney flap. 6 Experimental and Numerical Study of a Two-Element Wing with Gurney Flap

Numerical Results: The numerical results are presented in Figs. 18 to 20 with respect to the 0 0.2 0.4 0.6 0.8 X/C1 comparison with experimental pressure -4 distribution. Computational results have failed to reproduce with accuracy the complex flow -3 exiting in this particularly wing. Although suction peaks are at same level, the numerical -2 p

calculation did not predicted well the large C laminar separation bubble (see Fig. 9) existing -1 at all incidence angle mainly due to the low Reynolds number of the experiment (1.3x106) 0 and the airfoil geometry which would induce laminar separation bubble. At the 1 flap there was another problem with the position Cp - Experimental Cp - Computacional of the suction peak and the separation point, probably due to the numerical solution of the Fig. 18 Comparison with numerical results, o o confluent boundary layer created by the main alpha = 8 flap at 0 , 1% Gurney flap. element wake. However, computation results were used to identify points of interest in the 0 0.2 0.4 0.6 0.8 1 flow around the wing such the Gurney flap area. -4 X/C Figs 21 to 24 show results by using the -3.5 computational flow visualization. The area of -3 interest shown in Fig. 21 is that at the flap -2.5 -2

trailing edge with the 4% Gurney flap. The p

-C 1.5 incidence angle is 8deg with the flap at zero -1 deg. Fig. 22 shows the two counter rotating -0.5 vortices and these results are in agreement with 0 previous works [4, 5 and 6]. It is clear from Fig 0.5 1 22 the different scale of the two vortices making Cp - Experimental Cp - Computacional a downward movement of the flow that is leaving the upper surface at the trailing edge. Fig. 19 Comparison with numerical results, This downward movement in conjunction with alpha = 8o flap at 0o, 2% Gurney flap. the existence of a vortex wake body displaces the stagnation point (or effective Kutta point) as 0 0.2 0.4 0.6 0.8 1 -4 X/C it can be seen in Fig. 23 thus increasing -3.5 effective camber and chord. The development of -3 the wake from a wing with the Gurney flaps can -2.5 also be visualized as shown in Fig 24. Wake -2 studies are an important issue in the application -1.5 of the Gurney flaps in aviation as they can -1 -0.5 create a stronger vortex shedding due to the von 0 Karman structure of the two counter rotating 0.5 vortices. 1 Cp Cp - Experimental Cp - Computacional Fig. 20 Comparison with numerical results, alpha = 8o flap at 0o, 4% Gurney flap.

7 F.M. CATALANO, G.L. BRANDT

Fig. 21 Definition of the area of investigation Fig. 24 Boundary layer and wake at the Gurney flap region. development, 4% Gurney flap installed

3 Conclusions An experimental and numerical analysis of the application of the Gurney flaps in a two element wing was performed. The quasi two- dimensional wing had a non-conventional configuration as the flap dimensions was of 45% of the total chord. It was used three Gurney flap heights 1%, 2% and 4% of the reference chord. The results showed that the Gurney flap positioned at the flap trailing edge can increase Fig. 22 Counter rotation vortices at Gurney lift for most of the incidence tested. The Gurney flap flap is less effective when large turbulent separation occur at the flap and main element. The use of Gurney flap at the trailing edge of the main element is highly dependent on the gap and overlap optimization. The numerical simulation using k-ε turbulence model which is a non-linear Reynolds Averaged Navier-Stokes model (RANS) did not predicted accurately the pressure distribution when compared with the experimental results, mainly due to the difficulties in simulating of the large laminar separation bubble at the main element. However, with more grid adjustment and refinement good results could be achieved. Numerical simulation of the flow at the region Fig. 23 Downstream and downward where the Gurney flap was installed was displacement of the Kutta point. qualitatively very important to explain the effect

8 Experimental and Numerical Study of a Two-Element Wing with Gurney Flap

of the Gurney flaps in the entire flow around the wing. The low cost and mechanical installation simplicity allied with its significant impact on aerodynamic performance make the Gurney flap a very attractive device to be used in subsonic aviation.

References [1] Liebeck, R. H. (1978), Design of Subsonic Airfoils for High Lift, Journal of Aircraft, Vol 15, No. 9, pp. 547-561. [2] Giguere P., Lemay, J., and Dumas, G., Gurney Flap Effect and Scalling for Low Speed Airfoils, AIAA Paper 95-1881, Jun.1995 [3] Myose R., Papadakis M. and Heron Ismael, A Parametric Study on the Effect of Gurney Flaps on Single and Multi-Element Airfoils, Three- Dimensional Wings, and Reflection Plane Model. AIAA 97-0034, 35th Aerospace Science Meeting & Exhibit 1997, Reno Nevada USA [4] Ross JC, Storms BL, Carrannanto PC. Lift-enhancing Tabs on Multi-element Airfoils. Journal of Aircraft 1995; Vol 32 No3 649-655. [5] Jang C. S., Ross J. C., Cummings R. M. Numerical investigation of an airfoil with a Gurney flap, Aircraft Design Vol1 (1998) 75-88 [6] Carrannanto P. G., Storms B. L., Ross J. C., Cummings R. M., Navier-Stokes Analysis of Lift- Enhancing Tabs on Multi-Element Airfoils, Aircraft Design Vol 1 (1998) 145-158 [7] Lemes, R.C., Catalano, F.M. Experimental Analysis of the Confluent Boundary Layer Between a Flap and a Main Element with Saw-Toothed Trailing Edge, 24th International Congress of the Aeronautical Sciences ICAS 2004, Yokohama Japan.

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