Clark Y-14 Wing Performance: Deployment of High-lift Devices (Flaps and Slats)

An airplane's speed is relatively low during takeoff and landing. To generate sufficient lift, it is necessary to increase the wing area and/or change the airfoil shape on the leading and trailing edges of the wing. To do this, slats are used on the leading edge, and flaps are used on the trailing edge. The flaps and slats can move into or out of the wings. Deploying the flaps and the slats has two effects; it increases the wing area and the effective camber of the airfoil, which increases the lift. In addition, the deployment of flaps and slats also increases the dragof the aircraft. Figure 2 shows cruise, takeoff and landing configurations of a wing with a flap and a slat.

Figure 2. Various wing flap and slat configurations.

During flight, the wing of an airplane is continually subjected to a resultant aerodynamic force and moment, as shown in Figure 3(a). The resultant force, R, can be decomposed into two components. Typically, one component is along the direction of the far-stream velocity, V_∞, which is called drag, D, and the other component is perpendicular to the direction, which is called lift, L.

The moment, M, moves the nose of the airplane up or down, thus, it is called the pitching moment. In wind tunnel testing, the normal and axial forces are typically measured directly. The normal, N, and axial forces, A, are related to lift and drag through the angle of attack, α, as shown in Figure 3(b). The angle of attack, is defined as the angle between the far-stream velocity direction and the chord of the wing airfoil.

Figure 3(a). Resultant aerodynamic force and moment.

Figure 3(b). The decomposition of the resultant force, R.

The two force pairs can also be expressed as follows:

where α is the angle of attack.

The non-dimensional lift coefficient, C_L, for a wing is defined as:

where L is the lift, is the dynamic pressure based on the free-stream density, ρ_∞, and airspeed, V_∞, and S is the reference area of the wing.

Similarly, the non-dimensional drag coefficient for a wing is defined as:

The resultant aerodynamic force from lift and drag is located at a point on the wing (or airfoil) called the center of pressure. However, the location of the center of pressure is not a fixed location, rather, it moves based on the angle of attack. Therefore, it is convenient to move all forces and moments to approximately the quarter chord point (a distance 1/4 of the chord length from the leading edge). This is called the pitching moment about quarter chord, M_c/4.

Figure 4. Pitching moment about quarter chord.

The pitching moment coefficient, C_M,c/4, about quarter chord is defined as:

where M_c/4 is the pitching moment about quarter chord, and c is the chord length of the wing.

Wing performance relies on the Reynolds number, Re, which is defined as:

where the parameter μ is the dynamic viscosity of the fluid.

In this demonstration, the performance of a Clark Y-14 wing with a simple flap and a simple slat is evaluated in a wind tunnel, as shown in Figure 4. The wing is installed to a device called a sting balance, which is shown in Figure 5 and measures the normal force, N, and the axial force, A.

Figure 5. Clark Y-14 wing with a flap and a slat.

The results of the clean wing configuration are shown in Table 1. Figures 6 - 8 show all three coefficients vs angle of attack, α, for all four configurations. From Figure 6, both the flap and slat enhanced the lift coefficient, but in different ways. Comparing the clean wing and the slat lift curve, the two curves are almost overlapping at low angles of attack. The clean wing lift curve peaks to about 0.9 at 12°, but the slat curve continues to rise to 1. 4 at 18°. This indicates that slats can be used to increase lift. When comparing the clean wing and flap lift curves, the flap increases lift over the entire angle of attack range. And if both the slat and flap are deployed at the same time, the effect is cumulative and the maximum lift is even higher.

In comparing the drag coefficient for each configuration in Figure 7, the drag coefficient increases dramatically when both the flap and slat are deployed. Finally, as shown in Figure 8, the pitching moment coefficient goes into the negative regime when the flap is deployed. This means that the center of pressure shifts towards the trailing edge with the flap deployed.

Table 1. Experimental results for the clean wing configuration.

Angle of attack (°)	Lift coefficient, CL	Drag coefficient, CD	Pitching moment coefficient, CM,c/4
-8	-0.022	0.015	-0.129
-6	-0.029	0.014	-0.059
-4	0.096	0.016	-0.059
-2	0.208	0.011	-0.054
0	0.353	0.006	-0.065
2	0.460	0.004	-0.053
4	0.548	0.032	-0.051
6	0.708	0.015	-0.062
8	0.789	0.025	-0.061
10	0.849	0.031	-0.061
12	0.873	0.045	-0.056
14	0.856	0.058	-0.089
16	0.803	0.080	-0.125
18	0.803	0.092	-0.128

Figure 6. Lift coefficient vs angle of attack, α.

Figure 7. Drag coefficient vs angle of attack, α.

Figure 8. Pitching moment coefficient vs angle of attack, α.

Table 2. Parameters used for calculations.

Parameters	Values
Air density, ρ	0.00230 slug/ft3
Water density, ρL	1.935 slug/ft3
Gravitational acceleration, g	32.17 ft/s2
Viscosity, m	3.79 x 10-7 lbf s/ft2
Free-stream airspeed, V∞	60 mph
Reynolds number, Re	1.56 x 105
Chord length, c	3.5 in
Wing area, S	35 in2

Lift generation can be enhanced by the deployment of high-lift devices, such as flaps and slats. Most airplanes are equipped with flaps, and all commercial transport airplanes have both flaps and slats. It is critical to characterize the performance of a wing with flaps and slats during aircraft development.

In this demonstration, a Clark Y-14 wing with a flap and a slat was evaluated in a wind tunnel. The forces and moment measurements were collected to determine the lift, drag and pitching moment coefficients of the wing with and without flap and slat deployment. The results demonstrate that the lift coefficient increases when the flap and slat are deployed. However, this also resulted in a dramatic increase in the drag and pitching moment.