Leading Edge Devices

In order to avoid leading edge separation, particularly at low Reynolds numbers or for airfoils with relatively sharp leading edges, special high-lift devices can also be incorporated into the leading edge to supplement the benefits of trailing edge flaps. These are illustrated in Figure 3.35. The fixed slot and extensible slat have been in use for some time, whereas the Kruger-type nose flap was first employed on the turbojet transport.

As the name implies, the fixed slot is just that—a narrow channel through which the air can flow from the lower surface to the upper surface. This channeling of the flow allows the airfoil to operate at higher angles of attack before the upper surface of the leading edge separates than otherwise would be the case. Increments in C(inax of approximately 0.1 or 0.2 are achieved by the fixed slot. It is a moot question as to why this delay in the separation occurs. As in the case of slots with trailing edge flaps, the explanation has been offered in the past that the flow through the slot feeds energy into the slower moving boundary layer, thereby decreasing its tendency to separate. More recently, however, in a Wright Brothers’ Lecture (Ref. 3.16) Smith, in examining numerical results on multielement airfoils, concluded that im­proved stall performance from slots is most likely the result of more favor­able pressure gradients being produced on one airfoil element by the other.

The extensible slat is similar in its performance to the slot, but it is

Figure 3.35 Various methods for delaying leading edge separation.

considerably more efficient because it can be positioned to optimize its contribution to C|max. The mechanically extended slat is finding increased application, particularly with the use of thinner airfoil sections for high-speed applications. Figure 3.36 presents some data on slats taken from Reference

3.17. Here a NACA 64A010 airfoil was tested using a slat in combination with split and double-slotted trailing edge flaps. The slat is seen to improve C(max significantly, producing increments in C;max of approximately 0.9, 0.8, and 0.6 for the no-flap, split-flap, and double-slotted flap configurations, respectively. Unlike the trailing edge flap, the primary effect of the slat is seen to be an extension of the lift curve without the slat; that is, opening the slat does not change Ci by a large increment at a fixed angle of attack. The same is true of leading edge flaps and is not unexpected in view of Figure 3.32.

The performance of a leading edge flap is presented in Figure 3.37 for the same airfoil as for Figure 3.36. Comparing the two figures, it is obvious that the two leading edge devices are nearly comparable in performance.

Figure 3.38 shows a section of a sophisticated Kruger-type flap. As this flap swings down and forward, it assumes the curved shape that is shown. With this optimum shaping, its performance probably exceeds to some extent

Seotion lift coefficient,

Figure 3.37 Effect of leading edge flap on NACA 64A010 airfoil with and without flaps.

Figure 3.38 Flexible fiberglass leading edge flap used on the Boeing 747 and YC-14 airplanes.

the data presented in Figures 3.39 and 3.40. Figure 3.39 (taken from Ref. 3.18) shows that without a trailing edge flap the Kruger flap gives an increment in Cw to the 64-012 airfoil of only 0.4. However, the plain airfoil has a higher Cw to begin with than that of Figures 3.36 and 3.37. Hence, the total C(max for the Kruger-flapped airfoil without a trailing edge flap is about the same as for the other two leading edge devices. However, with the split flap, the Kruger flap produces a combined Clmax equal to 3.0, which is 0.3 to 0.4 higher than the corresponding data of Figures 3.36 and 3.37.

The data of Figure 3.40 (taken from Ref. 3.21) are based on Kruger’s original work.