Active high lift devices

In addition to the passive devices described above, the engines can be used to help maintain flow attachment. The upper surface boundary layer can be

sucked away by placing a suction slot on the upper surface as illustrated in Fig. 3.17(a), or near the trailing edge. This method has the added benefit that it helps to maintain a favourable pressure gradient, and hence, a thin laminar boundary layer, over a large proportion of the wing surface, thus reducing drag. A major problem with suction, however, is the tendency to ingest foreign objects, and to clog the intake slots or holes.

Surprisingly, a similar effect to that of suction can be achieved by blowing air into the boundary layer, as shown in Fig. 3.17(b). The high energy of the surface flow over the flap helps to prevent separation, and also, in effect, draws the air from upstream. The favourable pressure gradient that this produces induces a smoother thinner boundary layer over the forward portion. The high energy air may be bled from the compressor of a gas turbine engine.

Using the engine to blow gives far fewer problems than using suction. Blown flaps were used on the carrier-based Buccaneer shown in Fig. 3.7, in order to achieve the low landing speeds required for deck landing. They were also used on the F-104 (Fig. 8.8), which had extremely small wings, and required a high landing speed even with the blown flaps. In the event of an engine flame-out, and failure to relight (a fairly common occurrence), pilots were advised to abandon the aircraft rather than attempt to land.

Really dramatically high lift coefficients can be produced by means of an air jet at the trailing edge, as illustrated in Fig. 3.17(c). This jet flap works by entraining the upstream air. The flow is induced around in a strongly curved path. A small amount of downward thrust may also be produced by the jet directly, but this is a secondary effect. The Hunting H-126 experimental aircraft was flown successfully using the jet flap principle, and CL values as high as 7.5 were obtained (see Harris 1971). One problem with the jet flap, as with other really effective devices, is that strong pitching moments are produced.

These, and many other active high lift coefficient production methods have been demonstrated on experimental aircraft, but despite the investment of vast sums of money in their development over a period of more than fifty years they have so far found little favour in the mainstream of production aircraft. This is principally because of the weight, cost and complexity involved, but also because there are problems of control and stability associated with very low speed flight. The tail surfaces need to be large to provide sufficient force to keep the aircraft under control. These large surfaces produce a drag penalty at high speed, and can cause problems of stability, as discussed later. Notice the very large fin on the short take-off and landing (STOL) C-17 in Fig. 10.20. An excel­lent account of early research in boundary layer control is given by Lachmann (1961).

The simplest practical method of using engines to help the lift generation process is to place the wing in the wake of propellers or fans. Alternatively, the propellers or jet engine can be placed so as to either blow, or suck air over the wing. The problem with the latter method is that the propeller or fan is then placed in a highly non-uniform flow, and thus tends to run inefficiently, with an undesirable alternating load.

Using propeller wash is the preferred method for commercial STOL (Short Take-Off and Landing) aircraft at the time of writing. The DH Canada Dash-8 (Fig. 13.4) takes some advantage from propeller wash, and uses slotted extending rear flaps with leading-edge slats. This relatively conservative approach to high lift coefficient generation nevertheless gives the aircraft a remarkably short landing and take-off, while maintaining a simple design.