7- 1-1 Function of the Flaps and Control Surfaces

As has been explained in Sec. 7-1, the tail surfaces of an airplane serve a twofold purpose, namely, to stabilize and to control the airplane. In general, the tail surfaces consist of a fixed part, the stabilizer, termed a fin on the vertical tail and a (horizontal) tail plane on the horizontal tail, and a movable part, the control surface, termed an elevator on the horizontal tail and a rudder on the vertical tail. There is another set of control surfaces attached to the wing, termed ailerons; see Figs. 7-1 and 7-3. The tail surfaces, with the control surfaces fixed, serve to stabilize the airplane. The corresponding aerodynamic problems have been discussed in detail in Chap. 7. The airplane is controlled by deflection of the control surfaces. Control about the lateral axis is accomplished with the elevator, that about the vertical axis and the longitudinal axis with the rudder and the ailerons.

The geometry of the tail surfaces and of the ailerons is that of an airfoil with a flap (flap-wing) as shown in Fig. 8-1 (see also Fig. 2-24). The aerodynamic effect of the control surfaces consists of an additive lift produced by their deflection. This lift, acting on the tail surfaces or the wing, respectively, controls the airplane. The aerodynamic forces acting on the control surfaces generate a moment that, referred to the control-surface axis of rotation, is termed control-surface moment or hinge moment. The effect of the control surface should be strong enough to generate an additive lift that, for a given control-surface deflection, is as large as possible. At the

same time, however, the hinge moment should be as small as possible so that the forces needed for the operation of the control surfaces also remain small. A control surface in the form of a simple flap as shown in Fig. 8-1 has relatively large hinge moments. Efforts have therefore been made to reduce the moments required to move the control surfaces. This has been accomplished by means of so-called control-surface balances, as shown in Fig. 8-2. The most important types of aerodynamic control-surface balances are the inner balance (nose balance) as shown in Fig. 8-2д, the balance tab as shown in Fig. 8-2b, and the outer balance (horn balance) as shown in Fig. 8-2c. In all cases of control-surface balance, it is important that the lift increase caused by the control-surface deflection (control – surface effectiveness) should, if possible, not be reduced by the control-surface balancing.

The airfoil with control surface of Fig. 8-1 may serve two purposes: first, to control the airplane, and second, to be used as a landing device. In the latter case, its effect is to increase the maximum lift of the airplane, thus holding down the landing speed. This lift increase is usually accompanied by a drag increase. In Fig.

7- 3, several designs of such landing flaps are shown. In the arrangements of Fig.

8- 3a-e, the flaps are attached to the rear end of the wing, whereas in Fig. 8-3/ and g, flaps are shown in front of the wing (slat, nose flap). Some of these arrangements are also employed as take-off assistance to reduce take-off distance.

Finally, a few more forms of flaps may be mentioned, namely, the system of a

Figure 8-2 Various forms of aerodynamic control-surface balances, (a) Inner balance (nose balance). (b) Balance tab. (c) Outer balance (horn balance).

Figure 8-3 Several control-surfaces and flaps, (a) Cambered flap. (b) Slot flap, (c) Double-section wing. (d) Fowler flap, (e) Split (spreader) flap, if) Slat, (g) Nose flap.

brake flap (air brake) on the upper and lower sides of the wing (see Fig. 8-28). They have the shape of a rectangular plate and are set normal to the flight direction. It is the function of the air brakes in their extended position to increase strongly the drag of the airplane, thus reducing considerably the speed and generating a steeper glide angle (brake effect).