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Slab, all-moving and all-flying tail surfaces
For subsonic aircraft, it is normal to have a fixed tail surface and movable elevators as seen in Fig. 10.5. Supersonic aircraft, however, are usually fitted with an all-moving or slab tail surface, where pitch control is obtained by changing the incidence (inclination relative to the fuselage) of the whole horizontal tail surface. A slab tailplane may be seen on the F-18 in Fig. 10.6. This arrangement is advantageous for high speed aircraft, because in supersonic flow, changes in camber do not significantly affect the lift. Deflection of a conventional hinged elevator does produce a change in lift, but this is mainly because it effectively alters the angle of attack, as shown in Fig. 10.7. As we described in Chapter 5, a supersonic flow can readily negotiate the sharp change in direction produced by the inclination of a slab surface, and the plane slab surface produces less drag in supersonic flow than a cambered one.
The slab tailplane has also become popular on light aircraft. This is partly because greater control forces can be produced by moving the whole surface,
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Fig. 10.6 An F-18, with slab tail, twin fins, and full-span control surfaces on the wing The engine exhaust nozzle in the minimum-area convergent configuration for subsonic flight |
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Fig. 10.7 In supersonic flow, deflection of a conventional camber-change elevator produces an increase in lift mainly because the angle of attack is effectively changed. The camber-change produces little effect. The plane slab surface produces less drag. In supersonic flow the air can negotiate the sudden change in direction (a) Camber-change control surface (b) Incidence-change slab control surface |
and partly because it enables the tailplane to be moved out of a stalled condition, if this should inadvertently happen in a violent manoeuvre.
Another variant is a tailplane which is provided both with elevators, and a means of changing the tail incidence. This feature is commonly used on large T-tailed aircraft. The variable incidence action is usually employed to trim or balance the aircraft for steady flight, with the elevator being used for control in manoeuvres. This mode of operation requires a separate control for the incidence-change mechanism and elevators. An alternative arrangement is to link the incidence and elevator mechanisms. The use of combined camber and incidence control, enables the surface to produce greater force than could be provided using either control separately. The maximum force that can be produced by a control surface is often limited by the onset of stalling of the surface. The improvement in control obtainable with such ‘all-flying’ tails has to be balanced against the increased complexity and weight.
