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Unconventional control surfaces
The use of multiple roll-control surfaces is advantageous, partly for reasons of safety, but also because conventional outboard ailerons may become just too effective at high speed, and can induce unacceptable wing bending and twisting moments. On many aircraft, including large airliners, a set of high speed ailerons may be fitted inboard of the usual low speed ones, as seen in Fig. 10.10. This reduces the amount of span available for installing flaps, however, and one method of overcoming this problem, is to arrange at least one of these sets of control surfaces as so-called flaperons, where differential movement has the same effect as ailerons, and collective movement produces the effect of flaps. Flaperons are used on the F-16.
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Fig. 10.15 Trailing-edge elevons on the delta-winged Concorde, shown drooped with power off |
On delta-winged aircraft, trailing-edge elevons are fitted, as on Concorde (Fig. 10.15). Elevons are trailing-edge control surfaces which act as ailerons when operated differentially, and as elevators when operated collectively (i. e. both moving in the same direction).
One problem with delta-winged aircraft is that trailing-edge control surfaces cannot be used as flaps, without simultaneously behaving like elevators; producing a nose-down pitching moment, which has to be counteracted in some way. This is another reason why a canard foreplane is desirable on delta-winged aircraft. A combination of leading-edge flaps and elevons may also be used.
A final variant is the taileron used on the Tornado aircraft shown in Fig. 3.14. The slab tail surfaces can be operated differentially as ailerons, or collectively as elevators. Tailerons have a number of potential advantages. Like inboard high speed ailerons, they produce a smaller rolling moment than outboard wing mounted ailerons. They reduce the bending stresses on the wing, and allow more room on the wing for flaps. Notice the full-span flaps on the Tornado in Fig. 3.14.
When several sets of roll control surfaces are installed on one aircraft, the task of sorting out which surface to use in any particular condition is generally too much for the pilot to cope with, and the selection is normally made automatically. In most cases, the pilot has some selection override capability. Davies (1971) gives a good account of roll control surface operation on typical airliners.
