Basic Flight Mechanics by Ashish Tewari

Author:Ashish Tewari
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

Some airplane’s have a canard instead of a tail, which is a tail-like surface located forward of the wing. The elevator is then either a part of the canard, or the entire canard. There are some high-speed aircraft without a tail or a canard, e.g., the French Mirage fighters and the erstwhile Anglo–French Concorde supersonic airliner. In such a tail-less design, the pitching moment is provided by control surfaces called elevons located at the trailing edge of the wing. If the left and right elevons are deflected symmetrically, they cause a pitching moment to be generated by the wing. Of course, for a better effectiveness, the moment arm of the elevons should be reasonably large. This is ensured by a small aspect ratio and a high leading-edge sweep angle of the wings in a tail-less airplane. If the left and right elevons are deflected in opposite directions, they create a rolling moment about the center of mass like a pair of ailerons (to be discussed later).

In a level flight, the changes in the airspeed are produced by the thrust and the drag, while those in the altitude are generated by the lift. Since the drag (and thrust) are about one-tenth of the lift for the steady and level flight of a well-designed airplane, a variation in the airspeed takes a much longer time to produce than that in the altitude. Consequently, a pilot relies mainly on the pitch control which determines the angle-of-attack (thus the lift) in order to control the airspeed required for level flight at a given altitude.

The engine power controls the thrust, and is determined by the throttle input supplied by the pilot. If there are several engines, each of them must be producing exactly the same thrust in order that there is no lateral torque on the airplane. Otherwise, the airplane will depart from a steady and level flight. That is why each engine must be throttled separately. A four-engined airplane such as the Boeing-747 has four throttle levers to be adjusted every time the thrust has to be varied. This is not only cumbersome for the pilot, but also takes some time to implement, mainly due to the engine dynamics involved. A large jet engine, such as that of a Boeing-747, could take several seconds to produce the variation in the thrust demanded by the throttle input. Even if the thrust could be changed instantly, it would still take a long time for the airspeed to change to a desired new value. Therefore, controlling the airspeed is rarely performed by the throttle, and instead the pitch control is utilized. However, the vertical speed (or the rate of climb/ descent) is much more easily controlled by the throttle rather than the pitch control, because it is directly determined by the “balance of power” between thrust and drag. Thus, a quick change in the altitude (at nearly the same airspeed) is performed by making throttle adjustments.

While discussed here in the context of steady and level flight, the pitch and throttle inputs can be applied to control any flight situation.

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