Elevator

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Movement caused by the use of elevators.

Elevators are flight control surfaces, usually at the rear of an aircraft, which control the aircraft's orientation by changing the pitch of the aircraft, and so also the angle of attack of the wing. In simplified terms, they make the aircraft nose-up or nose-down.[1] (Ascending and descending are more a function of the wing—aircraft typically land nose up.) An increased wing angle of attack will cause a greater lift to be produced by the profile of the wing, and a slowing of the aircraft speed. A decrease in angle of attack will produce an increase in speed. The elevators may be the only pitch control surface present (and are then called a slab elevator or stabilator), or may be hinged to a fixed or adjustable surface called a tailplane or horizontal stabilizer.

The rear wing to which elevators are attached have the opposite effect to a wing. They usually create a downward pressure which counters the unbalanced moment due to the airplane's center of gravity not being located exactly on the resulting centre of pressure, which in addition to the lift generated by the main wing includes the effects of drag and engine thrust. An elevator decreases or increases the downward force created by the rear wing. An increased downward force, produced by up elevator, forces the tail down and the nose up so the aircraft speed is reduced (i.e. the wing will operate at a higher angle of attack, which produces a greater lift coefficient, so that the required lift is produced by a lower speed). A decreased downward force at the tail, produced by down elevator, allows the tail to rise and the nose to lower. The resulting lower wing angle of attack provides a lower lift coefficient, so the craft must move faster (either by adding power or going into a descent) to produce the required lift. The setting of the elevator thus determines the airplane's trim speed - a given elevator position has only one speed at which the aircraft will maintain a constant (unaccelerated) condition.

In some aircraft pitch-control surfaces are in the front, ahead of the wing; this type of configuration is called a canard, the French word for duck. The Wright Brothers' early aircraft were of this type. The canard type is more efficient, since the forward surface usually is required to produce upward lift (instead of downward force as with the usual empennage) to balance the net pitching moment. The main wing is also less likely to stall, as the forward control surface is configured to stall before the wing, causing a pitch down and reducing the angle of attack of the wing.

References

  1. Elevator (Wordnet, Princeton University. Accessed 29-01-2009.)