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Unformatted text preview: AA 311 Lecture 21: Longitudinal Control We next turn our attention to control of longitudinal motion, particularly control of the pitch attitude . For wings-level, equilibrium flight at constant altitude, is equal to the pitch angle . Thus, we are interested in controlling , and hence the lift generated by the aircraft. Figure 1: Standards for stick and control surface deflections . For conventional aircraft, there are three primary control surfaces which provide moments about the three coordinate axes. Figure 1 depicts the elevator, rudder, and ailerons, as well as a pair of canards, which are somewhat less common. The figure also depicts the sign conventions for the various stick, pedal, and actuator deflections. Aerodynamic control surfaces are generally small lifting surfaces which generate small local forces. Each of these forces acts through a large moment arm to generate a control moment about the aircraft center of gravity. Table 1 shows the effect of positive control surface deflections on the aerodynamic moments. For example, a positive elevator deflection (as indicated in Figure 1) results in a negative moment about the lateral axis: M e < 0, as we shall verify shortly. Because these control surfaces are simply lifting surfaces, everything we know about wings can be applied to our study of actuator effects. Table 1: Effect of control surface deflections on the aerodynamic moments. Actuator Angle to be Symbol for Moment Sensitivity Name Controlled Actuator Deflection to Actuator Deflection Aileron a L a > Elevator (or ) e M e < Rudder (or ) r N r < We previously assumed that the lift of the aircraft was a function only of its angle of attack . Now, we recognize that deflections e of the elevator will also affect the lift. We continue to consider static (equilibrium) flight conditions, however, so we ignore other dependencies. Assuming the control deflections are small ( e 1 radian), we may write (for the entire aircraft) C L = C L + C L e e. 1 The coefficient C L e = C L e is typically positive, but small relative to C L . Thus, the effect of positive (downward) elevator deflections is a small increase in the value of C L ....
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- Fall '09