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Effect of altitude on short period pitching oscillation
The damping of the SPPO depends largely on the pitching moment produced by the tail surfaces as a result of the rate of change of pitch angle. Therefore it will be altered by changes in the tailplane effectiveness for a given amplitude of the motion. If the aircraft is flying at high altitude, the density is reduced. Thus, for a given aircraft attitude, and hence lift coefficient, the speed of flight will have to be higher to maintain the required lift from the wings. Thus a given rate of pitch will result in a smaller angle of attack as far as the tailplane is concerned, and its effectiveness will be reduced (Fig. 12.5).
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Fig. 12.5 Effect of altitude on damping As altitude increases a larger air speed is required. A given angular velocity results in a smaller inclination of the relative air stream and a smaller change in tail lift (a) Low speed (b) High speed |
This is not the only problem associated with altitude. Many aircraft in high altitude cruise will be flying transonically. This condition encourages movement of the shock system on the top surface of the wing in response to small changes in aircraft attitude. This leads to a further deterioration in stability.
