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Fig. 12 shows the condition when the rear end of the body is elevated by depressing the elevator flap T. This occasions an upward tail force that turns the wings down through the total angle (i'). With the former loading and speed, the angle of incidence is still (i) degrees with the new flight path f-f, the new flight path being at an angle (c) with the horizontal F-F. The body is turned through angle (i'), but the angle (i) with the flight path f-f is still constant with equal loads and speeds.

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Longitudinal Stability

Assume a machine as in Fig. 9 to be traveling steadily along a horizontal path in still air. A sudden horizontal gust now strikes the machine from the front, thus causing a sudden lift in the main wings. As this gust strikes the wings before the tail, the tail will stand at the old altitude while the wings are lifted, thus giving the position shown by Fig. 10. After passing over the wings it lifts the tail, this effect probably not being sufficient to restore the wing and the tail to their old relative attitude since the gust generally loses velocity after passing the wings. A head gust of this type often strikes the front wings diagonally so that it never reaches the tail at all. To remedy this upsetting action of the gust, the pilot must move his rear elevator so that the elevator is in the position shown by Fig. 12, that is, the flap must be turned down so as to raise the tail.