The Aeroplane Speaks. Fifth Edition

Now, we will suppose that a gust or eddy throws the machine into the lower position. It no longer travels in the direction of T, since the momentum in the old direction pulls it off that course. M is now the resultant of the Thrust and the Momentum, and you will note that this results in a decrease in the angle our old friend the neutral lift line makes with M, i.e., a decrease in the angle of incidence and therefore a decrease in lift. We will suppose that this decrease is 2°. Such decrease applies to both main surface and stabilizer, since both are fixed rigidly to the aeroplane. The main surface, which had 12° angle, has now only 10°, i.e., a loss of one-sixth. The stabilizer, which had 4° angle, has now only 2°, i.e., a loss of one-half. The latter has therefore lost a greater proportion of its angle of incidence, and consequently its lift, than has the main surface. It must then fall relative to the main surface. The tail falling, the aeroplane then assumes its first position, though at a slightly less altitude. Should a gust throw the nose of the aeroplane up, then the reverse happens. Both main surface and stabilizer increase their angles of incidence in the same amount, but the angle, and therefore the lift, of the stabilizer increases in greater proportion than does the angle and lift of the main surface, with the result that it lifts the tail. The aeroplane then assumes its first position, though at a slightly greater altitude.