Fig. 6., Fig. 7., and Fig. 8.
In the same way, the machine having no lifting tail is longitudinally unstable, for, being balanced on its centre of pressure which would be coincident with its centre of gravity and probably about 2 feet from the trailing edge of the plane—it may assume any position (Figs. 9, 10, 11 and 12), and still be in equilibrium, when it is evident that the proper position (Fig. 9) is only maintained by the constant control of the tail elevator.
Fig. 9., Fig. 10., Fig. 11., Fig. 12., and Fig. 13.
Now take the case of a machine having a low centre of gravity. Its natural position is shown at Fig. 13, and it is at once evident that any other position such as Figs. 14 and 15 could not be maintained for a moment, since the weight being at an angle, must inevitably drag the machine back to its natural position (Fig. 13). In the same way with regard to longitudinal balance, a machine with two lifting surfaces such as Fig. 13, is in its natural position with the centre of gravity perpendicularly under the centre of pressure, any other position, such as Fig. 17, A, is impossible, as the gravity pull must drag the machine along the dotted line till it resumes its proper and natural position (B).
Fig. 14., Fig. 15., and Fig. 16.
The next difficulty is in the banking or tilting caused by the turning of the machine in going round a curve. In a very interesting discussion carried on in the “Aero,” it was stated that a low centre of gravity machine could not bank up, as the pull of gravity acting on the low weight would prevent it. It was also stated by another writer that the machine would bank up too much and slide down sideways, because the greatest weight having the greatest momentum would swing out too much. There is evidently some confusion here. Let us consider the question.
In turning there are three forces to take into consideration:
(1) The centrifugal force, which tends to make the machine fly off at a tangent to the curve at which it is turning.