Fig. 28—Action of aeroplane.
Action of an Aeroplane. Before starting to glide it is perhaps well to understand how the machine operates and supports its passenger. The illustration shows the cross section of an aeroplane moving forward through the air in the direction indicated by the arrow. The front edge of the aeroplane is elevated so that the surfaces form an angle with the horizontal. The front edge enters practically still air and causes it to follow the curve of the planes and leave at the rear in a downward direction. Since the action and reaction of two forces are always equal and opposite, there is a force exerted against the aeroplane causing it to rise.
A sky-rocket is caused to ascend by the reaction of gases formed by burning powder escaping downwards through a small hole. The aeroplane, by means of its curvature directs the air downwards and so rises itself.
The planes pass so rapidly on to new and undisturbed bodies of air, and stay over one body for so brief an instant, that there is no time to completely overcome the inertia of the air and force it downwards. This may be likened to a skater moving swiftly over very thin ice which would not bear his weight were he standing still, but since he is moving so rapidly, that any one portion of the ice does not have time to bend to the breaking point, is supported.
Equilibrium. A glider will remain in perfect equilibrium only so long as the centre of gravity of the machine and operator fall in the same vertical line as the pressure exerted by the air. If the former is forward of the latter, the machine will incline forward and travel downwards. If the centre of gravity is to the rear of the centre of upward thrust exerted by the air, the head of the machine will rise, while if it is to either the right or left side, the machine will lean or turn over respectively to the right or left.
Fig. 29—Ready to Start
The centre of pressure on the plane is somewhat in advance of the actual dimensional centre of the plane. This is due to the curvature of the plane and also to the disturbing action upon the air of the front edge.
To make a glide, carry the machine to the top of a slope. Have two assistants hold the ends of the lower plane. Get in underneath and stand up between the arm pieces. Grasp the front horizontal beam of the lower plane and lift the machine until the arm sticks are snugly under the arm pits as in the illustration.
If necessary have the two assistants prepared to run a short distance with the machine, but as soon as you are in motion you will be relieved of all weight and surprised at the lift exerted.