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May, 1911   AVIATION   31

pressure, then the model will fall backwards and land on its tail, when dropped. If W is arranged so that P and W are in the same vertical line, then the model will settle on an even keep to the ground. If the model is thrust forward in the air, it will be found that W will have to be adjusted until P and W are in a vertical line, to secure equilibrium. We have here a lever of the first class, where BG is the fulcrum, with GW as the weight lever arm and PB as the power lever arm. When we have equilibrium, the power applied at P multiplied by PB is equal to the weight W multiplied at GW; i.e.,
W x WG = P x PB

In the case of the soaring bird the center of pressure P is to the rear of the center of gravity W. Hence, P x PB is greater than W x GW, and the tendency of the pressure at P is to pitch the bird forward. Hence, as the air strikes the wing the body rotates around the leading edge G, as a fulcrum, and the wing assumes a negative angle. Owing to inertial of the machine, it does not instantly drift, but it rotates and assumes a negative angle, the force of which is resolved as in Figure 7.

[[image - drawing of a curved wing]]
Fig. 7

DO, the normal component of the wind's pressure, is resolved into a lift OE, which is equal to and opposite the weight W. and a drift, OL. Now, the drift OL, instead of being directed to the rear, is directed forward, owing to the rotation of the wing to a negative angle. This rotation also brings the center of gravity W and the center of pressure O into the same vertical line. The rearward drift is thus converted into forward drift or advance, and the gird is thrust forward by the wind coming from the front. When in this position, the wind now strikes on the top of the wing, and, on account of the inertial due to the momentum forward, the center of gravity rotates downward around the leading edge of the wing, and again assumes a positive angle, and the action of the normal component of the

[[image - drawing of a curved wing]]
Fig. 8

wind's pressure is as in Fig. 8. Here DO, the normal component of the pressure of the wind on top of the wing is resolved into the vertical component OE, and the forward drift or advance OL. The bird is thus alternately rotated upward and downward around the leading edge of the wing, and in each case it is thrust forward by the component OL. The position of the wing in each case is greatly exaggerated in order to clearly show the principle. It is very difficult to detect this oscillation during soaring flight, but it can be very easily seen during beating flight. The above principle of action exists during beating flight, and it is in this way that the wing acts as a propeller and drives the bird forward. A second reason why rearward drift does not develop, lies in the fact that the line of least resistance is forward in the plane of the wing instead of rearward. In Fg. 9, the state of affairs with regard to the air currents existing around a wing is represented. Owing to the shape of the wing, the front or leading edge divides the air, part being thrust upward and part downward. This gives
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