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30  AVIATION   May,1911

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[[image - photo of Mr. H. La. V. Twining]]

SOARING FLIGHT
By H. La V. TWINING
Head of Physics and Electrical Engineering
Los Angeles Polytechnic High School
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So far we have been dealing with flat planes. Lilienthal found that curved surfaces were more efficient than planes, and by experiment, he determined the relation between the lift and drift for planes and curves.

[[image- diagram of straight lines and curved surfaces]]
Fig. 5

In Fig. 5, let AGB be its chord. Let CF be a horizontal line, and let AB be parallel to CF. Then AB is said to present a neutral angle of incidence to a horizontal wind. If the curved surface AGB occupies the position CGD, then the angle between the chord CD and the horizontal line CF is the angle of incidence of the plane to a horizontal wind. This angle, DCF, is called a positive angle. If the curbed surface occupies the position EGF, then EFC is the angle of incidence, and it is called a negative angle. If a perpendicular be drawn to the chord at its deepest part of the curve, such as K in the surface AGB, then K divided by the chord AB is called the camber of the surface.

Upon examination, the wing of a bird is found to be attached by its leading edge, forward and above its center of gravity. This wing is kept extended and maintained against the force of the wind by the muscular exertion of the bird. As the wind strikes the under surface of a curved wing, the first effects are similar to those produced upon an aeroplane. A lift and a drift is produced and the bird ought to be driven to the rear, but, owing to its inertia, it stands for an instant against the impact of the wind. Owing to the structure of the bird machine, as mentioned above, the following results take place:

[[image - drawing of cross section of a wing]]
Fig. 6

In Fig. 6, Let AB represent a cross section of the wing, and W a weight, representing the center of gravity of the body of the bird. Let this center of gravity be attached to the wing at B, the leading edge of the wing, by a bent wire G. Let P be the center of pressure, due to the action of the air upon AB. If a model be build with a cross section like the figure, and W be adjusted along OG, certain things can be clearly demonstrated. When W is forward of the center of pressure P, and the model is dropped from a height, it will pitch head foremost and strike the ground. If W is adjusted so that it is back of the center of pressure
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