It has been long known, principally through experiments on soaring, that a large, if not by far the largest, part of the supporting force obtained by birds in regular flight, is probably furnished by upward air pressure on their wings, regarded as planes moving horizontally, with their surfaces slightly inclined to the direction of motion.

Langley's “Experiments on Aero–dynamics” furnish some numerical data for estimating the power required to sustain an aeroplane of given weight, propelled horizontally by a known force, and he applies these to the determination of the problem whether this could be effected by screw propellers, analogous to those of a ship, actuated by machinery of existing type.

In the summer of 1898 I constructed a full sized gliding machine with which I made many short flights, ranging from fifty to one hundred feet. This machine was constructed on the plans of the Herring soarer used by Mr. Octave Chanute in his experiments on the borders of Lake Michigan during the summer of 1896, and, although I was not able to make so long a flight as Mr. Herring did, I acquired some knowledge of the management of the apparatus and the difficulties to be overcome. I used the Lilienthal form of curve, which is the arc of a circle, and the height of the curve is 1/12 the length of the chord.

My experiments showed not only that the problem of stability is a very great one, but that it ia very necessary to find some more efficient form of curve than the circle, in order to reduce the power necessary to drive the machine through the air at a sustaining speed. I, therefore, determined to experiment with various surfaces, until I found one offering much less resistance than the one I was using.