Wilbur Wright to Octave Chanute

Dayton, September 26, 1901

Your letter 25th rec'd. We should be glad when convenient to you to have an opportunity to copy Mr. Huffaker's notes. Our own notes were quite brief in most cases and in some were not recorded at all. We only put down what we considered of real importance.

The kite test of Aug. 9th, to which you called my attention in Chicago, I find is not in my notebook. On examination I find that it is among the memoranda you wrote out for me at Kitty Hawk. It was as follows: wind 10 meters, angle 10 1/2°, pull 18-20 lbs., weight 108 lbs. Except as to wind velocity this agrees quite closely with our recorded test of July 30: wind 7.5 meters, angle 10°, pull 18 lbs., weight 98 lbs. Why the higher velocity of the test on the hilltop did not produce a better angle I do not know. All our recorded tests were made on the level plain.

I note what you say about the speed, angle, and ratio of wing surface to weight of the Lilienthal machine as reported in the Revue [de l']Aéronautique. I find that Bretonnière reports an observation of a gliding stork which in still air made a descent of 10° at a speed of 22 miles per hour, which is about equivalent to the Lilienthal performance. (I find on looking up the weight of the stork that it spreads .71 lbs. per sq. ft. as against 1.46 in the Lilienthal machine, so that the case is not as similar as I supposed when I wrote the preceding sentence.) I find that Kress, Wellner, and Herring, however, all confirm a support of about 1.4 lbs. per sq. ft. at a speed in calm air of 20-22 miles. I confess that this is a puzzle to me, especially in view of the fact that Prof. Langley and also the Weather Bureau officials found that the correct coefficient of pressure was only about .0032 instead of Smeaton's .005, so that the support reported was nearly or quite equal to the maximum normal pressure at 90°.

I am arranging to make a positive test of the correctness of the Lilienthal coefficients at from 4°-7° in the following manner. I will mount a Lilienthal curve of 1 sq. ft. and a flat plane of .66 sq. ft. on a bicycle wheel in the position shown. The view is from above. The distance from the centers of pressure to center of wheel will be the same for both curve and plane. According to Lilienthal tables the 1 sq. ft. curve at 5° will just about balance the .66 sq. ft. plane at 90°. If I find that it really does so no question will remain in my mind that these tables are correct. If the curve fails to balance the plane I will cut down the size of the plane till they do balance. I hope to make the test on the first suitable day. If you have any suggestion to make regarding it, or any error in the principle employed to point out, I should be very glad to have it.

As soon as I have made this test I will revise my manuscript and forward it at once.

[P.S.] I wish to call your attention to the following points in connection with the Lilienthal glide above-mentioned. (1) It is stated that the surface was horizontal; therefore the only propelling force was the tangential, which at 9° is .042 of the normal; which in this case is identical with lift and amounts to 220 lbs. But .042 × 220 = 9.24 lbs. to overcome the head resistance of framing and man's body. This under the circumstances is a lower estimate than any estimate of 10+ lbs. for the resistance of framing and operator in our 1901 machine. (2) If at 20 miles per hour he made a glide at a descent of 9°, with an angle of incidence of 9°, at 22+ miles his angle of incidence would have been reduced to four degrees and his angle of descent to a little over 5°. Did he find in actual practice that so slight an increase of speed almost doubled the length of his glides?

Reply from Chanute to Wilbur, September 29, 1901