Chicago, March 16, 1905
I have yours of 11th inst. The Zahm experiments covered all resistances (head, drift, skin friction) of the soaring buzzard, the motive power being furnished by the ascending trend of wind and hence measured by Rv. Your formula wv/ac does not apply as it takes no account of the head resistance, which varies in birds.
I do not find my measurements of the flight of the sparrow, but my present recollection is that his speed is 25 miles an hour, and he goes about 10 feet before he acquires it.
The most suitable ground in Illinois for practice I believe to be the large flat top of the sand hills at Dune Park. They are easily accessible to one who knows the secret path in the swamp, and are quite desert; the only objection I can see is that the fine sand might cut the machinery. If circumstances are such that it is not advisable for you to resume experiments near home, I would suggest that you come to Chicago and go down with me to look the ground over. Give me a few days' notice.
I believe flapping wings to be more efficient than screws, because they utilize the currents produced by the upstroke.
Wilbur Wright to Octave Chanute,
March 26, 1905