Octave Chanute to Wilbur Wright

Chicago, February 16, 1905

I have been quite sick for over a month and confined to the house by the grippe.

You asked me in your last letter whether I had definite information regarding the tip to tip measurements of a crow, and this is the first moment that I have had energy enough to look it up.

The best information I have is that given by Mouillard in his measurements of birds:

Wt. In Surface Spread Width Propor-
lbs. sq. ft. of wings of wings tion

Egyptian rook 0.869 1.533 2. 75 ft. 0. 59 ft. 4.66 to 1
" crow (corax) 1.353 2.198 3. 52 " 0.656 5.37 " 1

Cayley mentions the "rook" and the "crow" indiscriminately and he estimates it to weigh 1 lb. per square foot of surface, which probably obtains after the bird has had breakfast.

In this connection I may mention a discrepancy on page 25, Aero. Ann., 1895, in which Cayley's model is said to have descended a hill at an angle of 18º, while in the reprint of the same paper in Report XI, Aeronautical Soc., Gt. Britain, page 69, it is given as 8º.

I undertook in my book, page 39 and following, to compile the estimates heretofore made of the power exerted in flight by birds. A compendium follows:

W. Smyth 160 ft. lbs. per min. per lb. = 206 lbs. sup[porte]d per H.P.
Alexander 270 " " " " =122 " " " "
Tatin 33 " " " " " =1,000 " " "
" 197 " " " " =167 " " "
Richet 105 " " " " =314 " " "

My own observations that the buzzard generally descended 1 in 10 while gliding at 16 miles an hour, or 23.47 ft. per second, led me to assume that the resistance was 1/10 of the weight, so that for one pound we would have (23 47 x 60) /10 = 140.8 ft. lbs. pr min.

Hence: 140.8 ft. lbs. pr min. per lb. 234.4 lbs. supported per H.P. But if there be an ascending trend of wind it is possible to do still better, for then the bird can assume a negative angle to the horizon and be supported.

Prof. Zahm measured the resistance of my buzzard weighing 4.25 lbs. and set at a negative angle of 3 in a current of wind (artificial in tunnel) at speeds of 6 to 15 ft. per second. He deduced that the resistance varied as the 1.81 power of the speed, and that at 15.52 miles per hour, or 22.76 ft. per second, it was 0.27 lbs. Whence power (0.27 x 22.76 x 60) / 4.25 = 86.88 ft. lbs. per min. pr lb. Hence: 86.88 ft. lbs. per min. pr lb. 380 lbs. sup[porte]d per H.P.