Dayton, March 11, 1905
Your letter of March 6th has been received. Regarding the computation of the power consumed by the buzzard, as measured by Prof. Zahm, I do not understand from your letter that anything more than head resistance and skin friction is included in your estimate. If so, of course the total power consumed must be greater.
The power consumed by any bird or flying machine may be figured from the formula wv/ac, in which w = weight, v = velocity, 1/a = ratio of drift to lift, and 1/c = efficiency of the screws or wings as propellers. In the case of the crow flying at 34 ft. per second, or 2,100 ft. per minute, I would fix the value of l/a at 1/8, and 1/c at 1/.75; when we have (1 x 2100)/(8 x .75) = 350 ft. lbs. per pound of weight.1 The minimum value of l/a may be rendered independent of velocity by regulating the size of the wings. The value of 1/c is about the practical limit of the efficiency of screws under usual conditions, and I see no reason for believing that wings are more efficient than screws, as propellers. It is quite incredible that, when flapping, the wing can be kept at the optimum angle at every point, as in soaring; and there are losses due to the fact that the pressure is not vertical throughout the stroke. Although I think 25 percent a fair estimate of the probable loss from both sources.
Birds unquestionably develop power many times greater than is consumed by our Flyer, per pound weight. if you will fix in your mind the distance within which a small bird acquires full speed, say 30 miles an hour, and then figure the power necessary to accelerate its weight to this velocity, I think you will be astonished. I know I was almost dumbfounded, especially in view of the fact that the power available for acceleration is over and above that used in flying. I shall be curious to know what distance you fix upon as that within which a sparrow acquires full speed.
The German patent office has now held up our application on the ground that the principle of twisting the wings is disclosed already in Moedebeck, page 331, where it is stated that the wings are distorted or twisted to steer to right or left. We had been congratulating ourselves that this had been overlooked by them. We fear that it may interfere with our being granted a broad claim on twisting the wings. It will certainly require careful handling to get around the objection and, as we have the cited passage only in German, it would be useful to have the original English of these few sentences, beginning with "at a negative angle of 7 degrees" and ending with "Fig. 105 shows this apparatus in the air." The intervening sentences are all that we need.
Are you acquainted with any prairie land in Illinois which you would consider suitable for a practice ground? Something neither too accessible nor inaccessible?
1 Chanute notes: "94.3 lbs. sust[taine]d per H.P."
Octave Chanute to Wilbur Wright, March 16, 1905