# Does a 1lb crow expend only 7.68 Watts to fly at 37 km/h?

The Wright brothers and the 1 pound crow that flies at 37 km/h expending just 7.68 W

In an 1905 letter to Octave Chanute, Wilbur Wright evaluated the power, used by a 1 lb bird to fly at 34 ft/s, utilizing a relation that can be rewritten as:

eff x Power = Thrust X Speed = Drag x Speed,

where: eff (the efficiency of the crow) = 75%, Drag/Lift = Drag/(mg) = 1/8 and mg = 1 lbf.

In consequence, Power = (1 lbf x (1/8) x 34 ft/s ) / 75% = 7.68 W.

"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.",
Letter of Wilbur Wright to Octave Chanute, Dayton, March 11, 1905

7.68 W/lb seems a quite low figure as long as, it's generally accepted that any model plane with less than 50W/lb is going to struggle (see The Watts Per Pound rule)

Does a crow only expend 7.68 W/lb or at least it is possible to build a model plane able to fly at 37.3 km/h (34 ft/s) developing just 7.68 W/lb shaft power?

• A crow doesn't have a shaft to power. A prop is not very efficient either. Oct 12, 2015 at 13:17
• ratchet freak, read carefully the text I quoted because W. Wright gives there some explanations that will clarify your remarks. Oct 12, 2015 at 13:31
• Simple answer: we suck compared to nature. Although in our defence, nature has had more practice. Oct 13, 2015 at 23:59
• Nature never invented a wheel though. Sep 12, 2017 at 3:31
• I came here expecting an eventual comparison of the crow to a European swallow carrying a coconut. Learned some historic trivia instead. Sep 12, 2017 at 9:48