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THE POPULAR SCIENCE MONTHLY. 



CORRESPONDENCE 



THE FLYING-MACHINE PROBLEM. 

 Editor Popular Science Monthly : 



SIR : To the fjreater part of Prof. Le 

 Conte's article, " The Problem of a 

 Flying-Machine," in the November number 

 of " The Popular Science Monthly," I give 

 hearty assent, and yet I can not admit that 

 his premises warrant his very discouraging 

 conclusions. lie shows clearly that as the 

 animal, flying or walking, increases in size, 

 the ratio between power and weight grows 

 smaller, until finally the limit of muscular 

 strength is reached; or the "weight over- 

 takes the utmost strength of bones to support 

 or muscles to move." He shows that, among 

 mammals, this limit was probably reached 

 in the gigantic dinosaurs; and that the largest 

 flying-birds, such as the turkey-cock and con- 

 dor, are "evidently near the limit," and that 

 the ostrich and emu have passed it, and 

 hence arc unable to fly. 



He then speaks of the wonderful efficiency 

 of the animal machine as a means for turn- 

 ing heat into work. "Nerve-energy acting 

 through muscular contraction, and supplied 

 by the combustion of foods, such as oils, fats, 

 starch, sugar, and fibrin, together form the 

 most perfect and efficient engine that we 

 know anything of ; i. e., will do more work 

 with the same weight of machinery and fuel. 

 ... A bird is an incomparable model of a 

 flying-machine. No machine that we may 

 hope to devise, for the same weight of ma- 

 chine, fuel, and directing brain, is half so 

 effective; and yet this machine, thus perfected 

 through infinite ages by a ruthless process of 

 natural selection, reaches its limit of weight 

 at about fifty pounds. . . . The smallest 

 possible weight of a flying-machine with 

 necessary fuel and engineer even without 

 freight or passengers, could not be less than 

 three or four hundred pounds " ; and hence 

 Prof. Le Conte concludes that "since the 

 animal machine is far more effective than 

 any we may hope to make, therefore the 

 limit of weight of a successful flying-ma- 

 chine can not be more than fifty pounds," 

 and that a '■Hruc flying-machine, self-raising, 

 self-sustaining, self -propelling, is physically 

 impossible.^'' 



Can this be so ? Is the animal machine 

 more effective than any we can hope to 

 make ? Will it necessarily " do more work 

 with the same weight of machinery and 

 fuel " ? Does the limit of weight in a flying 

 animal mark the limit for a flying-machine ? 

 At the risk of not being considered "a true 

 scientist " I must decidedly dissent from these 

 views; I can not look upon machine-flight 

 as a real impossibility, similar to the produc- 

 tion of perpetual motion or of a self-support- 

 ing arch of indefinite length. 



Before making a comparison between the 

 power of birds and motors, we must get some 

 idea of the power exerted by the former. 

 How much work must a bird of given weight 

 actually do in order to raise himself from 

 the ground and fly ? It is well known that, 

 once in the air, the power required is very 

 much less than that necessary for rising. 

 How much less is uncertain, but in a brisk 

 wind an eagle, or condor, or albatross will 

 circle around for hours, hardly ever flapping 

 his wings, and seemingly the only work is 

 that due to muscular effort in keeping the 

 wings outstretched. The work done in get- 

 ting up is, then, the greatest the bird or 

 machine would be called upon to do. This 

 work will evidently depend upon the ratio of 

 the wing-surface to the weight ; with wings 

 only a square foot in area, the most power- 

 ful condor could not fly ; and the greater the 

 wing-surface, provided the muscles are 

 strong enough to manage it, the less the 

 power required. 



This ratio has been measured on many 

 birds. The vulture, for example, can spread 

 0"82 of a square foot for each pound of 

 weight, and, assuming the entire weight to 

 be thirty pounds, the total wing-surface 

 woidd be 24"6 square feet. 



We now have a ready means for calculat- 

 ing approximately the maximum power such 

 a bird must be able to exert. In order to 

 rise vertically, he must force the air down- 

 ward until the reaction is equal to his weight. 

 Calling V the velocity of the air in feet per 

 second. It the reaction in pounds, v) the 

 weight of a cubic foot of air, A the area of 

 wing-surface in square feet, and g the ac- 

 celeration due to gravity, we may make 

 use of the well-known formula for reaction : 



Awv^ 



A/ — 2. , from which, sub- 

 \ Aio 



stituting value?, we find the necessary veloci- 

 ty to be about twenty-three feet per second. 

 The work done in giving the air this velocity 



Rv 

 would equal -— -, or three hundred and forty- 

 five foot-pounds per second, equivalent to 

 about six tenths of a horse-power. It is here 

 assumed that the air is driven downward in 

 parallel streams ; but a bird's wing would 

 naturally send off a part in other directions, 

 and consequently the power necessary would 

 be somewhat greater. Allowing twenty-five 

 per cent for this and other losses, we see 

 that a vulture weighing thirty pounds would 

 not need to exert more than three quarters 

 of a horse-power, and this only for a few 

 moments while rising. 



Suppose, now, our flying - machine to 

 weigh six hundred pounds, or twenty times 



