Date: 22 Jan 15
From: Frank Gue, B.Sc., MBA, P.Eng.,
2252 Joyce St.,
Burlington, ON L7R 2B5
905 634 9538
For: Editors, The Economist, London, UK
Re: “Flight details”, Feb. 17
The finding that birds’ wings create more lift on the downstroke than on the upstroke is not in the least “extreme” but is a necessary condition of ornithoptic (flapping) flight.
In the telling aphorism of Wolfgang Langweische (Stick and Rudder, 1944), “The wing lifts the airplane up by pushing air down,” Any bird does likewise. To generate lift, the upgoing wing would have to be so angled as also to generate a backward lift component, which would oppose the thrust required to propel the bird forward; this is unthinkable in any man- or nature-made flying machine. Therefore, since the upgoing wing must not push air either up (defeating its purpose) or down (imposing unacceptable reverse thrust), the only alternative is not to push it either way, which is what all-wise Mother Nature has evolved birds’ wings to do.
If the experimenters were to photograph a bird flying through a visible haze, as Ludington did (“Smoke Streams, 1943), they would find that the vapors behind the downgoing wing would be angled sharply downward (“downwash” in the jargon), while the the vapors behind the upgoing wing, though disturbed, would not be angled at all. Under control of its on-board, on-line, real-time, digital-proportional computer and instant-acting servomotors, the wing (and any tip feathers, individually) flies at its continuously varying “zero-lift angle”, displaying a sophistication we poor human engineers have not even approached.
Integrated across the time of any single downstroke, the force would indeed, and necessarily, be equal to twice the weight of the bird, because the average of zero for half the time and twice the bird’s weight for the other half has to be equal to the bird’s weight. Further, the bird’s weight multiplied by distance flown in a measured time yields the horsepower (birdpower?) the bird is developing.
If you have an engineering problem, check Mother Nature first. She likely solved it millions of years ago.