Thank God for Kutta-Joukowski


Know that a bird’s flight feathers are analogous to the blades on an airplane’s propeller?  It is actually the other way around of course, birds came before airplanes after all, but that is the manner in which the astonishment came to me.

And I’m not the Lone Ranger.  Long before Kitty Hawk there were attempts at manned flight designed around the avian wing.  The problem was propulsion.  Wright Brothers, or someone else, would have been aloft sooner had they understood how birds do more than just glide.

OK, you’ll remember that the mechanics of flight revolve around a curved surface – an airfoil.  As it moves through air (or water – think penguins) the molecules flowing over the curved top must move faster than those with the shorter path to travel below.  This creates a drop in air pressure above and lift*.

Well, the outermost part of a wing – the hand wing – is composed of stiff slightly pointed primaries which are longitudinally asymmetrical.  When a bird in flight flaps downward the narrower portion of the primaries curve creating airfoils and voila forward ‘lift’ occurs.

The several primaries on both wings of a bird combine into an analogue for a multi-blade propeller.  One on each wing.  Try it yourself next time you find a feather.  Hold it by the bare part of the shaft and move it through the air as had its original owner.  You won’t take off, but you’ll get the idea.

My favorite bird?  Cooper’s Hawk.  It is incredible to watch them Top Gun song birds.  Cuts bird seed budget line item way back.

*Bernoulli’s principle, developed in the eighteenth century, explains the ramifications of the pressure differential, but not why the air moves faster on top than underneath.  Explanations of flight and lift always bothered me because I was unable to get that part.  I’m happy to report that the Kutta-Joukowski theorem, developed in the twentieth century addresses that aspect.  It is complicated and I don’t completely understand, but feel better to know that I might one day.

3 Responses to “Thank God for Kutta-Joukowski”

  1. agierke Says:

    I’ve always liked albrecht durer’s painting Wing of a Blue Roller.

  2. ben colenso Says:

    The flow of Air entering just in front of the airfoil has to equal the flow of air just leaving the airfoil . In accord with this requirement the air separates at the leading edge at a specific time and recombines at the trailing edge at a specific time. And we know that the curved top of the wing is a longer distance for the air to travel than the lower surface. So the upper molecules of air have to cover more distance in the same amount of time than the lower molecules. Because of this they have to speed up to cover a longer distance in the same amount of time as the lower path.They also thin out in density across the greater distance because the molar flow rate is constant.
    I hope that rather rough explanation is helpful. ? U stated above that you we unsure why the air moves faster on top. cheers

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