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Re: The Aerodynamic Origin of Bird Flight



Re flying fish never flapping, what about hatchet fish? (note that I'm not saying that their flapping generates a lift increment -- but, they do flap)

In your blog, you state:

As a wing at speed comes within its chord length to a flat surface a pressure wave compression occurs beneath the wing. This allows more lift to be created at lower speeds than clear air Bernoulli lift. This can routinely observed watching pelicans slide along a swell front without flapping. <

This brings up three points.

1) the compression at low HAG -- about 80 years ago a couple of Japanese researchers conclusively demonstrated that lift at altitudes less than the chord can be either lower or higher than same speed lift at altitude and that changes in height that are considerably less than the chord will cause the lift differential to change back and forth from an incement to a decrement and vice versa. The animal can't depend on a lift increment at low HAG.

2) Pelicans aren't using the compression effect. They are using the rising air off the leading side of the advancing swell in a manner similar to the utilization of orographic lift (ridge lift).

3) Since it is virtually impossible to define the boundary conditions (location of stagnation line, upper and lower travel path times, etc.) adequately enough to allow the Bernoulli principle to be used to quantify lift, other methods of quantification are usually used.

All the best, and I enjoyed your blog,
JimC