Why do birds have so much rough surface (overlapping feathers) compared to aircraft? Isn’t that cause additional skin friction drag?
Could planes maybe benefit from this?
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It may seem counterintuitive, but by introducing a small amount of turbulence, this interrupts the slower boundary layer which is directly in contact with the wing, thereby delaying local airflow separation and aerodynamic stall. Essentially, at the expense of a small amount of parasitic drag, the stall performance and control effectiveness of the wing can be improved.
Aircraft add small fins called Vortex Generators to achieve this effect on otherwise smooth wings, flaps, ailerons, rudders, and elevators.
Here is a link to check out: https://en.m.wikipedia.org/wiki/Vortex_generator
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6$\begingroup$ Also, the Reynolds number of birds and airplanes are very different. What works for birds may not be necessary or even helpful at other Reynolds regimes. What works for insects also doesn't work quite as well for birds. $\endgroup$– JZYLCommented Jul 8, 2020 at 16:59
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$\begingroup$ @JZYL you mean because speed of airplane is so much faster then birds? $\endgroup$ Commented Jul 8, 2020 at 17:07
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2$\begingroup$ Another factor is that bird wings (and to some extent the feathers) have a highly variable geometry that's under very precise control. The bird can thus position the wings to minimize drag (or whatever) as its speed changes. $\endgroup$– jamesqfCommented Jul 8, 2020 at 17:58
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2$\begingroup$ @AeronauticFreek reynolds number just tells us how (un)important the viscosity of the air is relative to the plane. And yes, it increases with both chord and speed. For larger, faster planes, the viscosity of the air is less important (though it still is very important). $\endgroup$ Commented Jul 9, 2020 at 4:42