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Gentlemen, Last night Myth Busters did a great experiment on a Buick with smooth skin and then golf balled it with hundreds of dimples. The result was a 15% decrease in fuel burn at 65 mph. MB does good science so I believe the result. Why are aircraft skins not dimpled? Ken
The dimples are for preventing separation by inducing a turbulent boundary layer. Most aircraft have turbulent flow over most of their surfaces anyway, partly because the Reynolds number is far higher, which means that, since they're larger and faster, viscosity is less important. You CAN have a lot of laminar flow instead of turbulent if all your aircraft's surfaces are very smooth and well shaped, but most metal aircraft don't meet this standard. If I'm not mistaken, at higher Reynolds numbers, even laminar flow is less prone to separation.
Still, you do see a line of bumps on some sailplane wings, fairly far back. Also, there are vortex generators on light aircraft (and others) which I don't think are entirely the same, but which stir up the boundary layer to prevent separation. I think in this case with vortices.
The Buick probably had lots of separated flow where there were sharp corners or sudden reductions in cross section. Still, 15 percent seems like a lot.
Perhaps a Cessna Skymaster with the rear engine out would benefit from dimples. With the rear engine on it helps keep the flow attached.
Until the advent of better airfoils, we would often use "turbulators" on model aircraft to improve performance. A turbulator is a discontinuity which would accomplish much the same thing as those dimples. Probably with less drag because it's only in one spot instead of all over.
nifty image of more and less separation here:
With attached flow, you get pressure recovery (i.e. more pressure) on aft facing surfaces, and so less drag.
I'm not a fancy aerodynamicist and I can't do all the math (although I can do some of the simpler stuff), so don't take my word as gospel.
Make it a habit to check your fuel gauges to ensure the tanks are even.
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