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Gentle Climb
12th Oct 2004, 11:19
Please don't flame me for this question BUT having watched a program about the bouncing bombs used by the dambusters, it was demonstrated that a dimpled surface creates less drag than a flat surface. I'm not a technophobe, but it struck me that there are elements of this that could be applied to particular aircraft surfaces. Is this already done or have I simply jumped to a wrong conclusion? Apols if this is the wrong forum.

john_tullamarine
12th Oct 2004, 11:41
The effect of dimpling is dependent on Reynold's Number (Re) and can precipitate an earlier transition from laminar to turbulent flow. Similar situation results in golf balls being dimpled ...

Not a great deal of use with an aircraft due to the larger areas and boundary layer thickness, not to mention the manufacturing and potential handling problems if dimpling were to be used, say, on wing surfaces due to in-service deterioration and asymmetric separation patterns.

Perhaps the currently practising aerodynamicists can offer more up to date comment ?

... and flaming is NOT a problem on Tech Log ...

Mad (Flt) Scientist
14th Oct 2004, 00:14
The main problems with boundary layer drag control methods have been practical. IIRC Airbus tried huge swathes of some kind of special 3M film on a A340(?) but the big problem was maintenance. Either the damn film peels off (and flaps around in the breeze) or you have all kinds of issues with baggage doors and access hatches and such. The film used was more rippled or grooved than dimpled, I think, but it was a vaguely similar mechanism at work.

And yes, applying something like that might be problematic to the wing; though I'd be far more concerned at attempts to maintain laminar flow characteristics than deliberately inducing turbulent flow; the former would probably break down far more catastrophically.

SR71
14th Oct 2004, 08:20
A golf ball is a bluff body whereupon separation is a major problem. If you can delay it by energizing the boundary layer i.e., provoking turbulence, the benefits are obvious.

The problem with commercial jets in the cruise is the opposite in the sense that you'd like to laminarize as much of the flow as possible as parasitic drag is a function of the shear stress which in turn is a function of the velocity gradient through the boundary layer. This also has the added benefit of reducing the pressure drag.

Boundary layer thickness tends to be proportional to 1/sqrt(Re).

Typical velocity gradients through boundary layers of both the laminar and turbulent kind illustrate that for the purposes of reducing the skin friction coefficient, you want a laminar boundary layer over as much of the wing as possible.

Here's a link to NASA's project: NASA F-16XL Laminar Flow Aircraft (http://www.dfrc.nasa.gov/Newsroom/FactSheets/FS-023-DFRC.html)

During the days of my research studies, there were other students doing similar kinds of things albeit on a wind tunnel scale.

The major problem?

The holes through which you exert the suction tended to get continually blocked with dirt/bugs etc etc

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