Aerodynamic impact of small, spanwise creases, gaps, channels etc?

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yankeeclipper

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All things retractable, detachable, openable, etc. invariably imply small interruptions in an otherwise (potentially) smooth body. So I'm wondering if there is a rule of thumb, or just common knowledge about how influential spanwise interruptions are, particularly if their width and depth are no bigger than the boundary layer size.
 

autoreply

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There's only one correct answer to that; "it depends".

On a turbulent airfoil with a raw surface like a C172, a bug (1/12" square) probably doesn't interrupt flow noticeably. On a smooth, short-chord laminar flow airfoil, a 1/64" bug likely causes transition in your boundary layer and a noticeable rise in drag.
 

JMillar

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I read something where Rutan was saying he lost something like 6mph just by going through a rainshower. Don't know any rules of thumb for you, but I suppose you should try to keep most of the flaws as far back on the wing as possible.
 

autoreply

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Some laminar airfoils are very sensitive to contamination. If you (like in a shower) contaminate the wing, you can get spectacular effects. The Janus glider for example almost triples his drag when seriously whet. Given that the wing is roughly 50% of the frontal drag and the frontal drag is roughly half the total drag, we're talking about a twelvefold increase in profile drag of the wing. Newer laminar profiles are a lot better and don't suffer so badly. Good for healthy adrenaline levels too...
On a typical C172 though I doubt you'll be able to notice a drag increase.
 

Topaz

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...On a typical C172 though I doubt you'll be able to notice a drag increase.
And that, of course, is the real issue. Do spanwise surface disruptions matter? Of course they matter. How much they matter depends upon how draggy the rest of your airplane might be.

Go look at a sailplane. Every single one of these things that can possibly be accessed from inside the aircraft without an external access point or door, is accessed from inside the airplane. Now go look at a 172. The nosegear has about as much drag as the entire sailplane you just looked at a moment ago. The impact of an access door? Negligable.
 
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