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Old Jun 04, 2012, 11:25 PM
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Question
Does drag scale?

I've been reading about gliding and the three main forces: gravity, lift, and drag. Drag seems pretty easier to understand. I can't move as fast and have to use more energy when I'm hip deep in water. Track stars wear smooth spandex suits. But, I was curious if it has the same effect at all scales, or is simply related to total surface area, regardless of scale.

More specific to what I'm trying to achieve, a small RC glider, how much of a factor is drag at a 28" to say 1m wingspan. I'd imagine for this really big RC gliders it matters a lot, but does it really matter for a tiny glider? That is, is reasonably smooth good enough? Or is obsessive sanding, varnishing, filling, polishing, etc. par for the course? Are we talking doing filets at every join, or "just slap it together and throw dammit!", or somewhere in between?

Thanks for answering all my noob questions everyone.
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Old Jun 04, 2012, 11:40 PM
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For smaller airplanes, drag will be less (they fly slower, so less drag. They weigh less, so less lift and therefore, also less drag). But (and this is a BIG BUT!), the relationship between lift and drag changes as size diminishes, with drag not decreasing to the same extent as lift.

Less total drag for smaller airplanes, sure. But lift/drag ratios don't scale.

To scale aerodynamic effects, you have to wrestle with the Reynolds Number (you can find the definition in Wikipedia). Reynolds Number compares the size of an object, with the density, velocity and viscosity of the fluid it's trying to move in. For small objects in air, the viscosity starts to really change things, compared to larger objects.

I'll let others chime in, now.

Yours, Greg
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Old Jun 05, 2012, 06:35 AM
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Joined May 2002
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Read here if you want your eyes to bleed:

http://www.grc.nasa.gov/WWW/BGH/reynolds.html

And here for a lighter explanation (not really):

http://en.wikipedia.org/wiki/Reynolds_number

And for a good read that doesn't really apply to our stuff:

http://jila.colorado.edu/perkinsgrou...lds_number.pdf
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Old Jun 05, 2012, 07:31 AM
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tom:

That last paper is fantastic! What a tremendous grasp of small, sticky stuff.

Thank you very much for putting up that link.

Yours, Greg
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Old Jun 05, 2012, 09:55 AM
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Thanks a bunch everyone. I don't mind reading until my eyes bleed at all. What's a big help is knowing the search terms to look for since knowing the correct jargon is really important. I'm sure I'll be posting more question in this thread!
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