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Old Jul 22, 2007, 12:45 AM
Neil Walker is offline
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Discussion

Stickers on molded wings - does it matter?


I have a nice 60" molded glider (Bird, HN 354) but it is mostly white with just a bit of red on the leading edge and tips. I'd like to put some sort of design over the top of the wing to aid in visibility - so obviously it would have to cover a significant area, for argument's sake let's say 1/4 to 1/3 of the surface. I'd like to do it in sign vinyl. I couldn't care less about the weight, but will the thickness of the vinyl and irregular pattern affect the airfoil in any way? Am I better off spending the time on a good paint job?
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Old Jul 22, 2007, 05:07 AM
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Unless you are squeezing the thing to its limit for competition, it won't affect it enough to be an issue.
Old Jul 22, 2007, 07:09 PM
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Thanks for the response. Someone else told me I should just try it and if I actually notice a difference then I should try out for the F3J team
Old Jul 22, 2007, 07:12 PM
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I remeber seen somewhere that the height of a decal must be smaller than boundary layer, and this is important on the first quarter of the chord where the flow is laminar. Aft the 25% of the chord the air is turbulent and maybe the decal will help adding turbulance.

If I remember well I would swear that I read it from a M. Drela note, but I could be wrong.

Take care.
Old Jul 22, 2007, 07:35 PM
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Try http://www.mh-aerotools.de/airfoils/ to find the height of the boundry layer
Old Jul 22, 2007, 09:15 PM
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Question: Do model planes achieve laminar flow over the wings at the Reynolds number and speeds they fly at?

If the wings do not achieve laminar flow then does the height of the boundary layer have any meaning?

Enquiring minds want to know. Thanks. -John Elliott
Old Jul 22, 2007, 09:35 PM
Neil Walker is offline
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Quote:
Originally Posted by Ricardo RW
If I remember well I would swear that I read it from a M. Drela note, but I could be wrong.
I remember this too from Dr. Drela but I couldn't find the original post...
Old Jul 23, 2007, 05:21 AM
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Quote:
Originally Posted by jbeanelliott
Question: Do model planes achieve laminar flow over the wings at the Reynolds number and speeds they fly at?
Actually model aerodynamics often has the inverse problem: making sure the boundary layer transits to turbulent early enough to avoid laminar separation.
Old Jul 23, 2007, 11:41 PM
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Do it AGAIN! This time BETTER!
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If you look for sign vinyl make sure you ask for high performance or cast. These should be about 2mil in thickness and have a glossy surface.
If you get calendared or indoor grade vinyl it will be 4-6mil and may or may not be glossy depending on brand and quality. The calendared vinyl is streched to thickness during manufacture and is less stable as sun and heat can cause it to pull up edges. However, it will stick to foamies better

Ken
Old Jul 24, 2007, 10:46 AM
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If any blisters remain under the trim you can remove them by pricking them with a pin and flattening the blister with a warm covering iron. Don't overdo it and melt the foam! Try on scrap first. This will keep things smooth Bill
Old Jul 24, 2007, 02:16 PM
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Quote:
Originally Posted by jbeanelliott
If the wings do not achieve laminar flow then does the height of the boundary layer have any meaning?

Enquiring minds want to know. Thanks. -John Elliott
Yep. The larger it is the thicker the airfoil will act. Your 'effective' airfoil shape traces the outside of the boundary layer (within about 80% of its thickness).
Old Jul 25, 2007, 04:37 AM
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Quote:
Originally Posted by Gary Warner
Yep. The larger it is the thicker the airfoil will act. Your 'effective' airfoil shape traces the outside of the boundary layer (within about 80% of its thickness).
Also, because the BL is resilient, small differences in airfoil shape won't get their full effect. Say you have an airfoil with quite a bit of aft camber (which makes for lots of lift) That little twist at the trailing edge will be obscured to the flow within the "mush" of the BL and you won't achieve full lift. This is one reason why max lift usually is lower at low Re.
Old Aug 14, 2007, 05:33 PM
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stickers on wing.


Simple test.Trim out glider to fly straight and level. Weigh stickers for ONE wing, add to one wing, add balancing weight to other wing.Fly it, see if it pulls to one side or is inclined to drop stickered wing first at stall.
Remove balance weight, add stickers to other wing, fly again.
I doubt there will be much difference, except that turbulation effect might improve performance slightly.


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