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Old Jun 29, 2014, 09:37 PM
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Why Not Cover Bottom of Wings on Balsa Micros?

I just finished my first balsa RC micro plane kit and was curious as to why the bottom of the wings are not covered such as their larger counterparts are? Is it due to weight or something else? Of course I was skeptical at first, but I must say, the plane flies really nice. Thanks for any input!
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Old Jun 29, 2014, 10:00 PM
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DGrant's Avatar
United States, CA, Clovis
Joined Mar 2004
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Probably in the design to create a massive undercamber, which creates lift and stability..

StevensAero kits are known for this... as well as every one of them flies very well from what I understand.. I've got the LiddleGee that has a wing thats only covered on the top.. and it flies great.. and they're specific in the instructs to NOT cover the bottom.

but in a nutshell... I believe the uncovered bottom wing is a part of the airfoil design.
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Old Jun 30, 2014, 08:21 AM
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Blacksburg, VA 24060 USA
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Mountain Models micros also cover only the top of the wing.

If you cover the bottom of the wing, the wing camber will be shallower, so the model may fly faster, and it may stall at higher speed. That's not a bad thing, unless you fly in a small space, where you need to be more careful about keeping your model under control. Also, covering the bottom adds weight, which will also raise the stall speed.

Jim R.
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Old Jun 30, 2014, 09:03 AM
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Tim Wolff's Avatar
United States, MI, Temperance
Joined Sep 2002
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Generally;
An under cambered airfoil generates more lift at low airspeed than other types. The trade-off comes as airspeed increases and the same model climbs like mad or uncontrollably if there is an abundance of excess thrust.
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Old Jun 30, 2014, 10:36 AM
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Beaumont tx
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XS climing with unercambered wings can be cured with downthrust. If the plane flies level with at mid-throttle neutral elevator you have the right amount. Downthrust also educes sall/spin on ake-off

howell
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Old Jun 30, 2014, 01:49 PM
Arrowhead
Joined Dec 2010
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It's all about the airfoil. Some micro kits do have fully-covered wings (the SA Air Camper and Hummingbird being two I know for sure). But, the un-covered wing as others have said is a high-lift, high-drag wing that is good for low-speed aircraft. Also, covering adds weight...
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Old Jun 30, 2014, 09:36 PM
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Thanks guys, the answers helped to clear things up. The plane is in fact the Stevensaero MicroMAX 103, a great kit and even better flyer.
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Old Jul 01, 2014, 11:08 PM
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Lost In OC's Avatar
United States, CO, Castle Rock
Joined Feb 2005
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I cover all the bottom sides of my wings. It tends to help when flying in a breeze. This in on 13" models by the way.
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Old Jul 04, 2014, 02:50 AM
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Delta, BC, Canada
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The late Jef Raskin did some testing with very small airplanes operating at low Reynolds numbers and found that at low Reynolds numbers a slightly cambered flat plate was the most efficient airfoil he found. I think he called it a 4-40 airfoil, that is 4% camber at the 40% mark. A thin flat plate was also good. Thicker airfoils like this one you would get if you covered the bottom just don't work as well at low Reynolds numbers.

Stevens Aero also takes advantage of a little trick. They curve the wing root and then don't put any curve supports near the wing tips. This means the airfoil changes from a thin curved plate near the root to a thin flat plate near the tip. This is good because thin flat plates have very benign stall characteristics. Thus, the root of the wing will stall before you get a tip stall. This is very clever.
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Old Jul 08, 2014, 03:48 PM
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Beaumont tx
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Park Scale models do this, too.

howell
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