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Old May 07, 2014, 03:47 PM
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Airfoil selection?

I am planning to build a motor glider. The goal is to have a plane that will stay aloft in very light conditions and be able to fly in moderate winds 10 to 15 mph with or without ballast.

It will have 3 axis control . The design I have in mind will use blue foam board hotwire cut to what ever Airfoil selection that should work best.

Wing span 70", 9" root, 5" tip, 490 square inch area. 10 to 1 aspect ratio. Straight trailing edge swept LE. 4" sweep.

Total length: 52.5 from prop to end of rudder. 36 square inch H stab and 24 square inch V stab. VH is about .36 & VV .023

Guess the weight to be 16 to 20 ounces.

What airfoils would be good for this model?


Thanks,
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Old May 07, 2014, 09:53 PM
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Might depend on the intended wing structure, if any. Are you intending to use spars?
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Old May 08, 2014, 01:15 AM
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Hi,
here some proposals:
Goe 795 (thin, straight bottom)
Clark Y (yes, this ancient desing )
FX-60-126 (hard to build in wood but still really good performance)
MH 42
HQ profiles with camber > 1.0%

Depending on the construction (wood, foam/basla, epoxi) the airfoil has less influence on the total performance than the construction precision .
Regards, Kurt
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Old May 08, 2014, 06:41 AM
Kurt Zimmerman ≡LSF 4461≡
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You can't go wrong using Dr. Drela's airfoils. You can find them on the CRRC website.
Kurt
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Old May 08, 2014, 07:20 AM
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Thanks guys. I am planning on cutting a foam core wing with a hot wire so the profile should be dead on if my templates are.

I will embed a 40 inch carbon tube spar into the center of the 70 inch span. I think that will be strong enough.



Which AG airfoil would be best for this size plane? There are a lot of Drela airfoils.

Would the root and tip need different airfoils since the chords are 9 and 5 inches?

Is this taper ratio optimal or should I increase or decrease taper?
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Old May 08, 2014, 11:38 AM
Kurt Zimmerman ≡LSF 4461≡
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this looks to be a good choice:

AG12 - AG13 - AG14

Composite HLGs, roughly equal emphasis on float and run;

XP3 (polyhedral version)
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Old May 08, 2014, 12:22 PM
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Several good choices on airfoils listed above.
Unless you are using a fairly high level of technology your size and weight goal will be difficult to make. I have done some hand layups with limited success. Got no where near where you want to go.
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Old May 09, 2014, 01:26 PM
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You should read the CRRC website and specifically look a the design pages as most are written by Mark Drela. There are even a few series airfoils where he describes the characteristics. It is worth a look.

Another floaty series of airfoils is the SD 7036-37-38. The 7037 was a very popular airfoil for thermal models around the turn of the century.
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Old May 09, 2014, 04:03 PM
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Do a search for Drela's AG44 -AG47 for. I couldn't improve on the airfoil at all over 6 months of trying for a 2M design at your wing loading-flying speed. HT8 for the stab.


Will the stab be mounted under the vertical?
If so, Vh of .36 is undersized. You may need to go stab vol .42-ish and the vertical .026ish.
Quick math says your wing to stab mac to mac is ~37"? That's very good for a 70 inch model!


Try to mount the stab as high as possible. If you can improve the A/R ( keeping area the same) to around 12, then it prob ok to go .40Vh (keeping distance the same)
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