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Old Nov 05, 2013, 09:34 PM
Registered User
Australia, SA, Adelaide
Joined Jul 2013
478 Posts
Question
Airfoils with lots of concave shapes cut out of bottom side

I have lots of spare foam to use my intention is to build a super slow flying plane, I'm not going for absolute efficiency.

I have some foam with holes cut out of the bottom side.

Name: foam with cutouts.jpg
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I'm about to cover it with paper but I just wanted to check if there is any use for it as it is. Would it just create too much drag to be useful?

I will be cutting it and reglueing or adding to it to form an angled undercambered design so this is not the final wing shape, but it shows the material.
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Old Nov 05, 2013, 10:43 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
11,313 Posts
That's simply not a wing section. Once you cover the open face with paper or whatever it'll be a slab.

As it sits now it would fly but it would be terribly draggy. And it would still behave like a flat plate but with a parachute tied to it.

If you want to create a very slow flying plane you need to use an airfoil that looks more like a banana. That is pointed or rounded along the leading edge, thin and highly arched and reasonably sharp along the trailing edge. To work well at very low speeds the arch on the lower side would want to be roughly 10% of the chord. So a 7 inch wide chord would have a roughly 5/8 to 3/4 deep arch located at around the middle of the wing's chord.

Folks have done things of this sort using the blue fan fold foam typically sold to use under laminate flooring.
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Old Nov 05, 2013, 10:49 PM
Registered User
Australia, SA, Adelaide
Joined Jul 2013
478 Posts
lol yes I know the picture is not showing an airfoil, I should have mentioned I am going to cut the foam to reglue it at an angle to form an angled undercambered design, that's in another thread though, I forgot to mention it again.

But once I do this, would there be any advantage to leaving the holes there or would the drag just be too much.

Also I also forgot to mention that I am building it from recycled materials which were free such as old packaging.
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Old Nov 06, 2013, 08:17 AM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,034 Posts
I was going to suggest cutting the full depth sections out, (top, bottom, and 5 verticals), and use them to cut some airfoil shaped ribs. And build an open frame wing the same as a balsa version.

You would still need something for a leading edge, a spar, (or 2), and a trailing edge, possibly some square balsa strip, (or something stronger depending on the size of the wing and wing loading).

Then cover the top surface with something, possibly paper or thin card, (if this is a recycling exercise), maybe leaving the bottom surface open if it was strong enough to survive flight.

I don't think I would try to make a wing out of the existing shape, ........unless stuck on a desert island and needed a 'kite' as a means of a signal.

Ex-packaging polystyrene is some pretty awful stuff, it's weak, and can be recycled foam with different densities. It's Ok as a nice thick block if needed, but shapes like you have I would have thrown away, ().

But there is nothing stopping you from trying, .........
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Old Nov 06, 2013, 11:20 AM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
11,313 Posts
If you can make the airplane out of that stuff light enough then it will descend slowly. But it won't so much glide and parachute down. Cutting and forming the trays into an arc will help but you'll want to cover up those pockets on the bottom and carve or hotwire the leading and trailing edges to make the shape at least a trifle more like an airfoil.

All in all I think it will still have a glide that is more like a modern parachute though.
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Old Nov 08, 2013, 06:38 AM
Registered User
Australia, SA, Adelaide
Joined Jul 2013
478 Posts
It will be easy to cover.

I might get a hot wire cutter in the future, but now I'm just experimenting with basic construction methods. For making an elaborate wing structure I'll probably buy depron rather than trying to use the left over polystyrene. But I really want to have some fun making some very basic designs out of some of this polysyrene foam I have.
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