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Getting a feather trailing edge on foam cores

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Getting a feather trailing edge on foam cores

Old 01-06-2020, 04:53 AM
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jester_s1
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Default Getting a feather trailing edge on foam cores

I want to cut my own cores for a future SPA project. In my last build, the factory made cores where left with a flat TE that required adding a balsa TE and ailerons. I guess they did that because tapering the cores to a perfect sharp edge is hard to do? So do you guys have any tips for me on getting a sharp trailing edge with foam?
Old 01-06-2020, 05:08 AM
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Propworn
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The foam trailing edge is to fragile stick with the balsa one.
One of the university Aerodesign teams insisted on trying the tapered foam trailing edge and it never worked out with the hot wire cut. When they still insisted they tried cutting and shaping it and it turned out looking like poo. It didn't even come close to looking like the airfoil they were trying to duplicate.
Even molded airfoil are not usually feathered but end in a blunt edge

Last edited by Propworn; 01-06-2020 at 05:19 AM.
Old 01-06-2020, 06:49 AM
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Philip Avonds is a well know championship builder and he says that the squared off TE is intentional so as to reduce flutter. I discovered this when building one of his F-104 kits. There is no other plane out there that deserves a feather edge more than the starfighter, but he insist it is not a good idea to have feathered TE's
Old 01-06-2020, 02:32 PM
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jester_s1
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I'm doing sheeted foam wings, so a feather edge will yield a 1/8 inch thick TE when it's sheeted. I guess keeping 1/16 or so wouldn't be so bad, as I could sand the sheeting down a little to thin it.
Old 01-06-2020, 07:29 PM
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speedracerntrixie
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Jester, the best way to achieve what you are looking for is to make your templates in upper and lower halves. When cutting the wing this would allow you to adjust the thickness of the TE to whatever you want. Coming to a sharp edge TE with your foam will never be perfect but as you said it won't matter after being sheeted. Like you said it will be 1/8" at that point anyways. To get a really sharp TE, I bring the sheeting 1/4" past the core, cap with 1/8" square bass wood and sand to the desired sharpness. For some added durability you can add a strip of fiberglass cloth between the sheeting but only if you are using epoxy to adhere the sheeting to the cores. I have used this method on many pylon airplanes.
Old 01-06-2020, 07:42 PM
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jester_s1
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Is there anything special to know about wire choice and heat? Thick wire vs. thin, go slower/faster?
Old 01-07-2020, 03:37 PM
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Originally Posted by jester_s1 View Post
Is there anything special to know about wire choice and heat? Thick wire vs. thin, go slower/faster?

Nichrome wire works well as does .020 stainless safety wire. Heat and speed are hard to nail down as bow length and foam density are big factors. It takes some practice to get repeatable results. PM sent.
Old 02-03-2020, 05:28 AM
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I use .020 stainless mig wire a small roll will last you and your friends a lifetime. Its cheaper and the wire thickness and its content is highly controlled giving the best overall heat distribution. I clean/polish the wire with a piece of scotch bright pad for each cut. My bows range up to 9 feet and free hand with the wire attached at one end and held at the other just about any length is possible.

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