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Author Topic: Reinforced Trailing Edge for a Solid Wing  (Read 1237 times)

Offline Dave Hull

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Reinforced Trailing Edge for a Solid Wing
« on: June 13, 2023, 10:48:33 PM »
In another post, there was a question about a plywood reinforced trailing edge. The thread was concerning making a solid wing more durable. This new thread will cover a method I used to easily create a reinforced TE.

I first tried this on a SIG Buster I was flying in Goodyear racing. The plane hunted badly, and was a bit heavy--but it sure did go! The thought was to try to improve things by using a new wing: lighter, thicker airfoil, but hopefully still rugged enough for the rigors of racing. It turned out well, so here are few details.

First, I assembled the wing. This one is a 3/8" balsa plank with higher density wood forward of the spar and lower density wood aft. The stub spar is tapered to reduce weight and also minimize the structural discontinuity you would get with a full section (constant retangular shape).  If you notch the rear panel you can epoxy in the spar and then true up the edge before gluing on the aft panel. Next install the tips. One of the purposes here is to use the orientation of the grain to avoid warping (cupping) of the wing. The tip as shown stiffens it up. With the tips installed, now true up the balsa leading edge joining surface. And glue on the "hardwood" leading edge. This one is probably spruce, but I like to use tulipwood as well. It has tight grain, and can be found with pretty low density for its strength. I've also used it for the stub spars and found it perfectly satisfactory.

To get the wingtips to match, I find it easiest to make a template and use it as a sanding gauge.
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Offline Dave Hull

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Re: Reinforced Trailing Edge for a Solid Wing
« Reply #1 on: June 13, 2023, 11:07:57 PM »
Once you have your wing blank assembled, you need your sanding jig to cut in the recess for the plywood trail edge reinforcement. The jig is shown in the previous picture. It is essentially a sanding block. But like most of sanding, it is how you employ it that makes the job either a success or a mess. (A life lesson in there somewhere....)  In this case, a fresh piece of fairly coarse grit open coat garnet paper is laminated to a very flat/straight block of sufficient length using 3M 77 spray-on contact cement. Either use an oversize piece of sandpaper and trim exactly to the edge of the block--or be the luckiest guy I ever met and align it right to the edge when you press it on. If the paper does not come exactly to the edge, the quality of the sanded recess on your wing will be poor and the whole effort is wasted.

Next, I clamped the sanding block to a guide fence using some small C-clamps. You want the sanding edge of the block to overhang exactly the dimension you want for your TE strip--and you must make sure that the edge of the sanding block is exactly parallel to the fence.

Use the edge of your bench as the runner for the fence. The edge of the bench needs to be flat and smooth. A coat of Johnson's paste wax is helpful for smooth operation. Now shove the fence up against the workbench edge, then slide the wing blank up against the fence. Hold down the wing blank once it is positioned correctly with your palm and putting some weight on it. Now slide the sander over your wing. Technique matters here. It's not just a happy-slidey thing. Think more like taking strokes with a wood plane; it actually shares a lot in common with a molding plane. Keep taking strokes until you reach the depth desired. If you used a straight bench edge and the fence is square, then holding it firmly against the bench as you work prevents tippy and sloppy results.
« Last Edit: June 13, 2023, 11:39:36 PM by Dave Hull »
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Offline Dave Hull

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Re: Reinforced Trailing Edge for a Solid Wing
« Reply #2 on: June 13, 2023, 11:10:18 PM »
After sanding your TE recess, it should look like this.

It is better to use a fairly coarse sandpaper to achieve a free cutting action. The scratches from the paper are going to be covered up by the plywood TE strip. If I recall correctly, this wing has 1/16" reinforcement.

The strip can be thinner or thicker with this technique. And, you could do it on both the top and the bottom. Maybe 1/32" on both sides would have been a better low drag solution. Or for that matter, you could go for 1/64" on both sides and get a very thin edge without sanding thru layers of plywood. The second side would have to be sanded in after all airfoil shaping, so you would be working with a thin edge at a slight angle. still seems doable with the setup shown. I would first glue in the bottom reinforcement before sanding in the recess for the second side.

When I glue it down, I put a clamping buck--for example, reusing the guide fence because it is straight--down over the edge with some waxed paper in between. I like to use Original Titebond, so mild clamping keeps things straight during drying. If you used epoxy I wouldn't worry about warping but would probably still clamp it to keep things aligned. My garage occasionally has very high Coreolis forces and things that are waiting for the glue to dry sometimes slide out of alignment because of butterflies in Brazil.

Another way to reinforce a thin edge if you are going to glass the wing is to lay in carbon tow behind the shaped wood TE in the sandwich. That was done on wood F2C wings for years and works fine. No plywood needed....
« Last Edit: June 14, 2023, 12:57:38 AM by Dave Hull »
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Offline Dave Hull

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Re: Reinforced Trailing Edge for a Solid Wing
« Reply #3 on: June 13, 2023, 11:23:42 PM »
This is what the new wing looks like mounted on the old airplane. The picture was taken at Kennedy Park in Napa, California on a cold and windy day. The fatter wing (and maybe a better alignment?) reduced the hunting and didn't seem to hurt the airspeed. But that's another story....
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Offline Lane Puckett

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Re: Reinforced Trailing Edge for a Solid Wing
« Reply #4 on: June 24, 2023, 09:33:25 AM »
Dave,
Thank you for the clear detailed information on how you reinforce the TE's.  It helped me a lot.

Thanks,
Lane
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Offline Dave Hull

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Re: Reinforced Trailing Edge for a Solid Wing
« Reply #5 on: June 24, 2023, 04:04:58 PM »
You are welcome, Lane!
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