Wing warping and bowing Written by Don Slusarczyk As seen in the WINTER 2022 issue of Park Pilot
>> The majority of model aircraft today are all made from foam. Carbon-fiber or fiberglass spars are typically glued to the LE (leading edge) and trailing edge or into molded slots in the foam. Throughout the past few flying seasons, I have noticed that the wing on some foam models warps on sunny days. I would typically see the dihedral droop or the upper surface of the wing bow downward with anhedral. This often happened on models with medium to dark colors that were painted on the upper-wing foam.
I suspected that it was because of foam expansion of the upper surface being in direct sunlight, so I did an experiment. I would flip the airplanes over, with the bellies facing up, and let the models sit in the sun upside down. With the sun shining on the underside, I thought that the bowing would go away, or at least be reduced. After a time, that is exactly what happened—the bow in the wing would reduce or go away. It was thermal expansion of the foam.
This bowing from the sun was bad on one particular model that I regularly flew, an Me 163 Komet made by TechOne that was painted red. Because it was a flying wing, the bowing would change the flight trim as the day went on. Each flight required it to be retrimmed when it was in the air. I started flipping it over after landing so that the sun was on the underside, and the bowing was not as bad of a problem; however, the underside of the Me 163 was also the same shade of red.
Why would it bow when it was right side up and not bow in the opposite direction when upside down? It has to do with the location of the carbon-fiber spar in the wing, which is not in the center of the wing on this model when viewed from the front. It is located closer to the underside of the airfoil. This caused an unequal reaction to the foam expansion, depending on whether the wing was facing up or down in the sunlight.
To help cure this imbalance, I decided to add a small carbon-fiber strip to the upper surface of the wing. I used a 0.8mm x 3mm strip. A slot in the upper surface of the wing was made with a new X-Acto blade using a straight-edge ruler as a guide. The carbon fiber was installed on the edge, vertically down into the slot, and then some thin CA was wicked into the foam to hold it in place. The spar addition is not very noticeable because it is only 0.8mm wide, but I could feel the added stiffness in the wing afterward.
At the next flying session,when the model was sitting right side up in the sun, almost no bowing occurred in the wing. I still could see a little if I sighted down the LE, but it was a great improvement.
On this model, the factory spar was located near the lower surface of the wing and that caused the imbalance. On models with no carbon-fiber reinforcement that are bowing in the sun because of coloration, I suggest adding spars to the top and bottom surface of the wing. If you add carbon fiber to just one surface, you run the risk of the same imbalance that I had with my Me 163.
A large carbon-fiber spar is not needed to fix this issue. A 0.8mm x 3mm strip is typically all that you need on a park fl yer model. The other benefit, in addition to reduced sun warping, is added strength to the wing (or stabilizer). On thin foam wings, the carbon-fiber strips will have to lay fl at on the wing surface because they cannot be buried into the foam vertically. I keep a selection of carbon-fiber strips of varying widths and thicknesses and adjust as needed for the specific model. Multiple pieces are sometimes needed to fix the bowing, with one spar near the quarter chord of the wing then another approximately 2/3 to 3/4 of the way back.
If you are experiencing flying surface bowing when your model is sitting in the sun, try adding a little carbon-fiber reinforcement in the right places to keep those wings and tails straight and true.
By Don Slusarczyk | don@slusarczyk.com
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