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  • Crossle32F commented on DavidB685's instructable DHC-1 Chipmunk2 months ago
    DHC-1 Chipmunk

    Hello, thanks for the response, we got our DHC-1 flying last night. Your info was really helpful. My son made weight reduction and wing redesign of this model his end-of-year school project for Grade 9 STEM and it turned out to be a really interesting exercise. We decided to try making a foam wing, he found that the typical blue Dow XPS insulating foam board was much stronger but only slightly more dense then the cheaper white EPS beadboard. We chose a Clark YH airfoil, downloaded the data free from the U of Illinois airfoil site, drew it up in SolidWorks to find the volume, and made root and tip templates out of aluminum sheet. We increased the span to 15" each side outboard of the fuse, which still looks reasonably scale but gives a bit lower wing loading. Hot wire cutting ...

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    Hello, thanks for the response, we got our DHC-1 flying last night. Your info was really helpful. My son made weight reduction and wing redesign of this model his end-of-year school project for Grade 9 STEM and it turned out to be a really interesting exercise. We decided to try making a foam wing, he found that the typical blue Dow XPS insulating foam board was much stronger but only slightly more dense then the cheaper white EPS beadboard. We chose a Clark YH airfoil, downloaded the data free from the U of Illinois airfoil site, drew it up in SolidWorks to find the volume, and made root and tip templates out of aluminum sheet. We increased the span to 15" each side outboard of the fuse, which still looks reasonably scale but gives a bit lower wing loading. Hot wire cutting the tapering section and planform was a challenge but after many attempts we got two good wing panels and a centre section. Following your example, we went with 6 degree dihedral using a 1/16 plywood spar tapering out to about half-span (just outboard of where the landing gear mounts). Also your ailerons were bigger, probably a good idea; ours were originally scale and did not give enough roll control. So we decided to extend the ailerons almost to the root instead, so we could run a single servo in the centre section operating short torque wires to each aileron. This worked OK. The whole wing assembly (without covering) was 46 grams, vs. 84 for the original. Other weight reduction in the fuse, and going back to the Park 250 motor (with a 6 x 4 APC Speed E prop and a 2S 800 mAh battery) brought the all-up weight down to 270g (0.6 lbs) including landing gear and some nose weight (down from about 400 g without landing gear, which was unflyable!). Static it drew 43 Watts, so watts per pound about 72 W/lb which is marginal. Wing area is 135 square inches, for a wing loading of about 10 oz/sq. ft. eCalc showed no warnings with this setup. Slow-fly prop might have been better? We put the landing gear on to be able to have a takeoff roll and build up some airspeed and control authority before taking off, since we had so much trouble handlaunching out original build.So, it lifted off well, flew, and landed, but according to my son was very twitchy and hard to fly! Our CG was right on the spar line (centre of lift) about 50 mm behind the leading edge and it should have been 40 mm, as you found, however we were reluctant to add more nose weight and were running out of time for the school project deadline. Given another day, we would have cut holes in the tail to reduce weight and get the nose weight out and the CG forward. This should solve the wild flight characteristics. Main lesson learned is to keep the empennage weight down; we built the tail first, skinning both sides of the vstab and hstab with balsa sheet and adding a lot of gusseting and glue filleting for strength, and this was the main mistake. Should have built the tail light, and reinforced later after getting far enough along to pre-assemble and find out how the balance and overall weight were shaping up.Anyway, thanks again for posting you project and responding! Hopefully our notes here might help others doing this sort of conversion. Next for us is a 30" BF109 from an old, abandoned kit we inherited.Cheers!

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  • Crossle32F commented on DavidB685's instructable DHC-1 Chipmunk3 months ago