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Building Tips and technical articles. => Building techniques => Topic started by: Gordon Tarbell on May 22, 2011, 04:49:41 AM
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Getting ready to start the engine crutch on this 746 Buccaneer,Mvvs 49 RE for power. The 1/16th" ply doublers are warped like a dogs hind leg. Tried steaming and straightening, no Joy! time to make new ones. All the ply I can find at LHS is not that straight and seems very heavy. Can I just use some thinner (1/32") ply and double it up with another layer of balsa to get the thickness, maybe lighter at the same time? I have some 1/32" and some 1/64" still I think. The full length fuselage sides are nice light 1/8" stuff. I don't want to try and use the original doublers for fear of having a banana shaped fuselage (already fought that battle with earlier plane).
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Lots of kits use the 1/32" ply for doublers on the full fuselage builds, it should be ok.
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Of course you can. First though, unwarped plywood is pretty much impossible to find. The good news is that by the time you epoxy it to the fuselage sides and glue beam motor mounts, and F-1 and F-2 bulkhead to it as well as a wing, it pretty much straightens itself out. Banana fuselages are more the result of poor technique or using dissimlar weights of balsa for the sides. Heavy?, it seems that way but the actual doubler will be much smaller than the sheet you are buying and weight forward of the CG isn't always a bad thing.
As for 1/32 Ply, sure you can use it. I wouldn't bother with the balsa sandwich though. I routinely use 1/32 ply in smaller stunt ships but the Buccaneer is a pretty large plane. Also,the MVVS is a heavy engine albeit very smooth. If you do go this route, do a good job of fiberglassing the nose section to at least the high point of the wing.
A better option may be to use the construction technique that Randy Smith uses on his designs- a balsa/carbon fibre veil/balsa sandwich. That is, make the doubler out of 1/8' balsa, epoxy it to the fuselage side with a piece of CF veil inbetween. I have crash tested this method on more than one occassion and it is as strong or stronger than the more traditional plywood doubler technique. Some say that balsa also absorbs engine vibration better than plywood. It will result in a slightly wider fuselage but that can be tapered into a spinner so that its visual effect is minimal. A pic of my Buccaneer is attached for you inspiration...now Git Er Dun! 8)
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Getting ready to start the engine crutch on this 746 Buccaneer,Mvvs 49 RE for power. The 1/16th" ply doublers are warped like a dogs hind leg. Tried steaming and straightening, no Joy! time to make new ones. All the ply I can find at LHS is not that straight and seems very heavy. Can I just use some thinner (1/32") ply and double it up with another layer of balsa to get the thickness, maybe lighter at the same time? I have some 1/32" and some 1/64" still I think. The full length fuselage sides are nice light 1/8" stuff. I don't want to try and use the original doublers for fear of having a banana shaped fuselage (already fought that battle with earlier plane).
I use 1/64 ply for piped 61's, so 1/32 is fine.
Brett
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Pete said it! Plywood is hard wood and transmits vibrations. I've used Randy's method for years, even with 3/32nd doublers and sides. A combination of Smith and Hunt technologies!
W
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Ward-O....While I understand the theory, we've had two recent cases where Expert class fliers built VERY nice planes, the fuselage noses soon went soft and wiggly, and the engines no longer ran properly. Both planes were built with 1/8 balsa sides and 1/8" balsa doublers, with CF Mat & epoxy between. The only guesses I can offer is that 1) maybe the balsa was too soft, since balsa is so variable in density and 2) maybe the epoxy soaked into the balsa enough that the bond failed (dry joint)? 3) Possibly the epoxy was bad, or mixed wrong, but I would bet not.
Meanwhile, planes built with 1/8" balsa sides and 1/32 ply doublers just soldier onward, and their engines run just fine. The concensus around here is that plywood doublers work fer sure. y1 Steve
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For .35 -.46 size planes I make doublers from two layers of 1/64 ply laminated together. Light, and they seem stiffer than standard 1/32 ply. Easier to cut out too, just use scissors!
Add some carbon veil in between and it would probably work just fine on the larger planes.
Cheers,
Mike
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The balsa CF sandwich is the route I'd take. Using the proper type, properly mixed and properly applied epoxy is key however. First do not use the epoxy from Big Box stores. Use any of the premium slow cure (2 hour working time minimum) and thin epoxies mentioned in other sections in this site. Apply epoxy to all facing surfaces let sit about 5-10 minutes (The reason for slow curing epoxy) re-apply and then scrape off the excess. The balsa sides should be evenly damp. Lay in the Carbon Fiber veil. Place sides on a flat hard surface and clamp between a board and working surface for 24 hours. Alternately you can use weights in leiu of clamps. When hardened (Post 24 hours) assemble and finish as normal. Make sure the engine and tank compartment compartment is fully sealed and fuel proofed.
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My present project is a kit (yes, folks!), Since I don't think 1/16" ply fuselage doublers are necessary, I traced them onto 1/64" ply. I've been using 1/64 for a long time with great results.
Floyd
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I also use 1/64 for doublers, even for models with a 72 4S. Then I use 0.5 carbon veil over the hole fuselage.
Martin