combatpigg

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I am building a RC SPEED plane and have just finished making the female mold halves for the fuselage. The upper half of the fuselage will be supporting the engine mount beams, wing mount and tail feathers. The lower half of the fuselage is basically just a fairing for the engine cylinder and tuned pipe. I need to watch the weight very closely and I don't have any idea what would be the best choice of materials to do the lay up with. Any suggestions ?

davidfee

Are the upper and lower halves going to be bonded together, or will it be a two-piece fuselage with the lower part (fairing) just attached with screws or something? It would be much stronger and lighter if it was one piece. In any event, there is no need for a lot of bulk.

I'd suggest an outer layer of 2oz glass and an inner layer of 4oz glass. This will make a strong and reasonably light structure. The 2oz will help give a nice finish while still adding strength, and the 4oz will give good strength at a reasonable weight. You can go with aramid or carbon, but it's not really needed, IMHO. Models are typically way over-built.

good luck,
-David

combatpigg

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Hi DAVID! Thanks for the reply, I will pick those weights of cloth and give it a try. The original idea was to go with upper and lower halves that could be seperated, but now I can see that idea getting too heavy. I will bond the upper and lower halves and cut in access panels. Can I use fiberglass for the motor mount rails and use wood screws to clamp down the engine?

davidfee

If your engine uses rail mounts, why not epoxy wood rails to the inside of the glass fuselage. Isn't that what you had in mind? I would think that would work great. A thin light-ply bulkhead behind the engine could really stiffen things up, if needed.

If you want to screw into glass/epoxy, you're better off drilling and tapping for machine screws rather than using wood screws.

good luck,
-David

combatpigg

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I figure that I could make a clay dam and pour in some JB weld, or glass fiber and resin. The engine uses #2 hardware to hold it to the rails. The space available for maple rails is slim, the sides of the fuselage are flush with the outside edge of the engine lugs. Ideally I would use #2 threaded inserts in a bed of resin or epoxy, but I don't think they are readily available. This is an extremely high rpm engine, so the clamping force has to be good. It's hard for me to picture plastic machine threads[2-56] being able to withstand the abuse before crumbling, but I have no experience in this area.

Ed Smith

I speak from experience here. It is bad practice to have a high performance tuned pipe engine sitting on wood beams. No matter how hard the wood the engine vibration will cause the wood to compress where the engine sits as well as loosen the wood screws. Absolutely no wood screws. The beams should be faced with at least 1/16 thick metal, and not just under the engine. I would never use wood beams for a high revving engine. Change the design and fit a firewall with an aluminum motor mount. You will not be sorry. If that engine has the slightest slack on the mount it will not pipe up.

Ed S

davidfee

Thanks for that input, Ed. I have some experience with 1/2A glow, but none with tuned pipe setups.

-David

combatpigg

My Feedback: (3)

Thanks for your help, DAVID and ED! I'm going to make some aluminum inserts and put them in a bed of epoxy. I'm working with a weight budget of 13-14 ozs RTF, which leaves me with a weight allowance of 7 ozs for the airframe. The wing weighs 3.5 ozs, so the fuselage is where the "make or break" work is.