- Oct 18, 2003
- Saline Michigan
This is serious thread drift, but it is your thread. Several comments:Okay. So, the epoxy runs into the honeycomb cells, so you need pre-preg, so you need big ovens, so that's out. That's the kind of info I was after.
It's going to be dark blue, so I'm probably going to need to do a cure at elevated temp (Aeropoxy at ≈250, I think). Will need to figure out how to do that.
I'm hoping Vne will be pretty zippy. Wouldn't mind 300 mph. Too early to know. Based on what you were saying, Bill, maybe I'll need to look at some thicker foam in the fuselage. 1/2"? Parts of the fuselage—e.g., turtledeck—will have much tighter curves, so I hope it's okay to switch to thinner foam in those areas. I'll get to that point sometime. Sounds like 1/4" in wing panels might be adequate. As you said, the carbon fiber fabric I use will influence things. Jim Marske talks some about the different recommended fabrics in his book.
Speaking of Jim Marske, I'm going to use his book in helping design the spar. I've gone through a few different ideas about how to make the spar(s). At this point, I'm thinking of making it one piece (if possible). The question is if the Graphlite rods are flexible enough to do the bends in the gull wing. Maybe I'll be able to achieve that by using a greater number of smaller rods vs. fewer larger ones.
Make sure that the foam will stand the cure and operational temperatures;
Vinylester resins have higher Tg than room temp cure epoxies without the hot cure;
Marske is a good place to start on design. Please read my stuff on beams and composites too. http://www.homebuiltairplanes.com/forums/showthread.php?t=28953 and http://www.homebuiltairplanes.com/forums/showthread.php?t=29030
Stress in an elastically bent piece is kappa*E*y, where kappa is 1/bend radius, E is Young's Modulus, and y is the distance from the neutral axis of the rod you are bending. So each rod in your bent spar will have built in stress going from compressive at the inside of the bend to tensile on the outside. Flight loads will be superimposed on top of those stresses. Your design will require a beefier spar than would otherwise be the case. In the extreme, you can have no load carrying capability at all. Using Jim Marske's 21Mspi for E, 200 kpsi compressive strength, and 0.06" thick rods, at 3.15" bend radius, you get to compressive strength of his rods just by bending. His small round rods will give more margin. If the scale bends result in rods being bent to these kinds of radii, you could "cheat" and make the radii larger. This may still be a much more acceptable solution than attempting wet layups of spar caps from carbon tapes...