Carbon fiber Armrest/extinguisher hold and wing tip beginnings
 

This the beginning stage of some new wing tips I'm working on for the RV7. The pictures show ABS sections produced in my 3D printer that will be joined together then molded. They will be made from carbon and cured in an autoclave. I won't claim they'll be any faster, but they'll look really cool. In my opinion any how.



The next idea came from the fact I wanted an armrest and also needed somewhere to mount a Halon extinguisher. It to is carbon, cured in an autoclave.

Nice
 

Nice work! I checked your profile and noticed you "own a composites company." Looks like you know what you're doing. Nice looking autoclave in the background! It's definitely not your everyday backyard workshop.

When I lived down there, Sun City was nothing but old folks.

Times have changed..........

Apparently they all moved to Sun City, FLORIDA - where you can legally drive your golf cart on public roads.

Cool looking parts! I bet that 3D printer is a lot of fun.

How are you going to handle the difference between the coefficients of thermal expansion of the aluminum and the carbon?

Large holes like for windshields? Or thick bond lines?

Thanks,
Dave

for the wingtips? because if so....umm....standard ones are fiberglass, so, same issue

In assume you're mainly talking about the wing tips? Good question though. My back ground for the last 15 years has been manufacturing carbon fiber products where this actual science has never been an issue. Two possibilites may eliminate this factor though.
First, the wing tips are going to be very thin. As a result the expansion may show to be minimal. Just a guess. I'll have to do some homework to actually find out what the difference will be.
Second, it if does show to be a problem. I can try a mainly glass laminate with a single layer carbon, if I want that look. Which I really do. Or just all glass.
Once again my experience isn't on the science end so I'm always open to suggestions from those that are.

2024-T3 has a CTE (coefficient of thermal expansion) of about 12.5 e-6 in/in/F, while carbon usually has a CTE close enough to zero to call it zero.

What this means is that if the two materials are fastened together and then the temperature changes, that they push against each other enough that they stretch or compress to be the same length. The forces can be significant.

Fiberglass, conveniently, has a CTE much closer to aluminum, and is inherently less stiff, often around a fifth of aluminum's stiffness, depending upon the fiberglass orientation. As you know, the carbon is about as stiff as aluminum, and can even be stiffer, with appropriate fiber orientation.

In any case, the equations for this sort of thing typically have the length drop out, but the various thicknesses, widths, CTEs and material stiffnesses are important. If you can manage to keep the temperatures moderate, that helps, too, but it's unrealistic for an airplane. Providing a means for one material to slide against the other is an oft-used approach for managing the issue. Oversize holes, similar to that used in windshield installations, is one way to handle it.

The reason I brought it up was to learn some of the tricks for dealing with the CTE mismatch, carbon versus aluminum.

CTE shouldn't be a problem - Run the carbon fibers at + and - 45 deg to the edge of the wing if you must.

I'd be more worried about galvanic corrosion. Include an electrically insulating layer of glass between the carbon and aluminum otherwise the aluminum will tend to sacrificially corrode over time.

All minor issues in the overall scheme of things though.

Jim Sharkey
RV-6 Snowed in at 1B8 for the winter :(

Looks like it would be difficult to pull my flap handle up!