Carbon Fiber Pushrods

[Floridagator]

Hi everyone:

I was reading a spec sheet the other day on the Extra 330SC, and happened to notice that it has carbon fiber pushrods. Neat.

Then I happened to stumble on an interesting website that sells all shapes and sizes of carbon fiber:

www.dragonplate.com

They have carbon fiber tubes up to 3/4" OD and 48" lengths.

I'll admit, I'm not a builder (just a builder wannabe) so I don't know what the standard pushrods look like. What's a typical diameter and length for something like an RV7 or RV8?

Has anyone tried to implement the carbon fiber pushrod idea before? Is the weight savings significant enough to even bother?

[Mike S]

Carbon and aluminum dont play well together.

Therefore, you would need steel or some other metal threaded adapters to put into the carbon tube.

Not too sure the weight savings would be enough to explore this territory.

[penguin]

I think Extras used steel push rods, so they may save a pound or two - they are also slightly larger aircraft, so greater weight savings than RVs. I don't think RV push rods are 3/4 dia, but if that's available then other diameters should be also. So you won't save much, and will need different end fittings, but they're experimental airplanes, so it a go.
Pete

[Greg Arehart]

I'm guessing that the cost of a carbon fiber pushrod would exceed the cost of the aluminum, and for not much weight savings. Coupled with the compatibility issue already noted (electrochemical reaction between carbon and aluminum) that would require (heavier) steel fittings, I'm not sure it would be worthwhile. But, that's what experimentals are all about.

greg

[Floridagator]

Originally Posted by penguin

I think Extras used steel push rods, so they may save a pound or two - they are also slightly larger aircraft, so greater weight savings than RVs. I don't think RV push rods are 3/4 dia, but if that's available then other diameters should be also. So you won't save much, and will need different end fittings, but they're experimental airplanes, so it a go.
Pete

http://www.extraaircraft.com/media/EA330SC.pdf

Look under the 'airframe' section.

Good feedback from all. Now I'm curious how they rods are terminated on the Extra.

I'm sure the cost/benefit analysis would show that it's not worth the effort, but it seemed like an interesting experiment.

[robpar]

When I was in school, it was 20 years ago I admit, we toured a military research base at Victoria BC and one of the researchers there was doing research in carbon fiber failure modes. The problem they have with carbon fiber, at least 20 years ago, was that carbon fiber failed instantly and without any visible warnings.

Aluminum stress fractures over time but the method is well understood and one can look for the signs. I have not heard of big advances in this area I maybe wrong but I do not think I would risk carbon fiber in such a critical piece as a control rod.

The solution for the military is extreme maintenance, something that is likely out of the reach for most of us as the expense would not be worth the benefits.

Bob Parry

[jsharkey]

It's common practice in the composites industry to co-laminate a thin layer of glass fiber on the surface of the carbon composite where it interfaces with an aluminum fitting to electrically isolate the two materials with different electric potentials. The joint is also "wet assembled" and sealed to prevent moisture getting between the surfaces and acting as an electrolyte. Both of these help stop the formation of a battery - at least for a while.

Carbon composite has around the same galvanic potential as silver so if it were a metal it would be on the noble end of the scale - meaning that just about anything below silver will corrode preferentially if in contact with it in the presence of an electrolye. Remember that zinc/carbon formed 1.5V D cells for generations.

Also I suspect that an Extra is short lived like an F1 or a Nascar racer - it's not meant to work in a flying school for 40 or 50 years like a C172 so worrying about what might happen years down the road isn't an issue.

Jim Sharkey

[dhammer]

Originally Posted by robpar

When I was in school, it was 20 years ago I admit, we toured a military research base at Victoria BC and one of the researchers there was doing research in carbon fiber failure modes. The problem they have with carbon fiber, at least 20 years ago, was that carbon fiber failed instantly and without any visible warnings.

Aluminum stress fractures over time but the method is well understood and one can look for the signs. I have not heard of big advances in this area I maybe wrong but I do not think I would risk carbon fiber in such a critical piece as a control rod.

The solution for the military is extreme maintenance, something that is likely out of the reach for most of us as the expense would not be worth the benefits.

Bob Parry

Things may have changed over the last 20 years. If you climb into a modern glider they can have spans of over 25 Meters with a thickness of about 6 inches. The only way they can make structures like that is with carbon fiber. I'll bet a solid steel spar wouldn't be as strong. I think that a wing is probabably a critical piece. How about carbon fiber props? The latest C-130H and a lot of other turbine powered planes have them. Helicopter rotors?

It has been my experience that an aluminum spar, prop, or anything metal will fail with a single bang. I've never seen that happen with any composit, carbon included. That stuff comes apart like a cable, one strand at a time, unlike aluminum like a chain; one big bang. Maybe he was talking about a single strand. Composites don't work harden either, so you'll never have any cracks to stop drill.

Don

[flion]

I don't have the engineering specs or anything, but speaking from my experience with R/C gliders (back when Rick Spicer was building his Synergy IIs with Kevlar to more recently when I lost a Stork that had CF wing spars and fuselage), a composite structure actually gives less warning than metal. Metal has a limit where it deforms before breaking entirely. Well, so does a composite structure, but it's a narrower window. And a broken metal structure tends to shear, while a broken composite structure does not. One of the F3B team's Synergys crashed (elevator failure) and the wing looked perfectly intact until they picked it up. Then it looked like a wing-shaped piece of kevlar cloth. The impact, pretty much a flat belly-flop, had fractured the resin throughout the wing.

That said, I like composites (except for working with them). And I think it would be possible to fabricate some very nice CF parts for the RV. But I'm not sure it would be worth the effort.

[videobobk]

I really like CF. I have what I think is the only CF canopy system on a 9A. It saved something like eight or nine pounds but cost something on the order of $250 for each pound saved. Considering that the plane cost about $50/lb, that is an expensive way to save weight. I would be pretty cautious of something as hidden as a control tube, but it would be an interesting experiment. It might be worthwhile. I would look first at the fiberglass parts--wheel pants, wing tips and cowl. Anyway, it's your call!

Bob Kelly