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  #1  
Old 03-28-2021, 07:01 PM
BillL BillL is offline
 
Join Date: Sep 2007
Location: Central IL
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Default Carbon rudder for RV??

I have spent too much time trying to research a comparison between my tall 7 rudder and a carbon fiber design. The objective would be the same area, dimensions, lower weight, higher strength, and much higher torsional stiffness with the CG closer to the pivot . . . all resulting in a more "balanced" part.

Have any of you carbon part/industry experienced members seen or done a comparison of this type?

Thanks,
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  #2  
Old 03-28-2021, 07:23 PM
Eric Minnis Eric Minnis is offline
 
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Location: Burlington
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I have a fairly extensive level of experience with aerospace composites and structures in the RD and production environments.

A common misconception is that carbon is lighter for a given structure than aluminum. With a part like a rudder, I doubt very seriously one could achieve any weight savings. Stronger would be heavier in that application IMO.

A more rigid rudder may also present challenges elsewhere.
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  #3  
Old 03-28-2021, 07:27 PM
David Paule David Paule is offline
 
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Quote:
Originally Posted by Eric Minnis View Post
I have a fairly extensive level of experience with aerospace composites and structures in the RD and production environments.

A common misconception is that carbon is lighter for a given structure than aluminum. With a part like a rudder, I doubt very seriously one could achieve any weight savings. Stronger would be heavier in that application IMO.

A more rigid rudder may also present challenges elsewhere.
Actually, in my experience, once the strength and stiffness has been matched, in a decent design, the carbon one is indeed lighter.

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  #4  
Old 03-28-2021, 07:50 PM
Eric Minnis Eric Minnis is offline
 
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Quote:
Originally Posted by David Paule View Post
Actually, in my experience, once the strength and stiffness has been matched, in a decent design, the carbon one is indeed lighter.

Dave
Old retired engineer
I would love to see a carbon RV rudder that was lighter and had the same structural properties as the original metal version. Not saying it can't happen, I learn new things daily.

I'm surly are not going to wet layup or oven cure it lighter. Even in the autoclave with proper cnc'd tools it would be a significant challenge.

I had a group of V tail Bonanza owners approach me about building new composite ruddervators due to the scarcity of the magnesium versions. We could have done it but at a cost that was more than some of the aircraft they were to be installed on. Would not have been lighter either.
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  #5  
Old 03-28-2021, 11:15 PM
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BJohnson BJohnson is offline
 
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Location: Federal Way, Wa
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Default Maybe but...

The current rudder design is very efficient. I’m a composites guy at heart but love the simplicity and durability of the RV’s. You could build a lighter rudder but the cost would not be very appealing to do it right. But I agree with Eric, hand layup or resin infusion like what you see on YouTube would not be lighter. More expensive materials and processes would be needed.
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  #6  
Old 03-29-2021, 11:46 AM
scsmith scsmith is offline
 
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Location: Ashland, OR
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One feature that will often yield a lighter part in carbon is to not match the full properties of the aluminum part. Often the aluminum part is built with minimum gauge constraints that make it heavier than is needed. For example the early rudders were 0.016 but developed cracks along TE fold and at rivets. So later rudders use 0.020. But other than stress risers and fatigue, the 0.016 was plenty strong and stiff. So one would not need to design a carbon rudder that replicates the full strength of the 0.020 aluminum. To do so would make the carbon rudder heavier than needed.

It is often, but not always the case that using carbon as “black aluminum” will end up being heavier. But with good design the carbon parts are lighter.

Just as a quick weight check, a 0.020 skin for a rudder weighs 1.44 lb (assuming a rudder area of 15”x48” just as an example). The same skin made with one-core-one carbon sandwich of 3k cloth weighs 0.9 lb.
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  #7  
Old 03-29-2021, 03:48 PM
BillL BillL is offline
 
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Steve, wouldn't the carbon part be much stiffer with the core? That is what I was thinking. The tall 7 has been seen in pieces (5 incidents). It is innocent until proven guilty (of course), but it seems to fail in bending along a line extending from the upper pivot bearing aft to the TE. Roughly. Some dynamic load, stimulation, resonance, flutter etc. Anyway, the whole line of curiosity was the possibility of a different, stiffer design. Although, the aluminum design is a torsional component (the aluminum one), it only has end caps, spar and TE connected with ribs for stiffening. It seems the shear loads would applied to a stiffer "skin" would render the entire rudder stiffer in torsion. When I clamped the tall rudder (.016 skin) to the bench with the pivot pins mounted to a stiff steel beam, it seemed like there was some low torque dead-band until the angular deflection was enough to stiffen the skins. Minor buckling?? Is that expected?

How are carbon parts fastened together, could two skins attach to a trapezoidal beam (spar) and then to themselves (TE) with adhesive? Or does adhesive have inadequate shear strength?
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RV-7
Lord Kelvin:
“I often say that when you can measure what you are speaking about,
and express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meager and unsatisfactory kind.”
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  #8  
Old 03-29-2021, 11:08 PM
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BJohnson BJohnson is offline
 
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Default Water intrusion

One ply of 3k fabric over core is not a good idea. Water intrusion into the core will be problematic shifting the balance, and could delam the skin if it freezes.
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