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Design => Stunt design => Topic started by: Ken Culbertson on March 20, 2018, 03:20:01 PM
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I am installing a floating bellcrank from Tom Morris and would like to know if anybody has used a solid carbon fiber rod for the center post? (It will be in a 1/4 aluminum sleeve) I have had a poorly installed metal one bend on a pull test a gaszillion years ago before carbon. Will it wear against a brass bushing? Can it be lubed? Am I an idiot?
Any advice will be appreciated.
Ken
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I am installing a floating bellcrank from Tom Morris and would like to know if anybody has used a solid carbon fiber rod for the center post? (It will be in a 1/4 aluminum sleeve) I have had a poorly installed metal one bend on a pull test a gaszillion years ago before carbon.
I'd think an aluminum tube could be lighter, although maybe harder to glue.
Will it wear against a brass bushing?
Yep
Can it be lubed?
I have lubed carbon axles. It didn't help much. I tried heat-shrink Teflon tubing, too. I forget whether that worked, but it was heavy.
Am I an idiot?
Not based on this inquiry. Had you considered Kevlar, that would be a different matter.
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Yep
That is the answer that I was hoping not to hear. Everything short of ball bearings wears to some degree. Any idea now bad? I like the idea of carbon fiber because it is stronger and will not stay bent if I hit it it hard...but it does cut rather easily against the grain and I have visions of little gremlins in there sawing my bellcrank axel in half on every corner.
Thanks for the advice - Ken
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just did one , 1/8 steel rod , doubled in center 1 1/2 in. , brass washer Ea End .
5/32 brass tube in 1/8 ply ea side ( top & bottom ), second 1/8 ply lam outside mounts , tube bearings are 3/8 ,
center rod ( pivot 0 is full depth of wing.
Swings Free, No Slack . Brass on steel is maybe self lubing , buts vasilined after passivating ( Boiling in B Soda Sol'n .)
Was conveniant for this design , long 1/8 ply spar joiners - undercarage mount mounts - belcrank mounts ; Doublers
to ease any doubts, just little tube braces really .
The cranks drilled 1/8 x 2 at pivot , secondary 1/8 rod was filed flat to mate my crossed intersection ' 8 ' hole .
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Been Figuring a outside take off / arm above / outside wing .
a z bend 1/8 or 5/32 wire pivot , second shorter wire as binding so cant screw loose ( control Failure ) .
Think it'd be a bit of a swine to assemble - sequential during construction - with instalation of the mounts . :P
It'd get the pushrod perpendicular to the flap horns , at neutral , and the pushrod pretty much horizontal on this ship .
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think Brass on Carbon would be better , self polishing . Alumn to soft , gall And Self Erode .
Wotter you gunna use for GLUE in there ? The Bellcranks glued to the toob ?? .
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You probly wanna look up suitable bearing material for C / F toob ; nylon of phenolic resin stuff maybe .
Avoid sharp edes & burrs ! , chamfer the edges of whatever the bush is , or radius .
Barreled Bearings were derigure for flexable shafts , in infernal combustion engines .
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This isn't my work, but I've done the install the same way using plywood!
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This isn't my work, but I've done the install the same way using plywood!
This has some posibilities. Love the way it transfers the load to the spars, hence the whole wing and the braces pretty much guarantee you are not going to bend the axel on a pull test. The only thing I question is why doesn't the axel extend through the spar cap? In one of those situations (that only happens to other people) where you lose tension and have to pray the bellcrank holds when it hits the lines on the other side, you are going to get the full initial force on either the up or down line. If the axel extends through the spar and is in some way attached to the fuselage you have more than just the glue joint between you and a rebuilding session. That would be my only change.
Ken
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I don't have numbers at hand, but if you had trouble with a poorly designed metal one, why not change the quality of the design rather than the material? Poor design is poor design and will screw you up no matter what; Good design should take the material into account.
CF is, pound for pound, stronger than steel. But it's very likely not so inch for inch. A good 5/32" music wire should hold things just fine.
I pull test my floating bellcranks and their supports by hanging about two or three times the expected pull of a pull test from them, while they're sitting in a shop vise.
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This has some posibilities. Love the way it transfers the load to the spars, hence the whole wing and the braces pretty much guarantee you are not going to bend the axel on a pull test. The only thing I question is why doesn't the axel extend through the spar cap? In one of those situations (that only happens to other people) where you lose tension and have to pray the bellcrank holds when it hits the lines on the other side, you are going to get the full initial force on either the up or down line. If the axel extends through the spar and is in some way attached to the fuselage you have more than just the glue joint between you and a rebuilding session. That would be my only change.
Ken
Ken,
Most of my axels do extend beyond the spar, in fact just above the sheeting a bit top and bottom.
I then Epoxy a piece of 1/16" ply over the axel and the ply is snug against the inner fuselage sides.
I solder a washer on the top and the bottom.
I believe I have photos in my builds of this.
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I don't have numbers at hand, but if you had trouble with a poorly designed metal one, why not change the quality of the design rather than the material? Poor design is poor design and will screw you up no matter what; Good design should take the material into account.
CF is, pound for pound, stronger than steel. But it's very likely not so inch for inch. A good 5/32" music wire should hold things just fine.
I pull test my floating bellcranks and their supports by hanging about two or three times the expected pull of a pull test from them, while they're sitting in a shop vise.
I suppose you are right about the metal vs. CF but I am stuck with 1/8 unless I want to rebush one of Tom Morris's bellcranks. It was a poor design by today's standards.