Bush gear bending fuselage problems

[1avidflyer 1983]

Sorry guys for the way we (maybe should say I) hijacked the other thread with the discussion about bending up the bush gear and such.  Paul,  I haven't ordered springs yet, thanks for that source.  It is a good bit cheaper than the Mc Master Carr prices.  I've been wondering about using multiple wraps of 3/8" bungee instead of springs. Of course the springs compress and bungee stretches so the legs going to the cabane would have to act just the oposite of what they do with the spring system.  With the bungees, it would be lighter, more adjustable (more wraps)  possible longer travel too.  That's what I'm thinking might be some of the advantages.  I could be all wrong on this though.  I know more knowledgeable people than me have designed the gear with springs for good reason.  Of course Cubs and such used bungees for how long also.   It would be nice to get some others into this discussion for a more rounded out pool of knowledge.   I think it would also be helpful if others who have had trouble with their bush gear would give some imput as well.  Take care,  Jim Chuk

[Av8r3400 1179]

I'm a little fuzzy on how the additional vertical member will help (per the sketch).

[1avidflyer 1983]

This is how I see it.  If you tied a long rope tight between two objects on the ground, and then pulled up on the rope, you can get an awful lot of pull on the rope.  It acts like a lever.  The cabane V acts the same way.  When the landing gear deflects in a hard landing, that pulls pretty hard on the bottom of the V.  The crosstube acts to keep the V from pulling the sides of the fuselage in.  If the crosstube had a verticle between it and the V it would have made the crosstube that much less likley to buckle under the load.  My thinking is that if you also tied it to the seat truss, you then gain all the strength from the seat truss also.  If the seat truss is infilled, it's pretty strong in its self.  I'm inclined to think that if you tied the V to the seat truss, you wouldn't even need the cross tube.  Not that it would hurt anything, but I think the seat truss is far stronger than the cross tube.  Good topic for discussion.  Jim

[wypaul 448]

If you look at the picture that I posted of the cabane failure you can see that the cross tube failed and deflected downward.  I think the simple added tube may have stopped the fuselage damage but without actual testing there is not a way to tell.  One thing for sure the cabane, which is a truss, would be stronger.  I am sure that infilling the seat truss would have been more important is limiting the damage to my fuselage and that is in the works.

 

Paul

Edited 2 Apr 2013 by Paul S

[FoxDB 17]

I had a similar failure of my fuselage due to Bush Gear. I feel the problem is the placement of the gear attach tabs directly under the longeron. This causes a bending moment to the tab which caused torsion to the longeron. If you look at the placement on a cub it is offset to the outside to put the V Leg load directed to the center of the longeron. My solution was to add a brace tube to resist this bending moment. Attached are pictures of the damage.

[1avidflyer 1983]

Thanks for posting the pics.  Do you have any pics of the repair you could post?  I think I understand what you are saying about not pulling on the center of the longeron.  You actually have leverage that twists the longeron.  Thanks for the info.  Jim.