Quote:
Originally Posted by Tango Charlie
You might consider hanging some weight hung off of a torque tube clamped to one end of the frame.
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Good suggestion. What I did with the fuselage for mine was brace one end of the structure (mine has no frame and is fully monocoque) with whatever I had laying around (which happened to be things like wood 2x6's. Then I hung a beam off the other end with weights on it, to twist one end of the structure relative to the other. I put dial indicators at each end (at the braced end to be sure it was not moving much) and subtracted the readings to get a deflection.
I did the same kind of thing (although differently really ... bad science) with the motor scooter frame from which the engine was stolen. I had no value for what was OK for torsional rigidity, so used the scooter as a minimum standard. The monocoque was more than ten times as stiff in torsion -- no surprise because in the scooter frame case, the single tube resisting torsion was about 2" in diameter, whereas the fuselage was about 24", albeit of a much lighter, lower modulus material (wood).
If you use the aluminum skin for triangulation, you may want to bond it in addition to rivets, etc. Otherwise the stresses concentrate around the corners.
On big bumps, the structure will try to bend in a way that puts both sides in compression up at the third tube up. On one side, this is resisted by a straight tube. On the other side is is resisted by a much more flexible structure (where the door goes) If the door has a beam in it, then you'll want a pretty good latch and hinge system to transfer the compression load without play.
If the door does not handle this compression load well, then the diagonals will tend to bend the second-tube-up on the door side. If this tube is stiffened by the aluminum skin below it, the local skin loads (near where the diagonals connect) can be higher than you might want, causing oil canning, etc.
All of which I mention because, perhaps not obviously, I've built structures in the past that ended up more flexible than I would have wanted. It is interesting to play with those cheap steel shelving units that are partially triangulated with a couple braces on the back. They at least don't simply fall over to the side as they could without the bracing, but remain very flexible in torsion (twisting the top shelf relative to the bottom).
There is nothing I've seen that indicates that you are unaware of this stuff. I'm impressed. But sometimes a second pair of eyes will pick up on something that might otherwise go unnoticed until the symptom shows up.