mast and airframe stress??

In reality the margins of safety will vary for different components of the aircraft. Those that are taking mainly static loading will need less margins than other components that are subjected to high/cyclic, so the designer will apply the necessary factors to satisfy strength/fatigue life requirements.

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Bensen's standard end design for 3/4" angle braces, for example, involves a taper and a 1/4" hole. The cross-sectional area lost is almost 2/3 of the total area of the extrusion. A 20% presumed loss at a welded joint looks almost trivial by comparison.

Doug,
Its rare (for me ) to find where a bracket brakes in the 1/4 in hole. But they certainly do fail where the angle tapers to the flat portion. Is that because the hole is usually sandwitched between the frame and bolt head and washer, or does the taper just bring the stress to that point?
 
Mark: You may be looking at stress concentration, which can cause fatigue and cracking in unexpected places. A couple things could be to blame.

First, there may be some periodic bending of the angle in use. That bending will use the "toe" of the taper as its fulcrum. The result will be work-hardening of the metal until it gets brittle and cracks.

It's also very important to create a true radius at the "toe." A sharp corner will concentrate stress. I have had good luck starting with a rat-tail file, then fine emery cloth wrapped around a 1/4" bolt, and finally buffing with a wheel and compound. The surface of the radius should look like a mirror when you're done.

In tension, there's no question that a Bensen angle end will snap through the hole. At that cross-section, there are only two strips of metal, 1/8" x 1/4" each, to bear the load.

The Gyrobee does not use the Bensen pattern. It employs 1x1 angle with no taper except for a cosmetic one at the very end, past the hole.

Incidentally, Bensen called out 6063-T5, square cornered, for all of his angle braces on the B-8M, except for the engine mounts and the mast-keel braces. Those were 6061-T6 with a corner radius. Probably everything should be.
 
That is one of the features in a Gyrobee that I have not considered as an improvement in the Bensen design. But now that you have pointed it out, I can't think of any Gyrobee braces failing. I went with round chromemoly on the KB 4 because I did not like the 3/4 angle. Thanks !
 
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