Autodidact
Well-Known Member
I've calculated the "I" about the principle axis where "I" is at a minimum for a 3/4"x3/4"x1/8" extruded 6061t6 angle section and compared it to the "I" for a round 6061 tube that is 7/8"x0.065" and has the same weight within 1%. For the angle, I ignored the fillets so as to make this a little easier - had I included them the angle's "I" would have been ever so slightly smaller. The Euler buckling formula is Pcr = (pi²EI)/(L²), and with everything else equal and since "I" is not raised to any power, the buckling load is directly proportional to the value of "I" for whatever section is being used. "Imin" for the angle section was (by my math) Imin = .003664 in^4, and for the tube it was I = 0.013653 in^4, or about 3.73 times larger meaning an almost 4 times larger load capacity.
The angle is sometimes very inexpensive, but the tube is much stronger and a larger diameter tube such as 1"x0.035" has an "I" of 0.012368 in^4, still 3.375 times larger than the angle and at 64% of the weight but for approx. 3 times the cost. Even 7/8"X0.035" tube (0.0081606 in^4) is still 2.23 times stronger and approx. half the weight of the angle but slightly more than 3 times the cost.
For an ultralight it would seem that the tube is the better option, but angle has been used for this successfully. For an ultralight that needs to be aerobatic, maybe tube is best. This is for a fuselage truss and the weight gain is a relatively small percentage of the overall empty weight. For a larger plane, the angle might work OK if it is much cheaper than the tube; it depends on the angle section, some are expensive and some are dirt cheap.
My original reasoning for wanting to use angle was to easily use solid rivets. But now that I am more motivated to use the tubes, I can see a way to use the solid rivets in conjunction with AN3 bolts to make a structure that satisfies my sensibilities. The AN3 bolts to go through the tubes with two washers and a jam nut weigh 0.02lb each. two bolts per joint is 0.04lb, four joints per bay is 0.16lb, and say 12 bays for the fuselage is 1.92lb for the bolts - not so bad. I would press the tubes into a square-ish section only at the joints and use a short piece of tube for a spacer, and the rest of the gusset could be solid riveted with a bucking bar inserted into the end of the tube for that short distance. I'm still daydreaming about this...
The angle is sometimes very inexpensive, but the tube is much stronger and a larger diameter tube such as 1"x0.035" has an "I" of 0.012368 in^4, still 3.375 times larger than the angle and at 64% of the weight but for approx. 3 times the cost. Even 7/8"X0.035" tube (0.0081606 in^4) is still 2.23 times stronger and approx. half the weight of the angle but slightly more than 3 times the cost.
For an ultralight it would seem that the tube is the better option, but angle has been used for this successfully. For an ultralight that needs to be aerobatic, maybe tube is best. This is for a fuselage truss and the weight gain is a relatively small percentage of the overall empty weight. For a larger plane, the angle might work OK if it is much cheaper than the tube; it depends on the angle section, some are expensive and some are dirt cheap.
My original reasoning for wanting to use angle was to easily use solid rivets. But now that I am more motivated to use the tubes, I can see a way to use the solid rivets in conjunction with AN3 bolts to make a structure that satisfies my sensibilities. The AN3 bolts to go through the tubes with two washers and a jam nut weigh 0.02lb each. two bolts per joint is 0.04lb, four joints per bay is 0.16lb, and say 12 bays for the fuselage is 1.92lb for the bolts - not so bad. I would press the tubes into a square-ish section only at the joints and use a short piece of tube for a spacer, and the rest of the gusset could be solid riveted with a bucking bar inserted into the end of the tube for that short distance. I'm still daydreaming about this...
