Ok so is aluminium stronger then steel. NO aluminium is more rigid then steel, and is stronger then steel by weight comparison only. There is really no such thing as structual aluminium, some aircraft grade aluminium is significantly stronger. But is still not as strong as steel, the strength from the aluminium frame in cars and airplanes comes from the engineers and there ability to design a monolithic "frame" and this is only done out of the need for weight savings.

Aluminium is a rigid alloy and does not compare to the yield strength of steel, which is why steel is used in allot of applications where movement can distort a frame, because steel can come back into "plane" on a repeatable basis. thats not to say alu. can not be used in allot of applications and have advantages over steel due to weight savings. The reality is that the stresses a cataraft frame can endure will never approach the structual limits of either metal.

Something else to note would be the weld strenght comparison between alu. and steel. This is the biggest problem I see with aluminium frames vs. steel, and is where you have no comparison to steel, as I mentioned in an earlier post but did not specify the weld issue. Steel welds are significantly stronger then alu. welds. In fact alu. welds are considered brittle and not to be used in a constant flex situation. Again this is not to say that a frame will ever be subject to forces significant enough to break a weld with either metal. This is why all connections in an airplane are mechanical aka pop rivets or bolts. Anyway I can elaborate further and would enjoy the debate if some of you are still on the alu. band wagon.

Not trying to be rude just informative.
Thanks,
H4F