horizenjob wrote:
This same issue shows up in a locost frame. An advantage for aluminum is that you could use a larger diameter tube which is longer and not worry about the buckling. I see this in the frame I am designing. There are places where a tube could be more then 50 times as long as it's diamter, which is much too slender. So more bays are needed to avoid this and it adds weight.
I don't think it's true that aluminum frame has no advantage, but an aluminum copy of a steel frame would have less advantage.
I think that fabric planes use steel frames because, in general, they were designed before good aluminum was available. WWII is when that started happening. I don't think 6061 was available until the mid 50's. For most people the idea of a plane made from aluminum monocoque was more attractive then a fabric plane. So in a way that's a more appropriate use of the material, Understanding the appropriate use is really a major key to a successful design.
I still think the main point to take away from it is one structure had a strength requirement and stiffness came second and on a car chassis typically stiffness comes first. A strong but light material is desirable for the first but the stiffness per given mass of material is desirable for the second (ironically both steel and aluminum fit this bill).
I did some quick numbers for Euler's buckling and a 1"x0.065" square tube commonly found on a Locost would have to be between 65" and 70" long and unsupported for buckling to be a concern. I don't recall seeing any unsupported lengths that long on your design (I thought you were going with larger diameter stuff anyways?) It still seems easier to add a diagonal or two rather than completely change the entire chassis material.
To keep the same theoretical bucking properties with aluminum versus steel, the aluminum tube's second moment of area would have to be three times that of the steel tube which isn't hard to do since everything is to the 4th power but its axial stiffness will still only be a little over half that of steel.
This is the kind of stuff I was referring to when I said:
Quote:
there is more to it then welding some aluminum tubes together.
Its doable but it takes work to do it right and the builder would have to decided at what point have they reach diminishing returns for their efforts.
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