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Replacing Alum Tube with 4130 Steel for Fuselage

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Tom Kay

Well-Known Member
Joined
Aug 10, 2007
Messages
409
Location
Canada
Hi;

I'm interested in getting to know welded tube fuselage construction a bit better, especially as compared to other possible methods. One example, of a plane that does interest me, is the Murphy Renegade, all aluminum frame.

With the Renegade, the fuselage is 6061-T6 1" diameter .062" wall round tubes for the 4 main longerons. With that it uses 3/4" square .035" wall for all of the cross pieces, verticals and diagonals. All the square tubes fit into Murphy's unique flanged tube. Short pieces of this flanged tube fit over the main longeron tubes, and are then Avex riveted together. (see picture 1).

The company claims that 4130 steel tube in place of the aluminum would be twice as heavy. Does this sound correct? If one were to use the same diameter, then maybe. But if you use 4130 Chrome-Moly in place of the aluminum tubes, wouldn't you shrink it down to match the original strength? This would make the steel tubes smaller, presumably. And that would also mean using smaller tubing for the cross pieces and diagonals.

Is there a kink in my think? Is there an obvious flaw I'm missing here? I know aluminum is lots lighter, but with properly sized tubes and good welds, wouldn't the weight of steel in its place end up comparable?

Why go this route? Someone mentioned the other day that aluminum starts to fatigue day one of its use, whether you approach the ultimate load capacity or not, whereas with steel, it won't fatigue unless you get fairly close to the max load carrying capacity, then it starts to fatigue. Is that roughly correct?

Thanks, Tom.
 
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