Square or round longerons?

[stick]

Hi everyone, first off thanks to Scott for all his work with sonerai.net, and to Jeff for setting up this forum.

I just got my plans and I'm wondering about the pro and cons in regards to square vs round tube longerons on the fuselage. So far I figure from my experience that the square is stronger but slightly heavier (dunno if it's enough to make a difference). Anyone have other input? ease of fitting parts/ bending?

Thanks

[oahupilot]

round is stronger then square, plus when you put the fabric on you won't have sharp lines/breaks in the shape of the plane. The round tubes will give the plane a better final shape when covered.

[schrome]

I disagree with Oahupilot: square is stronger than round. But it's a moot point because round is strong enough. As for visible lines in the fabric, the underside of the tail cone is the only place you would notice the difference. From 20 feet away I doubt whether anyone could say which type of tubing was used.

The advantage of square tubes is easier joints, i.e., simple miter cuts instead of fish mouths. But of course there is a small weight penalty. According to the AS catalog, the difference between square and round is approx. 0.07 lb/ft. Total length is about 64 feet, so that's 4.5 lb.

I used square but if I had it to do over I'd get or make a good tube notcher and use round.

Ed

[oahupilot]

Ed

how do you fiugre round is better the square? round does not have the stress risers at the corner of squares, round has better torsional rigidity, round has better buckle characteristics.

I should mention that I am comparing tubes of equal mass. If you just say a square tube of 1 inch width versus a round tube 1 inch diamter thats not a fair comparison the square tube is a larger tube by mass and can support more load.

[4trade]

Sharp corners add more drag for airframe= slower plane. If you build that plane for square tube, you build in more weight and drag.

Sq tube is easier to build, but those joints for round tube takes only couple hours more, not tens of hours.

[schrome]

Hi OP,

I didn't say better, just stronger. The comparison in this case is 0.75 x 0.035 square vs. round. They do not have equal mass. Square has the greater cross sectional area, which means it will tolerate a higher tensile load. It also has the greater moment of inertia and section modulus, which means higher bending loads. I'd have to blow the dust off the textbook and read about torsion, but in a welded truss I believe bending and tension predominate in the individual members.

Ed

[oahupilot]

Ed,

Like you said it has a greater cross area so its not an equal comparison, if you compare equal cross section of round and square, round wins out.

Round has equal resistance to bending in all directions perpendicular to the central axis of the tube, where square tube this is not so. Square tube has maximum resistance to bending perpendicular to its face only. Square is great for bridges or truss where the load is always perpendicular to the face, but in an airplane where you have multiple loads from different angles round offers a uniform strength.

in the end it doesn't really matter in the case of the soneria since both are strong enough, but I still think round is a stronger material for the sonerai.

[schines]

See what you started, stick?
Just kidding, it is great to learn something from those who have different experience and backgrounds. That is what makes this forum so good!
As for the original question, since both square and round tubing are allowed by the plans, either is strong enough, and the drag/looks difference seems minimal, too. The real issue, for me anyway, has not been mentioned yet - I chose round simply because it is about half the price of square! Having just (almost) finished a fuselage with round tubing, I can say that there were many times I thought about how nice it would be to have square tubing and avoid making all those fishmouth joints.

[stick]

Schines, you're right. My first real post and I'm already causing a ruckus. Next time I'll pick a less controversial subject.

Thanks to all for commenting. You brought up a few considerations that never even crossed my mind. Think I'll probably go for round at this point. Now to find a place to build!

[wafer]

I asked the same question after I got my LTS plans some years ago. At the time the plans were drawn, square tubing was priced competitively with round tubing and, as mentioned above, easier to cut/splice joints for welding the fuselage. Since then, square tubing increased in cost and it is simply not feasible when compared to round tubing.