Tubular boom from sheet?

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cluttonfred

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The simplicity of the tubular boom fuselage used in some older ultralights like the Hummer, Drifter, etc. is very appealing. That said, availability of the right diameter/thickness/alloy tube can vary a lot by location and over time. It's also probably not as simple as it looks as you need a thin tube with reinforcements at critical points if you are not going to accept the weight penalty of a thick tube from nose to tail.

hummer.jpg

What about making a tubular boom fuselage from rectangular metal sheets wrapped around circular or polygonal bulkheads, creating a series of nested tubes using the appropriate thickness for the stresses in each section? I could even see adding 90-degree flanges to the end of the sheets so that, as long as the bulkheads are located at sheet junctions with bent tabs toward the ends, you'd have access to both sides of the rivet holes for easy fabrication (rivet squeezer or pulled rivets) and inspection as you go.

Good idea or a waste of time?

Cheers,

Matthew
 

Riggerrob

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How about S-wrapping a single sheet of aluminium?
Start by making 100 degree bends in the middle of the sheet … spaced one diameter apart.
Then leave a sold web/wall down the center-line. If you wanted to get fancy, you could even press or turn a few lightening holes in the center web.
The first row of rivets could be hammered from both sides, but the last row would need to be pulled (e.g. Pop Rivet.
Injecting foam would suffice for compression loads instead of ribs.
Rivet a few aluminum doubler plates where other components bolt on.
If you want to thicken the top and bottom spar caps, just rivet on another layer of aluminum.
One advantage is that you could taper the cross-section from front to rear.
 

BBerson

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It's a good idea if you have a slip roller to coil the sheets.
I don't think flanges would look that great. Some gliders have external lengthwise flanges (Blanik, Pilatus, I think).
But not end joint flanges. Four foot tubes would be mailable. But my slip roller is only 3'.... So three feet it is!
Could be 2024, 7075 or whatever, depending how thick your slip roller can handle.
 

Victor Bravo

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The Pilatus glider fuselage is a pretty good structure. It (probably) allowed the controls, bellcranks, stiffeners etc. to be put in to each half before the halves were joined. A lot easier than crawling back there to buck rivets. Just like a plastic model airplane. And the glider was fully aerobatic.
 

BBerson

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Yeah, the Pilatus B-4 was relatively simple. The construction was as flawless as anything I have ever seen.
Instead of external flange rings, the connections can be thick ring doublers with several bolts. Much like the tailboom attach point of a JetRanger is bolts.
 

Hot Wings

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If you are going to all of the trouble to build from sheet why retain all of the disadvantages of a tube? Might as well build a flat sided structure. The BMG 6 is an example, though probably more complicated than needed.
Over View.jpg

A triangular boom is another option. No rollers needed, just a brake.
 

mcrae0104

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I see that I have once again come up with a solution in search of a problem. :p
I think it would be fun for the challenge. Plus you could tailor the taper and thickness at various locations. Never let practicality get in the way of a fun idea! All you need to do is scale up this procedure:
 

davidjgall

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Hovey's Wing-Ding II used a large doubler rolled around a tube to make the tail boom. You might dig up those old plans to see how it was done half a century ago.
 

wsimpso1

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Once you are fabricating a tube from flat stock, it might as well be tapered to tailor stiffness and strength, allowing a lighter boom than with a straight tube. Yeah it will look like an aft fuselage, but less weight is a great thing to get in exchange.

Billski
 

Mad MAC

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The other option to get a tail boom cheap is to find one from a time ex'd Robinson R22.

Just make sure no ones been near it with cherry rivets.
 

Mad MAC

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Because there is a push rod guide in the middle of the boom that can wearout before life limit. Replacement of the bush requires the dismantlement of the boom and its very hard to reassemble with all solids. They may from time to time get reassembled with cherries, unfortunately the fatigue environment of the boom means cherries are not suitable (smoking fasteners, loose rivets cracking, etc, etc).
 

proppastie

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Boku posted the tail boom for one of the HP gliders, it was sheet metal.
 

rv7charlie

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You might be surprised at the lengths to which people will go to save weight. Find some pics of an uncovered Bucker Jungmann steel tube fuselage. the longeron tubes drop in diameter at almost every intersection as you move toward the tail.

The 'rolled tube' fuselage concept is everywhere; RVs have them; Thorp T18s have them; Mustang IIs have them, etc etc. They're just tapered instead of constant diameter. Pulled rivets are everywhere, too (T-18, Sonex, etc). No reason I can think of to not 'roll your own', except the biggest show stopper for most of us: having the engineering chops to ensure that it's both strong enough and light enough. I also see no reason for the tabs; just lap the seams with properly engineered joints. The RVs have lap joints in the top skins.

Charlie
 

pfarber

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Monocoque fuselages were given up for a reason. Unless you go with ultra rigid composites, you need a pretty thick and heavy tube to do what a semi-monocoque structure will do.
 
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