spar design: single box vs. twin beam

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Peterson

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St.Rober, MO
While I still have a lot to learn, it would be cool to eventually build an aircraft of my own design. I see two patterns when it comes to wing design : a box design or a twin beam. What are the pros of cons of each?
 

Dan Thomas

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Sep 17, 2008
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My Jodel has a box spar. Big heavy thing that is easily the biggest part of the whole job. It takes all the lift and drag and torsion loads, it carries the main gear, and it is the seat. The fuselage is minor in comparison. The wing bolts to the fuselage with two massive bolts made from 4130 bar, and three small bolts at the ends of the root ribs and center of the trailing edge. Those two in the fat root ribs control the torquing of the wing. The ailerons are on false spars mounted on the outboard ribs.

That box has its drawbacks. There's a lot of wood in it (but it's very strong). Inspecting the interior has been impossible until now. I suppose one could drill small holes and ffed one of those new snake cameras into them, but some reinforcement around the holes might be wise, and there are numerous plywood diaphagms inside the box to maintain its cross-sectional shape that divide it into many compartments, so many holes would be needed. Repairing rot or other damage in that spar would be a real pain. All the ribs are glued to it, so the builder had better be careful about alignment. Can't just tweak a strut fitting to fix a wing-heaviness problem.

Big image here. Click on it to magnify it: https://s-media-cache-ak0.pinimg.com/originals/ab/6c/62/ab6c62142d9b0e6c7ee477aac0d95460.jpg


 

Pops

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My Jodel has a box spar. Big heavy thing that is easily the biggest part of the whole job. It takes all the lift and drag and torsion loads, it carries the main gear, and it is the seat. The fuselage is minor in comparison. The wing bolts to the fuselage with two massive bolts made from 4130 bar, and three small bolts at the ends of the root ribs and center of the trailing edge. Those two in the fat root ribs control the torquing of the wing. The ailerons are on false spars mounted on the outboard ribs.

That box has its drawbacks. There's a lot of wood in it (but it's very strong). Inspecting the interior has been impossible until now. I suppose one could drill small holes and ffed one of those new snake cameras into them, but some reinforcement around the holes might be wise, and there are numerous plywood diaphagms inside the box to maintain its cross-sectional shape that divide it into many compartments, so many holes would be needed. Repairing rot or other damage in that spar would be a real pain. All the ribs are glued to it, so the builder had better be careful about alignment. Can't just tweak a strut fitting to fix a wing-heaviness problem.

Big image here. Click on it to magnify it: https://s-media-cache-ak0.pinimg.com/originals/ab/6c/62/ab6c62142d9b0e6c7ee477aac0d95460.jpg


My Falconar F-12 was built the same way as the Jodel but heavier and stronger for bigger engines and higher airspeeds. It was stressed for +9 and - 6 G's if I remember correctly. I have a set of plans.

Dan
 

Kiwi303

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Jul 22, 2015
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En Zed. Aka The Shire.
I would like to try converting the Jodel box spar from Ply into Carbon over foam.

I'm sure it could come out lighter for the same strength...


I have a set of D.11 Jodel plans waiting for me in the Post office until I get down from the family farm back to town to collect them.
 

rbrochey

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Oct 17, 2010
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Gallup, New Mexico USA
I'm building the Falconar F10 which incorporates the box spar based on the Jodel... somewhat easier to build. That was one of the things I liked about the design was the 9G wing that you can detach (with a few helpers) and haul the plane down the highway in a full size pickup. And it's an wood aircraft which is another very appealing feature. The plans are very precise and the support from Chris Falconar is 2nd to none. Love that box spar!
 

Pops

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I'm building the Falconar F10 which incorporates the box spar based on the Jodel... somewhat easier to build. That was one of the things I liked about the design was the 9G wing that you can detach (with a few helpers) and haul the plane down the highway in a full size pickup. And it's an wood aircraft which is another very appealing feature. The plans are very precise and the support from Chris Falconar is 2nd to none. Love that box spar!
I built a large box on wheels that bolted to the fuselage at the wing attach bolts so I could transport the fuselage to the airport. Drove back home and loaded the wing. Raised the fuselage off the box and rolled the wing in place on the wheels and bolted up. Picture of the Falconar F-12 on left.

Dan
 

mcrae0104

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While I still have a lot to learn, it would be cool to eventually build an aircraft of my own design. I see two patterns when it comes to wing design : a box design or a twin beam. What are the pros of cons of each?
What material will you be using?

Also, when you see a rear spar on a design, rarely is it acting as a beam; usually a single bolt connects it to the fuselage and its purpose is to transmit wing torsion.
 

rbrochey

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Oct 17, 2010
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Gallup, New Mexico USA
I built a large box on wheels that bolted to the fuselage at the wing attach bolts so I could transport the fuselage to the airport. Drove back home and loaded the wing. Raised the fuselage off the box and rolled the wing in place on the wheels and bolted up. Picture of the Falconar F-12 on left.

Dan
That's a beauty Dan!
 

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