Spars in a box

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cluttonfred

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A few years ago in the Bending-beam spar carry through schemes thread, Himat said “Make the overlap vertical and the spar stubs can go into a box that carry most of the load with all the surface of the spar caps to transfer the load. The pins/bolts then only lock the parts in place.”

To me, that sounds like a very interesting way to reduce the load concentrations that would be very suitable for a simple wood-and-fabric design. Does anyone know of an example of a cantilever wing (in any material, though I like wood) in which the spars fit in a box structure in the fuselage as Himat described?
 

mcrae0104

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To me, that sounds like a very interesting way to reduce the load concentrations that would be very suitable for a simple wood-and-fabric design.
Sure, it would reduce the concentrated load at the pin. Do you really need to though? And how much additional weight will it cost to build such a box than deal with loads at the pins? Consider how the fuselage feeds the load into the frame/bulkhead that carries the spar box, and you might decide load-bearing pins aren't so bad.
 

cluttonfred

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Thanks, guys. I am still trying to understand this approach as I generally imagine metal fittings as the primary way to connect fuselage and wings. This approach seems to use fittings only to hold everything in place and I am looking for examples. I do have Sky Pup plans so I will take a look.
 

wsimpso1

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I never did see how that could be any good. If it goes in and out fairly well, it will have a little slop and will then tend to load along a line at each end. If it is fairly tight, you will have the devil's own time getting it in and out.

Pins or bolts with matched and well designed bushings makes tons of sense to this mechanical engineer.
 

Chris Matheny

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I've toyed with the idea but you'd have to have a constant radius to avoid edge loading. Hardware looks better every time I look at it.
 

cluttonfred

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Sorry, that quote was from the larger conversation about bending beam arrangements as used in some sailplanes. As I am sure you know, the wing-fuselage attachments in a bending beam setup are essentially hinges that support vertical and drag/anti-drag loads and the left and right wing spars are attached to each other for the bending loads. These are the options we discussed, and it was the vertical overlap that brought up the spar box idea.
bending beam ideas.jpg
 

mquinn

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RV-12 has what I think you are looking for. I worked with a friend building one and we got the wings fitted just recently.
 

TFF

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In wood, I think it would be very challenging to make the spar. The laminations are going to be numerous and thick at that end of the spar. I do not think wood would like the carry through taper. It’s going to want to be full width to where it terminates.
 

Mcmark

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The Giles aerobatic aircraft are composite but the wing spars fit in the box and overlap horizontally.
 
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