3 Piece Box Spar Wing Modification For Single Seater SSDR

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buckie555

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I'm in the process of building a wood and fabric single seater of an established design that currently has a 1 piece cantilevered wing of box spar contruction. I am keen to modify this to be a 3 piece construction with a fixed centre section and two outboard sections, each approximately 2.5m long.

I realise that I will need to do a full structural analysis of my proposed modifications, however for now I would just like to get a rough ballpark idea of how heavy these modifications are likely to be. I am building to quite tight weight requirements and want to check that it's in the realms of being feasible for my airframe weight budget.

I have looked at a few different established 3 piece wooden spar designs such as the Chilton DW1 and the Falconar F11 however they have slightly different spar construction to mine. Namely solid plank spar and split caps.

My box spar is constructed of full, constant width, caps top and bottom that taper in thickness (height) and faced on either side with a ply shear web.
To give a sense of scale the spar is approximately 75mm constant width and where I envisage the joins, the spar cap thicknesses are approximately 35mm and 30mm top and bottom respectively with a spar cross section height of 150mm.

From what I've seen externally, I believe the Colibri MB2 3 piece wing was a similar construction but I could be mistaken.

My initial thoughts are for a fairly standard arrangement of vertical straps made of 4130 or 2124, reinforcement blocks, reinforcement ply doublers, etc for both the main box spar and, to a lesser extent, the rear drag spar.

Has anyone got drawings of the MB2 wing join arrangement or for that matter any other wooden aircraft of similar spar construction to above. I have an idea of how to proceed but I would like to see a proven example before I start doing structural calcs and FEA etc.

Any guidance, insight, gotchas or pointers to resources would be very much appreciated.

Thanks, Neil
 

mcrae0104

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I would like to see a proven example before I start doing structural calcs
Take a look at KR-2s. More images available from the google.
1623120839484.jpeg
I suspect your caps will need to grow in depth near the joint to accommodate the bolts (whether a single row per cap, or a double row as seen above). Your weight penalty will be the extra wood, the plates, and hardware. Perhaps also whatever scheme you cook up for the skin closure as well. Not a terrible price to pay if you don’t have space to fabricate a single, long spar.
 

Hot Wings

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Good example.
Keep in mind when doing the calculations for the wood/bolt bearing load that you can't use the total area of contact like we tend to do with thin ridged metal bolted joints. Long skinny bolts tend to bend and the wood crushes/deforms more closer to the metal strap.
Hardwood doublers may be worth the weight in this area?
Phenolic inserts for the bolt holes is another option that has been used by others.
 

fly2kads

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The Taylor JT-1 Monoplane and the Turner T-40 are two other examples that I know of that use a similar type of joiner between the center section and the outer panels. The Mono uses a box spar, and the steel straps have two rows of bolts and lightening holes. The T-40 has a laminated plank spar, and the steel straps have a single row of larger bolts. The T-40 fittings would be a little faster to make. Neither one uses the vertical section of the steel fitting pictured above.
 

buckie555

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Those examples were exactly what I was looking for, thank you. That KR-2s solution is the first one I’ve seen where the upper and lower tie bars are joined together. I like the simplicity and the assuredness of alignment between the two it provides. I take your point Hot Wings regarding the area. I can see that the relatively soft spruce within the straps sandwich would provide for lower bolt preload than would otherwise be the case, so we can’t bank on the shear on the strap faces taking the load but instead the shear within the array of pins (bolts). My initial thought was that the pins elongating their bores and deforming the caps could be the limiting factor. I see there’s a trade-off between bolt size and number to balance their weight, minimum distance to the cap edges and gaining enough cross-sectional area to take the shear and not deform the bores. I like the idea of inserts. Perhaps ash cylinders or maybe even aluminium.
 

cvairwerks

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Ignore the mess in the warehouse, but here's a couple of photos of the spar joint for my Fairchild AT-21. The one photo is of the inboard, lower joint on the left side forward spar, while the second is the outboard forward spar. I don't know if the outboard is an upper or lower section. The joint for the rear spars are identical, but smaller. The forward straps are about 27 inches long, and the spacing at the joint bolt is about 8 inches. The forward spars have an upper and lower set at each joint and the rear spars likewise. There are some steel tubing splices that go with the bolts between the wing sections, but there are boxed and buried for now.
Forward.jpgOutboard.jpg
 

cvairwerks

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Tim, same style, with the exception that there is a built up "I" section that bridges the front bolts and the back bolts, that resides in the gap between the spar ends. As to the bushings, same style, but I'm not sure if they are magnesium or aluminum.... Lots of corrosion on them and the bolts will not press out. Somewhere, I've got the drawings for the PT bushings and I'm hoping the AT drawing is on the microfilm. Making them is going to be a job for a cnc lathe with a bar feeder or an old style screw machine!
 
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