Center Section Flooring

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Varmint
Posts: 20
Joined: Thu Nov 05, 2020 2:13 am

Center Section Flooring

Post by Varmint »

Does anyone know all the functions performed by the plywood floor of the center section?
Does it just support the fuel tank or is it also needed to resist loads in the center section and keep it "square". I could see this particularly the case in the 3 piece wing, not so much the one piece wing where the spars already resist moving opposite directions due to the drag/anti drag cables in the wing.

I ask because I have read that some have put two fuel tanks, one in each wing root, instead of the center section to add some fuel capacity.
If this is done, is the heavy plywood floor of the center section still required?
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Richard Roller
Posts: 240
Joined: Mon May 22, 2017 11:14 am
Location: Olathe, Ks.

Re: Center Section Flooring

Post by Richard Roller »

It's my understanding that the plywood floor does resist drag loads in the center section. The one piece wing has no drag/anti-drug wires in the center section and the cabanes will not react the loads.

Maybe Oscar will weigh in on this, he has the background to explain it.
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taildrags
Posts: 637
Joined: Tue Mar 07, 2017 10:39 pm

Re: Center Section Flooring

Post by taildrags »

In my opinion, the plywood floor in the centersection is a good and useful thing, but not for structural reasons. Going back to the 1932 Flying & Glider Manual, the wing was one-piece and that plywood is called out as 1/16" thick "for supporting tank and bracing center section". Several different photos in that manual show the centersection cabanes braced with a single pair of X-brace wires between the forward and aft cabanes on the starboard side to prevent them from pivoting forward or aft, while in other photos there are tubular diagonal cabane braces going forward the way most Air Campers are being built these days. The X-brace wire method makes it a lot more difficult to get a front-seater in and out of the cockpit. While the 1/16" plywood floor in the centersection can provide diaphragm strength to resist differential movement between between the port and starboard cabanes, it's not shown to have any stiffening members behind it so it wouldn't be able to resist a whole lot of that movement without buckling or tearing out at the fasteners. The Manual does not provide any indication of how the plywood floor supports the fuel tank.

On the other hand, the Improved plans clearly show diagonal cabane braces to hold the centersection rigidly square, which means the plywood floor does nothing in the way of resisting drag forces, whether symmetrical or asymmetrical. In the Improved plans, the plywood is called out to be 1/8" thick, which would give more diaphragm strength and stiffness if it were needed, but it isn't since the cabanes are braced against any drag/anti-drag forces on the centersection. In the Improved plans, the centersection fuel tank is clearly shown to be supported by 3/4"x3/4" wood supports fastened to the faces of the spars, so the plywood doesn't support the tank at all either.

The reason I like the plywood floor there (and my airplane doesn't have it but I wish it did) is to allow that compartment to be used as an accessible space to stow things, a la Mike Cuy's airplane. In the 3-piece wing supplement, an aluminum access panel is detailed if the builder wants to incorporate one, and even if you're building your plane with a centersection tank, having the plywood floor in there would make it easy to convert the space to a stowage area if you or a future owner wants to move the tank out of the centersection either to a tank up front in the fuselage, or a pair of them out in the wings.

Oscar Zuniga
Medford, OR
Air Camper NX41CC, A75 power
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