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Wag 2+2 wood wing concerns

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Joined
Nov 24, 2020
Messages
28
Location
Clarkston WA
I mentioned in an introduction thread that I'm scratch building a Wag Aero 2+2. I have the fuselage and empennage essentially done except for pulley installations. I built one wood wing last year and was getting ready to start the other.

I spent a lot of time working up what I thought of as improved flaps. The original design does not have flaps, but there is a provision for them if you want. Just a sketch.

My idea was to increase chord and width. I fiddled around fruitlessly trying to see if the additional forces would be a concern. Finally, after reading all sorts of threads here, I ended up reading texts on statics, stress of materials, wing analysis, etc. I'm no engineer, but I learned a lot.

So, the problem: I did a basic static stress analysis on the spars of the Wag design. I drew a free body diagram of the wing using basic assumptions. Then I used the standard beam formula and calculated moment of inertia of a rectangular spruce spar and ran the numbers. It is supposed to be able to go to a GW of 2200. I used 3.8 G and factor of safety of 1.5.

Boom.... Bending moment at the lift strut was over 70,000 inch-pounds. The front spar is .75 X 6.25 at 190 inches long with two 3/32 birch ply doublers at the lift strut. Rear spar is .75 X 4.5 at 190 inches long. Bending stress worked out to between 11,000 psi to over 14,000 psi on the front strut, depending on where I put the center of lift.

ANC-18 lists Spruce's fiber stress value at 6200. So the calculated bending stress is around double what the spruce is able to handle.

To see if I had the concepts right, I did the same thing with my '46 Commonwealth Skyranger. it has spruce spars, etc. At gross weight of 1450 and applying the same 3.8G and 1.5 FOS, the bending stress at the lift strut was (drum roll) 6200. Just like ANC-18 says.

I can go into more detail later if people are interested. But I'm wondering if anybody has ever heard of this concern coming up before on a Wag 2+2.

I admit I harbored suspicion in the back of my head for a long time. The wood wing looks like the original Piper cub wing with a few more braces. But I focused on other things thinking the design must be fine. Quite a few have been built. I should have looked earlier, but it was only recently that I made the time to even learn at how to look at these things.

But now I'm just not confident in the wing. In my first-run envelope back calculations (i.e. Excel cobbled spreadsheet), it looks like the wings at 2200 GW can handle 2.8 G with no FOS.
 
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