My Wooden Airplane
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2/8/2012 - 2/12/2012

Wing Struts

These were kind of fun! As you can see, they are hollow, airfoil-shaped aluminum extrusions. I got them from a place called Carlson Aircraft (http://www.CarlsonAircraft.com). At each end is a solid piece of aluminum approximately 1 inch square by 7 inches long.

I did have a few issues, however. The aluminum blocks are all straight except for the top of the rear strut. The plans show that it is cut at an angle, but they don't say what that angle is supposed to be. (And those particular drawings are not done to scale, so I couldn't simply measure it off the plans.) What I ended up doing is making a mock-up out of wood, and then transferring that to the aluminum.

Also, the bottom of the rear strut actually attaches to the front strut (as opposed to being attached to the fuselage). The plans show a bracket that is is 6 inches long, but they don't say how wide it is. I guessed, and my first attempt ended up being about 1/4 inch too narrow. (The rear strut actually came into contact with the front strut.) Also, although it makes sense to use two brackets (one on the top and one on the bottom), the plans don't specifically say whether I should use one or two brackets. I ended up using two, but that required a bunch of washers to space the rear strut equally between them.

Finally came the time to set the "wash-out". Wash-out is an intentional twist to the wing so that the tip is at a lower angle of attack then the root. This ensures that the wing root stalls first, and allows some degree of controllability during the stall. (If the tip were to stall first, the plane might become uncontrollable and eventually end up in a spin.)

The plans specify a washout of 2.0 to 2.5 degrees. My original plan was to build the wing flat (with no washout) and then force the wing to "twist" by varying the length of the wing struts. This would have worked out beautifully if there wasn't any plywood sheeting on the leading edge of the wing. But when I attached the sheeting, I noticed that the wing became much more stiff and a lot harder to twist. So, when it came time to set the washout, I supported the wing tip with a ladder at the trailing edge, and put about 20 pounds of weight on the leading edge to get the right amount of twist. This seemed really excessive to me, so I called the designer, wondering if I might have screwed up.

He assured me that while it is preferred, washout isn't really required, so long as I didn't have any "wash-in" (where the tip is twisted up so that it has a higher angle of attack than the root). So I removed the weights and used the contour jig to verify that I don't have any wash-in, and set the lengths of the struts accordingly.

Total Time: 13.0 hrs.