EAA Sport Aviation - January 2010

Bending Cap Strips

BY JOE NORRIS

WOOD IS A WONDERFUL material for building an aircraft. It has “memory.” ;at is, it can be deflected to an amazing degree and will still return to its original shape. But this memory can sometimes cause the builder problems during construction. One example is when building wood wing ribs. ;e top and bottom components of the rib, called cap strips, are curved to give the wing its proper airfoil shape. Of course, the pieces of wood used to build the rib are all straight sticks. Sometimes builders are able to force the straight cap strip pieces into the rib jig and hold them in place while assembling the rib, but it would be better if the cap strips had some curve to them before assembly. Here’s an easy way to pre-form the cap strips.

You’ll need a large block of wood that will become your forming jig. A 4-inch by 4-inch post works great for this. Take a length of the 4-by- 4 and draw on it an arbitrary curve. You’ll want the curve to be fairly gentle on one end and a bit tighter on the other. ;e amount of curve is not critical and doesn’t need to be a tight radius. After you’ve drawn your curve, cut the curve with your band saw.;View Illustration 1 to see what your block will look like after it’s been cut.

;e top cap strips for your rib will probably need to be curved a bit more than your bottom cap strips. (;e bottom ones may even be straight.) ;e amount of pre-set curve in your cap strip will depend on how far into the block you put the wood. It might take a couple of trial-and-error attempts before you find the exact curve that works best for your project. ;e curve doesn’t have to be exact, because all you’re doing is making it easier to fit the cap strips into the rib jig. ;e jig will hold the cap strips in position for the appropriate airfoil shape when you assemble the rib. You need to soak your cap strips in water to make them flexible and ready to be pre-curved. You can

do this by leaving them in water overnight, or you
can heat up water to near boiling and soak the cap
strips for a half-hour or so and they’ll be ready.

Once you determine how much curve you want to put in the cap strips, you can mark a line on the inside of your block where the end of the cap strip needs to be positioned to get a consistent curve. You can then line up a number of cap strips in the block and clamp the block halves together, trapping the cap strips in the curved jig. Illustration 2 shows the cap strips clamped in the block using a vise. You could also use C-clamps or bar clamps, or you may have another way to clamp the blocks together while the cap strips dry. Just don’t clamp the block together so tightly that you crush the cap strips. You need to put only enough pressure on them to get them to take the curve.

Leave the cap strips clamped in the block until they’re completely dry. ;is will probably be at least overnight, but don’t be in a rush. A full day might be needed to get the cap strips to dry thoroughly. Once the cap strips have dried you can remove them from the block; you’ll find they now have a nice, gentle curve that will fit in your rib jig much more nicely and make assembling your rib a snap!