N6408C Bearhawk 4 Place Builders Log: The Skylight

The skylight... Most taildraggers I've flown have a skylight, or "green house". The Bearhawk plans and manuals do not account for a skylight, so some modifications are necessary to install one.

There are four stringers at the top of the cabin for a fabric covering. The two middle stringers sit up higher than the two outboard stringers, a.k.a. the Bearhawk Hump. So my modification will require that I lower the two middle stringers. At least that's what I started when I began the mod.

Here is an in depth discussion on the subject at the Bearhawk Builders Forum.

I decided to go with 1/16" polycarbonate (Lexan) for my skylight material. Yes it scratches easier than acrylic (plexiglass), but polycarbonate is virtually indestructible and won't crack if installed correctly.

You can see in this picture the two middle stringers are a small diameter round tube with supporting stand off tubes holding their shape. The two outside stringers are 1/2" 4130 square tube. I removed about 3/4" from the middle two standoff's. At the suggestion of another BH builder, I could just "squeeze" down the stringers and weld them back in place, creating the lowered effect I wanted.

After standing back and scratching my head for 30 minutes, I decided I did not like where this was going. How was I going to fastened the polycarbonate to these skinny middle stringers? I liked how the two outside stringers (1/2" square tube) could be drilled with holes for AN3 bolts. So I rummaged through my scrap metal bin and found two sticks 1/2" 4130 thin wall tube that were identical to the two outer stringers. I decided to shape these to match the outer ones using two bolts on my welding table. I slowly worked the tubes every 1" until I got the curve I needed.

Using string I aligned the new tubes to the outer tubes.

Two items are in this picture. 1.) I used 3/4" aluminum angles to create a termination and attachment point for the rear of the skylight. 10-32 Nut plates and 426 flush rivets will accept the 10-32 SS screws. The fabric will also terminate and wrap around these angles. 2.) I used a piece of 1/2" square stock and drilled guide holes for the square tubes for spacing and consistency. This was my drilling jig for the remaining holes.

It's now time to start sizing and drilling holes in the polycarbonate. It's important to use drill bits specifically for plastic. I clamped the polycarbonate to the stringers and made my first 3/16" hole at the rear center location, and then out from there one by one. After each hole was drilled I inserted a bolt and finger tightened a nut. This kept the holes lined up and the poly formed nicely.

With the bolts temporarily in place. I purposely cut the poly sheet larger than needed. I will trim to size once the wings and wind screen are installed.

Here is the poly with a layer of the protective coating still in place. I over drilled the 3/16" holes to 7/16". This will allow for expansion and contraction of the poly, as it is known to do. For the final installation, large SS washers will be used with the 10-32 bolts.