Improving strength and beauty in the cabin wing root of an ultralight aircraft

Both of our demonstrator ultralight aircraft (Belite Trike and Belite Superlite) made it home from Oshkosh and are going through a small amount of improvement and upgrading.

I decided to improve the rib attachments from the fuselage to the Lexan upper cabin.  This is an area that is subject to abuse, because every time the wings are folded/unfolded, the cables drag through this root area.

We had been using some carbon fiber ribs for this interior cabin area, and as it turns out, they are not as ideal as I’d hoped.  The movement of the cable over the ribs was cracking these two interior ribs.

So, the first thing we did was remove these two ribs from the left and right side at the top of the cabin.  As a result, the upper lexan windshield lost its support:

In order to replace them, I started with two our our CNC cut baltic birch plywood ribs.  Here is what one of them looks like, before modifications:

It’s a good looking piece of wood.  In order to fit in the upper cabin area, the tail of the rib needs to be cut off.  And each of the round spar attachment areas needs to be enlarged.  So we lopped off the tail and enlarged the spar holes.  And we glued on a reinforcement piece of plywood.  Now it looked like this (one rib shown next to original CNC rib, showing tail cut off and spar holes enlarged):

Another reinforement ‘plug’ also needs to go in each end of the rib, but that won’t be visible until later.  Here’s two modified ribs, showing left and right ribs:

There are two steel ferrules on each side of the upper cabin which line up with pins from these ribs.  We place the rib up to the ferrules, and mark the locaiton of the alignment pins before drilling out the holes.

Now it’s time to mark the locations of the alignment pins.  Here’s the front end of the rib, with the hole location marked.:

And here is the rib, test fitted with the two 1/4 inch pins inserted in the holes. You can see the rib fitting perfectly under the upper cabin lexan windshield.

Here’s another shot of placement being tested:

After fit is verified, additional wood plugs to the rib.  They are fitted and glued in.   Then excess wood, pins, and glue is removed using a scroll saw and sander, providing a final rib:

Let’s look at the ends from the other side as well.  The pin is super strong (although this is not a load bearing point) and the whole assembly is looking sharp:

After placement back in the wing, a carbon fiber strip is used as a load washer across the length.  Screws are drilled and used every 3 inches.  The lexan needs to be trimmed, but here’s how it looks:

So let’s remove it, sand it up and stain it, so the wood will last many, many years:

And let’s install them back in the airplane upper cabin.  To do this, as I said, we used a #8 screw with flat head every 3 inches.  We also used a strip of carbon fiber 1 inch wide, acting as a load washer across the length.  The lexan was ground off (which I actually did on a test fit BEFORE I stained the wood) and the whole thing is looking very sharp, and very sturdy:

After wing reinstallation, here’s a photo of the cables running from the wing, through our new improved cabin root rib:

All very sharp looking.

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