wing root skin fit-up

Productive day at the shop today.

First order of business, sorting the parts we ordered from Aircraft Spruce.  Some hardware machine screws, lock nuts and castle nuts for Ron.  I also ordered 4 drain valves for the wing tanks (2 for me, 2 for Ron).  I wish this pandemic was over.  I hate paying shipping when I could drive to Brantford to get stuff - probably cost more in gas than shipping, but who doesn't like going to an aircraft parts store?!?!

One of the appealing features of the 750STOL is the visibility from the cockpit.  Most high wing airplanes have a wing spar/structure that carries through the top of the cabin or the cabin roof itself is solid.  Look at the roof between the wings on this stock photo of a Cessna 172: 

Compare that to the 750 which has a clear panel roof - the visibility from the 750 cockpit is amazing:

How does the 750 design accomplish this?  The wings taper sharply at the root, but the curvature of the upper wing surface creates a very complex curve.  To make this work, the root skins need to a way to mount at the varying taper angle from the tank rib to the root laterally and from the main spar to the rear channel as well.  I needed to make a root skin support angle out of 025.  I started with a piece of 025 strip and bent it to a 30 degree angle along the length of it.

I marked the angle 400mm up from the end and used the dead blow hammer to flatten from the end to the mark.  The goal here is to go from completely flat near the rear channel to a full 30 degree angle at the 400mm mark.  It went easier than I thought.

Next I marked the location for fluting bends between rivet locations on the tank skin.  It takes a bit of tweaking to get the flutes just right, but eventually the angle starts to get close to the curve required and I could tuck it under the tank skin to get the first rivet in near the reach channel:

​The strap duplicator is used to match drill the holes in the tank rib:

Continue to massage the flutes and work the angle until it matches the top of the nose rib:

It took a lot of fussing with flutes, adjusting here, tweaking there to get the angle to lay straight and flat.  I soon realized the cleco I used on the taper side at the spar doubler was preventing me for properly adjusting the flutes.  Once I removed that one cleco, it was much easier.  Unfortunately that misaligned the hole slightly, but I can recover from that when I upsize the holes in final drilling.

Rivets across the angle and into the nose rib.  That's better:

Root angle, looking forward:

Sometimes it's easier to take the plans out of the binder and put them beside me on the floor cutting mat instead of walking back and forth to the bench.

With the root skins laid out on 016 sheet, I used the bender as a straight edge and cut the templates out:

First fit up of the root skin looks promising.  The instructions aren't very clear on how this interacts with the nose skin, but my measurements look good.  The root angle also seems correct:

Laid the skin back on the bench and used the template cut out on the CriCut maker to lay out the curve that sits inboard and will match the curvature of the roof line and the offset of the root rib:

Finger clamps at the rear channel and spar, along with some wide masking tape to get the first cleco in the rear channel:

With all the holes drilled and clecoed along the rear channel, the skin appears to be fitting well:

Happy with how the root skin is fitting up, but I got a bit or work yet to do on it, including rivets along the root rib and the root angle.

Progress is progress.  I've got some more hardware for the fuel tanks, and I expect the pressure transducer I ordered online to arrive this week so I can begin testing it's suitability for fuel quantity measurement.  Soon I'll be able to flip the wing over on it's back and work on the bottom skins.

Thanks for following along - more to come soon!