I went with a Gretz pitot mount, which has all of the hardware included in the package and works perfectly with the GAP 26 pitot tube I'll be installing. The kit consists of the mast itself, a backing plate to secure the mast in place with screws/nutplates, a washer plate that acts as a spacer for the dimples and allows washers to be installed to change the angle of the tube as needed, and the necessary screws, rivets, and nutplates.
The only piece I had to fabricate myself was an angle to attach the backing plate to one of the wing ribs for strength and to allow the skin to remain flush to the rib with the weight of the pitot tube pulling the skin down.
Using the suggested hole pattern in the instructions as guidance I picked out a piece of scrap aluminum to use for the angle and cut it to size. The most difficult part of the whole process was bending it to a 90-degree angle.
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| I clamped the piece over a thicker angle to give it a nice sharp radius as it got bent. |
The backing plate is blank, save for the cutout for the pitot mast and the screws to secure it, so I needed to drill all of the holes myself. I chose to mount the tube where most builders locate it - one bay outboard of the aileron bellcrank/tiedown bracket bay, on the outboard rib. I clamped the backing plate in place on the inside of the lower spar flange making sure I had adequate edge distance for the holes, then match drilled it to the spar.
I then removed the plate and marked and drilled the rest of the holes. I slightly modified the hole pattern suggested by the instructions to accommodate the custom angle I had made. From here I only loosely followed the instructions and ended up doing things mostly in my own order.
After drilling and cleaning the parts with acetone to remove the markings, I clecoed the bracket back onto the spar. My next task was to match drill the angle I had made to the outboard rib, but I needed to make sure the backing plate would be laying flush against the skin before I could determine the proper location for the angle. I had bent the bracket to match the curve of the skin a bit more, but just to be safe I clamped a scrap angle over top of everything to act as the "skin" and keep things flush.
With the "skin" in place I clamped the angle to the backing plate and to the rib for drilling. I had pre-drilled holes in the angle earlier to use as a guide for drilling the rib.
Once the angle was securely fastened I removed the backing plate for better overall access while drilling.
To finish this part of the job, I secured the backing plate back in place and match drilled through the backing plate into the angle. I then removed the entire assembly and turned my attention to the wing skin.
The hole for the pitot tube obviously needs to be made in the skin for it to be installed, and this is definitely the most nerve-wracking part of the whole process. To start I checked the fit of the pitot mast to the backing plate and washer plate. I'm glad I did, because both of those cutouts needed to be enlarged a bit to allow the base of the mast to sit flush against the plates.
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| The red sharpie marks areas I needed to trim a little to allow the mount to sit flush against the plates when installed. |
Once I was sure the shape in the backing plate was correct, I clecoed the plate to the skin with some shims to act as the "spar" and marked the hole for the mast.
A bit of measuring and remeasuring to make sure things were correct, and I match drilled all of the holes for the backing plate into the skin.
Now for the truly fun part - cutting the hole for the pitot mast. I removed the vinyl and retraced the cutout from the backing plate, and I even set the pitot tube itself on the skin and used a purple sharpie to trace the inside of it as best I could just to make sure things were accurate. Once I had psyched myself up I started the cutout by drilling the perimeter of the smaller traced shape.
From there I made liberal use of the dremel tool cutoff wheel to start removing more material. It's very messy at first but it's easy to smooth out later.
Little by little I made the cutout bigger, constantly laying the back plate over top of it to check the fit.
Once I got close to my traced line I clecoed the backing plate back in place to get the final shape as close to the original cutout as possible.
After some more cutting with the cutoff wheel and a quick switch to some of the sanding bits for shaping the sharp angles and hard to reach areas, I was eventually able to slip the pitot mast through the hole.
A quick shift of the skin and the mast fits flush to the inside of the skin. Perfect!
Only thing left to do to these parts is to prime and dimple as necessary and then they'll be ready for final assembly. Of course, I have an inordinate number of things I have to do before I get to that point.
