19 – Fitting Dynon Pitot Tube Mast
It’s been a while since I’ve penned an update on the build progress … I’ve been distracted by a major decision … I’ve just retired!
I’ve had a wonderful, rewarding and privileged aviation career, both military and civil. I’m leaving my airline a little bit earlier than I had planned, but with all the downsizing caused by Covid 19 I have decided to handover to the youngsters!
So lots more time to concentrate on building an RV14 … : )
Having fitted the tanks, I thought it was about time to fit the pitot tube mast. Garmin do not market one, but there are various other options, and I liked the look of the Dynon mast with its neat welding and anodised finish.
Some people have mounted the mast outboard of the aileron bellcrank bay, but I decided to stick with the Vans position. This also means that the mast is inboard of the tie down loops, which might just make things easier when attaching rope etc on landaways.
On the other hand, it might be harder to route the pitot and AOA tubes to clear the aileron pushrod, but others seem to have managed just fine, so I’m hoping for the same.
Dynon do provide some rudimentary instructions for fitting the mast, and a template. However, the template has not been printed to the exact scale, so I ended up scanning it and then adjusting the print size to match the real world. Also the rivet positions along the front edge DO NOT match the RV14 spar, so you’re on your own for that bit.
First of all, having decided on the mast position, I clamped it in place and then match drilled through the spar. Aligning the front edge with the line of set rivets on the spar seemed about right.
The main reason for using the template, suitably adjusted, is to work out where to cut the clearance hole in the precious lower skin!
Having placed the mast through the template cut out hole, I marked where I had just match drilled …
… then I carefully aligned the template with the holes in the skin.
It goes without saying, but I will anyway. Be very careful you choose the correct lower skin and also the correct place to position the template!
I used spray adhesive to ensure the template didn’t move once positioned.
I drilled the attachment holes in the skin using the template positions.
As suggested by Dynon I used a step drill to begin the process of removing the cutout …
… and then a Dremel cutting disc to join the two holes.
I left plenty of excess material to file away progressively.
My half moon file worked well for this, and it didn’t take very long …
… to end up with a nice fit.
The moment of truth comes when you cleco on the lower skin to the wing and see if the mast front holes all line up!
Phew!
Then it was a simple process to match drill through the previously drilled holes in the skin into the mast base.
Some holes needed a 6″ long drill bit to ensure clearance.
The Mast came with a little angle bracket which is attached to the adjacent rib, to provide additional rigidity.
But how to fit the flipping thing???
After some head scratching, (I end up doing lots of that), I decided to clamp it to the rib with the bracket base over the specified hole in the mast base, and then match drill.
Access is difficult, hence the mirror which I used to check everything before drilling, and also to take this picture.
The bracket needed trimming to clear the pitot mast weld.
Having chosen a suitable place and drilled the holes in the bracket flange which will be against the rib, I then clecoed it into place & match drilled through into the rib.
Nobody mentioned you have to be a contortionist to build an RV!
The Garmin GAP 26 Pitot Tube is designed to fit the AN5812 pitot tube mount standard. Although the profile of the Dynon Mast matches this, I found it needed a tweak.
Basically it was slightly longer and narrower than the Garmin profile … I found that by squeezing it in the vice I was able to gently coax it into the correct shape.
I also filed a taper along the inside edge to enable the Garmin mast to seat snuggly.
The final head scratch involved working out how to mark the position of the attachment holes.
In the end I just marked and measured!
I haven’t yet decided whether to countersink these holes, or whether to use dome headed screws.
A countersink screw never really fits well on a curved surface, but would be less draggy I guess.
A decision for later on.
5 comments on 19 – Fitting Dynon Pitot Tube Mast
Hi Steve, Congratulations on your retirement. Great tips, much appreciated. I started my RV14 last year. Keep up the good work.
Marty(C777 Ret to Oz)
Thanks Marty, wilco! Enjoy your RV14 build … it’s very absorbing : )
You didn’t mention it, but it looks like you countersunk the holes in the mast base to match a dimple in the skin. You must have done that with an extension that works on a countersink cage, yes?
Hi Chip
Yes I did countersink the holes, and I’ve just been looking back at some pictures to try and remember how I did it! I took a picture illustrating the problem with the cage, but for some reason I didn’t take one of the solution … hmm. And I’m sorry, hand on heart I just can’t remember. There were a few situations in the build that I resorted to to careful “free-hand” countersinking due to access issues, and used the Cleaveland SPEED DEBUR AND COUNTERSINK SET 8” extension shaft. The thread fits the countersink bits.
If you have a 90 degree tight space drill set, there is a threaded extension usually in those kits that can be used for the countersink.
Great job on the install. I’m about to tackle it myself.