Trent's RV14 Build: OP - Pitot Tube Mast

I have decided to install a heated pitot tube, which meant i needed a pitot tube mast. I chose to go with the Gretz mast, as this can take either a Dynon or a Garmin pitot tube (i will likely go Garmin).

Location, Location, Location

Earlier, when planning locations for wing systems, i left myself 2 locations where the pitot tube could go.

1. At the stock Van's location - one bay inboard of the aileron bellcrank. The PROs: Good access due to an inspection panel on each side, clear of any tie down ropes. CONs: possible interference between the wiring / pitot lines and the aileron pushrod.

2. At a location which is 1 bay outboard of the aileron bellcrank. The PROs: no interference with the aileron pushrod. CONs: possible interference with the tiedown ropes.

In the end i decided that the interference with the aileron pushrod would be a problem i would encounter only once - in the build stage. Whereas interreference with the tie down ropes could be an issue i faced daily.

So: I decided to place the mast one bay inboard of the aileron bellcrank, in the right wing.

Gretz Kit

The Gretz kit consists of a nicely made steel pitot mast, a 'washer plate' and a doubler. The doubler gets riveted to the spar at the front side and also to the bottom skin. I have to fabricate a bracket which attaches to the skin and doubler, and ties the doubler to the closest rib. The mast fits through a hole in the doubler plate and the washer plate, and a hole i need to make in the bottom skin. Nutplates are installed on the mast itself and the mast is held in place with #8 screws into these nutplates. The washer plate is to give some space for the dimples formed in the skin/doubler plate.

Here you can see how the parts go together.

Parts Prep

As always, work starts with deburring. I had to enlarge the holes in the doubler plate and the washer plate to fit around the pitot mast. I also had to adjust the depth of the joggle on the doubler plate so the doubler plate would sit flush with the bottom skin.

The hole in the doubler plate was enlarged to fit the mast with a sizeable gap.

This shows the gap with the factory produced joggle.

This was after the joggle was increased

Forming the joggle - i used 2 scrap pieces the same thickness as the spar flange

I used 2 pieces of scrap the same thickness as the wing spar flange - had to cut one out to fit around the pre-formed screw dimples.

The final location of the pitot is up to the builder - however there needs to be a bracket made up to tie the doubler into the closest rib. The easiest thing would be to butt the doubler up against a rib - however, this would have meant only 3 rivets holding the doubler to the wing spar flange. I elected to move the assembly inboard slightly, and utilise 4 holes. This meant i had to make up a custom 90 degree bracket out of some scrap 0.032" sheet, as standard alu angle was not big enough. I cut this to 3/4" but this was not long enough and resulted on only 11/64" edge distance to the holes. This was then remade, and cut long enough to meet edge distance requirements, and will be eventually installed with pull-rivets to the web of the adjacent wing rib.

The doubler was installed using 4 wing spar flange holes.

A custom made 90 degree angle bracket was fabricated.

A rivet hole pattern was marked out on the doubler, and drilled to #40

The first bracket was not long enough and the holes did not have enough edge distance.

The second bracket was much better. I trimmed the washer plate so it didn't interfere with the angle bracket.

To match drill the angle bracket to the rib, i first drilled 4 x #30 holes in the rib so the eventual rivets would not interfere with the bottom skin riveting - the holes were aligned to be between the rib to skin rivet locations. I then mounted the pitot tube assembly in place and used a straight edge to make sure the doubler lower face was flush with where the bottom skin would be. It was then a simple case of match drilling the angle bracket to the rib, clecoing as i go. I drilled to #40 then enlarged the holes later, once match drilled to the angle bracket.

4 x #30 holes marked and drilled in the wing rib.

Making the BIG CUT in the bottom skin.

This was pretty nerve wracking! I started off by mounting the doubler to the bottom skin using a spacer for the 4 holes where the spar flange would be. I could then match drill the #40 holes to the bottom skin.

Since the #19 screw holes were already dimpled, this made drilling the wing skin for the screw holes a bit more challenging - however i was able to think out of the box and used some 3/16" brass tube, which happened to fit a #21 drill bit inside. I sanded down the outside so it was a tight fit in the dimples. This allowed me to centre drill the oversize dimples to #21 accurately. I then updrilled to #19.

The #21 drill fitted inside perfectly.

Nicely match drilled holes through the dimples.

When doing the parts prep, the doubler would not fit over the pitot mast at all. I had to file it a fair bit to get it to fit. It was a bit of a guessing game as you had to get the mast to fit in the holes, however also have the 4 x mounting screw holes somewhat centred. In the end i filed this so it has a clearance fit on the mast - with a fairly large gap all the way around. This meant that the hole in the doubler was not an exact close fit to the mast, and was therefore no good for marking the hole in the bottom skin.

What to do!

Given my nervousness in cutting a hole in the vary large, expensive, very hard to ship lower skin, i decided a practice piece would be a good idea. This would allow me to both practice how to cut such a large hole, as well as provide a very nice filing template to get the pitot mast fitted perfectly.

So i dug out a piece of scrap which was the same thickness as the skin, and match drilled the doubler in the same fashion as i did the wing skin. But the problem remained, how do i accurately cut a hole which is the same shape as the pitot mast (plus a tiny gap) and is also perfectly centred on the mounting hole locations?

The answer - a joggle stick! What is a joggle stick? It is something i learned from watching a bloke build a boat. You basically use a stick with a sharp point on one end and a unique shape or pattern on the other end. You go around the part and touch the pointy end on the part at one end, then trace around the unique end. You can then remove the part, place the unique end exactly where you traced around and place a dot at the pointy end. What you are left with is a perfect drawing of the part shape.

So i made up a "joggle stick" using a small piece of sheet with 2 holes drilled in it. I then went around the part and marked the 2 holes, or one hole and the end of the sheet. I could then remove the part and get an accurate drawing of the shape of the pitot mast. I was then able to cut this out, align the paper template with the mounting holes on my aluminium template and draw an accurate hole shape in an accurate location! Thank you Steve from Acorn to Arabella!

The 'Joggle Stick' in action to trace the shape of the part.

The 'joggle stick' was then placed back over the marks, and a dot placed at the end. I could then join the dots to get an accurate shape of the part.

Once i had drawn the accurate shape on the aluminium template using the paper template, i marked a line 4/32" inside, and drilled a bunch of #12 holes around this line. I then filed the hole very carefully so that only 1/2 of the diameter of the line was remaining and with multiple test fits to the mast, was able to get a very accurate aluminium template with a very slight gap around the mast. I used #8 screws when doing this test fit, in case the clecos had some play.

Once i was happy with the aluminium template, i was able to cleco this to the wing skin, mark the accurate hole and begin to drill and remove the material within the line in the same manner as i did for the template. I filed the skin hole until i had only 1/2 of the blue sharpie line remaining then finished it up with 320 grit sandpaper.

And it came out great!