CHAPTER 22 - Making a Pitot Mount
I am planning to build my RV complete with instrumentation for IFR flight. One of the requirements, then, is for a heated pitot tube. With that in mind, I purchased a heated pitot head from Aircraft Spruce, along with some extra tubing couplers and a short length of streamline tubing (1 foot is the shortest length that Aircraft Spruce will sell).
After studying the plans and my newly acquired parts, I set about trying to
design a mount. I was torn between the desire to have it hold up to high Mach
slipstreams (really, Van is working on the RV-6T(urbine), and I intend to
convert once he gets the bugs out
Primarily, the mount consists of doubler plates riveted to the top and bottom skins, with the streamline tubing piercing both the doubler and skin on the bottom. The streamline tubing 'floats' between these plates, attached to them by brackets fabricated from .063" angle. The 1' length of the tubing was just enough to give the recommended clearance between the wing surface and the pitot head. The mount was positioned outboard one bay from the access hole, to clear both the access and the tie down bracket. It was also positioned just aft of the spar with the doubler plates being it's only attachment to the spar. The top and bottom attachment gives it it's stiffness; only twelve rivets attach it to the spare, meaning that any separation will likely not damage primary structure (though it will probably make a mess of the wing skins.)
I started by making 3"x5" doubler plates of .040 material. One plate had centerlines marked and the outline of the streamline tubing marked and cut out of it. I also fabricated the angle brackets from 3" lengths of angle stock. These were notched and bent to conform to the streamline tubing, with the other surface flat to mount to the doublers. They were then predrilled with #41 holes. The brackets were attached in pairs to the streamline tubing and held in place with rubber bands; the doublers were clamped roughly into place on the spars. The mount tubing was put in place and the pitot head inserted, then the assembly was trued with a level and protractor and clamped into final position.
Now the doublers were drilled to the spar and the alignment checked again. Bars were clamped across the ribs, as shown in the picture, to help position the doublers. When all was correct, the brackets were drilled and clecoed to the doublers and mounting tube (#40 to the doublers, #30 to the tube). The entire assembly was removed from the spar and the mounting tube marked along it's leading edge for the hole for the pitot tubing. I cut a smaller hole aft for the electrical connection, and inserted a grommet. The mounting screw holes were marked for the pitot head, and then the whole thing taken apart so that the mounting tube could be drilled and cut. While they were apart, the doubler plates were marked around the edges, except for the spar edge, and drilled for the rivets that would attach them to the skins. The brackets were then reattached to them and any clearance for rivets needed was cut into the brackets.
With the mounting tube fabricated, a short piece of 1/4" tubing was flared at one end at attached to the pitot head with a coupling nut. The mount was re-assembled and attached to the spar, and the pitot tubing routed along the spar to emerge through a lightening hole near the mount. The pitot head was inserted into the mount, bending the tubing just enough to get it through the opening and then getting both tube ends into proximity. They would be joined with a coupling, which was used to mark the tubing for cutting. Once cut, coupling nuts were put over the ends, flares made, and the connection checked. I also checked to see how accessible it would be through the access hole and reaching through the lightening hole (tight but possible).
The pitot tube was uncoupled, and the pitot head was removed. I disassembled the extension tube and then reattached it with sealing compound in the threads. You might want to substitute Proseal for a more permanent assembly. I then reassembled the pitot head to the mount and reattached the tubing to get it aligned correctly. Finally, I removed it again and tightened the coupling nut, being careful not to let the tube rotate. The coupler was also permanently attached to the pitot line, which was then attached to the spar with clamps fabricated per the plans.
The electrical runs were made at this time. I used spiral wrap to bundle the wiring, and routed it through snap bushings. I left enough slack at the pitot heat wires to be able to pull the head free from the mount before disconnecting it.
The rest of the procedure overlaps the wing skinning process, but I'll detail it here. Drill and attach the skins with the mount removed. Once the skins are attached, cleco the doubler plates in place on the spar, then back-drill the edge holes (NOT the holes for the angle brackets) through the skin. You could use a shim plate to make up for the thickness of the spar flange, but I saw no noticeable rippling or denting without it. On the bottom skin, mark and trim the hole for the mounting tube. Remove the doublers and dimple countersink. You will probably want to machine countersink the angle brackets (I did), but not where they attach to the mounting tube. Clean up and prime the mount pieces, and rivet them together. Be SURE you use flush rivets to attach the mount brackets to the doubler plates, so that they will lie flat to the skin.
I will be riveting the top skin first, so I will attach the mount at that time, riveting the top doubler to the spar and skin. When I add the bottom skin, I will reach through the access hole to buck the rivets for the bottom doubler. Then I will run the wires through the mount and attach the electrical connector. After the wing is complete, I will attach pitot head, first plugging in the connector. Then I will make the pitot line connection through the access hole. It will have been a lot of work, but I think it's worth it.