Tools used were my band saw, my disk sander, a hand-held flat file, my scotch brite wheel, metal straight edge/ruler, hole punch, ultra fine blue Sharpee, my cordless drill, electrical tape, 3 in one oil, the following drill bits: #51, #36, and 9/64ths, a 6-32 tap with small handle, my table vise, 3 blocks of wood, a small level, and my old pitot tube. Who would have thought that so many tools would be needed just to drill and tap a few screw holes!
Thanks to my friend and fellow RV-8 builder Steve Riffe, who is about to finish building and take his first flight in his new airplane, I used his procedure from his kit log to mark, drill, and tap the new screw holes, and all went pretty well.
This time there was no drill press involved. I marked line around the mast that would ensure that the old screw holes would be completely removed. I had to dust the cobwebs off my band saw and set it up for the cut by using a small 2x4 piece of wood to set the odd-shaped pitot mast with its mounting flange onto so that I could run the wood and the mast through the saw. The cut was raw and somewhat jagged, and would require some file-finishing, leveling, and smoothing before it would be ready for the new screw holes and the new pitot tube.
I tried to use my disk sanding wheel on my electric sander but it was so clogged with old wood dust that this was not very effective. The end result was a smoother edge on the newly cut mast, but it was not very straight. Out came the hand-held flat file. I used the level and eyeballs to check for flatness of the newly cut edges, noted the high and low spots, and started slowly running the file over both sides of the top of the mast. I managed to get it very flat and square again, but also know that I had file marks that I would need to get rid of, so over to the scotch brite wheel I went. I was able to polish the file marks out to remove any potential stress risers on the end of the mast. I then used the old pitot tube to trial fit it into the mast. this was easier than using the new pitot tube because it no longer has the long tubes and electrical wiring in the way, since all this had to be removed as part of the pitot replacement procedure from Dynon. Never thought I would be using it again, but it came in handy for trial fitting the pitot into the newly cut mast.
Here is a pic that does not show all of the tools I used, but close:
Once I was satisfied, I replaced the old pitot tube with the new one, verified the fit, and taped it in place on the mast. Then I measured the same 11/32 inches at the base of the pitot tube mounting flange and re-verified the marked lines for the proper location of the 4 holes. I put tape on the end of all drill bits so that only 1/4 inch of the bit was exposed so as not to drill too deeply and damage the tubing internally. I then used the hole punch to set starter holes in the mast for the drill bit. I sandwiched the tube and mast assembly between two small pieces of wood and secured the assembly in my table vise, being careful not to apply too much pressure to avoid damaging the pitot tube. All you need is enough to hold the assembly so you can drill the 4 mounting holes.
I started with the #51 drill bit to drill pilot holes into the pitot tube. Wow, say that 5 times real fast! Here is a TIP: When drilling, the metal extensions inside the pitot tube that provide the material for the screw threads extend inward only so far. If you have drilled what you believe to be 1/4 inches into the pitot tube and the drill bit has not broken through this material, keep drilling just a bit more until you feel the bit go completely through. Be careful not to go too far. Doing this ensures that the tap will be able to properly set the threads in the hole. When I did this the first time long ago I don't think I drilled one of the holes all the way through, and my tap then stripped several threads as I tried to force the tap to go in further when it had no more room to do so.
I did all the drilling by hand using my cordless drill, going slowly and ensuring that the bit was as level as I could make it, and that it also followed the contour the curvature of the pitot mast assembly. this is NOT a very easy thing to do because the front and back ends of the tube and mast are shaped like a big fat wing for obvious reasons. So the curvature is different from front to back. If you align the drill bit with the metal extensions in the top of the tube you will be fine.
After the 4 pilot holes were drilled I switched to the #36 drill bit and re-drilled the holes again. Once this was done the holes in the pitot tube were ready for the 6-32 tap. I did all that by hand as well, and even took the tube out of the table vise and held it in one hand while tapping with the other. I used 3 in 1 oil to lube the tab, and made sure I applied the tap a little at a time and then backed it out, cleaned up the sludge, and the tapped it a little bit more. I did this until I felt the tap free up, which is a sign that all the threads in the material have been formed properly.
The most concerning part of this process, which I have reported on in my earlier posts when I tried this the first time, was tapping the two forward screw holes. The tap has to extend rather far into the inside of the pitot tube to properly tap both forward holes, and the tip of the tap comes very close to the internal metal air lines that provide the airflow to your instruments. I am still concerned that the tap may have scored the front part of the front tubing, but I did not feel it hitting anything, so perhaps I am OK here. The tap is tapered on the front, so maybe that also helped prevent any compromise of the forward tub as well. All I know is that the tap comes really close to this tube by the time you have properly cut the threads on those forward holes, so be careful.
With all four holes successfully tapped, I final drilled the mast holes using the 9/64ths drill bit to make a clearance hole for the 6-32 x 1/4 inch long screws. I successfully screwed in all four screws and verified that they all tighten up as expected. YAY! no stripped threads this time! Then I removed the screws and place the pitot tube back into the mast and screwed them together again to ensure proper hole alignment through the mast and to re-seat the threads in the new holes. I am goin g to order specifically sized counter sunk screws from Aircraft Spruce and will counter sink the holes in the mast once I have those screws in hand. Right now I am using the stainless steel pan head screws that I had to trim down because they only come in 3/8 inch sizes and are too long. These add a small amount of drag as the screw heads stick out into the slip stream. I will now use the flush head screws now that I know this part of this tedious process is behind me.
The finished product, on a mast that is about a half inch shorter now:
The location of the screw holes also came out much more symmetrical than my first attempt. Its still not quite perfect but is much better than the last time. The inlet hole of the pitot tube ends up being 6 inches from the bottom of the wing - exactly where I expected it would be. I'm glad to get this small step completed, and hope I never have to do it again! I still have the tubes to trim, and tubing flanges and NPT hardware to set up, as well as mounting the tube to the wing, but that will be done later, after other wing assembly steps have been completed.
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