01-10-2013: 2.5 hrs
It is time to install the pitot tube mount. I bought SafeAir1 pitot mount a while ago. I glanced through the instruction, and decided to install the mount at the next bay outboard of the aileron bellcrank,like most other RV7 builders did. The instruction is for RV7, and I was not thinking too much and went ahead with what I thought it should be done. I aligned the dash line on the paper template with the spar flange edge (which is not correct), and aligned the center mark line on the paper template with the second rivet outboard of the rib, then marked the cutout area on the skin. Made cutout on the skin, no problem, the fit is good. Then I realized that I shouldn't align the dash line with the spar flange, instead I should align the paper template edge with the butt line between leading edge skin and bottom skin. The way I did, there are two problems, one is the pitot mount plate will interfere with the rivets between leading edge skin and spar, and the side edge of the mount plate will interfere with the rib. Also, the dash line does not represent the start of the transition bend on the mount plate, so the bend will interfere with the spar flange. The first two issues are easy to address, I just trimmed away extra material, edge margins can still be maintained. The bad fit between spar flange and mount plate bent is a tough one, I don't want to touch the spar, so I filed down more material around the bend so the spar flange can fit. But the draw back is the thickness at the bend is less the 0.063" plate thickness, maybe only half of it. Did quick calculation assuming 50 lbs handling load (the aero load is going to be very small), the stress on the bend with thinned thickness is still ok.
Calculation assumption:
Abuse loads: 50 lbs applied fore-aft
Pitot tube to mount plate distance: 7"
Distance between forward and aft fasteners on mount plate: 3"
Mount plate thickness: 0.063" for moment of inertia calc
Spar flange and mount plate rotational stiffness: 100 in-lb/rad
Bottom skin/mount plate joint rotational stiffness: 10 in-lb/rad
Results: Mount plate will see about 26 in-lb moment at the spar flange/mount plate joint, and shear force of 103 lbs total. Bending stress is about 50ksi for 0.032" thickness. OK for most aviation aluminum.
Just to make myself feel better, I will add a angle between the plate and adjacent rib
Pitot and mast fit perfectly
The edge of spar flange is marked, and aligned with the line on template (This is not correct, should aling the edge of the template to the edge of the bottom skin. I had to trim the mast plate the way I did it)
Bottom skin is cut for pitot mast
Pitot mount is at typical location for most RVs, just outboard of the aileron bellcrank
The skin and pitot mast fit is very good
This is how much I trimmed the plate
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