Thursday, March 14, 2013

AOA Tubing Connection Method Finalized

Work on another project has prevented me from spending much time in the shop these past few days and likely for a few more as well. However, was able to spend a few minutes in the shop this afternoon looking at various options for making the angle of attack sensor port connection in the left wing.

For the time being, rather than drill a 1/4” hole into the skin for mounting a sports ball inflation needle, I’ve elected to initially install a flush rivet in the previously drilled 1/8” hole for the AOA port. The hole was dimpled for a flush rivet which will have the mandrel pounded out thus making it a hollow port. If keeping this small port free of debris proves problematic in the future, I still have the option of drilling the port larger so a sports ball inflation needle can be used and a protective cap screwed onto the threads whenever the RV-12 is on the ground.

Once settling on the use of a flush rivet for the AOA port, the next hurdle was determining how best to connect the tiny Tygon tubing running through the wing to the rivet. A few different rivet and tubing combinations were examined. I have flush rivets in both 4-3 and 4-4 varieties so set a couple of rivets in a piece of scrap to determine:
A. How difficult will it be to drive the mandrel out of the set rivets without deforming the metal around the rivet.
B. How much rivet material will there be to connect tubing onto.

One would think the 4-4 rivet would seemingly have the longest shop head and thus more metal for the tubing to slip over … to a degree it does, but as the 4-4 rivet crumples, it tends to bulge to one side making the shop head slightly lopsided when compared to the 4-3 rivet. I’m speculating this is because the rivet is way under its minimum grip range (the wing skin is only .020 inches thick). In the second photo bellow, one can see how the 4-4 rivet (the one on the left) is lopsided ... it even leans a little but that is not noticeable in the photo. At any rate the decision was made to proceed using a 4-3 flush pop rivet.

                                              On the left, one can see the lone 4-3 flush pop rivet with the mandrel
                                              driven out which will now serve as the AOA sensor port.

The simplest method of connecting the tubing to the rivet worked out to be using a small piece of 1/8” ID vinyl tubing as an interface. The tubing slips over the rivet’s shop head nicely and makes a nice tight fit and the small Tygon tubing can be forcefully slid into the larger tubing a little ways.
AOA port connection prototype - 1/8” ID vinyl tubing slid over the rivet’s shop head (mandrel removed) and the small Tygon tubing is forced inside the larger tube making a tight fit.

During final assembly, once the tubing pieces are connected to the rivet, a non-reactive silicone sealant will be used to seal the connections so they are air tight and hopefully also prevent the tubing from sliding off the rivet over time. Elected not to complete the tubing to rivet interface at this time because the shop was quite cold … will wait for a warmer day to use the silicone or at least heat the shop up a bit so the silicone cures properly.