Work hardening and pitot tubes

[jwilbur]

I asked this on the Dynon forum yesterday but my impatience compels me to ask it here as well.

I'm installing a Dynon pitot/AOA in my RV10 and because I don't know much about metallurgy, I have to ask this question about work hardening. The instructions state that the tubing coming out of the pitot is not annealed and will therefore work harden very quickly and so each section of tubing should be bent only once.

Because I put my pitot mount where Van's plans said to put it (one bay inboard of the bell-crank - lesson learned) I have an issue of keeping the tubing from interfering with the push-rod and still having the pitot removeable. After much deliberation and trial and error with scrap tubing I feel I have a good solution. But it requires a more-or-less complex sequence of bends. I did a decent job of it on my actual pitot the first time, but not perfect. It required several small hand adjustments before it was just right to where I can easily get it in and out and keep it secured well away from the push tubes and bell-crank. Essentially I route the tubing forward against the spar and at the same time outboard and upward to emerge in the bell-crank bay above the bell-crank itself (relative to a mounted wing, not a wing upright in the cradle).

I don't know what the implications of work-hardening are here. I'm sure my adjustments moved some parts of the tubing small amounts multiple times. So what are the dangers of these tubes being "work-hardened" and is there a way I can know if I have ruined these tubes?

Here's a picture showing the bends which resemble a sort of cork-screw if you could see it from a couple angles:

 

[rleffler]

You will know if you have problems bending it to the correct angle or if when you are bending you get a crack. I wouldn't worry too much. Just try to do you best to get it right on the first attempt. If it takes a few additional tweaks you should be ok.

Worse case, you have plenty of length. Just cut off below the problem area and put a fitting on. Your pitot won't care if one tube is a little shorter and the other is a little longer.

 

[jwilbur]

For the record I've abandoned this method and have cut the aluminum tubes short, concentrating instead on routing the plastic tubing. Although the shape of this was good for insertion, it would have been next to impossible to secure everything properly from the access panel.

Thanks for your reply, Bob.

 

[rleffler]

For the record I've abandoned this method and have cut the aluminum tubes short, concentrating instead on routing the plastic tubing. Although the shape of this was good for insertion, it would have been next to impossible to secure everything properly from the access panel.

Thanks for your reply, Bob.

Have you tested how far the heat travels up the tube?

I have a different brand that uses copper and it gets warm. Just to be totally safe, I ran aluminum tubing to the wing root, then transitioned to the plastic tubing.

 

[AAflyer]

Length of pitot tubing

I, too, am researching plumbing techniques for my pitot tube. I just bought the Dynon heated pitot/AOA and have the SafeAir plastic tubing installation kit for my QB RV-8. I was curious why I couldn't just trim the aluminum tubing that connects to the plastic tubes so that I could eliminate all the hassle of bending the aluminum. Seems like the flexible plastic would solve so many problems here, but your last post, that mentions the aluminum getting hot made me stop.
Maybe my question will be found in the installation instructions, but;
How short can you cut the aluminum tubes?

 

[rleffler]

I'm not familiar with the Dynon pitot, but I've done extensive testing with the Gretz pitot.

I would just hook the pitot up to a power supply. Make sure the pitot is held with something that won't burn or melt. The mount for the plane works just fine for this.

See if and/or how far heat gets transfered up the lines.

We are building experimental aircraft aren't we? Experiment!

 

[Bill.Peyton]

I cut the aluminum even with the connector. In this way, there was no need for any significant bend of the aluminum. You really don't want to install the pitot until after the plane is on the landing gear, it would be too easy to damage. On the other hand, you need to have the plumbing taken care of prior to permanently attaching the lower skin. here are a couple of photos during my build

 

[jwilbur]

Have you tested how far the heat travels up the tube?

Very good point. I did not run the experiment myself. However, I did do some research before cutting the tubes and found this builder's log who seems to have done some very thorough testing (scroll to 2009.07.03): http://www.kalinskyconsulting.com/rvproj/pitotstatic.htm

This report coupled with no warning in the Dynon manual and that many builders have cut the AL tubing short (and without any complaint that I have seen) gave me some confidence to cut the tubing. I've cut it so that the fittings just peak through the top of the mount.

 

[AAflyer]

Cutting heated pitot tubing

Seems like the way to go.