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Pitot mount location

Dennis Harm

Active Member
Building a 14A and am wondering where to mount the pitot tube . I have the gretz mount and don?t want it to interfere with the tie down ring and rope, thanks Dennis
 
Building a 14A and am wondering where to mount the pitot tube . I have the gretz mount and don?t want it to interfere with the tie down ring and rope, thanks Dennis

I mounted mine just aft of the spar and adjacent to the next rib inboard from the tie-down location for the express reason of avoiding interference with the tiedown ropes. If you're using your own tiedown stakes you can usually locate them to avoid having the rope interfere with the pitot, but most ramps have tiedowns that are spaced to accommodate much larger aircraft, and the tiedown ropes tend to end up going way outboard from the ring on an RV.

The downside of locating the pitot inboard of the ring is that the aileron pushrod is just above the mount, so you have to use right angle fittings to attach to the pitot & AOA lines where they come up through the mount into the wing. Did that on my -8A and doing the same on my -14, but it might be a better option to move the pitot several bays outboard from the tiedown...far enough to avoid interference with the ropes, but not too close to the wingtip since accuracy is affected especially at high AOA near the tip.

Also if I had to do it over I'd consider mounting the pitot on the right wing if you're planning to put the magnetometer in the left wing location per the plans. That gets all the magnetically 'noisy' stuff (pitot heat, aileron trim, and aileron servo) in the right wing and should leave the left wing a much 'quieter' environment for the magetometer.
 
I mounted mine one bay outboard of the access panel closest to the tie-down. (next to rib just outboard of the access panel). This allowed me to use the rib to help anchor/support the pitot mount, run the pitot/AOA tubes and wiring into the bay for access, use the access plate to mount the pitot heat controller, etc.

It's outboard of the tiedown, but according to my advisor (Vic Syracuse) this shouldn't be a problem most of the time.

This blog post and next couple after show my mounting: https://turnerb14a.blogspot.com/2018/01/january-18th-update.html
 
I also went 1 bay outboard of the access panel on the lt wing. It?s a little tricky getting it in and out, but you don?t do that frequently. I like czechsix?s suggestion of moving it to the right wing - this would have eliminated issues I had with pitot heat interfering with the magnetometer, and the wiring changes would have been minor.
 
Definitely outboard, outboard of the bell crank stuff too. If you get it outboard of all of that aileron pushrod and bell crank linkage it'll be easy to install. I think on mine it's maybe 1 bay inboard from the wingtip. That way I could easily reach it with the wingtip off, and it's clear of everything. Once it's mounted you'll probably never touch it again anyway, and the airflow is nice and clean out there.
 
I also went 1 bay outboard of the access panel on the lt wing. It?s a little tricky getting it in and out, but you don?t do that frequently. I like czechsix?s suggestion of moving it to the right wing - this would have eliminated issues I had with pitot heat interfering with the magnetometer, and the wiring changes would have been minor.

Another way to eliminate or greatly reduce the magnetic interference from the pitot heat power (10A, more or less) to the magnetometer in the ADAHRS would be to run +12V and 12V return wires as a closely-spaced pair to the pitot tube or pitot tube heater controller. SteinAir sells a twisted pair of 14 AWG red and black wires that they say is ideal for this purpose: https://www.steinair.com/product/mil-spec-wire-14-ga-red-black-twisted/ I think you could use a jacketed or sleeved pair of untwisted wires and achieve almost the same effect. The magnetic field generated by 10A of current in the +12V wire is cancelled by the opposite magnetic field generated by the 10A of current flowing the opposite direction in the 12V return wire. I have tested this with the compass sensor (magnetometer) in my phone and in a Garmin hiking GPS receiver and the results look good. I'm using this approach in my airplane for the landing/taxi lights (about 10A max), nav/strobe lights, and pitot heat (about 10A max). Three extra 14 AWG wires in the left wing at $0.42/ft + tax and shipping from SteinAir.

For a high-school physics understanding of how this works, view the lesson on the magnetic field generated by current flowing through a wire at https://www.youtube.com/watch?v=wZlHLHnZSRg and then add a second wire adjacent to the first wire (maybe 0.5 cm separation) with the current flowing in the opposite direction (make the current in the return wire negative) and calculate the total magnetic field at some distance (e.g., 10 cm) from the pair of wires.
 
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