Pitot Mounting Advice Needed

I need to know if I'm about to make a grave mistake in mounting my pitot.
I bought one of those Evan Johnson VFR mast mounted pitot's in a group buy a while back. It's very similar to a Saf-air heated mast mount pitot. It's instructions suggest mounting as close to Van's location as possible but I would really love to move it out one bay towards the tip to give clearance to my tie down ring since tie down points at airports vary a lot and don't want a rope rubbing against the mast. I can't for the life of me imagine a problem moving it out a little but thought it prudent to run it by you guys before I fire up the drill.

Pitot mounting

I mounted mine 1 bay outboard too. I don't want it close to the ropes. I'm not flying yet but I figure, a: it's not closer to propeller blast and b: The tip is in pretty much in the same location from the leading edge and from the bottom surface of the wing, and c: parralle to the bottom of the wing surface. I would think that's all that matters.

Bevan
RV7A wiring
so not flying yet, only in my dreams !!

Evans pitot - done!

I bought an Evans pitot tube.
Here's my kitlog entry for when I fitted it:-

http://www.mykitlog.com/users/display_log.php?user=jpreou&project=45&category=1382&log=13097&row=215

mounting that mast

I bought the SafeAir mount for a Dynon AOA probe and mounted it one bay outboard of the Vans location on my -8 QB wing. With the hole cut in the skin and the mast cleco'd in position, which includes clecoing the mount along the trailing edge of the main spar, and withthe skin all cleco'd in place, I was concerned about lack of rigidity. I discussed this with my PFA inspector, who agreed with me, and we then engineered some extra stiffening inside the skin to provide a more rigid attachment.

What I have now is a skin doubler, flanged at each end to rivet to the adjacent ribs and rolled at the front edge to rivet to the spar, plus a flange along the trailing edge which is just behind the rear edge of the mast mounting. Added a few ounces but the mast is now much more rigid than it was.

I haven't seen or heard of other folk raising this particular issue so may be I was just being picky and the solution is engineering overkill....

Chris

Another way to stiffen Pitot mount

I also have the Safeair pitot mount and installed per their instructions, one bay out. I added a column stiffener to the aft corner of the mount, away from the corner furtherest from the rib. It's a piece of .030 aluminum, bent into a 90 degree channel and riveted on the same rib, but near the top of the wing skin. Here's a poor picture of the installation and of the fabricated stiffener.

I also have a pitot from evan that i mounted one bay out. Made a doubler out of .040 ( probably overkill).


Also do a search on "pitot" in the forums, lots of good stuff from Kevin Horton that will make you feel better about your decision.

I don't think you have an issue. I mounted the Gretz mast one bay out, actually two ribs over from the Van's location with the plumbing reachable through the inspection hole at the bellcrank. I seem to recall that the Gretz instructions suggest putting it there. I believe from a functional point of view the vertical position is more important and the opening on the pitot tube I think was suggested to be 6 inches below the wing to avoid airflow interference. The Gretz hardware, instructions, etc were very user friendly and made the installation straigtforward. I've put the heated Dynon pitot on the mast with the controller just inside the bay with the bellcrank.

I did Gretz one bay outboard also

The "Pitot one bay outboard" is a fairly standard mod, so I think you're fine there.

Another vote for the Gretz kit. It had bracketry and a doubler as part of the kit. VERY simple to install. I installed a Cessna heated pitot one bay outboard from Van's instructions and ran Al tubing over to the inspection panel and converted to the Safe-Air flex tubing there with a AN bulkhead fitting.

NYTOM said:

It's instructions suggest mounting as close to Van's location as possible but I would really love to move it out one bay towards the tip to give clearance to my tie down ring since tie down points at airports vary a lot and don't want a rope rubbing against the mast.

Click to expand...

Actually Tom, if you want to make sure that the pitot does not get snagged by the tie-down rope, put it right where Van's suggests. RV's are relatively short-winged airplanes. There are very few airports where you're going to have tie downs that extend from the ground outboard. Every tie down I've ever seen extends from the ground inboard. Thus, if you put the pitot outboard of the bellcrank bay where the tie down is, the tie down rope is much more likely to snag the pitot than if you put the pitot where Van's suggests... one bay inboard of the bellcrank and tie down. (Perhaps this is different on the 6A?) In any case, I hope this helps. I was making this same decision, with the same concerns you have, about 2 years ago.

I would also follow Chris' advice above regarding adding a doubler to your mast. I'm using the SafeAir mast and followed their directions, but now wish I hadn't. I think a think .032 doubler between the skin and mast plate (and/or tying it to the adjacent rib) is good insurance. I'm not sure why they don't suggest it in their plans. It's not that its real floppy installed per the instructions, but it's not real stiff either.

Good luck.

NYTOM said:

I need to know if I'm about to make a grave mistake in mounting my pitot.
I bought one of those Evan Johnson VFR mast mounted pitot's in a group buy a while back. It's very similar to a Saf-air heated mast mount pitot. It's instructions suggest mounting as close to Van's location as possible but I would really love to move it out one bay towards the tip to give clearance to my tie down ring since tie down points at airports vary a lot and don't want a rope rubbing against the mast. I can't for the life of me imagine a problem moving it out a little but thought it prudent to run it by you guys before I fire up the drill.

Click to expand...

Pitot tubes, unlike static ports, can be mounted in a wide range of locations and still provide accurate pressure. Moving it left to right or fore and aft make have no difference to its accuracy. You just need to keep it well clear of the prop wash, out of the wake of anything ahead of it, out of the boundary layer, and reasonably well aligned into the local flow.

Why I love this site.

Thanks to all of you for your expert help. I especially like the advice on doublers and extra bracing for the mast. It seems like that thing could really vibrate and flex the skin in the wind and cause some cracks to form. On the ancient 6A I'm building Van calls for the pitot they provide (a bent piece of tubing) to be installed through the spar flange at the front edge of the bell-crank access panel on the inboard side of the tie down ring. I could install the Evan Johnson pitot on the inboard side also to clear the tie down but then I thought about the possible problems with the moving control rod and pitot tubing loop. I never even thought about the prop blast screwing up the readings. I only post a question on this site when I'm really in doubt and so far I've only received quick and very accurate solutions to my problems. Its like having you guys are sitting around in my shop watching TV and having a beer just waiting to help.