Spanwise pitot position

I'm close to the stage on my RV-7 build where I will need to finalize the position of the pitot tube, and I'm looking for a bit of advice. I have a SafeAir mount, and a heated Garmin GAP26 pitot, although I think the question probably applies more broadly.

My main concerns are making sure that I get an accurate reading (which as far as I can tell mainly means making sure the pitot is in undisturbed air and parallel to the flow), avoiding any fouling of the aileron pushrods or bellcrank by the internal tubes or wiring, and perhaps most tricky, preventing potential damage to the expensive pitot (or even worse the wing) due to entanglement with a tie-down rope or chain.

I note that the Garmin instructions simply say "mid-span", and that the Safeair instructions suggest mounting one bay outboard of the bellcrank bay. The Vans plans have the pitot slightly inboard of the tie-down ring location. This makes sense to me because given the narrow wingspan of the RV aircraft it's much more likely that the rope or chain will extend outboard from the tie-down than inboard. To make the thread most useful, perhaps I can ask others to comment on the three possibilities as follows:

1) Has anyone tried mounting the pitot inboard of the aileron bellcrank? Would this cause inaccuracy due to propeller slipstream? Does it solve the tie-down-fouling issue?

2) Has anyone tried mounting their pitot further outboard than the bay next to the bellcrank? This would make riveting and maintenance more difficult, but might solve all the issues. Could inaccuracy be introduced by being too close to the wingtip where air starts to travel spanwise and leak around the tip?

3) Should I just stop worrying and mount it where the SafeAir instructions say? (In the next bay outboard of the bellcrank, but as close as possible.) Is anyone who has experience with this location able to reassure me about tie-down damage?

Thanks in advance to anyone who responds. I have seen some other threads on the subject but am not aware of a definite conclusion being reached (perhaps that never happens....)

Anywhere outside the prop blast is fine. My Moni Motorglider had the pitot mounted on the end of the stabilizer (V-Tail) and it worked just fine.

The reason for mounting close to the bell-crank is easy access through the inspection panel. Do NOT mount it in line with where the tie-down rope will be.

Mounting close to the tip will not have an adverse effect.

I have mounted them in the outboard bay and installed from the wingtip with the tip off. Worked great.

Inboard of the bellcrank (Dynon pitot tube/AOA)...just bent the aluminum lines to clear all pushrod hardware and whatnot, then transitioned to the plastic Safe-Air tubing. No issues and it's always clear of the tiedowns.

Easier to do than it first appears.

ETA:

Should I just stop worrying and mount it where the SafeAir instructions say? (In the next bay outboard of the bellcrank, but as close as possible.) Is anyone who has experience with this location able to reassure me about tie-down damage?

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I would NOT do this...yes, it might be okay and there will be tons of people here who say they did it and it was fine, but all it's going to take is ONE mishap with a rope or (yikes) chain to possibly mess up your nice new Pitot tube.

I have mine mounted ON the belcrank inspection plate both on the -8 and Rocket. No issues.

Jesse said:

I have mounted them in the outboard bay and installed from the wingtip with the tip off. Worked great.

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The Grumman series have their pitot located right at the tip even further out than mentioned by Jesse.

The tip shape is essentially identical to the early RV-4 and RV-6 designs along with stubby square wings.

Would appear to ba a lot of spanwise flow at the tip on the Grumman.

IIRC, if you want "undisturbed air" you have to position the inlet about 18" ahead of the leading edge

Spanwise flow or not, the Grumman pitot tube location works quite well, as near as I can tell from cross-checking via GPS ground speed tests.

Canadian_JOY said:

Spanwise flow or not, the Grumman pitot tube location works quite well, as near as I can tell from cross-checking via GPS ground speed tests.

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Yes, according to the certification test in the POH less than 2 kts error from 50 to 180 kts.

Actual damage from tie-down ropes/chains?

Thanks everyone for the thoughts so far. Has anyone got any salutory tales of actual damage from tie-down ropes or chains? Especially if you can remember where the pitot was on that aircraft....

Dumb Idea?

I am preparing to install the wing ribs on my -14A project. Planning ahead for wiring and pitot line routing through the ribs brought me to the question as to where to place my pitot tube. I have read many threads on this subject and have a pretty good idea of pros and cons for the most used options.
My two preferred options:
(1)If I place the pitot in the plans location just inboard of tie down eyebolt, there will be no issue with tie-down ropes contacting the pitot tube. However, with this option it does make possible future maintenance of the connection fittings a little more difficult due to the aileron bellcrank assembly and torque tube locations. I know many builders have gone this route and had no problems.
(2)If I place the pitot just outboard of the tie down eyebolt, there are definite concerns with tie down ropes coming in contact with the pitot tube. However, it does appear to make for a "cleaner" install and future possible maintenance. The pitot line connecters would be easily accessible from the access panel for the aileron bellcrank assembly.

I don't have much light airplane flying experience so I'm not that familiar with the frequency in which the airplane actually gets tied down. Reading others posts, it appears some do not put the wing tie down eyebolts in place until they are actually needed.
Long story short, I prefer option 2, but don't like the tie down issue. So here's my dumb idea.
If I use tie down eyebolts that are approximately 4.5" long, that would, for most cases, allow the tie down rope to clear the pitot mast. Of course these eyebolts would only be installed when needed and removed prior to flight.
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This is a mockup of my preferred location. The rule with the blue tape simulates the tie down eyebolt. Here the bolt would be approximately 4.5"
[URL=http://s791.photobucket.com/user/shawnspics/media/RV-14/DSCN4003.jpg.html]
[IMG]http://i791.photobucket.com/albums/yy196/shawnspics/RV-14/DSCN4003.jpg[/URL][/IMG]
This shows the tie-down rope clearance when the ramp tie down point is approx. 7.5' from the wing eyebolt.
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This measurement shows the distance of the pitot tube from the bottom surface of the wing. I would consider cutting my pitot mount off by 1" which would give additional clearance.
I wasn't sure the actual height of the wing bottom to the ramp, but after looking at pictures it appears to be about "thigh high" on most people. I used 34" for my setup.
Additionally, one issue that this setup may have is increased force on the tiedown bracket (riveted to spar web) due to the lengthened eyebolt. However, under those type of loads, would the bolt bend before damaging the bracket? That's for someone much smarter than me to figure out.
Practical or not? Don't worry about offending me

Toobuilder said:

I have mine mounted ON the belcrank inspection plate both on the -8 and Rocket. No issues.

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Hu.... me too.

Mudfly said:

I am preparing to install the wing ribs on my -14A project. Planning ahead for wiring and pitot line routing through the ribs brought me to the question as to where to place my pitot tube. I have read many threads on this subject and have a pretty good idea of pros and cons for the most used options.
My two preferred options:
(1)If I place the pitot in the plans location just inboard of tie down eyebolt, there will be no issue with tie-down ropes contacting the pitot tube. However, with this option it does make possible future maintenance of the connection fittings a little more difficult due to the aileron bellcrank assembly and torque tube locations. I know many builders have gone this route and had no problems.
(2)If I place the pitot just outboard of the tie down eyebolt, there are definite concerns with tie down ropes coming in contact with the pitot tube. However, it does appear to make for a "cleaner" install and future possible maintenance. The pitot line connecters would be easily accessible from the access panel for the aileron bellcrank assembly.

I don't have much light airplane flying experience so I'm not that familiar with the frequency in which the airplane actually gets tied down. Reading others posts, it appears some do not put the wing tie down eyebolts in place until they are actually needed.
Long story short, I prefer option 2, but don't like the tie down issue. So here's my dumb idea.
If I use tie down eyebolts that are approximately 4.5" long, that would, for most cases, allow the tie down rope to clear the pitot mast. Of course these eyebolts would only be installed when needed and removed prior to flight.

This measurement shows the distance of the pitot tube from the bottom surface of the wing. I would consider cutting my pitot mount off by 1" which would give additional clearance.
I wasn't sure the actual height of the wing bottom to the ramp, but after looking at pictures it appears to be about "thigh high" on most people. I used 34" for my setup.
Additionally, one issue that this setup may have is increased force on the tiedown bracket (riveted to spar web) due to the lengthened eyebolt. However, under those type of loads, would the bolt bend before damaging the bracket? That's for someone much smarter than me to figure out.
Practical or not? Don't worry about offending me

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I would not be comfortable with a long tie down eyebolt. That would be quite weak IMO and would compromise the purpose of the tie down.

Removable tie downs

I think removing the tie down eyelets for each flight would be a PIA.

If you had a hangar and only flew local then you could leave them off but then again, if you had a hangar you wouldn't be lashing it down anyway so no interference issue in the first place.

However, an RV-14 is made to go somewhere so it will be tied down often. Murphy dictates that if they are removable, at some point they will be left behind.

runt said:

I think removing the tie down eyelets for each flight would be a PIA.
However, an RV-14 is made to go somewhere so it will be tied down often. Murphy dictates that if they are removable, at some point they will be left behind.

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Mine have been removable for 24 years. It's just about as easy to unscrew them as to untie them. Haven't left them behind yet.

Mel said:

Mine have been removable for 24 years. It's just about as easy to unscrew them as to untie them. Haven't left them behind yet.

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Nowhere near as long, but same here...they live in my Bison Mountain flight bag behind the seat, along with my tiedown ropes and travel chocks.

I second the comment on the long tiedown idea above...sounds like something to bend.

Why not just put them where the plans have them (outboard of the pitot tube) and quit fussing with it? That location seems to work fine for everyone else, and there's zero chance of catching a rope or chain on the pitot tube.

Long eyebolts will bend with near certainty and possibly damage the wing in the process. I have bent some of the standard Vans eyes with the winds I get.

Mine also come out for every flight.

Toobuilder said:

I have mine mounted ON the bellcrank inspection plate both on the -8 and Rocket. No issues.

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Ditto on my -7A.

I had a judge at OSH ask me why I "put the Pitot in the wrong place". With my puzzled look he pointed out that mine was outboard of the access panel "where Vans intended in the plans". He then pointed out two -10's on the row behind me that sure enough had theirs inboard of mine. I told him that it worked fine and had great access from the end of the wing with the tip off.
Anyway, that bugged me and made me question myself if I had simply made an error in following the plans. The rest of the time at OSH I looked at every 10 I could find finally finding a couple that had it in the same place that I did, but we were certainly the minority.
When I got home I pulled out the instructions for the Gretz mount and they clearly advised to position the mount as far outboard as feasible which had obviously led me to put it where I did. Next time a judge asks, I can provide a confident and coherent answer.

Mine is outboard from the belcrank bay, but on the outboard side of the next bay, unlike the mockup by the OP.

I have had no issue with tiedowns because the RV is so short that the tiedown ropes normally go forward from the eyebolt to the ground and pass the pitot with many inches of clearance.

Pitot Location

One more data point.

My pitot tube is not the Vans bent tube style but an L-shaped and heated style. It is located at the outboard side of the next bay outboard from the bell crank.

I carry the tie-down eyes in the bag with the ropes and chocks and only put them in when parked outside. Normally the plane is hangared.

There has been no issues with ropes getting too close to the Pitot tube.

Bevan

Thanks

Thanks for the input guys.
After consideration, I think I will be sticking with the plans location. My "extended" eyebolt idea may not be practical and could even possibly cause damage to the eyebolt, at the least, or worse the bracket and spar web under occasions the plane is tied down under high winds.
I think if I could mount the pitot mount on the outboard portion of the next rib bay over like rvbill and Bevan showed (-7 looks like), it would work ok. Unfortunately, on the -14, the rib in that location is reversed and the rib flange faces inboard. The dynon mount, and I think the Gretz also, requires a piece of angle be mounted to the side of the rib to support the pitot bracket.

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Here's the rib setup and orientation for the -14. The paper on the floor is the template for the dynon pitot mount if mounted outboard go the tiedown bolt. You can see the rib on the outboard portion of this bay has the flanges facing inboard making it difficult to properly mount the bracket if I move further outboard as others have done. Sounds like the orientation of that rib is reversed on the -7 making it a more viable option. The blue tape on the spar to the right of the inspection plate shows the plans location for the pitot.
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While I like the cleaner install and ease of future maintenance to the pitot line fittings mounting outside the tiedown bolt (left of inspection plate), I think going with the plans location (to the right of inspection plate) is going to have to work for me. I just hope no pitot maintenance will be required once everything is closed up

Getting back to post #11 for moment, Mudfly, sooner or later, the tiedown line will knock that pitot tube for sure.



- Airports don't seem to have a standard for tiedown spacing.

- The airplane positioning isn't always perfect.

- In a wind, airplanes can rock and move around alarmingly. I don't mean a gentle wind but one strong enough that you by golly need that tiedown to work.

The other comments about long tiedown rings are quite valid, too. I just wanted to comment on this.

Dave

Dave, keep in mind, that the tie down ropes (at the wings) will angle forward to a hard point AHEAD of the wing. This will also help with clearance of the pitot tube located outboard of the tiedown. Check out an EAA diagram here.

https://www.eaa.org/en/airventure/e...~/media/189C3D7FF47946808AE19C12F7D500D1.ashx

Bevan

This thread got me thinking,,,

Now the smoke detector is going off! Anyway, let's say that I wanted to put the pitot tube on the inboard edge of the wingtip. How far forward would it need to extend to be in undisturbed air? I understand the potential issue with klutzes (such as myself) walking into it, so the problem may be academic. But, if it's short enough, I might be tempted to try it.

Do you have landing lights cut into the leading edges near the wing tip?

Bevan

judoka5051 said:

Now the smoke detector is going off! Anyway, let's say that I wanted to put the pitot tube on the inboard edge of the wingtip. How far forward would it need to extend to be in undisturbed air? I understand the potential issue with klutzes (such as myself) walking into it, so the problem may be academic. But, if it's short enough, I might be tempted to try it.

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If it's a pitot probe only, as in the standard Van's design, it doesn't need to be forward of the wing. Provided it is outside the boundary layer, and is approximately aligned with the local flow (say +/- 15deg, depending on the shape of the pitot head), it will measure total pressure quite accurately. The main requirement for undisturbed flow is for the static port, which on Van's aircraft is separate from the pitot probe.

For all you ever wanted to know about pitot probe configurations, see Gracey, "Measurement of Aircraft Speed and Altitude". It's available for free download on the NTRS at http://hdl.handle.net/2060/19800015804.

The standard location near the main spar, a few inches away from the lower surface, and parallel to the surface and the chord line is a good one. You don't need to go to the trouble of moving it forward, unless you're worried about it being in separated flow during inverted stalls or similar.

Landning light? Yes I do.

I bought the plane as a partially built kit, and it has cutouts for lights on both wings. Personally, I wouldn't have got that route, but since the hole's already there.... Thanks for the idea Bevan.

Thanks for the link Dan. I was concerned mainly about being far enough forward to get out of the compressed (or whatever the term is) air at the leading edge.

Pitot/AOA

dmn056 said:

If it's a pitot probe only, as in the standard Van's design, it doesn't need to be forward of the wing. Provided it is outside the boundary layer, and is approximately aligned with the local flow (say +/- 15deg, depending on the shape of the pitot head), it will measure total pressure quite accurately. The main requirement for undisturbed flow is for the static port, which on Van's aircraft is separate from the pitot probe.

For all you ever wanted to know about pitot probe configurations, see Gracey, "Measurement of Aircraft Speed and Altitude". It's available for free download on the NTRS at http://hdl.handle.net/2060/19800015804.

The standard location near the main spar, a few inches away from the lower surface, and parallel to the surface and the chord line is a good one. You don't need to go to the trouble of moving it forward, unless you're worried about it being in separated flow during inverted stalls or similar.

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You discussed pitot probe only configuration and that is great info.
I'm using the Garmin GAP 26 Pitot/AOA probe. The instructions state that "optimal AOA functionality results when the tip of the GAP 26 probe is located no further back than 25% of the wing chord length from the leading edge".
Here's a thread that discussed that in more detail..http://www.vansairforce.com/community/showthread.php?t=137712&highlight=pitot+tube+location
I will be using the plans location for mounting my Pitot/AOA probe. However, because I'm looking for any reason to delay priming wing ribs, I have looked at this more than I should in the last couple of days.
Just for discussion purposes, has anyone considered mounting the probe on the front side of the spar? I don't know about other models, but the -14 has an access panel for the stall warning assembly just forward of the spar. You could possibly mount the probe in the next bay outboard (that would be directly forward of the tiedown eyebolt on the -14) and still have easy access via the access panel. One problem I noticed (on the -14 anyway), is you would have to drill a small hole in the spar in order to get the two flexible lines and wiring to the aft ribs where they could run inboard.
Like I said, I will be using the plans location, but for someone smarter, and more creative maybe something to look consider?