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  #1  
Old 06-10-2018, 09:55 PM
tmillican tmillican is offline
 
Join Date: Jun 2009
Location: Columbus, TX
Posts: 95
Default Airspeed low: Pitot obstruction?

Posting this on the steam gauges forum because both EFIS and mechanical ASI showing same indications. On my flight today, I noticed my airspeed reading unusually low. (RV-8A, O-360, Dynon pitot/AOA probe.) Flying today at 3000ft, 24", 2400 RPM indicating 125 knots on both my backup airspeed indicator and my Dynon D180. Engine seemed to be running fine. Climb performance was fine. Groundspeed was 180 with the dynon showing a 32 knot quartering tailwind (which I don't know how to interpret given the fact that I don't trust the IAS from that instrument). Landing over the fence at 70, I must have really been doing 90, because I sure felt fast.
So before the return flight I looked (of course) and didn't see anything in the pitot tube. I sucked on it (yeah--in thinking about it now--I'm kinda grossed out about it too)--I felt a little bit of something come out. I probed it with some safety wire, both the main hole and the little hole on the bottom--whatever that is. Return flight, same indications.
I could call the avionics guy out (there is no one at our airport), but this is something I should (in theory) be able to fix--with a little help from my VAF friends.
Thanks in advance.

Troy
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  #2  
Old 06-10-2018, 10:56 PM
Mark Albery's Avatar
Mark Albery Mark Albery is offline
 
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Location: Fremont CA
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Airspeed errors are more often due to static issues rather than pitot.

A relatively easy check on the pitot is to make a manometer attached to the pitot head from clear tubing. A bit tricky with a heated or AOA head, but quite easy with a bent tube. A leak will show up as the airspeed drops after you raise the liquid head and the two sides of the manometer go level.

Static errors are quite often visible by the altimeter indication changing as you accelerate on take-off or decelerate on landing.
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  #3  
Old 06-11-2018, 04:59 AM
F1R F1R is online now
 
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If you had a static line open as in totally disconnected, inside the cockpit, you would see a false LOW airspeed and false low altitude from ram air vent pressure raising the cockpit pressure slightly above accurate static air pressure.

In this same situation, (totally open /broken/ disconnected static fitting inside the cockpit) as your air speed decreases -as in an approach for landing, the air speed and altimeter will read closer to correct. - as you have less ram air pressure from reduced forward speed.

Last edited by F1R : 06-11-2018 at 07:39 AM.
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  #4  
Old 06-11-2018, 02:12 PM
tmillican tmillican is offline
 
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OK. This may very well be the problem.
I've noticed, too, that my Dynon used to always have the altimeter setting somewhat near normal when I cranked up the airplane--maybe the last setting but I never noticed--now it seems to always be set to something reading about 2000-3000 feet HIGHER than field elevation. Could that be the same issue?
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  #5  
Old 06-11-2018, 02:18 PM
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N941WR N941WR is offline
 
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I had a crack in my pitot line that caused what you are seeing.
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  #6  
Old 06-11-2018, 02:44 PM
vic syracuse vic syracuse is offline
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Default Not correct

Quote:
Originally Posted by F1R View Post
If you had a static line open as in totally disconnected, inside the cockpit, you would see a false LOW airspeed and false low altitude from ram air vent pressure raising the cockpit pressure slightly above accurate static air pressure.

In this same situation, (totally open /broken/ disconnected static fitting inside the cockpit) as your air speed decreases -as in an approach for landing, the air speed and altimeter will read closer to correct. - as you have less ram air pressure from reduced forward speed.
This is not correct. An open static system inside will show an INCREASE in airspeed. Don't believe me, just crack your alternate static vent sometime while in cruise.

I use to tease kids when were flying that we were going to go into warp speed and I would tell them to watch the airspeed indicator. I would put a little G on the airplane and crack the static line at the same time. Instant airspeed acceleration.

Sounds like you have a pitot line leak.

Vic
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  #7  
Old 06-11-2018, 04:27 PM
F1R F1R is online now
 
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Quote:
Originally Posted by vic syracuse View Post
This is not correct. An open static system inside will show an INCREASE in airspeed. Don't believe me, just crack your alternate static vent sometime while in cruise.

I use to tease kids when were flying that we were going to go into warp speed and I would tell them to watch the airspeed indicator. I would put a little G on the airplane and crack the static line at the same time. Instant airspeed acceleration.

Sounds like you have a pitot line leak.

Vic
It really depends on the cockpit air pressure relative to the static air pressure at the altitude you are at.

Lets say you have 2 air speed indicators in your NON pressurized cockpit.
Your cockpit pressure, if you have forward facing air vent intakes, will have slightly higher pressure than the atmospheric pressure at the same altitude, due to the ram air effect.

Both air speed indicators are properly connected and leak free to the pitot tube. However only one is connected to the static system. The second has it's static port open to the cockpit. If the cockpit air pressure IS above the static air pressure, the open static port air speed indicator should give an indication that is slower than the one that is plumbed to the actual static ports. Taken to the extreme, if the cockpit pressure was equal to pitot pressure, your indicated airspeed would be zero.
Any increase in cockpit pressure above static pressure will reduce the indicated airspeed of the open static port altimeter.

If your cockpit pressure is lower than the static air pressure then I would expect your observation to be correct.

Last edited by F1R : 06-11-2018 at 04:31 PM.
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  #8  
Old 06-11-2018, 04:54 PM
vic syracuse vic syracuse is offline
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Default Not to argue...

Yes, but we are talking about RV's here. With pressurized airplanes it is a different story.
Really trying to not confuse the gentleman with the problem so he doesn't go chasing the wrong system.

I even doubt you could pressurize an RV cockpit with all of the vents open.

Vic
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  #9  
Old 06-11-2018, 05:26 PM
F1R F1R is online now
 
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I was trying to help the opening poster, and based on my actual observations in my rocket with the heat muff air feed, front cockpit cold air NACA vent and the passenger seat underwing NACA air vent all with forward facing/positive pressure intakes, it makes about a 15kt IAS (conservative) difference at 200 kts. I intentionally keep one mini format EFIS static port open just in case icing ever blocks the static ports.

For accurate air speed, that leaves me a TSO'd air speed indicator, and 2 more Efii all properly plumbed and leak free and in agreement with each other. At approach speeds the difference is much reduced and negligible, in my ship.

Based on my experience, and the OP's reported numbers, I would go looking for a static system leak.

Last edited by F1R : 06-11-2018 at 06:30 PM.
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