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Airspeed Low

Jcurry

Well Known Member
Hey Guy's,
I have a 2011 RV 12. My airspeed is reading 10 knots low at all airspeeds. I have the Dynon D180. I did the airspeed leak down check with the syringe and it passed easily.
I've checked the tubing for defects and checked the connections at the Dynon. My tubing goes from the pitot tube directly to the Dynon (no module).
Any ideas would be appreciated.
 
What does the static port look like?

I assume you have a 3 or 4 leg ASI calibration flown at several airspeeds?
 
I have not flown any "calibration " legs. The airspeed does not come alive until right at rotation. Then it is 10-15 knots low when cross checked against 2 GPS at all airspeeds.
 
Low airspeed

I have a D180, did you do the zero airspeed calibration?
Check the D180 manual.
Dave
 
Last edited:
Jost a thought. Have you checked to make sure that the dimple drilled into the pitot tube for the screw that secures the pitot tube inside the gear reduction unit has not become breached by the screw?
 
Jost a thought. Have you checked to make sure that the dimple drilled into the pitot tube for the screw that secures the pitot tube inside the gear reduction unit has not become breached by the screw?

I checked the dimple and it was fine. Thanks.
 
Is there any part of your system that was not part of the pressure boundary during your syringe test?

I have a D180 and D100 combo. I notice that the D100 comes alive about 15 knots, but the D180 doesn’t until about 25 knots. There is also a difference of a few knots between the two nits IAS. Not really applicable to your issue, but thought I would mention it.
 
I have not flown any "calibration " legs. The airspeed does not come alive until right at rotation. Then it is 10-15 knots low when cross checked against 2 GPS at all airspeeds.

Am I misreading your post? It sounds like you are trying to reconcile gps groundspeed with indicated airspeed while on takeoff roll. If that's the case it won't work unless the wind is dead calm.

As others have hinted at, try flying the test pilot school Excel spreadsheet routine so often referred to here on VAF.

Thanks,

Joe
 
I have not flown any "calibration " legs. The airspeed does not come alive until right at rotation.
This doesn't sound right.
Then it is 10-15 knots low when cross checked against 2 GPS at all airspeeds.

At what altitude? At 5000' or so I'd expect IAS to be 10% or so lower than average ground speed.
 
Hi All,
Sorry for the confusion. I am not trying to resolve an airspeed vs. GPS disagreement. My airspeed is without a doubt reading 10 to 15 knots low. I believe
I have narrowed it down to a leak in the LH static port. I'm going update my D180 and see if anything changes. If not, I will be gaining access to the tailcone. Unfortunately, I do not have the split bulkhead...
 
Easier to do the following test. Disconnect (or plug) the static vents. Open the static line somewhere inside the cabin. Go fly. Problem fixed? (Try with air vents open, and closed.) Cabin pressure is usually different than true static, but not enough to decrease airspeed 15 knots.
An easier, alternative test is to set your altimeter to field elevation, then carefully do a high speed low pass 50’ over the runway. Does the altimeter read field elevation plus 50’? If so your static system is okay.
 
Group - -does the 12 use the same rivet crown shape for the static ports? Historically (here on VAF), it is some issue with the static port that has caused drastic AS errors.
 
Group - -does the 12 use the same rivet crown shape for the static ports? Historically (here on VAF), it is some issue with the static port that has caused drastic AS errors.

I believe this is true. But the vans’ rivet heads always work, It’s the other, fancier designs, that give trouble!
 
Easier to do the following test. Disconnect (or plug) the static vents. Open the static line somewhere inside the cabin. Go fly. Problem fixed? (Try with air vents open, and closed.) Cabin pressure is usually different than true static, but not enough to decrease airspeed 15 knots.
An easier, alternative test is to set your altimeter to field elevation, then carefully do a high speed low pass 50’ over the runway. Does the altimeter read field elevation plus 50’? If so your static system is okay.

Hi Bob,
Great info, I'm going to give that a try.
Thanks
 
Group - -does the 12 use the same rivet crown shape for the static ports? Historically (here on VAF), it is some issue with the static port that has caused drastic AS errors.

Hi Bill,
Thank you. It was your first reply that got me looking at the static system. I believe that's what may be causing the issue.
 
Easier to do the following test. Disconnect (or plug) the static vents. Open the static line somewhere inside the cabin. Go fly. Problem fixed? (Try with air vents open, and closed.) Cabin pressure is usually different than true static, but not enough to decrease airspeed 15 knots.
An easier, alternative test is to set your altimeter to field elevation, then carefully do a high speed low pass 50’ over the runway. Does the altimeter read field elevation plus 50’? If so your static system is okay.

Bob,
Would disconnecting the static line at the back of the D180 accomplish the same thing or is it imperative that I open the static line in the cabin?
Thanks
 
The instrument bay has cooling fans that will change the pressure.
Disconnect the static line from the back of the D-180. In its place, attach a spare tube and run it into the cabin.
 
When I had a pitot system issue on an Aventura HP I ran a temporary pitot tube and hooked it to a spare airspeed indicator vented to my cockpit for comparison. A little tough to do on an RV-12, but not impossible with a little duct tape and ingenuity.
 
Low airspeed issue resolved

After checking all of the pitot static plumbing and after deciding to stop 2nd guessing myself, I sent my Dynon in for evaluation. Turns out I had a couple of bad sensors. Works great now. Cudos to David Readout at Dynon as he was very prompt with communication, knowledgable, patient with me and had a fantastic disposition. Thanks to everyone here for your suggestions.

Jack
 
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