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Inaccurate Airspeed (Read 2436 times)
Aug 3rd, 2015 at 8:46am

Brian Reid   Offline
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I have a full Dynon Skyview setup in my World Aircraft Vision. There is also a backup mechanical airspeed indicator. The Skyview has consistently read about 7 mph (5 knots) faster then the mechanical ASI. When I did the last transponder check I also had them check which ASI is accurate. It turns out that the mechanical is the accurate one. Since they operate off of the same pitot static plumbing the inaccuracy has to be in the Skyview. I know this is just barely within the limits for ASI accuracy but I'd like them to be the same. Is there any way to adjust the Skyview ASI so that it would be accurate? Thanks.
 
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Reply #1 - Aug 3rd, 2015 at 5:51pm

Dynon Avionics   Offline
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So there are a few possibilities here. First, know that almost invariably (there are of course rare exceptions), when we get equipment back with problem reports like this, they test out fine. So first, leaks in the pitot/static system could cause oddities, though I'd expect that to have one of the instruments read lower than the other and the test set, not higher.

Next - if SkyView's IAS was wrong and always too high, you'd expect to see a fairly constant headwind indication when you don't have much actual winds aloft. Are you seeing that?

That said, you can try doing the zero pressure calibration on the ground as described in the installation guide (in a hangar, with absolutely NO wind). That's designed to set the "zero pressure" point, which would mostly affect low IAS indications. If that doesn't fix it, we'll need to see the ADAHRS to investigate further on our end.
 

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Reply #2 - Aug 4th, 2015 at 2:12pm

Brian Reid   Offline
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Thanks for the input.

I'll need to check about the in flight head winds. I've been flying almost fifty years and constant head winds are just normal. Grin

I'll also try the zero pressure calibration and let you know the results.
 
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Reply #3 - Aug 4th, 2015 at 5:05pm

mmarien   Offline
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Murray M.
Saskatoon SK CAN

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Brian Reid wrote on Aug 4th, 2015 at 2:12pm:
I've been flying almost fifty years and constant head winds are just normal.

So true Grin Grin
 

Glasair II FT - o320 - P-Mags - EFII - MTV 3 Blade CS - VP-X - Single 10" Skyview - Stratux Dual Band - zooooom ...
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Reply #4 - Sep 1st, 2015 at 1:28pm

Brian Reid   Offline
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To let you know the outcome:

1. no leaks
2. no unexpected headwind indications
3. performed zero airspeed calibration - appears to have solved the problem. Airspeeds are now within 1 knot.

Thanks for your help! You guys are great.
 
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Reply #5 - Apr 23rd, 2016 at 4:46am

jeffauen   Offline
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AirCam Twin

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Interesting zero calibration experience:

Symptom: My Skyview airspeed indication was 6MPH higher than the steam gauge. I confirmed the steam gauge was correct by flying on a calm day and comparing GPS (at 800 feet) and also by confirming known stall speed, clean.

BTW I'm flying a Lockwood Aircam with a cruise between 50 and 100MPH. Stalls at 39.

I called Dynon Tech Support. When I calibrated zero pressure, I failed to cover the static port (RTFM  Undecided). So I recalibrated. That brought the speed discrepancy down to 3MPH.

Here's the method I used to get them to match exactly:

Get to the calibration screen on the Skyview. Cover the static port. Put a soft hose on the pitot and block the end (the hose should be long enough to hold while you are looking at the Skyview config screen. Very gently squeeze the soft hose, so gently that there is no movement whatsoever on the Skyview or the steam airspeed gauge. The finger pressure on the hose is almost imperceptible. Hit the calibrate button and save. Test it. I got mine right on the first try. You may need to recalibrate if your squeeze wasn't perfect. I confirmed airborne with GPS and a stall series.

Speculation as to why this works:
A single pitot tube and static tube serves both gauges. The plumbing is teed near the tubes. The tube feeding the Skyview is 3 times as long as the tube feeding the steam gauge. Maybe the longer tube has a very, very small pressure differential?
 

Jeff
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Reply #6 - Jul 22nd, 2018 at 5:26pm

cjg3   Offline
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I did the zero pressure AOA/IAS calibration yesterday with hangar doors closed, pitot covered, and static ports sealed off. Today I flew the plane (RV-7A) on a Phase 1 flight and got high airspeed readings, e.g. a 57 KIAS stall when trying to do slow flight (should have been <50), 70 KIAS almost immediately when starting takeoff run, and 44 KIAS taxiing. Also the altimeter read ~100' low with the current altimeter setting.

The only thing I can figure is that it was very hot in the hangar-->90F--and that affected the calibration. Fortunately  the zero pressure calibration can be reset to default. I'll leave it that way, at least until more temperate weather.

There's no mention of ambient temperature limits in the HDX installation manual's calibration procedure, so if it's a factor, perhaps there should be.
 
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Reply #7 - Jul 31st, 2018 at 4:41pm

Dynon Avionics   Offline
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There aren't ambient temperature limits that would affect this. But to be clear, you want a draftless environment, but you also don't want to seal off pitot or static in any way that would allow pressure to be stored in any of the lines.
 

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Reply #8 - Jul 31st, 2018 at 4:42pm

Dynon Avionics   Offline
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The fact that you got such a high IAS immediately after starting the takeoff run makes me wonder if there's any odd pitot or static behavior. Is the pitot well out of the propwash? Are the static ports in known good places for the airframe? Does altitude behave normally when you start climbing?
 

Please do not use Private Messaging on form to contact. For private support:
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