Pitot guts

[IonSteve]

Apologies if this has been covered previously.

Question: Do the pitot and AOA lines in the Dynon pitot unit connect internally?  Phrased another way--should there be cross flow between the pitot and AOA lines?

I don't know how the Dynon AOA system works, so it may be a normal or correct circumstance that in my pitot boom there is cross flow/leakage between the two lines. I.e. if I block the two tubes on the back end of the boom and blow air into the pitot port, it immediately comes out the AOA port.

This is all pursuant to chasing down an incorrect air speed problem.  The static side of the system held manometer pressure with no problem, but when I tried to pressurize the pitot side the water column just bounced for a second and equalized immediately.  I then found that the air was going in the pitot port and coming right out the AOA port.

Thanks for your input--

Steve.

 

[Brantel]

The Dynon Pitot is vented. If you try to pressurize it, it will leak down. This is spelled out in the manuals. The best way to test your system with a Dynon Pitot is to take the line loose and test it from there on in.

 

[dynonsupport]

Both the pitot and aoa lines have an intentional internal leak to a common cavity, which is itself vented, which serves to drain moisture. That leak is not enough to affect airspeed appreciably.

 

[IonSteve]

Both the pitot and aoa lines have an intentional internal leak to a common cavity, which is itself vented, which serves to drain moisture. That leak is not enough to affect airspeed appreciably.

Okey doke, I understand the intentional internal leak--but the common cavity is not vented on my unit.  If I seal 3 out of 4 ends of the tubes and apply pressure the unit will hold pressure all day.  If required I can drill a little vent hole wherever I am advised to do so.

FYI, I got the unit (a D180) on a calibrated manometer this afternoon.  All the plumbing holds pressure fine, pitot, AoA and static.  The unit is consistently accurate within -0/+1 above 70 knots, -0/+2 from 50-70 knots, -0/+3 from 40-50 knots and kind of goes to hell below 40-ish knots. All good enough for me.

The current thinking re our inaccurate airspeed is back on the static source.  We thought we had a good static source but right now--according to classic airspeed trouble shooting--it is the most likely culprit.

The main symptom is our airspeed starts to hit a wall at about 85 knots indicated, even though we have established that we are going around 102.  Even diving from 102 knots up to 120 knots will only bring the indicated up to 95.  I.e. the faster we fly the greater the discrepancy.  This largely true regardless of density altitude, etc.

 

[dynonsupport]

Looking into whether I might be referring to an old construction style for the pitot that's no longer in use.

But given what you've described, that does indeed sound like a static issue. You might also confirm that you haven't swapped the pitot and AOA lines. Customers have done that on occasion, and you can get some really strange IAS readings.

 

[IonSteve]

Yep, made sure the pitot and AoA lines go to the right places. It was an early--and futile--hope that the problem was simple human error. It may still be human error, but not a simple one. Let me know about the common cavity vent. I will still work on the static source assumption.

 

[dynonsupport]

So if you plug both aluminum lines and one of the pitot/AOA snout ports, and blow in the other, you should get a small but detectable leak that either comes out the holes that are on the bottom of the tube, mid-probe (newer design) or out of the holes near the rear of the tube, on the bottom (older units). Those holes are there to help vent moisture. Not all pitot designs incorporate a leak by design like ours do, so if yours doesn't leak like that, it's not to our spec, but it's not all that different than many other pitots. We're happy to RMA yours, or if you decide to keep it as-is, just know that it won't drain moisture.