Pitot Problems

[johndavies]

I had my first flight yesterday (RV-7A) and the airspeed was reading 40 knots with GPS Groundspeed of 105 knots. This speed was constant accross a steam guage AS and my Dynon-100 and Dynon-180. I have the dynon heated Pitot. I hooked up the pitot head to a manometer as per Accepted Practises and all three instruments read 150 knots with 14.9 inches of water with no leakdown. I used tape to cover the Dynon Pitot drain holes. Back in the air I still get way off Airspeed readings (50-60 knots low but constant accross all three guages) With the external static dissconnected (using alternate static) the situation improves to 30-40 knots low. When doing the manometer test removing the drain hole tape causes an instant drop to 0 airspeed. I next flew using the regular static BUT WITH TAPE COVERING THE PITOT DRAIN HOLES. All airspeed readings are now normal and correspond to GPS readouts. The only conclusion I can come to is that the drain holes are just letting too much air escape. Has anybody else had this problem? Am I missing something?
Regards John Davies RV-7A C-FRVR

 

[jakej]

Are you absolutely sure the tubing from Dynon pitot is connected correctly - it's easy to do this incorrectly

Jake J

 

[johndavies]

Thanks Jake, but the fact that connecting the manometer to the pitot hole (and not the AOA hole) with surgical tubing and getting 150 knots indicated on all three instruments with 14.9 inches of water as per AC 43.13 with no leakdown should pretty well guarantee that the tubing is all connected correctly. The problem is that this requires the drain holes to be sealed off. As soon as the drain holes are unsealed the air pressure gushes out and airspeed instantly drops to 0. In the air all three instruments read 40-50 knots at 105Kn GPS (drains unsealed) and read correctly (+- 5knots) with drains sealed. Switching static sources (regular and alternate) seems to make little difference and Altimeter and VSI seem to be working correctly in all regimes. Sorry to be wordy but I wanted to cover all details as I am stumped on this.
Thanks, John

 

[johndavies]

I'm replying to my own post here - I am sort of waiting for Dynon Support to jump in - but it occurs to me it would be pretty simple to rule out the pitot head itself as the culprit. It is easy to plug up the pitot outlet and then connect the manometer to the pitot snout and measure the rate of the "leak" through the drain hole. All I need is some sort of figure from Dynon for the designed leak rate (so many inches water per minute say). It dawned on me that if the internal pitot tube was leaking (maybe where it joins the snout) at about the same rate as the drains were designed to leak then sealing them off would bring the pitot back to regular function. I wonder if Dynon has a leak rate figure so I can keep troubleshooting.
Regards, John

 

[khorton]

The only conclusion I can come to is that the drain holes are just letting too much air escape. Has anybody else had this problem? Am I missing something?

Is there any chance there is some sort of obstruction inside the pitot tube, somewhere between the inlet to the tube and where the drain is?  I think this would also give the symptoms you describe.  It might be worthwhile using some low pressure air to blow in the outlet to the pitot tube, to try to blow any foreign material out the inlet.  You wouldn't want to use much pressure to do this, as I don't know what pressure the pitot tube is designed to withstand.  450 kt CAS is only about 5 psi of pitot pressure.

 

[dynonsupport]

We don't have a leak rate I can easily give you.

Internally, the tubes go right from the front to the back. The drain holes on the outside aren't directly the ones on the inside, the ones inside are precision drilled into the tubes. I don't even know how it would be possible for them to be off.

Kevin's idea of something blocking before the drain is all I can think of as well. You seemed to prove that there is no leak in the system by doing the closed test. So the only way you can have an issue is if there is a restriction before the drain holes that make the drain holes a big ratio in comparison to the entry area.

I don't know how it could happen in production, but I guess it could a manufacturing defect, but if you could unplug the whole pitot and blow through it from the aircraft side outwards, it could help you figure out if it's free flowing.

Oh, and congrats on your first flight!

 

[jlakins]

Dynon
is it possible that he has the Pitot and AOA reversed.
If I read your description correctly then plugging the outside drain would allow the pressure to bleed through and pressurize the AOA line thus giving a airspeed indication.

Jimmie
RV-9A

 

[dynonsupport]

Jimmie,
Interesting idea and totally possible. If he's blocking both the AOA port as he pressurizes the pitot port, that could happen.

 

[johndavies]

Thanks for all the suggestions. I think the idea of a restriction/blockage before (or next to) the drain is the answer. I dissconnected the tubes from the wing side of the pitot and connected two tubes leading outside. Blowing down the AOA was easy but blowing down the pitot was very hard. I fooled with blowing both ways with only a little improvement so used some .032 wire to gently probe. There definately seemed to be some obstruction. I used the .032 to "ream" the tube a bit and then some .041 followed by plenty of air and miraculously the pitot now blows as easily as the AOA. I believe it was blocked when I received the pitot because I did notice that it was hard to blow through the pitot but figured "thats the way it is supposed to be". Kevin jogged my memory. I think some manufacturing debris may have got left in. I guess it wouldn't take much to equal the drain hole air volume. Anyway I have yet to try it as the weather was bad today but I am fairly confident given the difference in effort blowing down the tubes. Will report back after flying hopefully tomorrow. Thanks Guys, John

 

[johndavies]

Well that did it. It was definately a blockage and now the airspeed is working fine. Put in an hour of speed runs and the TAS seems to gel pretty well with GPS groundspeed although we had gusty winds so exact calibration was difficult. Thanks again for all the input,
John Davies