Dynon pitot probe for pitot only

[swill]

Hi all...

Although I dont have a Dynon display in the cockpit, I am using the Dynon AOA pitot probe for pitot (total) pressure only - meaning I don't have the AOA line hooked up.

I got my bird flying and we found that when flying with the chase plane at 140 kts, my EFIS airspeed is 135 kts and my mechanical ASI is showing 130 kts. (the difference between my EFIS and standard ASI gauge tells me that the mechnical must be 'broken' since they are both reading the same pressures).

Question - do I need to do anything to the Dynon probe in order to use it as a standard pitot probe only?? I squeezed the aluminum air line closed on the AOA side. That's about it. Tony at SafeAir1 said that I may be seeing slightly lower airspeeds due to the drain ports? Are they tied together in some way?

Thanks for all your help!
Scott Will
RV-7A
Flying!

 

[khorton]

If the Dynon pitot's drain system is well designed, it should have a negligible effect on the airspeed accuracy.  The pressure at the entrance to the pitot tube will be the same, even if there is a small amount of air going out the drain.  The only effect on the airspeed would be due to the amount of pressure drop from the pitot entrance to the drain hole.  But, they should have ensured that all passages along this route were large enough so the pressure drop would be pretty close to zero.

However, if there was some small orifice between the pitot inlet and the drain hole, and this orifice was also in the path from the pitot inlet to the ASI, this would be a problem.  There would be a pressure drop accross that orifice, and this would affect the airspeed accuracy.

I'm betting the Dynon folks aren't stupid, so they have almost certainly properly designed the pitot tube.

There are many more likely sources of your apparent airspeed errors than the pitot tube.

1. First, how do you know the chase aircraft has an accurate airspeed indication?  Many people come up with some very strange ways to check airspeed accuracy.  Many of these ways are not accurate.

2. You know at least one ASI has some errors.  They both may have some error.  Sort this out via a ground test, using either a water manometer (cheap) or have an instrument shop use a pitot-static test set (expensive).  Spreadsheets for water manometer height vs ASI reading - http://www.kilohotel.com/rv8/phplinks/out.php?&ID=331.

3. Check for leaks in the pitot and static systems.  A leak somewhere in the pitot system would require air to keep flowing though the pitot line, and the pressure drop would lead to an airspeed system that reads too low.  A leak in the static system could cause the system to over read, or under read, depending on whether the local pressure at the leak was higher or lower than the pressure at the static port.

4. Do flight testing to check for static system position error.  More info at http://www.kilohotel.com/rv8/phplinks/out.php?&ID=31.

 

[dynonsupport]

The Dynon pitot is a normal pitot, with normal pressure outputs. You can use the Dynon pitot with any ASI even without plugging the AoA tube, and we we can use a normal pitot with the Dynon EFIS if you don't want AoA.

Since you don't have a Dynon EFIS- any chance that the airspeed on the EFIS is actually off and not well calibrated? How do you know it's the pitot?