Dynon Pitot reassurance

[Simon Hitchen]

One credible source needed,
Has anyone installed a dynon pitot/Gretz mount in the bay inboard from the aileron bellcrank with satisfactory flight performance. This position puts the pitot 3 inches inboard of Van's recommeded position. I do see from extensive searching that alot of guys are mounting them in the next bay outboard but that involves tube routing issues and tie down interference concerns.

Many thanx.

 

[vlittle]

I installed my Dynon pitot inboard of the bellcrank and ended up with a 4% low TAS indication.

Since I also drilled and subsequently plugged the standard Van's pitot mount, I'm contemplating an experiment.

I'd temporarily remove the AOA line from my Dynon pitot and connect it to a Van's pitot tube mounted in the standard location. In the cockpit, I'd put a valve so that in flight I could switch between pitots. I think that would end any arguments pretty fast.

I'm in the middle of my annual right now, so results will be forthcoming later (Feb?). If anyone else can do this sooner, it would be interesting.

Vern Little

 

[Kevin Horton]

Vern - I'm very interested to learn the results of your experiment with two different pitot tubes, but I'll bet you a case of beer that the experiment you describe will show the two pitot tubes produce essentially identical results. Of course this assumes that there are no leaks in either pitot system, other than the very small leaks through the water drainage holes on the tubes.

 

[Simon Hitchen]

Thanx Vern,

I take it you're flying a 7?

 

[vlittle]

Vern - I'm very interested to learn the results of your experiment with two different pitot tubes, but I'll bet you a case of beer that the experiment you describe will show the two pitot tubes produce essentially identical results. Of course this assumes that there are no leaks in either pitot system, other than the very small leaks through the water drainage holes on the tubes.

Hey, I'm not taking sides on this, but willing to run the experiment. As for the beer, next time your out this way we can share a jug.

BTW, I'm flying a 9A.

Vern

 

[Simon Hitchen]

Thanx, the 9 obviously has a longer wing and longer flaps so the pitot mount is also further from the fuselage. I was worried about slipstream interference with the pitot on the short wing of the seven.

BTW Vern, during my searching I've found lots of threads for guys with the same airspeed error that you're experiencing. Seems quite often that the problem is with the mounting of the static port for what its worth. Let us know what the results of your test are. Thanx.

Anybody else with a 6,7, ir 8 mounted the pitot inboard of the aileron bellcrank bay?

 

[alpinelakespilot2000]

I installed my Dynon pitot inboard of the bellcrank and ended up with a 4% low TAS indication.

Vern-Just curious, did you put your's inboard or outboard of the hole Van's specifies for their pitot tube? In my -9, I put the Dynon/SafeAir mast in the bay just inboard of the bellcrank but right against the outboard rib. If anything, in this position it would be farther from any slipstream errors than would Van's position. Thus, I'm curious about exactly where in that bay you put yours. Thanks.

 

[vlittle]

Vern-Just curious, did you put your's inboard or outboard of the hole Van's specifies for their pitot tube? In my -9, I put the Dynon/SafeAir mast in the bay just inboard of the bellcrank but right against the outboard rib. If anything, in this position it would be farther from any slipstream errors than would Van's position. Thus, I'm curious about exactly where in that bay you put yours. Thanks.

Steve, that's where I put mine, as well (used the Gretz mount). I calibrated pitot/static using a manometer, and was within measurement error on the ground.

I'm using the Van's pop-rivet static ports. I did play around with washers over the static ports and dams in front of the ports, but in the end I couldn't trust my measurements... I didn't want to mess up altitude indications to correct the airspeed indication. Since I had no way of precise altitude measurements, I decided just to calibrate and apply the 4% correction.

I did change the angle of the pitot in the airstream by tilting the tip down a few degrees. My reasoning was that it would be more accurate at high angles of attack near stall, where airspeed indication is critical. This seemed to help, but only at low speeds.

Vern

 

[Kevin Horton]

Steve, that's where I put mine, as well (used the Gretz mount). I calibrated pitot/static using a manometer, and was within measurement error on the ground.

I'm using the Van's pop-rivet static ports. I did play around with washers over the static ports and dams in front of the ports, but in the end I couldn't trust my measurements... I didn't want to mess up altitude indications to correct the airspeed indication. Since I had no way of precise altitude measurements, I decided just to calibrate and apply the 4% correction.

I did change the angle of the pitot in the airstream by tilting the tip down a few degrees. My reasoning was that it would be more accurate at high angles of attack near stall, where airspeed indication is critical. This seemed to help, but only at low speeds.

Vern - Where is your OAT source? Are you certain that you have an accurate OAT indication? It would take about a 20 deg C error in the OAT to explain all of your TAS error, but maybe OAT error could be part of the problem.

If your airspeed error is completely due to static system error, then your altitude will also be in error. Making aerodynamic changes to correct the airspeed will also correct the altitude.

 

[vlittle]

Kevin, my probe is located underneath the HS.

I agree on the static position error... I don't want to go fix the airspeed problem until I determine if it's a static or pitot problem. 'Fixing' the static could cause altitude indication problems.

It seems like the only reliable way to calibrate the static in flight is to perform low approaches at different speeds to a number of different airports using current altimeter settings. Actually sounds like a lot of fun, as long as there are no birds on the runway.

Vern

 

[Kevin Horton]

It seems like the only reliable way to calibrate the static in flight is to perform low approaches at different speeds to a number of different airports using current altimeter settings. Actually sounds like a lot of fun, as long as there are no birds on the runway.

Vern - the laws of physics work in our favour for pitot systems. They work on total pressure, which remains the same as the accelerates and decelerates as it goes around the airframe. You have to work pretty hard to have a bad pitot source, unless you have a leak. About the only ways to screw up a pitot source are to have the pitot tube in the prop wash, or in the wake of something else, or in the boundary layer, or to have it grossly misaligned with the local airflow. Your pitot source is almost certainly not the cause of your problem, unless you have moved it quite a ways inboard. How far is it from the centreline of the aircraft, and how far aft of the prop plane?

TAS errors are most likely due to static source errors, static system leaks and/or OAT errors.

I've got lots of info on how to check static system accuracy on my web site. I need to update the static system calibration info to better cover the need to have an accurate OAT indication.