pitot systen position error

[n502pd]

I have been wondering for the last 25 hours, how do others deal with position errors on the air speed indicator? I happen to have steam gauge pannel, but I am pretty sure the digital pannels have the same concerns. so, what methods have users of both pannels used to assist with position error due to pitot tube placement and subisequent ASI indications?

[sonex1566]

You've got me thinking now! What sort of errors are you experiencing? I am assuming that your GPS ground speed doesn't seem to line up with your air speeds. I am almost ready to move mine out to the airfield for final assembly, I don't want to even contemplate trying trying to relocate the pitot and static tubes. I've only got a few hours up in a Sonex operated by the local SAAA here in South Australia, but I didn't pay much attention to the two air speed indicators. I was more concerned with just flying the plane and not breaking it! I am using a pitot and static supplied by Aircraft Spruce on mine.

[mike.smith]

FWIW, I have not experienced any measurable pitot error with mine in the location shown on the plans, in 5 years of flying.

[radfordc]

https://www.eaa.org/en/eaa/aircraft-bui ... tic-system

"If your airspeed reading is slow, it is because the static port is in a slight high pressure zone. This can be corrected by slipping a small "O" ring over the end of the static tube ahead of the tiny drilled holes (vents, that is).

Moving the "O" ring aft on the tube (in very small increments) will increase the airspeed - on the gauge (not for the airplane, amigo . . . sorry). Conversely, moving the "O" ring forward, away from the ports, will decrease the indicated airspeed in much the same manner. Very small changes in the "O" ring position make a noticeable difference in the indications.

What happens is that you are attempting to lower the static pressure "felt" by the static ports with the "O" ring interrupting the airflow, thereby causing a slight lowering in air pressure behind (downstream) the ring.

The closer the ring is to the ports, the lower the induced pressure.

If on the other hand the static-tube port happens to be located in an area of low pressure, moving the ring to a position behind the ports should provide a slight increase in the pressure felt by the static ports with results opposite to that described above.

Because a rubber "O" ring will deteriorate, it should be replaced with a metal ring having identical dimensions after the proper location is determined. A dab of paint or epoxy should hold it in place. One gent who does some wild gyrations in his modified Starduster has a big ol’ set’screw securing his "scientifically" located ring."

[builderflyer]

Joe,

Charlie is spot on.

My pitot-static system was built and installed per the plans. Through flight testing it was found that the pressure at the static ports was too low which resulted in the airspeed readings being too high by about 10% at 150 mph. So the solution through much trial and error was to place a collar around the static tube behind the static ports to raise the pressure just the right amount. The correct dimensions of the collar and its distance to be placed behind the static ports was determined through numerous test flights. The final result for my Sonex was for the airspeed indicator to read within 1 mph between 90 and 190 mph, with increasing error at or near the stall.

This has been my experience on my Sonex and, for whatever reason, may not be directly applicable to any other Sonex.

Art,,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,Jabiru 3300 #261

[n502pd]

Well, thanks for the comments, and the airspeed above,say 70 mph indicated and displayed on zero wind day gps tracking, was indeed very close with the plans built system. However, I really am not concerned about speeds well above stall, as not too many folks have had problems at those speeds with the old sudden stop!. My concern was that, at stall, with flaps or without, power on or power off, I had errors at actual stall between 7 and 12 % referencing gps, and airframe design stall speeds. that is where, IMHO, position error should be well understood and cared for, not at the high end of speed indications, for stall-spins close to mother earth. I was looking for a correction regimin that would give a very flat error percentage reguardless of indicated and actual airspeeds, across the full range of airspeeds seen. I too had built N502PD as close to plans as possibe, including the factory pitot static sensors, and their factory location, and suggested AS and static plumbing. Please keep the comments comming!!

[gammaxy]

I've mentioned this observation on a few other threads. The wing isn't the ideal location if you want a perfect measurement of static pressure since it is affected by the pressure distribution around the wing that varies with velocity and angle of attack.

I believe this causes the airspeed to read a little low near stall. It also causes the airspeed to read low during accelerated stalls--in my airplane it almost perfectly cancels out so the indicated airspeed during an accelerated stall is almost exactly the same as an unaccelerated one.

Because I understand the error and its consistency, I haven't felt a need to make any changes. Also, the ASI is pretty accurate during unaccelerated flight at more normal speeds.

The easiest way to see if your static system is sensitive to this is to watch the altimeter when you pull a couple G's. The altimeter will first show a rapid descent which disagrees with what the seat of your pants is telling you.

[n502pd]

OK, i will pay more attention to the altimiter when pulling a couple of g's. What I am looking at is the pitot placement relative to the wing and the pressure distrubtion as you mention allong with the allignment of the tube to the relative wind.

[radfordc]

Have you considered the Lift Reserve Indicator: http://www.sonexfoundation.com/Lift_Res ... cator.html

Dale Williams has one on his Sonex and loves it.

[builderflyer]

Joe,

You may be a prime candidate for an AOA system if your primary concern is in the slow speed arena. It's difficult, if not impossible, to obtain accurate indicated airspeed readings throughout our entire range of operation. Many of us want our calibrated airspeed to be as close as possible to our indicated airspeed within the range of speeds that we spend the most time at while flying our aircraft. That gives us a reliable number for Va, Vne, aerobatic maneuver entry speeds, and assessing the cruise performance on a cross country flight.

In your very early testing you will have stalled your Sonex and noted the indicated airspeed at which it occurs, whatever that number is. You then applied a safety factor to that airspeed, say 1.3, to determine a safe approach speed at that gross weight. You may have worked yourself up to doing accelerated stalls and observed the indicated airspeeds at which they occur and then applied an additional safety factor for those times you're hot dogging it at low altitude. So for your typical slow speed operations, having an accurate number isn't as important as knowing what that number is for your particular aircraft learned through testing.

But, again, if slow speed operations are your main concern, then go with an AOA system. As you likely know, that system will give you the most reliable information to operate your aircraft safely at slow speeds near the ground.

Good luck,

Art,,,,,,,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,,,Jabiru 3300 #261