Air speed off at low speeds

[n502pd]

I have made several flights to confirm that my steam gauge airspeed insturment shows approx 6 or 7 mph slow relative to average gps ground speeds below 70 mph, taking in to consideration winds. I have checked the instrument out of the a/c and it seems to be right on with a manometer system. Retested with the insturment installed in the a/c and still showed very close to perfect correlation with numbers on the manometer. A/C systems were checked for leaks over night and several were repaired, but the airspeed error below 70 mph still persists. Pitot and static ports checked for any unusual items, and none were observed: pitot and static ports are per plans. I really dont want to replace the ASI as it works almost perfectly at higher speeds. and of course the new ones are expensive.

Can anyone point me in a useful direction? maybe the nose end of the pitot tube needs tapering/champhoring (??), or the static tube needs more of a pointed nose to prevent disturbed air that close to the actual static ports? I dont have much experience here, but am open to suggestions.

As always,,,Thanks much!

[gammaxy]

I wouldn't worry about it as long as it is consistent.

The plans static pressure port is influenced quite a bit by the wing. This causes it to measure higher pressure than the actual undisturbed static pressure at positive loadings and large angles of attack. You can easily prove this to yourself next time you fly by pulling a couple G's from level flight and watching the altimeter initially show a descent despite your body feeling a vertical acceleration. This probably accounts for a few mph error at low speeds. The greater angle of attack might also be reducing the pressure sensed by the pitot probe a little by changing the relative direction of the airflow. Both combined might account for the discrepancy you are seeing. I haven't precisely calibrated my instruments at those speeds so I can't tell for sure.

I believe this error also accounts for about 50 feet of altitude error. I only notice it because in my recorded telemetry, my altimeter increases by about 50 feet while decelerating on the runway after every landing.

An interesting side effect is that stalls happens at pretty much the same IAS regardless of the number of G's being pulled.

[builderflyer]

Joe,

Your IAS may not be quite as far off at low airspeeds as you think. Did you determine your true airspeed and compare that with your average groundspeed? What altitude did you do your testing at? Did you make note of your pressure altitude and OAT at the time?

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

[n502pd]

These are both good replys! I knew I had forgotten something...true airspeed and altitude. I have not pulled any G's to speak of except once to check that the wings were still attached!! I will look at all the suggestions and post what I find. Thanks much!!

[builderflyer]

BTW Joe,

When I calibrated the ASI years ago I found that for most airspeeds the ASI read too high by about 10 percent. So at 150 mph, for example, the ASI read 165 mph and I thought I had the fastest Sonex out there at that time. But as it would turn out, the static source pressure was too low (I have the plans built pitot tube and static tube). After much trial and error I was able to reduce the static source error so that the IAS is now within 1 or 2 mph between 90 mph and 190 mph. But the ASI still reads about 3-5 mph low between stall speed and 80 mph. That's as good as I'm able to do. Let us know if you're able to do better.

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

[Concorde]

For those who have Dynon or Garmin AOA/Pitot tube , where did you put the static ports ?

[daleandee]

For those who have Dynon or Garmin AOA/Pitot tube , where did you put the static ports ?

I watched this thread and didn't want to derail it but I'll use your post to bounce an idea along ...

If you want some excellent information for low speed flying i.e. take-offs & landings, consider adding an LRI (Lift Reserve Indicator) to your panel:

https://sites.google.com/site/lrikit/

More information here:

http://www.sonexfoundation.com/uploads/Culver_Lift_Reserve_Indicator.pdf

Watch it as I make a landing approach:

https://www.youtube.com/watch?v=WjA7JDJVKeQ

Dunno if this helps -

Dale Williams
N319WF @ 6J2
Myunn - "daughter of Cleanex"
120 HP - 3.0 Corvair
Tail Wheel - Center Stick
Signature Finish 2200 Paint Job
157.8 hours / Status - Flying

[n502pd]

thanks Dale, I do have an AOA installed and it is set very conservatively now just to be safe until I can get the low speed ASI fully understood. I will then reset the AOA probe to read closer to actual stall parameters.

Now I have a question about the static probe. Plans have had me drill 4 very small ports, spaced 90 degrees from verticle, around the end of the probe about 1, or so inches aft of the tip (not at plans just now, so forgive me if the dimentions are not exact). since the wing AOA increases as speed decreases for level flight (as an example), the relative wind will tend to blow into the bottom opening of the static tube and increase the static pressure in the system. This, as I understand it, is undesireable for maintaining (theoretical) accuracy of the ASI. Is this correct thinking? Also, visualizing the end of the pitot tube, the cut end is not flaired...just cut straight off...presenting a flat surface to relative wind, and (maybe?) causing turbulance at the pitot entrance, therby(?) causing a lower pressure there? (I know these thoughts may be a bit of a reach, but afterall, there is that word...experimental...in the name!) I sure would like to have some better low speed accuracy. I am not sure my old 172 had this problem, as its pitot tube was well forward of the wing LE, and the static port didnt see any pressure increase with AOA change, as long as one didnt slip the craft. Has anyone relocated the static port to the fuse side, or even into the cockpit itself?

[n502pd]

BTW Joe,

When I calibrated the ASI years ago I found that for most airspeeds the ASI read too high by about 10 percent. So at 150 mph, for example, the ASI read 165 mph and I thought I had the fastest Sonex out there at that time. But as it would turn out, the static source pressure was too low (I have the plans built pitot tube and static tube). After much trial and error I was able to reduce the static source error so that the IAS is now within 1 or 2 mph between 90 mph and 190 mph. But the ASI still reads about 3-5 mph low between stall speed and 80 mph. That's as good as I'm able to do. Let us know if you're able to do better.

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

Thanks for the input Art! maybe I am not so far off from normal as I am thinking. But, I want to investigate this to see what I can do. Fly safe!!

[daleandee]

Now I have a question about the static probe. Plans have had me drill 4 very small ports, spaced 90 degrees from verticle, around the end of the probe about 1, or so inches aft of the tip (not at plans just now, so forgive me if the dimentions are not exact). since the wing AOA increases as speed decreases for level flight (as an example), the relative wind will tend to blow into the bottom opening of the static tube and increase the static pressure in the system. This, as I understand it, is undesireable for maintaining (theoretical) accuracy of the ASI.

Could it be that as the angle of the pitot changes and the bottom static hole gets more pressure that the upper static hole moves into a low pressure area and thus the overall change is minimal? Isn't that the reason for the multiple holes?

As some others have suggested ... go fly the airplane and don't focus on it too much as to the complete accuracy so much as the repeat-ability of those readings. The reason I use and recommend the LRI for low speed maneuvers is because the readings are repeatable for all wing loadings and bank angles. It really is the most used instrument on my panel.

But if you'd like to attempt to make your ASI more accurate you can always try the "o-ring" method. I've read a great deal about it but never used it. Here's a quote:

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.

From the EAA article:

https://www.eaa.org/en/eaa/aviation-communities-and-interests/homebuilt-aircraft-and-homebuilt-aircraft-kits/resources-for-while-youre-building/building-articles/instruments-and-avionics/installing-a-pitot-static-system

Best,

Dale Williams
N319WF @ 6J2
Myunn - "daughter of Cleanex"
120 HP - 3.0 Corvair
Tail Wheel - Center Stick
Signature Finish 2200 Paint Job
157.8 hours / Status - Flying