Pitot tube not reading at high aoa

Aaron H

Aaron H

Pre-takeoff checklist
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Aaron
Hi all,

Been doing a lot of short takeoff and landing a lately and have a started some off airport landings in farmers fields that I know.

When I do stall practice, my speed goes down to 38 knots pretty accurately. Below that is falls pretty dramatically until I stall.

I'm assuming the aoa after 38 knots and full flaps is to great for the pitot tube to accurately guage.

Does this sound like the problem and does anyone know of a solution?

I have an experimental lsa (eagle ea-100), so I change without issue

Any help would be appreciated

Thanks
 
You’d have to mount it on a tilting vane like some AoA indicators so it’s continuously getting direct airflow.

It’s a common problem with helos because of the differences in pitch throughout flight and the interference with rotorwash at slow speeds or high power climbs. Some cases you could be looking at a 30 kt difference between indicated and calibrated.
 
Just a swag, have you cleaned out the pitot tube.??

I once had an airspeed indicator on a C-152 quit reading below 55 indicated. A swap out with another ASI fixed that problem.

Other than that, someone with a ton more knowledge will chime in soon and let you know what you need to know.
 
Aaron H said:
Hi all,

Been doing a lot of short takeoff and landing a lately and have a started some off airport landings in farmers fields that I know.

When I do stall practice, my speed goes down to 38 knots pretty accurately. Below that is falls pretty dramatically until I stall.

I'm assuming the aoa after 38 knots and full flaps is to great for the pitot tube to accurately guage.

Does this sound like the problem and does anyone know of a solution?

I have an experimental lsa (eagle ea-100), so I change without issue

Any help would be appreciated

Thanks
Click to expand...
It is this way ih most aeroplanes. When someone with an experimental says that their plane stalls at 29 kts at 1,800 lbs, I guarantee that the indicated airspeed doesn't reflect reality. My Skyhawk's ASI got wonky when on the edge of a stall; it was quite good above that speed, which is all that's needed.
 
If that pitot tube isn't properly aligned it can cause misreadings at higher airspeeds than normal. If it's back under the wing it can give hokey info. Leaks in the system make errors.

More often it's a bad static reading. The airspeed indicator is a differential pressure gauge, with both pitot and static inputs, and a static port that gives poor static pressure in some attitudes will make the airspeed go off considerably. In the homebuilt world, getting a good, consistent static pressure can be a major challenge.
 
It was actually an SLSA but when the company went under it got changed to an ELSA.

I am assuming a lot of other manufacturers have the same issues.

does anyone recall any of the solutions that the other manufacturers came up with?
 
Does the kit manufacturer have any data on IAS stall speed? It’s not unusual for the airspeed to be inaccurate at high AOA. Example from a 172R, at max weight and full flaps, the airspeed indicator will show 33 at the stall, however the calibrated airspeed is 47.

Have you compared notes with other pilots who fly the same aircraft?
 
I just looked up the specs on the EA-100. It's published stall speed is 35kts or 40mph. I agree the ASI should read properly below the stall speed. I also agree the culprit could be either the pitot tube or the static port... The fact that this occurs only at high AOA leads me to think it is related to turbulence from the wing. Does it do the same with different flap settings? Look at where the pitot tube and static ports are located in relation to the wing downwash and see if one or the other might benefit from a different location. If not make sure the ports and lines are open and have no leaks. It is not much of a suggestion, but it is something.

You could try disconectng the static port at the outer vent as a test. Leave the tube open inside the cabin so it will sense the static pressure from inside, away from any turbulence. If that makes a difference the static port location is a factor. If not, then it might be the pitot tube location or type of sensor used.
 
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The pitot tube is out in front of the wing. You can see it if you soon in on the pilot side wing about half way.
I would assume the wing wash wouldn't be an issue.

The plane doesnt have a static port on the out side. It just reads from inside the cabin.

I don't notice it in any other flap settings that's why I am assuming it is the high angle of attack
 

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The Lion Air 737 crash FDR data shows the effect that AoA has on ASI reading. Obviously the numbers only apply to the 373Max and is at much higher airspeeds than are typical for light aircraft but I found it interesting. I think the issue is mostly the static port being affected but I am not sure. This is for 22 degrees AoA variation between Left and right side AoA reading.

The pitot pressure is read and then the computer applies a correction for AoA. In the case of the Left seat instruments the wrong correction was applied.

http://knkt.dephub.go.id/knkt/ntsc_aviation/baru/2018 - 035 - PK-LQP Final Report.pdf

This is from the Preliminary Lion Air report which I happen to have handy.

upload_2020-4-12_11-20-12.png
 
If you are tying to run on the edge, indicated airspeed seems like an indirect and pretty unreliable indicator. I would think that what you would care about is AOA.
 
Your picture shows the pilot tube with the cover in place. The tube itself protrudes nearly straight out from just below the forward most point of the wing cord, and at a level angle with the ground which is a slightly down angle compared to the wing cord.

One aerodynamic phenomenon that occurs is air that is being forced to move around an object affects the air upstream of it, thus causing this air to move also. This effect is most pronounced close to the skin of the object and the effect reduces the farther away you look at it. The air turning around the surface of a wing affects the air farther away from it than you realize, especially on the upper surface at low airspeed and high AOA.

Add to this that an airplane does not move through the air at the same angle as it is pointed. In level slow flight the nose is pointed up right much as the airplane flys a level track, this moves the stagnation point on the forward wing curve down changing the separation point of air that moves above and below the wing. The stagnation point would move further down as the power is reduced and the airplane descends at the same attitude. Air moving under the wing is mostly smooth, air above the wing is mostly smooth at cruise speed but becomes less so the slower the air moves. The point where turbulence begins also moves forward as the wing slows and the AOA gets higher. In slow flight there could be air turbulence a foot or more away from the wing on the upper side of the stagnation point.

Your problem appears to be that the tip of the pilot tube is feeling turbulent air in slow flight. If your tube was 1 inch lower at the front tip, or 1-2 inchs longer this problem should go away or at least be reduced. Pitot tubes are supposed to be mounted so the tip of the tube stays in air that moves under the wing, not above it. Yours can likely feel the air above the wing at high AOA.

The easiest way to test this is to extend the pitot tube about 2 inches and see if that helps. That would be easier than any other modification you can do.
 
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William Pete Hodges said:
The easiest way to test this is to extend the pitot tube about 2 inches and see if that helps. That would be easier than any other modification you can do.
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That tube does look to be in a poor spot. The old Cessnas had a plain aluminum-tube pitot that stuck out of the LE, but it was a lot longer than that.
 
I imagine it's just right on the edge of being okay all the time. You could try sliding a snug piece of longer tube over the existing one to see if moving the open end forward makes a difference. If it does then talk to your AI about a more permanent fix. Be aware that it could also make it worse too. Just be careful so you can takeoff and land safely if the ASI is inop or acts wonky. That shouldn't be a problem for you because it is already wonky now.

Capt Thorpe suggested you might be more concerned with AOA instead of airspeed. You have not indicated if your airplane has an AOA or LIFT indicator installed. If you do you could look at that during high AOA maneuvers, but you need to verify that it is calibrated properly for it to be of any real use. If you don't have one you can get one installed but it will likely be more expensive than fixing your existing problem, which you probably want fixed anyway. I know I would if it were my airplane. That's why I haven't mentioned it before.
 
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Even with dead nuts perfect indicated airspeed, if you are concerned about stalls, the number is going to vary. And, in an LSA one typically operates over a wider range of the useful loads than, say, a Bo. I notice day to day differences in performance due to full vs. empty tanks or, God forbid, a person in the right seat. So, the speed you learn while practicing pushing the edge while solo isn't going to work with a body in the other seat. AOA, on the other hand, should be more consistent.
 
Capt Thorpe,
I agree with what you are saying, and I have flown and a trained with an AOA meter for over 10 years. I don't think he is trying to push the edge, he simply noticed that the ASI goes wonky when doing practice stalls and he would like to resolve that problem with his airplane. Eagle aircraft is out of business, so as owner it falls to him to resolve. He wants it resolved even if he had an AOA meter.
 
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