Static Port / Airspeed Issues

[lr172]

During my testing, my TAS was reading 10 MPH high at top cruise speed. I did some research here and tried taping some o-ring material behind the static ports. It worked perfectly and my TAS now matches my averaged GPS speed. Fortunately I am getting Van's numbers at cruise

My problem is that it seemed to drop my low speed numbers as well. I was getting a Vs stall at 55 MPH and today I tested again and am getting 48 MPH, which is 7 MPH below Van's reference numbers. Is this typical when tweaking the static port? Does this point to a different problem, such as a leak? I tested for a leak in two ways and both passed (test via instruments at transponder check, as well as holding suction on static port with a very stable reading)

Larry

[n82rb]

My option is I want it accurate at slower speeds, who cares if it's high at cruise. I set power and get what I get and ground speed is my main concern. Getting it totally accurate throughout the range is about impossible in an analog system. I don't know if the efis systems allow you to tweet the scaling or not.

Bob burns

[DanBaier]

You're describing the difference between indicated airspeed and calibrated airspeed.

See http://www.vansairforce.com/communit...ad.php?t=59870

Dan

[Kevin Horton]

The air has to accelerate a bit to go around our fuselages, and that higher speed air has lower pressure, thus the pressure inside the fuselage is a bit lower than ambient pressure. Ideally, the pressure in the static system equals the free stream static pressure. A static leak leads to a too low pressure in the static system, which leads to a too high ASI reading.

Have you done a leak test on the static system?

[lr172]

Originally Posted by Kevin Horton

The air has to accelerate a bit to go around our fuselages, and that higher speed air has lower pressure, thus the pressure inside the fuselage is a bit lower than ambient pressure. Ideally, the pressure in the static system equals the free stream static pressure. A static leak leads to a too low pressure in the static system, which leads to a too high ASI reading.

Have you done a leak test on the static system?

Yes, I taped one side off and held a rubber tube over the other. I then created a suction and used my tongue to hold it. The altimeter stayed constant for 30 seconds. Now that I think of it, My altimeter needs vibration to give fluid readings. I will test again and try to observe via the EFIS altitude.

Larry

[flightlogic]

Larry, I think you need a real pitot static test. It is a simple procedure at an avionics shop with the tester. A thirty second test as described probably will not really tell you anything.

[lr172]

If my static was leaking enough to cause the ASI to read 10 MPH high, how far off would my altitude be? I compared my alt reading to a a WAAS GPS a couple of times (at fast cruise) when investigating this and they were pretty close (under 100').

Larry

[David-aviator]

Originally Posted by lr172

During my testing, my TAS was reading 10 MPH high at top cruise speed. I did some research here and tried taping some o-ring material behind the static ports. It worked perfectly and my TAS now matches my averaged GPS speed. Fortunately I am getting Van's numbers at cruise

My problem is that it seemed to drop my low speed numbers as well. I was getting a Vs stall at 55 MPH and today I tested again and am getting 48 MPH, which is 7 MPH below Van's reference numbers. Is this typical when tweaking the static port? Does this point to a different problem, such as a leak? I tested for a leak in two ways and both passed (test via instruments at transponder check, as well as holding suction on static port with a very stable reading)

Larry

Larry, there is nothing in the plan about tweaking the static ports. There are hundreds of RV's flying with reasonably accurate static pressure.

I would suspect a faulty airspeed indicator. Borrow one and hook it up in parallel with plastic tubing and set it in the passenger seat.

[RV7A Flyer]

You didn't say whether you have a mechanical ASI or an EFIS.

I know for my EFIS (Dynon), as tested by the test guy during static/alt/XPDR check, he tested the pitot/airspeed too...and it got *more* accurate at higher speeds. In fact, at Vne, it showed 0 error (which he said meant I had to fly as fast as possible all the time ).

[BobTurner]

Originally Posted by lr172

If my static was leaking enough to cause the ASI to read 10 MPH high, how far off would my altitude be? I compared my alt reading to a a WAAS GPS a couple of times (at fast cruise) when investigating this and they were pretty close (under 100').

Larry

This was pure luck. A perfect altimeter system will only agree with True (gps) altitude when there's a standard lapse rate.
That said, the ASI is more sensitive to static errors than the altimeter.