What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Static Port / Airspeed Issues

lr172

Well Known Member
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:D

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
 
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
 
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?
 
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
 
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.
 
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
 
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:D

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.
 
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 :) ).
 
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.
 
You didn't mention what type of static ports you have installed?

If they are the standard kit ports, you should not have to do any tweaking.

If they are not, any change you make to produce an error correction at a particular airspeed, may not have the dynamic influence at a different airspeed.
 
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 :) ).

I have both an ASI and EFIS. They read within 1-2 MPH of each other at both the high and low end.

Larry
 
You didn't mention what type of static ports you have installed?

If they are the standard kit ports, you should not have to do any tweaking.

If they are not, any change you make to produce an error correction at a particular airspeed, may not have the dynamic influence at a different airspeed.

They are the standard Van's Rivets provided with their static kit.
 
I have both an ASI and EFIS. They read within 1-2 MPH of each other at both the high and low end.

Larry

Then it's probably not an error in the ASI *and* the EFIS, and that leads to either a static error and installation error on the pitot tube, right? Since it would be very, very unlikely for both of your instruments to be in error by the same amount...
 
I tested the static again today. The EFIS is very sensitive, so I used it this time. Covered one port and pressed a vynal tube on the other sucked in some to get ~2000' altitude and locked the vacuum with my tongue. Once the altitude stabilized, it was rock solid. Held it for 30+ seconds until my tongue hurt.

I don't think that I have a static leak.

Lary
 
You didn't mention what type of static ports you have installed?

If they are the standard kit ports, you should not have to do any tweaking.

If they are not, any change you make to produce an error correction at a particular airspeed, may not have the dynamic influence at a different airspeed.

This has still been bugging me. I called Van's to get the correct rivet type and it turns out they are the rivets used for the baffling and I have many extra. I compared this to what is on the plane. Turns out the original builder who started the kit put the wrong rivet in. It had all of the other parts from Van's kit, so I assumed it was the correct rivet.:mad: It's dome is much higher than the suggested rivet (maybe .025 vs. .010) and I suspect this could be causing my issue. As soon as it gets above freezing around here, I will replace them and see if that solves the issues.

Larry
 
How about NO static system?

I bought a plane that doesn't have any static system installed at all! Guess what? No problems. I was very careful and bought a nice aftermarket system for the 8A I built, and expected to have to install one in this plane. I did notice that I can vary my altimeter by 25 feet by playing with the vents, but then it stabilizes. It's a 6 and I don't have the original plans (waiting for a dvd copy), was it 'optional' at some point?

Thanks, Lance
 
I thought that I would post a resolution for others to benefit from. On Friday I replaced the rivets with the Van's recommended baffle seal rivet. I flew today and did a four direction speed test. I ended up 1 MPH off the GPS speed:D I couldn't be happier to finally resolve this annoying issue. Mark another RV that works perfectly with the recommended rivet and location.

Thanks for the assistance with this.

Larry
 
I thought that I would post a resolution for others to benefit from. On Friday I replaced the rivets with the Van's recommended baffle seal rivet. I flew today and did a four direction speed test. I ended up 1 MPH off the GPS speed:D I couldn't be happier to finally resolve this annoying issue. Mark another RV that works perfectly with the recommended rivet and location.

Thanks for the assistance with this.

Larry

Good news.
Spread the word.
 
Back
Top