I have tested three different RVs and they all show anywhere from 8mph to 11mph errors in TAS computed from IAS when compared to GPS-derived TAS. Has anyone else seen the same thing, and what have they done to correct it? Is it a matter of placement of the static port or is it associated with the recommended rivet port?
Van's Air Force
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IAS errors
- Thread starter elippse
- Start date
I have a 8-10 kt error in my TAS at cruise due to a static position error. The error dissapears at 100 kts KIAS. I verified the error was related to static rather than pitot by using GPS altitude and my altitude hold. The magnitude of the error was verified using GPS ground speed with the widely available formula. I tested as follows:
1. Verified the GPS altitude was accurate while sitting on the ground. It was within 10' of field elevation.
2. Flew to 3000', engage the autopilot and accelerated to cruise.
3. The GPS altitude was indicating around 2870
4. Slowed to 70 KIAS and recorded both indicated altitude and GPS altitude while slowing.
5. They matched at 100 KIAS, after that GPS alt was higher than indicated alt.
6. Repeated the above, but while accelerating from 70 KIAS to cruise, same results.
7. GPS ground speed runs at cruise and 100 KIAS verified that TAS was reading 8-10 kts higher than GPS GS at cruise and GPS GS equaled TAS at 100 KIAS.
I have only identified the problem. I still need to solve it. I have Van's blind rivet port installed exactly where he specified on my RV-6A.
1. Verified the GPS altitude was accurate while sitting on the ground. It was within 10' of field elevation.
2. Flew to 3000', engage the autopilot and accelerated to cruise.
3. The GPS altitude was indicating around 2870
4. Slowed to 70 KIAS and recorded both indicated altitude and GPS altitude while slowing.
5. They matched at 100 KIAS, after that GPS alt was higher than indicated alt.
6. Repeated the above, but while accelerating from 70 KIAS to cruise, same results.
7. GPS ground speed runs at cruise and 100 KIAS verified that TAS was reading 8-10 kts higher than GPS GS at cruise and GPS GS equaled TAS at 100 KIAS.
I have only identified the problem. I still need to solve it. I have Van's blind rivet port installed exactly where he specified on my RV-6A.
Nuisance
Well Known Member
My airspeed indicator reads within a knot or 2 of the correct value at cruise as verified by 4-legged GPS runs.
I put the static port in the recommended position, but instead of the rivet head port, I used a port that is sold by STEC, and is used for their certified autopilot installations (obtained from my friendly avionics dealer).
We have had other threads indicating that the flush static ports that are sold by one of the tool vendors cause large errors as well, so I think the shape of the port is important. The STEC port is about 1/8" thick, and maybe 3/8" in diameter, with straight sides at a 30 degree angle and sharp corners...clear as mud right?
John
I put the static port in the recommended position, but instead of the rivet head port, I used a port that is sold by STEC, and is used for their certified autopilot installations (obtained from my friendly avionics dealer).
We have had other threads indicating that the flush static ports that are sold by one of the tool vendors cause large errors as well, so I think the shape of the port is important. The STEC port is about 1/8" thick, and maybe 3/8" in diameter, with straight sides at a 30 degree angle and sharp corners...clear as mud right?
John
Mine is accurate to about 1-2 knots using the stock Van's rivets in the stock location. I verified it using GPS several times during Phase 1.
Paul
Paul
RV7AV8R
Well Known Member
I have had a similar error in my RV7 and I noticed my altimeter has a similar error but I have not tested the ASI at 100kts vs GPS. The guys quoting accurate ASIs are flying RV8s. Is this a 6 and 7 problem? I put a static port on both sides, I think I will tape one off and see if it makes a difference. If anyone knows the solution I am all ears!
John Adams RV7
John Adams RV7
hevansrv7a
Well Known Member
Just to be a little different..
My error is about 4 kts, but the indicated is higher than the actual. This is based on the GRT using the self-calibration function for TAS. I used the pre-made pitot from Van's, installed in the place called out in the plans on my QB wing. My static ports are Van's "rivets", but they are just fwd. of the first bulkhead aft of the baggage area and another builder says I should have been another section further aft. At this point, don't know, don't care.
The GRT calibration is done (against GPS) by flying upwind where the difference between the magnetic and the GPS tracks is near zero, then flying 180 degrees from that. The calibration is done at several speeds and the device enters the data into a table. Autopilot is used for holding altitude and course. GRT, enroute, calculates wind speed and direction, HW and CW components. I have observed this extensively in many flight conditions and have come to the conclusion that my corrected system is accurate to within 1 knot. Not as precise as test pilots do, but good enough for me.
My error is about 4 kts, but the indicated is higher than the actual. This is based on the GRT using the self-calibration function for TAS. I used the pre-made pitot from Van's, installed in the place called out in the plans on my QB wing. My static ports are Van's "rivets", but they are just fwd. of the first bulkhead aft of the baggage area and another builder says I should have been another section further aft. At this point, don't know, don't care.
The GRT calibration is done (against GPS) by flying upwind where the difference between the magnetic and the GPS tracks is near zero, then flying 180 degrees from that. The calibration is done at several speeds and the device enters the data into a table. Autopilot is used for holding altitude and course. GRT, enroute, calculates wind speed and direction, HW and CW components. I have observed this extensively in many flight conditions and have come to the conclusion that my corrected system is accurate to within 1 knot. Not as precise as test pilots do, but good enough for me.
I've been doing some analysis of TAS tests with Jim Smith of Wichita on his RV-6 with 150HP O-320 using his new 3-blade ELIPPSE prop. His TAS from IAS was running about 11 to 13 mph high relative to GPS-measured TAS. He put a half-washer behind his static port and that has brought his error down significantly.
These errors are usually due to the static port being placed at a location where the pressure is lower (higher IAS) or higher (reduced IAS). To raise the pressure, add thickness close behind the hole in the port with tape or whatever, and to lower the pressure, add thickness just ahead of the hole in the port. On my friend's Lancair, we added about 4 thicknesses of transparent tape ahead of the port to get rid of a 4 mph low error. That's about the same amount I added on my pitot-static tube to get my IAS right on. It's also helpful to have ports on each side of the fuselage, joined with a "Y", to get rid of yaw-induced errors. My friend's Lancair has only a port on one side of the fuselage, and it affected the IAS in opposite directions depending on which direction a turn was made. My pitot-static tube has three equally-spaced holes around the circumference to minimize AOA and yaw errors.
These errors are usually due to the static port being placed at a location where the pressure is lower (higher IAS) or higher (reduced IAS). To raise the pressure, add thickness close behind the hole in the port with tape or whatever, and to lower the pressure, add thickness just ahead of the hole in the port. On my friend's Lancair, we added about 4 thicknesses of transparent tape ahead of the port to get rid of a 4 mph low error. That's about the same amount I added on my pitot-static tube to get my IAS right on. It's also helpful to have ports on each side of the fuselage, joined with a "Y", to get rid of yaw-induced errors. My friend's Lancair has only a port on one side of the fuselage, and it affected the IAS in opposite directions depending on which direction a turn was made. My pitot-static tube has three equally-spaced holes around the circumference to minimize AOA and yaw errors.
RV7AV8R
Well Known Member
Elippse, Thank you! This is a good suggestion on how to fix this problem! I will assume I will need to put the same "build up" of tape on both right and left static ports. In my case on the back half to increase static pressure.
I tested again today and verified that as I increase air speed the error between GPS altitude and Altimeter increased (the alitimeter reads high) and therefore so does my airspeed. It is good to impress friends going for a ride but drives me nuts.
Until your suggestion, I was stumped trying to think of a sloution that didn't involve drilling new holes all over in my painted airplane!
John Adams
I tested again today and verified that as I increase air speed the error between GPS altitude and Altimeter increased (the alitimeter reads high) and therefore so does my airspeed. It is good to impress friends going for a ride but drives me nuts.
Until your suggestion, I was stumped trying to think of a sloution that didn't involve drilling new holes all over in my painted airplane!
John Adams
Hi, John! I'm happy that this thread helped you! When you get ready to sell your plane and want to show your potential customer that you've got the fastest RV out there, just pile up the tape ahead of the port! Sometimes it's better not knowing about this stuff so when you brag on your plane's speed, you're not really fibbing! Paul
