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A.S.I error
- Thread starter ston
- Start date
- Joined
- Mar 23, 2005
- Messages
- 13,226
At what speed is it off? What are you comparing it to?
You can try running the "zero airspeed cal" option. Make sure there is no wind into the pitot at all when you do this.
You can try running the "zero airspeed cal" option. Make sure there is no wind into the pitot at all when you do this.
khorton
New Member
ASI errors have many possible sources.
1. The Dynon ASI could have large instrument error (not likely, but it is possible). This can be checked at an instrument shop, or you can do it using a water manometer. See the ground test section of my Determining Static System Error page.
2. The pitot or static system may have a leak. Do leak checks on both systems.
3. The static system could have position error. Does your aircraft have the exact same static system as specified by the designer? Many RV builders don't like the look of the pop rivet static source that Van specifies, and they decide to "improve" things by fitting a flush static port, without realizing that the shape of the static port is critical. They then find that the ASI reads 5 to 10 kt too low. I've got a web page that describes how to check for static system position error - Determining Static Source Error.
4. OAT error - if you used a test method based on GPS data to determine TAS, you need an accurate OAT to determine CAS. But it would take a very large OAT error to cause a 7 kt "error" in CAS.
5. Test method error - some people come up with creative (but mathematically unsound) methods to determine TAS. For example, if you do runs in several different directions, and average the GPS groundspeeds, the average groundspeed is not equal to the TAS. The best test method, in my opinion as a full time professional test pilot, is to do four legs in a box pattern. Record GPS groundspeed, GPS track, ASI reading, pressure altitude and OAT on each leg. Use the four leg section of the NTPS GPS PEC spreadsheet to deterimine TAS. Put the TAS in the column B of the PEC part of the spreadsheet. The IAS goes in column C, ASI instrument error in column D (put 0, if you haven't done an ASI calibration), pressure altitude in column F, altimeter instrument error in column G, OAT in column I, OAT instrument error in column J. Read the CAS in column Q, and the airspeed error due to static system position error in column R. Column T shows the altimeter error due to static system position error.
Contact me directly if you have any questions - khorton01 AT rogers DOT com.
1. The Dynon ASI could have large instrument error (not likely, but it is possible). This can be checked at an instrument shop, or you can do it using a water manometer. See the ground test section of my Determining Static System Error page.
2. The pitot or static system may have a leak. Do leak checks on both systems.
3. The static system could have position error. Does your aircraft have the exact same static system as specified by the designer? Many RV builders don't like the look of the pop rivet static source that Van specifies, and they decide to "improve" things by fitting a flush static port, without realizing that the shape of the static port is critical. They then find that the ASI reads 5 to 10 kt too low. I've got a web page that describes how to check for static system position error - Determining Static Source Error.
4. OAT error - if you used a test method based on GPS data to determine TAS, you need an accurate OAT to determine CAS. But it would take a very large OAT error to cause a 7 kt "error" in CAS.
5. Test method error - some people come up with creative (but mathematically unsound) methods to determine TAS. For example, if you do runs in several different directions, and average the GPS groundspeeds, the average groundspeed is not equal to the TAS. The best test method, in my opinion as a full time professional test pilot, is to do four legs in a box pattern. Record GPS groundspeed, GPS track, ASI reading, pressure altitude and OAT on each leg. Use the four leg section of the NTPS GPS PEC spreadsheet to deterimine TAS. Put the TAS in the column B of the PEC part of the spreadsheet. The IAS goes in column C, ASI instrument error in column D (put 0, if you haven't done an ASI calibration), pressure altitude in column F, altimeter instrument error in column G, OAT in column I, OAT instrument error in column J. Read the CAS in column Q, and the airspeed error due to static system position error in column R. Column T shows the altimeter error due to static system position error.
Contact me directly if you have any questions - khorton01 AT rogers DOT com.