Replacing an airspeed indicator

On Monday, March 10, 2014 9:11:34 AM UTC-4, son_of_flubber wrote:
Thank you Martin. Pointing out how the pieces fit together is very helpful. An annual test of the ASI over the entire range suggests that they have in the past found a few ASI that were not accurate over the entire range and that ASIs might go bad gradually over time. I wonder what those tests find in the field. (My question may sound like FUD, but I have a habit... people paid me to find defects in their technology.) Do I have it right that at a given point in time and altitude, the difference between TAS and IAS is constant over the entire range? I'm still figuring out how to use my LXNav V7 to check the calibration of my 15 year old Made in China ASI (and vice-versa, if I find a problem, either one of the devices could be the culprit.) On Friday, March 7, 2014 7:30:41 PM UTC-5, Martin Gregorie wrote: On Fri, 07 Mar 2014 13:45:53 -0800, son_of_flubber wrote: Is there any reason to suspect that an ASI (especially a non-TSO'ed one) might be more accurate in the middle of it's range? On this side of the pond the rules say that ASI calibration must be checked as part of the Annual Inspection and must be within 2kts throughout its range. The calibration check applies a known pressure to the pitot inlet and and records the ASI reading. This is done at 10 kt intervals from the ASI's highest indicated speed down to 20 kts. An ASI is a sensitive pressure gauge that happens to be calibrated in kts. The calibration is independent of the airframe. IOW, if the IAS in flight shown by a freshly calibrated ASI is not the same as independently measured TAS then the error is due to the placement of the static vent and/or the pitot. For example, its well known that pressure under the wing is above bulk atmospheric pressure, so a static vent anywhere near the underside of the wing will make the ASI read low. Similarly, a static vent placed on a convex bulge in the fuselage away from the wing will see a lower pressure and so will make the ASI read high. That's why the static vent is commonly halfway along the boom: at that point the boom is a straight taper and so has minimal curvature in relation to the slipstream while the vent is placed well away from the flying surfaces. -- martin@ | Martin Gregorie gregorie. | Essex, UK org |

Use of a simply made manometer should clarify the delemna of comparing 2 instruments, both of which have unlnown accuarcy.
UH