Tuesday, November 2, 2021

IAS accuracy testing

I am still searching for the elusive accurate IAS, or at least, a root cause.

First I checked a pressure sensor with a water manometer to be sure all was well, and my numbers came out accurate to within 0.1 inch of H2O, which is good:

Position 1: I set up a standard "UAV" Pitot tube with one of our pressure sensors and put it on the usual strut mount:

Position 2: I mounted the probe on a boom on the wingtip. This album shows more details of the process for installing the boom:

The results compared to the built-in IAS were as follows:

Built-in ASI
Probe Pitot
Position 1
Probe Pitot
Position 2

You can see that the second and third columns are very similar. So moving to a different position did not affect the readings very much. However, the readings remain quite a bit different from the built-in ASI.

I then did a quadrangle course comparing the built-in ASI to GPS speed. The data are as follows. The headings are not "real" but they are more or less 90 degrees to one another:
  • Approximate altitude: 5400 feet
  • Local static pressure: 84267 pascals
  • Local outside air temperature: 13.4 degrees C
Heading IAS (kias) GPS ground speed (kt)

Taking the average of the 4 ground speeds:

(79 + 90 + 89 + 77) / 4 = 83.75

Therefore our TAS was 83.75 ktas or 43.08 m/s.

At the given barometric information, air density is 1.0197 kg/m^3.

Dynamic pressure = 0.5 * 1.0197 * (43.08^2) = 946 Pa.

This corresponds to sqrt(2 * 946 / 1.225) = 39.3 m/s indicated = 76.4 kcas.

So at 76.4 knots calibrated airspeed, we have:
  • Built in ASI: 80 kias
  • Airball Pitot position 1: 72 kias
  • Airball Pitot position 2: 75 kias
It looks like the built-in ASI is way off. The Airball Pitot under-reads in Position 1 but shows improvement due to movement from Position 1 to Position 2.

So is that the answer then? Move the Airball to the wingtip and trust it, and know that the built-in ASI is super inaccurate? We need to do a bit more testing before we're sure. Specifically, at each of these airspeeds, from 40 to 100 kias (according to the built-in gage), we should do a full quadrangle. Stay tuned.

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