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 (kias) |
Probe Pitot Position 1 (kias) |
Probe Pitot Position 2 (kias) |
| 40 | 46 | 46 |
| 50 | 50 | 52 |
| 60 | 58 | 60 |
| 70 | 67 | 68 |
| 80 | 72 | 75 |
| 90 | 76 | 80 |
| 100 | 86 | 90 |
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) |
| 000 | 80 | 79 |
| 090 | 80 | 90 |
| 180 | 80 | 89 |
| 270 | 80 | 77 |
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.
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.