Saturday, January 11, 2020

Static source is not responsible for IAS problem

Today, I flew a short flight with an alternative static source made of a very long acrylic tube jury-rigged to the strut:

I wrote down the IAS from Airball and N291DR's ASI, and compared them with what I got when I was flying earlier (see this post) with the probe's original static source. This is what things look like (click on the photo to see it full size):

Airball reads lower than N291DR's ASI in all cases, and the alternate static source caused a tiny correction in the expected direction (Airball's reported IAS was higher, corresponding to a lower static pressure presumably unaffected by the mounting pylon), but ... it's not a significant change.

I jotted down some data from the flight:
GPS magnetic heading: 300°
GPS ground speed: 75 ktas
Airball IAS: 85 kias
N291DR IAS: 95 kias
GPS altitude: 3,271 feet
The flight was approximately 2200Z on January 11, 2020. My Foreflight wind briefing said:

So we can assume I had about a 20 knot headwind, traveling more or less straight into the wind, and low enough that my TAS and IAS were the same to a rough approximation. If my GPS ground speed is 75 ktas, then by these assumptions I would expect my IAS to be 95 kias.

And indeed the N291DR ASI showed 95 kias, whereas Airball read 10 knots lower.

   ~  ~  ~

At this point, given the validation I have done of the sensors and telemetry and everything else, I am very, very close to the simplicity of a water manometer attached to a piece of pipe sticking out in the wind. :) Therefore, the next thing to suspect is installation error -- i.e., that the airflow around that portion of the wing is sufficiently slowed that it reads 10 knots low.

I hope to design an experiment to suspend the Airball probe a couple feet in front of the wing using a boom securely attached to the struts. Stay tuned for more details.

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