Saturday, January 4, 2020

Debugging apparent IAS discrepancy

I am now full-on debugging the difference between our aircraft's ASI and the Airball display, to see who (if anyone) is "right".

First the obvious. Is the data shown on the Airball display the same as what I get by post-processing the logs? The answer is "yes". I looked at our recent flight video and compared it to the data when I post-process the logs and to direct cellphone footage of Airball taken by Melissa. Indeed, what I see on the Airball screen in flight is consistent with the entire chain of data manipulation and custody.

So, is the "fancy" math somehow wrong? I post-processed the data and calculated an indicated airspeed based on using the center hole pressure as a plain Pitot tube -- without any alpha/beta correction or anything. Just convert pascals to meters per second using the density of dry air, and then convert that to knots. Compared with the "fancy" equations, there is very good correspondence.

Zooming in on a region of the graph just to be sure, we further confirm that "the fancy math" is not our issue here:

So the math is reasonable, and it seems to match the physical pressure quite well, and we know our pressure sensors are good because we just tested them with a water manometer a few days ago.

The remaining hypotheses would be:

1. Aerodynamic errors caused by the design of the probe, or its placement on the aircraft.

2. The ASI in N291DR is wrong! :)

We don't yet know about #1. We would like to rig up a simple wind tunnel test. Meanwhile, we have a realization about N291DR:

N291DR does not have a static source

Correct. The altimeter and ASI in N291DR have a "static" source consisting merely of a stub of tubing that is open to the back of the panel. So this is definitely a place to start suspecting mischief. One would expect that the inside of the fuselage experiences suction, leading to an artificially low "static" pressure, which would cause both the altimeter and the ASI to read "high". Can we confirm this?

For that, let's look at this photo from last evening's flight:

Airball shows 76 kias, N291DR shows 87 kias. Both Airball and N291DR are set to the current barometric reading, 30.02 in Hg. Airball shows 5500 feet altitude, while N291DR shows about 5530 feet.

Could the altitude difference explain the IAS difference?

Using the international standard atmosphere:
5500' --> 82,742 Pascals
5530' --> 82,648 Pascals
So the reduction in pressure as measured by the altimeter is approximately 94 Pascals of "extra credit" that our ASI could be reading.

If we take 76 kias, convert it to a dynamic pressure (917 Pa), add 94 Pa, then convert that back to an IAS, we get 79 kias.

79 kias is clearly more than 76 kias, but it does not explain the "jump" in reading all the way to 87 kias.

We therefore do not yet have an explanation for the discrepancies. Our workup should include one or both of the following:

a) Fly a quadrangle course with N291DR, "calibrating" the Airball IAS as per the usual practice using GPS ground speed, and use that as a "source of truth" to find out what parts are in error.

b) Stick the Airball probe in a wind tunnel. We've shied away from that so far because we hoped to have a complete, well-instrumented experiment. But maybe it's worth 1 hour of wind tunnel time to just stick our existing probe, with our existing telemetry and everything, with the simplest possible setup to get a reality check.

Remember the Dynamixel servos from 2 years ago?  I just resurrected them. They are sort of shaky and wobbly but they would give us a reasonable way to position the thing. And they work, and they are ready today. Here they are mounted on a wooden pole, with the probe attached....

Stay tuned. We'll get to the bottom of this soon enough!

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