Integration test using water manometer

If you follow this blog, you know that we've done extensive water manometer testing of our system, especially the pressure sensors. However, in our quest to knock off hypotheses about the IAS discrepancy in N291DR (and as a way to, coincidentally, also build confidence in the components of our system), we decided to do a quick water manometer test of the whole system. Sort of like an integration test. Never hurts.

Sorry that we don't have pictures for this one ;) but what we did is apply pressure to the dp0 sensor of the probe, in inches of water, and read off the IAS from the cockpit display. This tests the whole thing -- end to end. If we convert applied pressure to IAS in knots, and compare to the reading on the instrument, we conclude that the full system is working correctly.

Probe position is not responsible for IAS problem

Continuing our experimentation, I decided to mount the probe on the end of a 3/4" fiberglass pole securely fastened to the struts of N291DR. Here are a few pictures of the mounting:

The result is that, while our AoA numbers near stall decreased (expected since the probe is relatively free from upwash effects), our IAS readings remained consistent. This is a graph of the results:

Note that, at low IAS, we report numbers higher than the N291DR instrument. This is to be expected. We are compensating for AoA and yaw angle, whereas N291DR's instrument is just a simple tube sticking out into the wind. At high angles, N291DR's instrument will read low.

The mystery continues. The next thing is to do a proper calibration using a quadrangle course with ground speeds and record the results. At this point we're just knocking hypotheses off the list, and some may argue we should have done that before doing the probe on the long pole experiment, but this yielded interesting results in and of itself. It taught us that, while the AoA values change, the IAS numbers remain pretty consistent regardless of where we stick our probe.

We did capture a video of the approach to a stall, which you might find interesting. Check it out here.

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.

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!

Test flight with leak-free probe

Using my Shapeways 3D printed leak-free probe nose and modifying a bunch of other parts, I made a leak-free test version of the probe. at the end of this post are a bunch of construction pictures.

A test flight in N291DR was delayed by the need to change out the spark plugs, but after that, I took off for a quick dusk jaunt. The IAS on Airball was still way under that displayed by the aircraft. This photo album contains the photos I took of both together.

Now my priority is to look for all the other sources of error. It may be a simple bug in my code :) but if that's the case, I need to make sure things are better tested. Stay tuned.

Debugging of leaky probe

To try to debug the low dynamic pressure readings, we pulled the probe we were using and did a water manometer test on the sensors:

Our test temperature was 15 degrees C. If the density of water at 4 degrees C is 1.0000, the density at 10 degrees C is 0.9997 and at 20 degrees C, 0.9982. Therefore, the density variation of water with temperature is not important. We used distilled water.

The following are our results:

Sensor  Applied   Result    Result
        (in H2O)  (Pa)      (in H2O)
------------------------------------
dpβ     -15       -3707     -14.90
        -10       -2487     -10.00
         -5       -1250      -5.02
          0         -13      -0.05
          5        1229       4.93
         10        2474       9.95
         15        3703      14.88

dpα     -15       -3733     -15.00
        -10       -2494     -10.02
         -5       -1245      -5.00
          0          -6      -0.02
          5        1238       4.98
         10        2477       9.95
         15        3707      14.90

dp0       0          13       0.05
          5        1226       4.93
         10        2470       9.93
         15        3700      14.87

These are all within 1% of target. The sensors must not be the problem. We now suspect plumbing.

The hose connections seem fine, but a bogo leak-down test (put your lips to the probe hole and suck, and feel the air blowing out with your tongue) indicates there is significant leakage. Why this started happening at some point in service, as opposed to immediately, is anyone's guess. But if going to a Shapeways manufactured probe nose fixes it, that's fine and well within budget.

Luckily, we have some of the noses we 3D printed at Shapeways years ago (remember that...?) and can put them back into service for a quick test, perhaps with some sort of adapter:

Stay subscribed for more details as fast as we find them out ourselves!! :)

Complete flight with video

Yesterday, Melissa (Product Marketing Manager for Airball) and I went for a flight in N291DR. I used the replay program I wrote recently to turn the data logs into a video. At the beginning, I blew into the probe to create an "artifact" I could use to synchronize the GoPro and data-generated videos, and create a picture-in-picture combination. And here it is:

There are some specific points of interest, so here are a few direct links to them:

• Takeoff and climbout from KRHV [t=566] 
• Steep turns: [t=1466s]
• Approach and low pass over a field: [t=1573s]
• Demonstrating AoA changing at near-constant IAS: [t=2016s]
• Approach and landing at KRHV: [t=3070s]

One issue we're having is that the probe "reads low" in IAS, and we suspect it's due to some leakage in the plumbing. It's a bit strange since this problem crept up in the past few weeks; it was not always this way. This may be due to jostled pressure hoses, or it may be that the natural porosity of the hobby FDM 3D printed parts are affecting accuracy. We have pulled the probe from service and are doing some tests. In the worst case, we return to making the probe noses using more fancy 3D printing techniques, e.g., sending them out to Shapeways. Stay tuned!