Sunday, February 16, 2020

Ideas for a next-generation probe

Losing the static probe

I have a friend who is an aeronautical engineer at nearby NASA Ames, at Moffett Field. He's been a great resource in helping me think about my probe. He has designed 5-hole and 9-hole alpha/beta probes for the NASA Orion spacecraft, so this sort of thing is not new to him.

Based on his feedback and my flight test data, it's clear I'll likely need to do some sort of calibration of the probe, based either on CFD simulations or wind-tunnel data, or a combination of both. Luckily, my Reynolds number range is not huge, so I should be able to get some pressure ratio calibration that will work for my entire range of dynamic pressures.

He has pointed out to me that, since I'm all in on calibration anyway, I don't technically need this bulky static pressure probe sticking out. I could measure any additional pressure between any of the holes on the ball nose. All I need is 3 pressures, to determine 3 unknowns (α, β, q). Once I determine q, I can subtract that from whatever the barometer reading is, from whichever probe hole I choose to connect it to, and get a "true" barometric reading.

Discovering the ESP32-WROOM-32U

While I was living under a rock, the world has gone all in on ESP8266 processors and Wi-Fi "Internet of Things" modules. The next step from there on is the ESP32-WROOM-32U which has lots of memory, can be programmed via the Arduino IDE, and comes with all the Wi-Fi I need. And it costs $4.50 per part. Yes, you read that right.

With all that memory, I can afford to embed the calibration tables in the probe itself, meaning that anyone can grab the probe, connect to Wi-Fi, and receive data sentences at 20 Hz in the form:



α = angle of attack
β = angle of yaw
q = dynamic pressure
p = barometric pressure
T = temperature

This means that people don't need to use "my" host Raspberry Pi software to get immediate, useable airdata.

A smaller form factor

Given all the above, it should be possible to fit the whole thing into a 1 inch diameter by about 8+ inch long housing, still with the transparent polycarbonate tube that is our trademark. :) If we add a USB interface, it should be possible for someone to get a probe, put it together, and then program it with the Arduino IDE themselves without having to take it back apart!

Recall we got ~ 12 hours of battery life with a 1 Ah LiPo battery. With a 3.2 Ah 18650 battery, regardless of how much power the ESP32 consumes to transmit on WiFi, it is very likely we could get similar if not better battery life.

The installation flexibility of such a small package would be awesome! You could simply zip-tie it to any sort of "curved" mounting ... we would not need to add any screw holes.

Wired installation

To install this in a wired fashion, we have an ongoing to-do item to prototype an RS-485 driver. With that and a standard R/C airplane "battery eliminator circuit" to step down the ship 12VDC to something near 5V, which we could then down-regulate to 3.3V, we'd have a wired system with the comms going over a single twisted pair. More on our experiments with that soon.

Of course you might ask: But the wired probe will not need Wi-Fi. Will you use a different microprocessor so that you don't waste all that Wi-Fi functionality? To which I answer: $4.50. I can afford to standardize on the part with the integrated Wi-Fi and just ignore the Wi-Fi, and it's still cheaper than most of my alternatives.

New "V block" mount in progress

One of the contributions of the Airballs to the universe is ways of mounting reasonably lightweight, compact junk to your plane in a secure but "tool free" manner. Our mounts so far have been 3D printed to the exact contours of the struts. We have versions for a Citabria, Super Decathlon, C172, and RANS S-6S.

This arrangement is very secure, but leads to a rather bulky 3D printed chunk though, and may not be universally applicable. This has led us to consider a more generic "V block" shape, where we augment the 3D printed parts with a piece of aluminum angle and add some padding. This is what we have so far. It seems just as secure as the custom-contoured version. Stay tuned for more details.

The "pitch string"

Yesterday I tried out this idea I've had for a while -- the "pitch string":

I attached a piece of fishing line vertically down the airplane's centerline, with tiny beads on it every 2 inches. Now this may not be the case for everyone, but for me, if I focus on the horizon, I see two equally clear images of the string (due to parallax). My aiming point is halfway in between them. So I can use these to watch the movement of the nose as I bank into turns, and keep track of the pitch of the airplane relative to the horizon.

My contention is that something like the pitch string, plus Airball, is the ideal setup for VFR flight instruction.

Already, even after all these hours of flight (well -- I mean, 300 hours is not a lot, but it's something), it helped me notice how I need to raise the nose in turns, and be more aware of adverse yaw. My CFIs always told me to pick a point on the nose or windshield and use it as a reference. However, without specific instruction as to which point to pick, I think my eyes tend to wander and I become more heads-down. Something that gives me a specific thing to focus on is super helpful.

An improvement may be that, instead of equally spaced beads, it should have just one "bead" that the pilot can move up and down to act as a reference. For training airplanes used by many people, I can imagine a "ruler" aligned edgewise so it doesn't obscure the view, but with graduations on it so each pilot can reset the "bead" to their preferred location, might be a useful thing.

Saturday, February 15, 2020

New probe is very consistent with existing ones

Today I did a flight with one of our new probes. Compare the "new" one (top) with the "old" one (bottom) here:

You can see that the new one has a bit of a longer static tube, but is otherwise quite similar to the old one. The readings are super consistent with what we had before. This graph shows the data from the old probe in two separate experiments, versus the new probe:

So basically, anyone can build a probe according to our specs with their 3D printer and some electronic muckery, plop it on their plane, and without any additional messing around, get consistent results. This is HUGE. :)

The next step is calibration, of course. More on that soon.

Sunday, February 9, 2020

Sunday, February 2, 2020

Two more systems!

As of last night, after diverse trials and tribulations, we have two more Airball systems:

Apart from the various noodling around we've been doing with verifying the airdata on the plane, the problems with this particular build-up is that we seem to have gotten a bad batch of TCA9548A I2C mux chips, and messing with that issue wasted a bunch of time. But we're back on track, and newly confident that, if we assemble our things right, they actually do work properly. Yay.

The first system we hope to give to a friend who has a background in aeronautical human factors, and it looks something like this:

You can see the probe and display, obviously, and our current prototype for our "V block" shaped strut clamp that should be compatible with a large variety of airplanes.

Stay tuned for more news!