Monday, January 18, 2021

Mechanical airball crufty prototype

In between bouts of work on "serious" things, I whipped together a sorta-prototype of the mechanical Airball concept, using two Hitec HS-81 servos I happened to have around, and a bunch of 3D printed parts. Here's the obligatory CAD:

I used glass beads (6/0 size) as ferrules for fishing line; this tended to have a bit more friction than I would like due to the sudden bends and means I should probably use pulleys instead. Here are some photos of the thing:

I failed to account for the tensions in the lines and how they would interact, which means that the left/right travel of the "ball" was limited. A future prototype should make a more full analysis of the ranges of motion desired, and also greatly simplify the paths of the "lines" controlling the ball so as to make it easier to put the thing together. All good lessons learned.

This is a brief video where you can see me moving the pulleys by hand and moving the ball around. Clearly there's too much friction to expect anything more from this particular prototype.

Friday, January 8, 2021

Mechanical airball

Here's a crazy idea that I invite any of you crazy folks out there to try.

So the trick with cockpit displays is ... brightness. And this makes it hard to make a general purpose HUD, because you need to collimate a super duper bright, yet high resolution, display, and super-impose that against the background. But mechanical things don't have that problem. And in fact, with tiny motors and microcontrollers, we can actuate mechanical things pretty easily. Could we build a subset of the Airball visualization by mechanical means alone, and put that in the pilot's field of view?

To get our minds thinking, consider the following:

The instrument is basically a small rectangle of Plexiglas with a physical circle behind it. The circle can contain an RGB LED so it can change color and brightness with airspeed, or it can be a chunk of some material. Two "cables" (really, tiny pieces of fishing line) hold it up from above, and these are either themselves elastic, or the slack is taken up by some spring or rubber band or something. Two other "cables" (more fishing line) hold it from below, and a microcontroller controls their lengths so as to put the ball in any (X, Y) position you ask it to.

Now, if this can be actuated "quickly" so its response time is "live", sort of like a real windsock, then it can replace or augment an Airball display in the panel.

I'd probably use it to augment an Airball display -- because the Airball display receives raw AoA and yaw values, etc., and contains the "settings" that determine what is stall, what is best climb rate, what is best climb angle, etc. Then it can send simple commands to the mechanical device to go to (X, Y) coordinates based on the fixed mechanical fidicials printed on the Plexiglas. And it can tell the device what color to flash the RGB LED (if installed) for the specified airspeed.

Alternatively, all you want is one-time "programming" of the set-points for a given airplane, so it can be standalone. Maybe you program it using some sort of app, and a USB or serial connection, during a test flight, and then you set it running.

Think about it! I probably don't have time to build it, at least at the moment, but if you're interested, it should be a cool little gadget to make! If I can help you make it, let me know!

Monday, January 4, 2021

Boards came in the mail!

 Today, a shipment of boards came in the mail, via DHL from my friends at

The first was a series of 5 populated probe boards, with bugs ironed out and improved placement for the pressure sensors. The PCBWay folks decided to throw in all the "extra" parts that they didn't use for the build, as well:

Next was a set of 5 unpopulated copies of the new display board, with a stencil:

And finally, I also got a pile of components for the display board from Mouser, with more on the way:

The new probe boards work great. I'm currently 3D printing some components for a new probe and should have it running soon. We're getting really close to being able to make "kits"!! As I said, this one has better component spacing, and I also found 3D models of the Honeywell pressure sensors (here, under "Resources") so I can model them in CAD much better now:

Friday, January 1, 2021

Rough "calibration" using the car

Today, with Melissa as PIC and me as flight test engineer, we drove up and down Hwy 101 to take some data. Our goals were:

  • Validate the test infrastructure -- servos, driver code, Python post-processing, etc.
  • Get a super quick order of magnitude check on theoretical vs. actual data
We succeeded in both, which is great. However, do note as you look at the below that our data is utter dreck. It is the worst data in recorded history. It's like all of 2020 condensed into a CSV file and turned into our data. It's the kind of data you'd find at the bottom of a 40 year old outhouse, after....

Ok, you get the idea. Poor data is poor. The reason, we believe, is that we were taking data in the midst of traffic. That's fine. We don't claim to be doing science here. This is just a way to get some numbers so that, when we show Airball to someone, they don't look at us funny and suppress a laugh. We need our IAS (especially) and alpha / beta (why not?) numbers to look non-parodic.

First of all, here is the obligatory shot of the probe and its bandana flag, flapping bravely in the wind:

The entire ball o' wax -- data and Python -- is in the airball-data Git repo. First we plot the values of (dp0/q), (dpA/q), and (dpB/q). (These values are defined in a previous blog post.)

Although the data is really poor, you can see that the "shape" of the data sort of mimics what we would expect. With that in mind, we adopt a hypothesis that the data is equal to the theoretical values with a constant scaling factor applied. If we plot this scaling factor versus the sum of the squares of the errors between data and scaled theory, we get:

It appears that if we multiply the theoretical values of (dp0/q) by 0.5, and multiply the theoretical values of (dpA/q) and (dpB/q) by 0.7, we get the best fit.

We will plug this into the firmware, and proceed with construction and flight testing. Meanwhile we will see if we can get access to a wind tunnel, or persevere with our testing on the car. Stay tuned!