Part 3 - Measuring the relative wind

by Ihab Awad, Founder of Airball.aero

If you missed it: Part 2 - The relative wind.

Last time, we talked about the relative wind. In the 1940s, getting a good measurement of this or any other aerodynamic quantity was rocket science. Think of the piece of metal tube sticking out of a Citabria or Aeronca to get an idea of the technology of the time.

But actual rocket scientists at NASA had a solution -- the Q Ball. This flew on the nose of both the Bell X-15 and the Saturn V rockets, and consisted of a ball with a series of holes, and motors to move it around. The air pressure on the holes would tell the system if the ball was facing directly into the wind, and the motors would rotate it as needed. And so these vehicles had an exact measurement of where the air was blowing from, and how hard.

This is what the Q Ball looked like on the nose of the X-15. If you zoom in, you can see the tiny holes on the metal ball. And the X-15 control panel had lots of tapes and readouts with technical terminology in Greek alphabet to display all this information!

It turns out that, if we don't plan to be supersonic, we can achieve the same effect with a stationary ball with holes in it. If we carefully measure the pressures at the different holes and do some math, we can "sniff" the direction and strength of the relative wind. And in fact, this is also common: lots of instruments used for flight testing employ this technique.

These instruments often need fancy calibration and cost thousands of dollars. How can we make them cheaper? Well, three ways. First of all, if we make the "ball nose" larger, then we can get reasonably consistent results without having to do fancy calibrations. Second, we can 3D print the parts rather than use expensive machining and welding. And third, hobby electronics and sensors are amazingly cheap these days! We did all these things, and created the Airball probe. Here is a prototype, and our design for a more productionized version:

This sends out relative wind information 20 times per second, and the battery lasts all day. It's all Open Source, meaning the blueprints and software are out there for anyone to use, free of charge. Here is our 3D printer cranking out a set of "ball noses", and a couple of probes kitted out on our workbench.

Our "ball nose" is 2 inches in diameter, which is no larger than action cameras that people stick on their airplanes all the time. We fly with this kind of setup regularly on our test ship, N291DR, using a 3D printed strut mount of our own design. It works pretty slick.

So there is no barrier to this information being at everyone's disposal. But what then? Do we all get X-15 control panels with Greek letters on them? We don't think so. We'll talk about that next time.

Next time: Part 4 - Displaying the relative wind.