Saturday, February 10, 2018

Successful first test flight

Well we did it! Thanks to my friend KC Budd who flew our hardware on his Cessna 172, N2720L, we have a successful first flight!

We flew KPAO KHAF, stopped for lunch, then returned KHAF KPAO. The air was pretty bumpy in places, over the hills and near the ground at KHAF. We did enough maneuvering to calibrate the fiducials on the Airball display, and observed how it performed in stalls.

Now the main goal of this test was to see if we have a project, or if we're barking up the wrong tree. I had no idea what the actual data would look like -- the AoA and yaw may be so bunched up on one end of the scale or the other that it would be useless. The airspeed feedback due to the ball size may be bogus. The whole concept may be bogus. Who knows until you've tried it, right? I have done some testing on the old Android app with my other friend's RV-9A, getting data from his Dynon avionics, but this was the first time with an actual probe -- and a homemade one at that. Would this be a complete dud and should we pursue something else, like quilting?

Well so KC's initial feedback is that, to a first approximation, we have a project. I concur, but then I'm obviously highly motivated. :) The AoA varied enough in all phases of flight to give useful feedback, and the airspeed feedback from the ball size helped situate us. Climbing at Vy with the AoA pegged on the Vy symbol was pretty cool. Cruise looked like cruise. Stalling looked like you'd expect.

There is still some human factors work to be done to determine if applying some nonlinear transform to the vertical scale a little bit would make it more information-dense, but for now, this thing looks like data rather than nonsense. May it continue to be so.

There are videos, which I'll link to as soon as they are uploaded from my phone.

This is our route (outbound and return). You can see the wiggling we did outbound, which was when we were testing stalls and calibrating the display:

This is what the data for the whole flight looks like. About 2/3 of the way through you can see a sudden vertical drop in temperature; this is when we switched the system off at KHAF and went to eat lunch, then came back, switched it on, and flew home:

I will be analyzing this data and posting updates as I go. One thing that was interesting is that there was a yaw "bias" in the instrument, which caused us to always be off-center. My assumption is that this was due to a combination of the flow field around the fuselage, and the mounting of the probe. It was therefore hard to try out the slip indication. One important question will be whether a constant offset can indeed correct for this bias, or if it's nontrivially related to airspeed, power setting and other factors.

Here is the back of my car as I got all my stuff together:

Eventually I got the probe mounted. You might recognize the mount from the renderings I posted yesterday. This mount worked really well and was very rigid. I'm going to make a couple of improvements to it then post the design here, and the 3D printed parts on Thingiverse, since it might be useful to some people in its own right:

We mounted the display on the windshield with a Ram suction mount:

And here we are flying (note the yaw bias):

Here is the probe flying over the ocean:

It also works over mountains:

And it can even work near the coastline, in between ocean and mountains:

which surely you must agree makes it a very versatile probe indeed! :)

More soon; stay tuned!

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