Flight tests with the UAV DevBoard

After my last UAV project went up in flames (literally!) I’ve shifted gears and designed a dedicated autopilot (AP) test vehicle. She is a tailless plane with removable wings for transportation and has plenty of room in the fuselage with easy access.

For the autopilot, I’m using Bill’s UAV DevBoard with a modified AileronAssist code for handling the elevons. This modification is a work in progress by Ben Levitt and he is including a lot of great features. You can check out the latest versions of the code here. The tailless had her first autonomous flights this weekend and I’ll be posting all of the flight logs on this blog. So far, I’m really pleased with the AP’s performance. Not only does the DevBoard and software perform well, but Bill also provides a wealth of information on the theory of his methods as well as the “how to” to get the hardware and software installed and running. I know this is beginning to sound like a commercial for Bill’s stuff but if you do a search on this website, you’ll find nothing but praise for Bill and his accomplishments. And to top it all off, he’s always willing to immediately chime in and help whoever asks for assistance.

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  • Wow I would have thought a flying wing would be super skiddish without winglets. The paths looks really nice. My devboard is on the way for my wing! I joined the google group and am looking over the beta code. There is a possiblity I'll hit you up with a few questions once I start tinkering with my elevon setup. Keep up the great work! Your paving the way for us other wing guys.
  • More yaw stabilizing tests. This time I attached vertical fins just in front of the propellor on either side of the fuselage. They worked fine but produced a lot of prop noise. The first picture shows the WP path with fins attached and the second is with fins removed and the CG moved forward 1/2". You can clearly see the benefits of vertical stabilizers on a flying wing for close, WP flight. The winds were calm.


    Vertical stabs removed--------

  • Bill had wondered about the tailless AP performance without a vertical stabilizer. At the time, I said "No way it could fly", but the seed was planted, the seed grew I did a little more research on yaw stability and finally couldn't resist giving it a try. A video of the flight can be seen here.
    Unfortunately, adding the onboard camera introduced a variable (mass distribution) that had a significant effect on the results

  • I'm getting a better understanding now how the altitude hold works. Altitude held closer to my target of heightmax. Just a little more tuning-----

  • Thanks for the comments Bill. It sounds like I'm pretty close to understanding this. The one thing I'm still not clear on is what happens if your flying at min throttle, and pitch for min throttle, but you're still descending. At some point, does it switch from min throttle and pitch to max throttle and pitch, or is there some in-between?

    My initial tests were with the stock values in AA8b. Thinking that the default 35% min throttle wasn't holding altitude, I changed it to 60%, since that's really about right for cruise. The next test was worse, and I now realize it was because 60%, and 0 pitch gives a pretty healthy climb, so I was bouncing off the ceiling faster than before.

    The next test will be 60% and -2 degrees for the min throttle settings, and I'll reduce the pitch even more if needed to keep it from hitting the ceiling. I also reduced the max throttle pitch from 15 to 10 degrees. I was concerned that the high climb rate of 15 might be coasting up to the ceiling, particularly with the lag and inaccuracy of GPS altitude readings.

    As a thought for future hardware/software versions, the MK (MikroKopter) allows you to assign a pot on your radio to any parameter, so you can vary the setting in-flight. This is used mostly for tuning, and once you get it set to the best value, you land, read the setting, and enter it manually. Something like that would be really handy for adjusting the minimum throttle pitch.

    Thanks,
    Rusty
  • T3
    Rusty,

    You are right, the key to getting altitude hold to work the way you want is to set the minimum throttle, pitch at minimum throttle, maximum throttle, and pitch at maximum throttle.

    As you point out, the minimum settings determine whether or not the motor cuts off. Most of the flying I do, I want the motor to cut off and see if I can thermal. So, I set the pitch at minimum throttle to zero. That way, the plane can "punch through" the altitude hold ceiling and continue to climb on a thermal.

    If you want to keep the motor running, you have to keep the plane from "punching through" the ceiling. To do that, you need a slightly negative pitch at minimum throttle, so that the plane does not get to be more than 10 meters above the target.

    So, in the case where you would like to keep your plane near the target altitude with the motor running all of the time, you need a negative pitch at minimum throttle, and a positive pitch at maximum throttle.

    The maximum throttle settings are easy to figure out. You set them for the behavior that you want during climb out. You set them first, until you get the climbout the way you want.

    Then, you set the minimum throttle and pitch to get the control behavior that you want at cruise.

    There is also a "glide pitch" setting that you use to determine what the plane will do if it climbs on a thermal.

    Best regards,
    Bill
  • Hi Bill,

    In looking at the way the altitude hold works, I'm thinking that the key is getting the minimum throttle, and pitch at minimum throttle set correctly. I guess ideally, if you could give it a cruise throttle, and pitch setting that resulted in perfectly level flight, it would just fly at that without any oscillation in altitude. That's going to be impossible, but getting close shouldn't be too hard.

    Settings that result in a climb (too much power/pitch) will trigger the motor cut when it gets too high. I'm not completely clear on what happens when the settings result in a loss of altitude. Does some extra throttle and or pitch kick in as you drop below the target altitude? By "some", I mean that it doesn't go to max throttle and the max throttle pitch setting does it?

    I guess my main question is whether it would be better to shoot for settings that will result in a slight climb, or a slight descent. I'm thinking descent, to avoid the throttle off situation.

    Thanks,
    Rusty
  • T3
    Bryan,

    The min height that you enter does not work the way you think it does. Min height simply sets a lower bound on the target height that you dial up with the throttle. Except for the oscillations (which we can fix), what you saw is pretty much what I would expect.

    Bill

    Bill
  • Bill,
    Looks like the max values were dead on. I had max height set at 90m which is just under 300 ft. I'm just not sure why the min height appeared to be @ 190 ft instead of the entered 130 ft (40m). Part of the problem is my power loading. I downsized the motor but the power loading is still more than I need so when full throttle is activated, there is too much thrust. The only time I use full throttle is during the hand launch.
    Rusty,
    I'm glad you got those switches figured out! Looking forward to more flight reports, especially when you get your new GPS data logger.
  • T3
    Hi Bryan,

    Me again. I forgot that your units are feet, not meters, so the amplitude of the oscillations are not as large as I thought. The are not much greater than 10 meters, actually.

    I think what is going on is that we need to turn the altitude control gains down a bit, particularly the throttle.

    Best regards,
    Bill
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