Hello everyone,
Increase Throttle = No Tail Hold
This should be a pretty basic setting to change but I can't locate it anywhere. I'm running the Pixhawk on my Trex and when I quickly increase the throttle, the tail gives out horribly and the heli does some 360's. The heading hold mode seems to give out when the throttle is increased. I am looking for the parameter that will increase the gyro gain value for the pixhawk internal gyro. So far I have played with the Rate Yaw PID values in the "Extended Tuning" tab, but haven't fixed the issue yet. Right now, my PID values for Rate Yaw are: P=0.2 and I=.02 and D=0
Any recommendations?
Thank you very much
Evan
Replies
I think changing the head speed on a UAV heli is a very bad idea.
Generally once the head speed is changed, the PID gain becomes wrong. For example increasing the head speed will introduce the direct increase of the loop gain in your system. So you may bring your heli into oscillation by simply increasing the head speed, then it may go out of control, or simply has a very bad response to your stick input.
Further more, increasing the head speed will increase the torsion applied on the heli frame, which needs to be compensated by the tail thrust. Although the thrust of the tail is increased with higher rpm, it's not a linear relation between the increasing of torsion and the thrust. Controller will compensate it, however by increasing the head speed, you have screwed up the control system because the gain is changed, then you have a bad tail.
That's why DD tail performs great. The rotational inertia of the heli is constant, once the DD tail is fine tuned, the head speed will no longer cause any blow-out, it may still cause oscillation of rolling and pitching if the head speed jumps, say, from 2000 to 3500rpm, but the tail is still under control.
Try to find a suitable HS, then keep it governed, it's good for the whole system, more robust and more predictable. UAV is a different story, the normal 3D heli setup is not optimal in this case.
Yes, you are correct, technically the gains need to change. I have found that tuning for the higher speed, does allow acceptable performance at lower speeds. Maybe not idea, but not dangerous in any way.
Eventually, I want to try have "flight conditions" where you have multiple gain settings for different conditions.
I'm already using DD VP tail.
Yes it looks like if the head speed is not changing too much, tuning for maximum HS is still acceptable most of the time, although the heli may feels a bit "lazy" and the tail is about at its limit.
The "flight condition" seems a great idea, it solves this problem. Perhaps it's also helpful for multirotors that is used to pick up and drop payloads frequently, like the "beer delivering drone" :)
Yeah, you definitely can't play with the head speed too much if you have conventionally driven tail. I actually had a problem with this 2 weeks ago. My 450 has a motor intended for 4S, but I'm running 3S with stretched blades. Did it to reduce head-speed. But then I forgot that, after flying my big heli, I had left the ESC setpoint knob at 50%. I was having tail blow-outs real bad, and trying to figure out what was wrong. Finally I figured it out. One of those "duh" moments.
My big heli with the DDVP tail, I can turn down the main rotor to 50% if I want.
Evan, what helicopter are you using, and what is the main rotor speed?
Reading about your troubles, I should point out that there is an issue with helicopters using reduced main rotor RPM. The problem is that as you reduce the main rotor RPM, the tail rotor RPM also drops. This results in reduced tail rotor thrust. At low speeds, you will usually find that the tail rotor thrust can't prevent the rotation of the helicopter. This is commonly called a "blow-out".
I actually experienced this on Sunday, and it took me a while to figure out what was going on, even though I knew about the potential problem. My 450 heli is currently set up with a motor intended to run on 4S (2700KV) but I am still running it on 3S (normally 3600KV). I do this for reduced rotor speed because I get an extra few minutes of flight time. I'm also using longer main rotor blades, so it still has plenty of lift.
Generally, this works OK, though the tail is a bit soft. However, the problem that happened, I was flying 3 different helis. On some of them, I actually run a reduced governor setting, even as low as 50%. I had finished flying that helicopter, and forgot to turn the knob I use for the Ch8 RSC Passthrough back up to it's normal setting of 90%.
So now my 450 heli had a slow motor and then also was running only 50% maximum speed. I didn't even notice. But I was getting the tail blowing out all the time. I played with the gains, and played with ColYaw. Nothing worked. Then I realized my mistake. Turned the rotor speed back up, problem gone.
This is part of the reason why I created the code for the Direct Drive Variable Pitch tail. I can turn the main rotor speed WAY down, while keeping the tail rotor speed high enough for good control. It works great.
Anyway, not saying this is your problem, but just something to think about. If you find that you can't get the settings right for the tail to hold, you should always consider physical effects. Helicopters are complicated machines, and setting them up to work right involves more than just getting the flight controller settings right.
Hey Rob,
Thanks again for all the great information. I think you might be on to something. I think it may definitely be attributed to head speed, especially since when my battery starts to get very low (I know I should stop testing before this happens, but...) and the rotor speed drops off a bit, this blowout becomes even worse, almost a constant dead spin.
I am trying to figure out how to increase the RPMs of the motor while keeping the collective pitch the same? I tried playing with the throttle curve in the radio's menu, but that seems to act more like a pitch curve when watching the servos on the swash. I also tried playing with the Throttle Hover slider under the basic tuning menu, but that didn't seem to help much either.
FYI, we are using a T rex 250 for testing purposes before we switch it over to a T-rex 600 for a competition we are entering. I'm not sure exactly what head speed we are running on the 250. I do know that before we incorporated the Pixhawk into the system, the helicopter flew fine though using the 3GX flybarless unit it came with so everything mechanically should be ok.
Cool, glad to hear you're starting out with a little heli.
How exactly is your ESC throttle being controlled?
We just did some more testing and kind of tricked the system by arming it normally and then turning the aux3 knob up a bit till right before the blades begin to spin with no throttle stick input, then backed it off just a tad and proceeded to fly. This gave the throttle stick some extra RPMs before liftoff and the tail finally help solid during pitch pumps! Now I just want to figure out a better way to increase the RPMs since there's probably a better way and I think we could even use a bit more RPM if possible.
Any thoughts on this? I hope that made some sense
Actually, it didn't make a lot of sense to me. Spektrum "programming" makes my head explode because it's not logical. They try to make it easy by restricting what you can do, but I find it more confusing. It sounds like your Ch8 is actually linked to your "throttle" or collective stick, and then you're adding to it with that Aux3 knob? That's not right. Your Ch8 knob should be the only thing controlling Ch8 output.
Your best bet might be to use RSC_Mode 2, and then set the RSC_Setpoint to 1900 or something like that. That should give you 90% motor speed, which should work if your heli is mechanically setup properly. Then, your Ch8 in the Tx will act as an on-off switch for the motor. This assumes you have a good governor ESC.
Yes, I think Jim's put you on the right track here. Thanks Jim!
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