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12-11-2011 08:03 AM

JohnnyCrash

Re: mCPX cyclic pitch trim change with decreasing battery charge

Quote:

Originally Posted by DoubleCH

Hope I pointed in the right direction for your investigation. I wouldn't change the FET though. Even running normal mode and reduced headspeed (lower throttle curve) for indoor, I don't find the FETs to be nearly as warm as the motor heat. There isn't much more room to raise headspeed as the stock PWM duty cycles are more biased to full throttle (i.e. even normal mode runs almost full throttle). One easiest test is to try flying without canopy. That should bring main rotor wash thru' the 3-in-1 board to keep ambient temp. Maybe that's the reason I don't see cyclic drift. I peeled out the canopy "windscreen" long ago to improve cooling.

Removed canopy and flew though a couple batts, no improvement... Starting to look like this is something I might just have to live with. Dang.

Well, thanks everyone for the help, I'll probably get on the blower to Horizon and see if they'll send me a new control unit. May as well try and get a spare part out of this

Maybe the last thing to try is adding weight to the nose to make the heli slightly nose-heavy. I have a 1" longer tail, and moved the battery forward 2mm, but maybe more is needed. The trim drift was a problem even with the stock tail, though. It seems like a control problem though, flybarless setups shouldn't care about center of gravity--it just takes more power to fly an unbalanced heli because of the constant corrective cyclic input. I could try foam vibration dampening mounts for the circuit board too.

Peace

Ian

12-11-2011 07:28 AM

JohnnyCrash

Re: mCPX cyclic pitch trim change with decreasing battery charge

Quote:

Originally Posted by DoubleCH

DoubleCH,

I tried running at full throttle and it didn't help. I guess the motor produces enough heat that the MOSFET temp doesn't make much difference. I also crashed in the process, so I had to spend my "d'oh!" for a new swashplate. D'oh! Well, I've got a spare baseplate for the swash now :-l

I also straightened the feathering shaft. Blade tracking is great now, but doesn't help the trim drift any. Yaw and roll trim are still stable.

You're right in that removing the canopy helps more than anything! I might try pulling the 'glass' out of the canopy as well... 'adding lightness' never hurt. Have you noticed that the heli is much quieter without the canopy? Maybe it just provides more surface area for sound production, acting like a speaker cone...

Getting there!

Ian

12-10-2011 04:01 AM

DoubleCH

Re: mCPX cyclic pitch trim change with decreasing battery charge

Quote:

Originally Posted by JohnnyCrash

DoubleCH,

Thanks for the heads up man! I owe you a cold one even if it only helps a little

Peace!

Ian

12-10-2011 02:18 AM

JohnnyCrash

Re: mCPX cyclic pitch trim change with decreasing battery charge

Quote:

Originally Posted by DoubleCH

Personally, I don't have any cyclic drift towards end of battery, but I've read a lot on HeliFreak that people figured it was temp deviation which the gyro chips are sensitive of. Some has eliminated (or partially so) the issue by putting foam behind the 3-in-1 board to isolate heat from the motor.

DoubleCH,

That's really interesting! I know the gyro ICs are supposed to be heat sensitive, but I figured they'd be temp compensated internally. I know the first flight of my EXI 450 on a cold day involves yaw trim twiddling because the Detrum GY48V is picky about temperature.

There are several things happening that heat the circuit board up in flight. I first reduced the severity of the problem by running the rotor head faster--and also noticed longer flight times. Obviously the rotor and/or motor is/are more efficient at higher speeds, so there is less heat output--longer flight times from lower energy consumption, and less cooking of the main board, either due to motor heating or motor control MOSFET heating.

I could try using lower resistance MOSFETs (I have some), or improving the gate drive of the existing ones to lower heat output. I'll probably try raising the headspeed some more first--because it's the easiest

At 100% throttle the MOSFETS have no switching loss, so that eliminates most of the MOSFET heating.

I also noticed that landing the heli and waiting a minute or so removes the trim error--but taking off again and flying a while causes it to come back. This happens regardless of state of battery charge. It definitely sounds heat related.

Thanks for the heads up man! I owe you a cold one even if it only helps a little

Peace!

Ian

12-09-2011 10:35 PM

DoubleCH

Re: mCPX cyclic pitch trim change with decreasing battery charge

12-09-2011 05:02 AM

JohnnyCrash

Re: mCPX cyclic pitch trim change with decreasing battery charge

Quote:

Originally Posted by Froghammer

Probably your easiest way to check it out is to swap two of the servos. See if you start having to trim the aileron. If not, and it's still affecting the elevator, then it's a different problem and start attacking that.

And your heli doesn't have any vibration problems or anything? From what I've noticed, those usually affect the tail more than anything else (like a bent feathering shaft, for instance...) but it might be able to mess around with the 3 axis gyro.

Also, go through your radio and make sure you have no subtrims or anything like that going on. Sometimes it's easy to accidentally change a setting in there and miss it.

That's how I'd do it at least.

Well, I dug through all the subtrims and only had a collective pitch positive offset (these puppies tend to come from the factory with too much negative pitch). I cranked up the swashplate links a couple turns. All subs and lever trims are now zero. The heli behaved the same way.

I swapped two of the servos-no effect. All servos work very well. I checked the blade balance--fine. The cyclic pitch still creeps backward as the battery discharges. D'oh!

The heli flies very well otherwise, this is driving me bananas! The only thing that helped was raising the head speed--I now reach 75% throttle before enough collective is applied to lift off. With the higher headspeed, only two or so clicks or forward cyclic trim are needed to prevent backward creep throughout a battery charge. Also, my flight times are about a third longer! It seems 'normal' sounding headspeeds require too much collective, reducing the efficiency of the rotor, leading to shorter flight times. I bet it could stand to be higher still. The mCPX may be a little low on power when flown stock--I'm surprised I get longer flights by running the blades fast. I wonder if the best flight times would result from running at 100% throttle all the time!

The heli flies really well, which is pretty remarkable considering it has been crashed probably fifty or more times (no exaggeration), and 10-20 of them were really hard. The trim creep is still there, unfortunately--maybe I'll check the feathering shaft, it's the one thing that's still in rather rough shape after so many crashes

Ian

12-07-2011 08:33 PM

Froghammer

Re: mCPX cyclic pitch trim change with decreasing battery charge

12-07-2011 08:02 PM

JohnnyCrash

Re: mCPX cyclic pitch trim change with decreasing battery charge

Quote:

Originally Posted by Froghammer

Yeah- no trim on flybarless helis. Ever. Or subtrim.

It sees it as input and will try to go in the opposite direction (as you're finding out...).

I neutral the trim when connecting the battery, and it flies fine at the beginning of the charge, but as the battery runs down, the heli acts as if it's applying its own back cyclic trim. I have to add forward cyclic trim on the transmitter to prevent the heli from creeping backwards. As the battery runs down, more and more forward trim is needed to hold a stationary hover.

It isn't because I'm using trim at the beginning of the flight--they're all centered.

Ian

12-07-2011 07:58 PM

JohnnyCrash

Re: mCPX cyclic pitch trim change with decreasing battery charge

Quote:

Originally Posted by bartman

My manual states
You will not need to set any trim; the flybarrless design of the mcpx renders trim unnecessary.

Not sure if that helps? I know if I try to lift off slow mine will roll over all by itself . If you spoolup fast it just goes into hover.

The flybarless control unit doesn't seem to like very low head speeds, but this is a design quirk and not a problem per se. The heli is fine once the head approaches flying speed. The other axes never need trimming.

Thanx,

Ian

12-07-2011 07:55 PM

JohnnyCrash

Re: mCPX cyclic pitch trim change with decreasing battery charge

Quote:

Originally Posted by waylander

It's possible that the elevator servo is starting to die and you can only notice it when the power is lower or check for some binding in the servo as well since at higher power the servo might be able to handle the binding but not as it loses power.

Thanks, I cleaned the servos and they weren't that dirty... they all work well. I suppose I could try swapping servos to see if it moves the problem to another axis... That may give some indication of where the problem lies. The problem has been there for a long time now, I've just noticed it more & more as I've gotten better at flying the thing. I have to give it a click or two of forward cyclic trim every minute of flying. It interrupts my flight and gets annoying.

Peace

Ian

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