mystery drop in rotor rpm

[moyesboy]

since the aquatic demise of my S&T quark, i'm running an ACE 40A ESC (from a miniT).
Its been great. I've been flat lining 100% and geting 6 min flight with rotor rpm at 3000+ and its not been a problem for many flights (over 40, definitely). I was happy with this setup and giving it quite a hard time with no problems.

I am currently breaking in two new revo 2100 packs and while only stooging about in idle1, (100, 80, 100) with about 2800 rpm I'ver had the rotor rpm drop suddenly, close to the beginning of the flight. no lipo alarm, so its not pack volts. Landing stopping, starting (without powering down) and it goes back to normal. ITs happened twice, both times with one of the new packs. There is still about 2400 or so rpm and it hold steady, but of course I put it down straight away. This time (2nd time) I have made a note of which pack it is on, see if its the same one next time. I also checked the ESC temp (not hot) and the motor (not really hot). After the last incident I bounced the heli up and down to load the motor and switched into idle2 a few times and I couldn't reproduce the problem. After 3 minutes of that the motor was pretty warm, the pack cool and the ESC not all that warm.
no problems shown with the pack on the charger and its the same state as its mate after 4 minutes of flying.

ESC is prime suspect, though surely if its overloading I'd be able to overload it deliberately.

Any ideas? I'm worried becuase it happening when I am not loading the system hard at all, and I fear the effect might be more catastrophic when I do!

[funfreex]

Gordon, I had the same happen to mine whilst hovering a few weeks back! I was in idle up 2 at the time and had a bit of height so flicked back to normal, closed throttle to try a reset and opened again but still it just wound down. Luckily it still had enough rpm to get it on the deck. The problem has not duplicated since!

[moyesboy]

I don't know how theESXC is working, I assumed it would get hottest when the most power is going through it. Maybe when it is running at less than full power, and the battery volts are max, its actually getting hotter then when its going flat out with the battery volts pulled downa bit. Both times its with one of my new packs a the start of the flight in idle 1 just hovering - and in the evening too, not hot weather. also after landing its recovered and gone back to high speed for the balance of the flight without a problem.

[SRH990]

The ESC will get hottest running below 80% throttle, 100% is actually best if the gearing is right. The ESC cannot actually reduce the voltage to the motor, it just switches it on and off rapidly to achieve the same effect, the lower the % throttle the more switching it has to do & the harder it works.

[Ade_Law]

id say it more likely the motor. if you have a dicky bearing it could go stiff when its hot or it could be the magnets demagnetising.

Ade

[moonmanflyer]

I had a similar thing happen on my trex 450, belive it or not, the belt had slipped on to the outter part of the drive gear and caused so much strain the head speed dropped by half, so might be worth a check.

[moyesboy]

The drop in magnetism with temperature is a gradual thing (coincidentally I've found out lots about rare earth magnets and temperature for a project at work). This is a sudden drop to a constant rpm - in line with ESC overheating and a temperature activated auto shut down power reduction.
I have just taken the motor out and it does sound like its had a hard life (which it has) - although it feels pretty smooth when it is spun under power it has a rough sound - and there is a different sound to it when it starts up on the heli. Last good crash it stripped a main gear (16T pinion on this motor, so a lot of force to do that) I'd put the different sound down to the new main gear.

Is all a bit weird that the power train seems happy to have a hard flight but not an easy one.

Doesn't seem to be any way to get the motor apart (medusa) to change the bearings.

No belt tracking probs, and after stopping it starts up again good.

[robgt]

Is all a bit weird that the power train seems happy to have a hard flight but not an easy one.

It's trying to tell you something Gordon
Beat it up!

Cheers,
Rob

[moyesboy]

yes, all the stooging about it putting it to sleep.
4 and 5 cycles on the new lipos now, when can I start hammering them?

[robgt]

I usually put 5 or 6 "half" cycles on new packs before starting to use them for a full cycle. I then go another 3 or 4 cycles before I start hammering them.
I might be being overly cautious with the packs, but it seems to work for me (so far).
Cheers,
Rob

[moyesboy]

I just put the motor back in, turned 180 degrees so it loads the bearing outer race the other way (as if that would change anything ). 4 min ground run with periodic bursts of loads of negative showed no issues, but blew all sorts of things about the room (I'll go outside next time, its all too scarey indoors).

The other change that might effect this is I'm running a setof align night blades (blind ones ) that definitely load the motor more - so I'm running just over 80% throttle with more load but getting good revs due to the new batteries (though my year old revo performs exactly the same...).

Nice looking evening tomorrow so I will give it some hammer and see what happens.

[moyesboy]

Ran three revos through it on idle up 2, 100% flat line - swappng the packs with minimal cooling time between.
Repeated with two packs at dusk. Tried to keep it working hard to the best of my ability and it didn't miss a beat.

So I'll carry on - my worry is I'm taking it somewhere hot at the end of the month...hopefully there will be some warm weather to test it out in before I go.