I had to mount the battery of my RK Zero in the nose. I'm using a stock LiPo battery and E-Flight 5 amp ESC

I can't get enough takeoff power to lift it into the air, even though the battery has been fully charged. When I apply full throttle, the motor isn't cranking a whole lot of power. Any suggestions?

From : Don Stackhouse

It could be a calibration issue with the ESC, although that's unlikely. A bad battery is also possible, and it's possible that the motor might have been damaged during soldering.

You will need a tachometer and a voltmeter to properly sort out what's going on. Those are two very useful items to have in any electric flight toolbox, and a worthwhile investment.

Check the battery voltage after a full charge. It should read close to 8.3 volts, or 4.15 volts per cell. If it doesn't, then you're not getting a full charge. One possible cause of that is insufficient volts from the big battery that's supplying your charger. If it is less than about 11.6 volts, then the Apache charger will try to put in as much charge into your flight battery as it can, but it will still not get the battery up to a full charge.

Once you've verified the battery indeed has a full charge, check the prop RPM at full throttle. You should get around 5000 to 5100 prop RPM on an MPS-1A and a 2-cell Li-poly. If you don't, you then need to figure out if the problem is with the speed control or with the motor. Unsolder one leead from the motor to the speed control, then connect the battery (with correct polarity!) directly to the motor, bypassing the speed controller. Once again, you should get about 5000 to 5100 RPM. If you do, but didn't when running with the speed controller in the loop, then the speed controller is probably the problem.

However, if you get less than 5000 to 5100 RPM when connected directly to the battery, then the problem is probably your motor. Its a rare occurrence, but it does happen. It's typically the result of using poor soldering technique or a not fully heated up iron when soldering the capacitor and ESC leads to the motor. This causes the soldering operation to take too long to complete, which causes more heat energy to soak into the terminals on the motor. They get hotter, and cause their sockets in the plastic end bell to melt, which then lets the terminals shift in their sockets and lets the brushes lose some of their contact force against the commutator. In extreme cases it can kill the motor outright, but more commonly it just robs a big chunk of its power output. As you slowly bring up the power, the prop will accelerate to some intermediate RPM (3200 RPM was one example I've seen), probably at around half throttle. At that point the brushes start to bounce on the commutator, and any further increase in the throttle does not result in any more RPM. BTW, since power absorbed by a prop is proportional to the cube of the RPM, 3200 RPM is only about 25% power. That is definitely not going to be enough to take off.

Unfortunately, the odds of being able to repair the motor are pretty slim, even if you did have watch repair as a hobby once upon a time (which I personally did, and my track record with repairing these motors is still pretty awful). The best bet is to replace the motor.

BTW, a little hint: solder the capacitor leads to the motor terminals, then clip them off about 1/4" to 3/8" long. Solder the ESC leads to the clipped ends, not directly to the motor terminals. This reduces the amount of soldering you have to do on the motor terminals themselves, and thereby reduces the chances of overheating them.

Don Stackhouse
DJ Aerotech