I have one of your mps-2a power systems in a small parkflyer model. I would like to upgrade the climb performance by using a 3 cell lipoly of 250 or greater capacity instead of using my 2 cell 250. Will I greatly reduce the motor life by doing so?

From : Don Stackhouse

If you leave the throttle wide open all the time, yes. However, any reasonable usage of the throttle will not hurt the motor life measurably, as far as we can tell in any of our tests. Feel the motors right after landing form a typical flight. If they don't feel uncomfortably hot, you're probably OK. I have a lot of hours on these motors in MPS-2A power units running on 3 cells, and I do not notice any problems with motor life.

However, one of the drawbacks of twin motors is the balance between them. If you do have one go bad (a likely cause could be overheating the terminals on the motor when soldering on the capacitor and leads, which can then cause plastic end bell of the motor to melt around the brushes, which then allows them to shift and lose preload with the commutator), the other motor tries to carry the load all by itself. In that case, the good motor will quickly burn out, while the sick motor continues to limp along, looking like it's running OK but not actually making the power that it should. If you then install a new motor in place of the now-burnt-out formerly good motor, you still have the same problem, and the new motor will burn out in short order.

Get in the habit of feeling both motors occasionally after a fairly long flight. They should both be at about the same temperature. If one feels significant;y colder than the other, then the cold motor is going bad (or is gone altogether).

The other more objective test is to slip one of the motors aft, out of the front frame, and pull it to one side to that the pinion gear is clear of the propshaft gear. Make sure the gear teeth are clear of each other, if they slip back into mesh while running, it could severely damage the gear. You can leave all the electrical connections connected. Now, run the motor at full throttle just long enough to read the prop RPM. Next, put the first motor back into the front frame and get the gears back in mesh, then pull the other motor out of mesh and re-run the RPM test. The prop RPM's between the two motors should be nearly equal. If one is substantially slower, then that motor is going bad. Replace it before it causes the other motor to fry itself.

Don Stackhouse
DJ Aerotech