I have just 2 more questions. I think I'd like to run 2 of your MPS-2B engines in Parallel in my model.
Would I be safe running this on a Pixie speed 7-p controller?
From : Don Stackhouse
Yes. The only question is whether your battery can handle the currents. Also, the MPS-2B uses the little 5-4.3 prop, while the MPS-2A uses the 6-5 prop (that's the only difference between the two). Because of the smaller prop, the -2B only pulls about 18 watts at full throttle, and typically a bit under 2 amps
You could even run a pair of MPS-2A's in parallel on the Pixie 7-P with no trouble, as long as you battery can handle the current. Four of the -2B's in parallel would probably be pushing things a little, but you could probably get away with even that as long as you used a little common sense with the throttle.
Also I have a 1300 Lithium Polymer battery at 11.1 V would this voltage be too high for this motor or do I need to go down to a 2 cell for it?
We run the two-motor systems on three cells all the time. The max currents and watts I mentioned for the -2( ) systems are based on using three cells. If you want to run continuous full throttle, they are going to get hot, and presumably there will be some shortening of motor brush life. However, in our experience in the models we fly, there is almost never any need for full throttle with so much power available. We typically cruise at about the 50% throttle stick position including most aerobatics, occasional bursts to about 3/4 throttle for extreme maneuvers, and only use full throttle briefly just to show folks what it looks like. Even doing ROG's off of fine gravel, the plane will be airborne and climbing nearly vertically before we can get the throttle all the way open. Flying electrics has always been about "energy management", and using the power available as efficiently and conservatively as possible. Electrics teach you to fly with finesse, not brute force, although I'm sure that some folks who have watched me fly my Fokker Triplane with an MPS-2A and a 250 mah 3-cell pack might think otherwise!
The "-P" in "Pixie 7-P" stands for "programmable". The Pixie can be programmed by motions of the throttle stick while in programming mode for a number of different parameters including voltage cutoff. However, we've found that the factory default settings work fine, and that the low voltage cutoff is pretty much superfluous anyway. The airplane will tell you when it's time to land, unless it's really ridiculously overpowered. The power absorbed by a prop is proportional to the cube of the RPM, while the RPM delivered by the motor is linear with the battery voltage. As the battery voltage decreases during the flight, the rpm drops, causing a decrease in watts drawn by the prop, forcing you to increase throttle position to sustain the same level of performance. Normally this isn't noticeable until the last few minutes of the discharge cycle. At that point you'll find you need 3/4 throttle to get the same level of performance that previously only needed half throttle. Soon after that you'll need full throttle just to sustain flight. Very shortly after that, even full throttle won't sustain altitude. With a two-cell pack that typically occurs at around 3.0 to 3.1 volts per cell, and with a 3 cell pack it happens at around 2.7 volts per cell, both within the safe limits for the cells.
Also, per the guys at E-tec, it's safe to run down to about 2.35 volts per cell under load (i.e.: throttle open), because the cell voltage will rebound to 2.7 volts as soon as you close the throttle. Therefore, we use 2.35 volts per cell as the basis for setting the low voltage cutoff in the ESC.
You're very welcome! Please don't hesitate to ask if you have any more questions!
Don Stackhouse
DJ Aerotech
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September 5, 2008
