I've just finished my Curtiss Wright Junior, and have a question about the cutoff voltage to use in the Pixie 7P for your 2-cell LiPo pack. In the Pixie docs, it says to leave it at the factory default, 4.7 volts, for a 2-cell lithium pack. But that's a lot lower than the usual 3 volts per cell recommended as the "bottom" for these cells. I understand that under load the voltage can be pulled a little low due to the cells' internal resistance, but is it really OK to go that low? What do you guys use?

From : Don Stackhouse

The factory default. It's actually OK (according to our distributor for these cells) to go to 2.7 volts per cell, and that's with no load on them. Under load they can drop to 2.35 volts per cell and still jump back to 2.7 volts per cell as soon as you unload them.

In any case, you won't get that low unless you're thermalling a lot. The RPM you get from the motor is proportional to the voltage you feed it from the battery. As the pack discharges, the voltage drops from initially about 4.2 volts per cell, then quickly levelling off around 3.7 volts for most of the flight, then decreasing towards the end.

However, the power absorbed by a propeller is proportional to the cube of the RPM. By the time the voltage drops to around 3.0 to 3.2 volts per cell, the RPM will therefore be down to around 70%, and that means that the power the prop is pulling from the battery will be down to the cube of that, or only about 34%. The plane will get very sluggish over the space of the last couple of minutes so that with the Junior on a 2-cell pack you will need about 3/4 throttle to cruise, then lose so much power that it can no longer sustain altitude even with full throttle.

Unless you get so high in thermal lift that you can run the battery dead with the throttle at idle, you won't be able to fly long enough to run the batteries dangerously low. From that standpoint, the low voltage cutoff in the speed controller is really just a backup. The airplane's performance in flight will tell you when it's time to land.

Don Stackhouse
DJ Aerotech