Electric pre-rotator figures

Jazzenjohn said:
The biggest r/c brushless motors are near the minimum size needed for a prerotator based on the very few posts of what people are using now. The plusses are that they are light weight, efficient, and compact. The minuses are that the big ones usually run at much higher voltages than typical gyro electrical systems, they are expensive, and they don't like to pull from a dead stop or slow speeds.
Lithium batteries are definately up and coming. They have a high power to weight capacity, but they recharge slowly (at this time) and, if done incorrectly, are suseptable to start on fire. They WOULD NOT hook up to any 12v alternator and charge up safely. They require special circuitry to charge safely.

All very true - you'll be running 30+ volts at those power levels. You're looking at around an hour to charge the batteries, and on a dedicated LiPo charger off the aircraft. The hard core aerobatic guys are usually getting 10+ minutes of flight time per charge, should let you get several starts anyways.

Seems like it might be worth pursuing for the ultralight crowd, may let you stay within the weight limits where nothing else would.
 
I still have huge reservations about using brushless/lipoly for direct drive because it's poor pulling power from low rpm's However, It might be a good choice for Ralph's electric prop style prerotator. Some of the larger rotating can brushless motors are made to have a prop directly mounted on them. The li-polys weren't available at that time so the package could be made smaller and lighter albeit more expensive and longer to charge. There's quite a bit of info on Ralph's site for assembling such a system. I like the idea of a timer for arming and for the prerotation but I don't think I'd be able to duplicate that without mucho help. I think I'd be able to put it together using remote control. I wonder if a folding prop would work on it. There are quite a few of them out now.
 
Last edited:
To give an idea about the differences between Ralphs VDP batteries and what's available today, The batteries he used were 1500 mah rc packs 14.4 volts. $20 each pack or $40 each side total $80. Today the same packs can be found for $10 each or $40 for all batts. Lipoly 3100 packs (necessary because of the current draw) are double the capacity but cost $153 each for 14.8 volts each side or $306 total for all Batteries. They would give 6 starts instead of 3. They old weight is about 12 oz per pack or 48 oz total vs. 18.5 ounces for lipoly saving almost 2 pounds of weight from your battery pack ... and your wallet.
 
Last edited:
Top