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Part 2: The Power System

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Two Astro Flight ``05'' cobalt motors. The one on the left is geared down to spin large props at low RPM's, the one on the right is direct-drive.

Most electric kits by Goldberg and Great Planes come with a $10 motor that they expect you to use to power the plane. You've probably all seen such planes - hand launched, struggling to climb, way underpowered. Why are they so anemic? Let's summarize some basic electric rules of thumb to illustrate why these planes fly so poorly, and what we can do to make them better.

Putting this together, we can see why electrics with cheap supplied motors often come up short in terms of performance. In the case of the Mirage, the usual flying weight with the supplied motor would be around 2.5 - 3.0 lb. At a minimum, we would ask that the motor supply about 125-150 watts. But most of these motors are capable of handling only about 100 watts, and would start smoking at 150 W. This is not really enough. To add insult to injury, most of these planes (GP Cub, CG Mirage 550, GP PT-Electric) have low wing loadings, fly slowly, and would benefit greatly from motor gearing and swinging large props at low RPMs. But they're all sold without gear drives and feature little 8x4 props. Even gearing these motors to swing 10'' props would yield more ``effective'' power. The one case where direct-drive motors make sense is the GP Electrostreak. This little aerobatic plane flies almost decently with the stock motor, and really comes alive with more capable direct-drive motors.

So how do we improve the situation? Adding a gear drive to the stock motor will help, but many opt to replace the motor entirely. I've chosen the 10x6 - 10x8 propellor series for the Mirage (ideally, an 11'' prop would yield best vertical performance, but there's not enough ground clearance for such a large prop). To swing this prop at a healthy clip, I'm using a trusty Astro Cobalt 05 motor with a 2.2:1 gearbox and 10 sub-C nicad cells. This will spin a 10x7 prop at approximately 8500 RPM, for a power output of about 250 watts. At a flying weight of about 3.5 lb, this is a power loading of 72 watts/lb. Looking at our rules of thumb above, this will yield very ``glow-like'' performance, especially considering that this plane is a floater to start with!

This kind of figuring is by no means limited to electrics. A typical sport .40 glow engine produces (at actual flying RPMs) about 0.7 HP. This converts to 520 watts. To comfortably fly a plane at 75 W/lb, the maximum flying weight should be about 6.9 lb. This is within the range of most .40-sized models.


next up previous
Next: Until Next Month... Up: Review: The CG Mirage Previous: Review: The CG Mirage

Craig Kulesa
Sat Jan 30 23:41:54 MST 1999