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Part II: Design Examples

In last month's newsletter, I motivated why electric power is a natural choice for multiengine models, and described the basic design of a twin-motored model. This month, let's see how it all fits together by highlighting a few examples.

Which motor? Which plane? Unlike glow motors, which develop a ``fixed'' amount of output power at a certain RPM, electric motors have a very wide power range. For example, an Astro Cobalt ``25'' motor, depending upon gearing, can swing a prop anywhere from 8-14 inches diameter on 10-18 nicads - yielding output power equivalent to a .15, .19, .25, .32, .40, or even a .53 4-cycle. This flexibility is a great advantage, but can also be confusing: Which model? Which motor? What prop? How many batteries?

Fortunately, there are relatively simple rules of thumb that nearly guarantee success with electrics. The first is: you need about 50 watts per lb of flying weight to fly comfortably (sailplanes, trainers, mild-mannered sport planes). Aerobatic/pattern planes should target 75 watts/lb, and if you exceed 100 watts/lb, you can fly like Franc does <grin>. For example, if you have a design that will weigh 5 lb all-up, you should power it with a motor capable of cranking out at least 250 watts for normal sport flight, 375 watts for aerobatics, and over 500 watts (if your name is Franc). Most airplanes (especially large ones) benefit substantially from gear drives since they often yield substantially more static thrust and propeller efficiency. Gear drives allow you to swing large props at lower RPM's, similar to (but more extreme than) 4-stroke engines. Most kits with supplied ungeared ``can'' 05 motors end up supplying only 30 watts/lb - the reason for many electric models' poor performance.

Estimating motor power: Wattage, or power, is simply the product of the battery pack voltage (which is roughly equal to the number of cells under load) and the current the motor is drawing (typically 10-20 amps for ``can'' motors, 25-45 amps for cobalt motors). So if you have a cobalt motor that draws 25 amps on 12 cells, you have approximately 25 x 12 = 300 watts. But the best way to determine a motor's power is to measure the voltage and current yourself!

Selecting a kit: When selecting a kit or plans for building, look for the following traits: First, is the kit well-designed, relying on good engineering rather than heavy ``box''-like construction? [Example: The Great Planes and Goldberg Cub kits weigh almost 2 lbs more than the Sig Cub, which uses more conventional and efficient stick construction.] Lighter planes fly better. Also, is there enough wing area to carry the load of nicads? A model with insufficient wing area will require more power to simply stay aloft, which leads to short flight times. A good electric plane will easily fly level at 1/2 throttle or less.




next up previous
Next: Example: Killer Bee Up: Multi-Engine Electric Planes Previous: Multi-Engine Electric Planes

Craig Kulesa
Fri Dec 4 18:35:52 MST 1998