Orme's law

Orme's law is a rule of thumb that helps modelers design an electric power system for a radio-controlled aircraft. It recommends using one NiMH rechargeable battery cell for every 35 square inches of wing area on sport planes, and one cell for every 50 square inches on trainers. Because a LiPo cell has roughly double the voltage, the number of cells is cut in half when using LiPo batteries.

Origin

The rule was created by Matthew Orme, who managed sales of Aveox motors to the hobby market and later developed a line of small, high-performance brushless motors at RazorMotors. He wanted a simpler way to recommend power systems to the many people asking for advice. Brushless motors are very versatile, which can make it hard for newcomers to decide on a system.

Need for Orme's law

The main purpose of Orme's law is to show that the electric motor is not the power source, unlike an internal combustion engine. With an IC engine, a specific size gives a specific power output, and manufacturers often recommend a 2-cycle glow plug motor by its displacement. Because electric motors operate over a much wider range, there was no good way to recommend one for a plane.

Orme's law makes clear that the battery pack is the power source and determines the available power. Once the power needs of a model are known, an electric motor can be chosen to match that source. For example, a 10-cell NiMH pack produces about 1 volt per cell under load, and at 30 amps that translates to 0.4 horsepower.

System selection

The rule makes system selection easy. It assumes the hobbyist uses 1700 to 2000 mAh cells and props for 4 minutes of flight at 30 amps.

A 2000 mAh battery is 2 amp-hours, or 120 amp-minutes, so 120 amp-minutes divided by 30 amps gives 4 minutes of full-throttle flight, with longer times at reduced throttle. For instance, a Citabria Pro electric conversion with 560 square inches of wing area needs exactly 16 cells according to Orme's law. Before this rule, much more work was required to design the power system.