Orme's law
A rule of thumb for sizing battery cells to model wing area.
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.
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.
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.
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.
- Originator
- Matthew Orme
- Application
- Electric power system design for radio-controlled models
- Nimh cell recommendation for sport plane
- 1 cell per 35 in² of wing area
- Nimh cell recommendation for trainers
- 1 cell per 50 in² of wing area
- Lipo cell adjustment
- Number of cells cut in half compared to NiMH, due to double voltage
- Assumed cell capacity
- 1700 to 2000 mAH
- Assumed flight time
- 4 minutes at full throttle (30 amps)
Lore & Background
The rule was developed by Matthew Orme, who managed sales of Aveox's motors to the hobby market and later created a line of 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, making it hard for newcomers to decide which system to use.
Orme's law addresses the fact that, unlike internal combustion engines where a given engine size produces a fixed power output, electric motors operate over a much wider range. The law makes clear that the battery pack is the power source, not the motor. Once the power requirements of a model are determined, a motor can be chosen to match the battery.
For example, a Citabria Pro electric conversion with 560 in² of wing area needs exactly 16 cells according to the rule. Before Orme's law, much more work was needed to design the power system.
Reader's Guide
Orme's law simplifies system selection by providing a direct relationship between wing area and battery cell count. It assumes the use of 1700 to 2000 mAH cells and a prop setting that draws 30 amps, yielding about 4 minutes of flight at full throttle. A 10-cell NiMH pack produces roughly 1 volt per cell under load, translating to 0.4 HP at 30 amps. The rule's significance lies in shifting the modeler's focus from the motor to the battery as the true power source, making electric conversions more accessible to hobbyists. It remains a practical starting point for sport planes and trainers, with a straightforward adjustment for LiPo cells.
Did You Know?
- Orme's law recommends one NiMH cell per 35 in² of wing area for sport planes.
- For trainers, the rule recommends one NiMH cell per 50 in² of wing area.
- When using LiPo cells, the number of cells is cut in half because a LiPo cell has double the voltage of a NiMH cell.
- The rule assumes 1700 to 2000 mAH cells and a 30-amp draw for a 4-minute full-throttle flight.
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