science

Orme's law

Orme's law is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Orme's law rather than just read about it. In short: Orme's law is a rule of thumb to assist modelers when they design an electric power system for their radio-controlled model. Orme's law simply recommends the use of one NiMH rechargeable battery cell for every 35 square inches (230 cm2) of wing area for sport planes.

Key takeaways

Orme's law belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Orme's law to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Orme's law from memory before moving on to harder problems.

Reference excerpt

Orme's law is a rule of thumb to assist modelers when they design an electric power system for their radio-controlled model. Orme's law simply recommends the use of one NiMH rechargeable battery cell for every 35 square inches (230 cm2) of wing area for sport planes. One cell for every 50 square inches (320 cm2) of wing area for trainers. When using LiPo cells with higher voltage, the number of cells is cut in half, since a LiPo cell has double the voltage.

Origin This rule is the work of Matthew Orme, who used to manage the sales of Aveox's line of motors to the hobby market, and later developed a small high performance line of brushless motors at RazorMotors. Matt wanted to come up with a simpler way to recommend systems to the many people who ask him for advice powering their models. Brushless motors are very versatile and it can be hard for newcomers to decide which system they should use.

Need for Orme's law The main purpose was to demonstrate to the modeler that the electric motor was not the source of power like an internal combustion engine. With an IC engine, a particular sized engine will have a particular power output. Typically a model airplane manufacturer will recommend a 2-cycle glow plug motor by its size (typically .40-.60 cubic inch for the average sized plane) Since electric motors operate over a much wider range, there was no good way to recommend an electric motor for a plane. Orme's law makes it clear that the battery pack is the power source, and it determines the available power, not the electric motor. Once the power requirements of any particular airplane model have been determined, an electric motor can be chosen to match the power source. i.e., a 10 cell NiMh battery pack produces approximately 1 volt per cell under load. At 30 amps, that translates to .4 HP.

System selection Matt Orme's rule makes system selection easy. Orme's law assumes that the hobbyist will be using 1700 to 2000 mAH cells and will prop for 4 minutes of flight, or 30 amps. 2000 mAH = 2 amp hours or 120 amp minutes, hence the 4-minute flight time (120 amp-minutes /30 amps) at full throttle, with increasing flight times at reduced throttle settings. Applying the rule using a hobbyist's Citabria Pro electric conversion, which has 560 square inches (3,600 cm2) of wing area, it needs exactly 16 cells according to Orme's law. Before Orme's law, much more work was needed to design the power for the system.

References

Worked examples

Example 1 — a first encounter with Orme's law

Start with the simplest possible case. Write down what Orme's law claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Orme's law before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Orme's law ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Orme's law

In research: Orme's law appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Orme's law in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Orme's law is common in secondary-school and first-year university syllabi. It links to neighbouring topics Model aircraft, Rules of thumb, so understanding it makes those chapters shorter.
In everyday life: Look for Orme's law outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Orme's law in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Orme's law means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Orme's law out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Orme's law in simple terms?

Why does Orme's law matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Orme's law?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Orme's law.