Inrunner
Inrunners spin fast but lack torque, often needing a gearbox.
An inrunner is an electric motor where the rotor sits inside the stator. This term is mainly used for brushless motors to set them apart from outrunners, which have the rotor outside the stator. Most electric motors are inrunners.
In drones and model aircraft, inrunners typically spin very fast—often up to 11,000 rpm per volt—which is too fast for most propellers. They also produce low torque. Because of this, inrunners are usually paired with a gearbox in surface and aircraft models to lower speed and increase torque.
Many inrunners are "ironless," meaning they lack an iron stator core. Instead, the wire is placed inside the can and held in place with epoxy or resin. Without an iron core, these motors have no cogging—they spin freely with no magnetic resistance when power is off. A well-made ironless inrunner is highly efficient, with almost no iron magnetization loss and very little windage loss. However, because there is no magnetic stator core, the motor has very low torque and a higher motor velocity constant (Kv, measured in rpm per volt) compared to an iron-core motor.
Modern brushless inrunners can use iron core windings to improve torque while still keeping high rpm capability. This design is more common in radio-controlled cars and trucks.
- Max rpm per volt
- 11 000 r/min per volt
- Common usage
- drones and model aircraft
- Typical application
- used with a gearbox to reduce speed and increase torque
- Ironless variant
- no iron stator core, no cogging, very low torque, higher Kv
Lore & Background
Inrunners tend to spin exceptionally fast, often as high as 11 000 r/min per volt, which is far too fast for most aircraft propellers. They lack torque, so most inrunners are used in conjunction with a gearbox in both surface and aircraft models to reduce speed and increase torque.
Many inrunners are "ironless," meaning there is no iron stator core to magnetize. The wire is run inside the can and held in place by epoxy or other resin material. Because there is no magnetic iron core, ironless inrunners have no cogging—they spin freely with no magnetic interaction when power is disconnected. A well-designed ironless inrunner is extremely efficient due to virtually no iron magnetization loss and very little windage loss. However, the lack of a magnetic stator core gives very low torque and a higher motor velocity constant (Kv) compared to an iron core motor.
Modern brushless inrunners can have iron core windings to address the lack of torque while maintaining high r/min capability, and these are more commonly used in radio-controlled car and truck applications.
Reader's Guide
Inrunners are significant in radio-controlled aircraft and surface models because they offer very high rotational speeds, but their low torque necessitates the use of a gearbox for most propeller-driven applications. The ironless variant is notable for its extreme efficiency and lack of cogging, making it suitable for applications where smooth, free-spinning operation is desired. However, the trade-off is very low torque and a higher Kv. Modern brushless inrunners with iron core windings have emerged to combine high speed with improved torque, particularly in radio-controlled car and truck applications. The legacy of the inrunner lies in its dominance as the most common electric motor type, yet its specialized use in model aircraft and drones often requires careful matching with gearboxes to achieve usable power output. The distinction between inrunner and outrunner remains a fundamental classification in brushless motor design.
Did You Know?
- The vast majority of electric motors are inrunners.
- Ironless inrunners have no cogging and spin freely when power is disconnected.
- Modern brushless inrunners with iron core windings are more common in RC car and truck applications.
Frequently Asked Questions
What is an inrunner in RC aircraft?
An inrunner is a brushless electric motor in which the spinning rotor is mounted inside the stationary stator. The term exists mainly to distinguish this layout from the outrunner, where the rotor wraps around the outside of the stator.
How does an inrunner differ from an outrunner?
The key structural difference is that the inrunner's rotor sits within the stator, while the outrunner's rotor surrounds it. As a result, inrunners typically achieve much higher rotational speeds but produce considerably less torque.
Why do inrunners usually need a gearbox in RC models?
Inrunners can spin at roughly 11,000 rpm per volt, a speed far beyond what most propellers can handle efficiently. A gearbox is therefore coupled to the motor to cut the output speed down and multiply the torque to a usable level.
What does 'ironless' mean for an inrunner motor?
An ironless inrunner omits the iron stator core entirely, which removes cogging and allows the rotor to reach a very high Kv rating. The trade-off is extremely low torque, so these motors are best suited to high-speed, low-load setups.
Where are inrunners most commonly used in the RC hobby?
They are a standard choice in drones, small model aircraft, and surface models where a gearbox tames their high speed. In fact, the majority of electric motors used in the hobby are inrunners, making them the default rather than a specialty option.
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