Outrunner-type motors are designed differently than the small electric motors you see in most devices. Outrunners have a split-barrel design that is comprised of a base (non-rotating) and a drum (rotating) that holds the internal permanent magnets and is fastened to the axle. The drum, axle, and internal magnets all form the Rotor. Inside the drum, and attached to the base, is the Stator and Coil Windings. The diagram below provides a cut-away end view and a side view of a typical Outrunner motor:
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Physical Dimensions
Outrunners come in an extremely wide range of physical sizes. On the small end, you have 1.5g motors that are the diameter of a US nickel producing over 20g thrust!. At the large end, you have outrunners weighing over 5 1/2 lbs. capable of flying 45lb. or larger models via direct drive. Outrunners allow you to spin park flyer props at performance speeds and thrust but also deliver enough torque right at the shaft to direct drive larger, higher pitch props for 3D applications. Although not as electrically efficient as their inrunner counterparts, outrunners are extremely versatile, quiet, and powerful.
Kv
The Kv rating on a brushless motor refers to the Revolutions Per Minute (RPM), Per Volt applied. So, a 100 Kv motor connected to a 1 volt battery would be turning 100 RPMs.
Outrunners historically carry lower Kv ratings than their inrunner counterparts, but I hesitate to make this an absolute statement. With the growing interest in Electric Ducted Fan (EDF) models, manufacturers have delivered some very high Kv outrunners to the market as well. Both inrunners and outrunners are available in models that exceed 100,000 RPM.