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R/C Electric Motors For EDF and Pusher-Prop Parkjets

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Axi: The first two digits of the number are the stator diameter in mm: the second two, its length. The full designation of the motor includes the number of turns of the winding, found after a slash. For example, the 2212/34 has a 22mm diameter stator that is 12mm long, and it has 34 turns of wire.

Mega motors: Similar designations - Mega 22/30/3 has a 22 mm-diameter case x 30-mm-long rotor, and a three-turn stator.

Aveox: An Aveox 27/13/3 seems to use the following: 27-mm diameter of the outside of the motor (not the rotor!) and 13 mm is the length of the rotor while three is the number of turns of the stator.

Hacker: A Hacker B20-26S is 20 mm in diameter, with the “S” indicating short length (L is for long) and is a 26-turn stator.

KV ratings: Kv gives the rpms produced by a motor per volt applied, i.e . if the motor has a Kv of 2000 and you run it on 6V it will turn at 12000 rpm.

Number of Turns: Turns are much like gearing! The higher the number of turns, the larger the propeller you can swing, however, the lower the maximum current it can withstand. Brushless motors that come in 1-, 2-, 3-, 4-, and 5-turn varieties are usually the internal-rotor type (but not always like the Hacker!). Brushless motors with high numbers of turns (say 10-30) are usually the external rotor (outrunner as we call it) type like AXI, Nippy, PJS, and the new little Hacker Baby.

GEARING: Gearing allows our motors to turn high rpm at low amps while swinging efficient propellers. For example an Aveox 1406/3 speed 600 size motor on 7 cells will turn an 8 x 4 prop at 13,000 rpm for a prop speed of 50 MPH while drawing 28 Amps. If you use a 3:1 ratio gear box with an 11 x 11 prop the rpm goes down to 5,100 while the prop speed goes up to 53 MPH and the current goes way down to only 15 Amps. The end result is a cooler running, more efficient motor and almost double the flight times. The drawbacks to gearing are that the gear box can block cooling air to the motor, add to the weight of the plane, and add to the length and sometimes circumference of the motor. The motor also has to be retimed to run in the opposite direction from the prop. If the plane has a narrow nose it will probably need a planetary in-line gear box. These gear boxes have little or no prop shaft offset, allow the motor to turn in the same direction as the propeller and are usually more expensive than standard gear boxes. Belt drives allow the motor and propeller to turn in the same direction, but can be large and heavy

Inrunner vs. Outrunner?

Inrunners are "standard" style motors. Meaning the can is stationary and the shaft spins inside.

The latest type of brushless motor available is the so-called "outrunner" like the Model Motors AXI types. At first sight these are rather odd. They are arranged the same way as a brushed motor with the coils in the center and the magnets on the can. is the CAN which rotates NOT the center armature. This means they are a bit tricky to mount since you obviously can't just clamp them down but it does have one BIG advantage. These motors generate much more torque than a conventional arrangement. In practise what this means is that they will turn a much larger and more efficient propeller without needing a gearbox. Gearboxes of course add complexity, cost and weight so that's a real advantage.

Other Resources:

Has produced a HUGE Database of Electric Motor info
The Great Electric Motor Test #5
Featuring: Hacker, Jeti Phasor, Krontronik, Mega, Model Motors (AXI), 
MP Jet, Uberall Nippy, PJS, Plettenberg, and Typhoon Motors
The entire Motor Test Results have been compiled in a 108 page PDF
available for downloading here