Brushless Motor Frequently Asked Questions

1. What is an outrunner motor?

The 'normal' type of motor that you are probably familiar with is referred to as an 'inrunner', and the armature (stator) of the motor rotates inside the motor while the outer case of the motor stays fixed. An 'outrunner' motor has an inner stator that is stationary, while the outer case rotates. This design allows relatively low RPM with very high torque, eliminating the need for a gearbox. Outrunners are also 'brushless', meaning their are no mechanical contact points to wear out. The only wearpoints in an outrunner motor are the two bearings on the shaft.

2. What is a KV rating?

The KV rating of a motor indicates the motor's 'no load' speed per applied volt. For instance, a 900KV motor will spin at approximately 900 rpm per applied volt. If you connect a fully charged Lipo battery (12.6 volts) to a 900KV motor it will spin at about 11,340 rpm (12.6 x 900) with no propeller attached. The KV rating is determined by the number and type of motor windings, and is a main factor in determining what size propeller can be used on a motor. The higher the KV rating, the smaller the prop that can be used, all things being equal. Compare the 2410 motors - they are identical except for different motor windings that give them different KV ratings: the 2410-09Y is 1100KV and spins an 11 inch prop, the 2410-08Y is 1260KV and spins a 10 inch prop, and the 2410-12D is 1530KV and spins a 9 inch prop (all using a 3-cell Lipo).

3. How do I choose an outrunner motor for my plane?

The 'watts per pound' method of choosing a motor is often used, but it's a little complex and confusing to some people. I prefer a simpler method - 'thrust to weight' ratio. For this method, you need to know two things: the actual amount of thrust a motor will produce, and the estimated weight of the plane. I provide thrust data for all of the motors that I sell, and the all up weight of a model is usually fairly easy to determine. Most models will fly with about a 50% thrust to weight ratio, so a 20 oz model would need about 10 oz of thrust to fly at a very modest level of performance. I prefer at least an 80% thrust to weight ratio, so a 20 oz plane would require 16 oz of thrust, and would fly rather nicely. Even better is a 100% thrust to weight ratio, and this is what I recommend for trainer type planes. You want plenty of thrust to get you out of trouble, and a low pitch prop to keep the speed down. For unlimited vertical performance and 3D flying, I suggest at least a 120% thrust to weight ratio, so you would want at least 24 oz of thrust with a 20 oz plane. This high level of performance is easy to achieve with todays brushless motors and Lipo batteries. When choosing your motor, keep in mind that outrunners that spin larger propellers are generally more efficient.

4. What is an Electronic Speed Control (ESC), and how do I choose one for my motor?

The ESC functions as the main control between the motor and receiver. It works like this: You send a signal to the receiver via your transmitter, the receiver then sends a signal to the ESC, and the ESC responds by providing the appropriate level of electric power to the motor. The battery pack plugs directly into the ESC, and the ESC usually powers the motor plus the receiver and servos via a Battery Elimination Circuit (BEC). This eliminates the need for a separate battery to power your receiver and servos. This sounds a bit complicated, but it's really pretty simple once you see the components properly connected. The ESC has a standard servo type plug that connects to channel 3 of the receiver, and there are 3 wires on the end of the ESC that connect to the the 3 wires on the brushless motor. There are also 2 wires for the battery connection on the ESC.  When choosing an ESC for your motor, you need to know the expected amp draw with the propeller you intend to use. You want an ESC that is rated for a higher amp draw than what the motor/prop combination uses. I provide amp draw data for several commonly used propellers for each motor.

5. How do I choose a battery for my motor?

Most of the motors I sell provide their best level of performance with 3-cell Lithium Polymer (Lipo) batteries. You will need a battery that provides at least as many amps as the motor/prop combination requires. It is best to choose a battery that is rated for quite a few more amps than what you expect to draw. The batteries physical size and weight needs to be considered, along with the battery capacity (mah). Higher capacity batteries provide longer flight times, but are also bigger and heavier.

6. Why does my motor vibrate at high speed?

This problem is usually caused by a propeller that is out of balance. For the larger slow fly propellers such as the GWS EP1047 and EP1147, you can usually do a fairly good job of balancing by simply using a very small screwdriver inserted through the prop hub to determine which end of the prop is lightest, and then adding some 5 minute epoxy on that end. For high RPM applications, it's best to use a prop balancer in order to obtain the needed accuracy.

7. My prop doesn't fit!

Most of the motors that have a 3mm threaded shaft come with very small prop nuts that do not fit the hub recess on some of the GWS props. The prop will fit anyway - there are two ways to mount them. You can put a prop nut on the shaft first, followed by the prop, and then a washer, and then the other prop nut. When you tighten the prop nut securely, friction will prevent the prop from spinning on the shaft. The other method is to simply put the prop right up against the motor bell flange. You simply put the prop on (with no nut installed first), and then follow it with a washer and the securing prop nut. The whole front of the motor (the 'bell' or 'rotor') spins anyway, so this works fine. Just be certain the bell flange set screws are very tight - something you should check anyway.