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 wear-points in an outrunner
motor are the two bearings on the shaft.
2. What is a KV rating?
The KV rating of a motor stands for RPM per volt, and 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 3 cell 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 Emax CF2822
motors: The CF2822 1300kv motor spins an 8” prop to achieve 24 ounces of thrust,
while the CF2822 800kv motor spins an 11” prop to achieve the same. (both motors are
using a 3-cell Lipo).
3. How do I choose an outrunner motor for my plane?
You've probably heard about the 'watts per pound' method of choosing a motor for your
model, but it's a little complex and confusing to some people. We 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. We
provide thrust data for the motors that we 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. We 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 we 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, we 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 level of
performance is easy to achieve with today’s 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. You might think of the ESC as a carburetor for 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 the throttle channel 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. We 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 we sell provide their best level of performance with 3-cell Lithium
Polymer (Lipo) batteries, but some motors perform best with higher (or lower) voltage,
so it's important to choose the appropriate voltage battery. 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 the motor
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.
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