Brushless motors tend to be
labeled in three different ways.
1. Can Size. This is based on a
system that Mabuchi Motor Company
came up with a while back for
brushed electric motors for R/C cars.
It has now made its way into the
brushless electric motors for cars.
Since some builders use these same
motors on R/C planes and helicopters,
you will see them used for multi-copters, as well.
Here are a few of the can sizes:
• 370 = 27 mm x 34 mm
• 380 = 27 mm x 38 mm
• 550 = 37 mm x 57 mm
• 700 = 40 mm x 71 mm
to spin slower and have more power
than inrunner motors. Most multi-copters are built using outrunners like
the ones shown in Figure 4.
2. Glow Engine Size. In this
case, the brushless motors are rated
at what an equivalent R/C gas glow
plug of a given displacement would
be. Since brushless motors don't really
have a displacement, it's only a
38 SERVO 11.2012
Propellers (like the ones shown in
Figure 6) come in many shapes and
sizes. Unlike R/C motors, R/C props
have a rating system that makes them
very easy to compare. An 8x4 prop
will have a diameter of 8" and a pitch
of 4". The pitch of a prop is the
theoretical distance in inches the prop
will move forward in one revolution. A
pitch of four means in a perfect
world, the prop would move forward
4" for each revolution.
A prop with an 8x6 rating will
give us much more thrust than a prop
with a rating of 8x4. The 8x4 prop will
also cause our motor to
draw more amps. Smaller
props are used on faster
motors, while larger props
are used on motors that
run slower. In addition to
the size and pitch, a prop
can also be designed to
run in a counter rotation.
These props will be
designated with an R
added to the rating
number. Sometimes a P (for pusher) is
used. For a quadcopter, you need two
normal props and two counterrotating props. This is done to
counteract the propeller torque effect.
Props have to be balanced. This is
something you must do to every prop
you place on your multi-rotor. In order
to balance a prop, you need a prop
balancer like the one in Figure 7.