Electric motors provide a clean and reliable power
source for models. Selecting a motor is not much different than selecting an
engine when you look at the fundamental requirements.
The basic principles that make aircraft fly should be
used to select a power plant for the performance desired.
Power to weight ratio and flight speed envelope make up
the aircraft’s performance.
This translates to thrust to weight ratio and pitch
speed. Considering power and propeller choosing an electric
motor doesn’t have to be a challenge.
The power loading is the first parameter to consider.
Power to weight ratio for electric models is generally
quoted in watts per pound (W/Lb).
This is basically a performance gauge.
More power available per pound results better the
aircraft performance. Power
loading hold true for models all the way up to full scale.
Some examples are listed below.
1 Horsepower (HP) = 746 Watts (W)
( HP )
( Lb )
( W/Lb )
chart shows that high performance requires a high power
loading. The Cub flies sedately at 40W/Lb and so will a model
with this power loading.
Follow the chart below for model aircraft.
Loading ( W/Lb )
|3D or High
are many aircraft designed for internal combustion or glow
engines. We can
also approximate the required power based on engine
survey of available engines resulted in the chart below in
terms of watt per cubic inch displacement(W/ci)
Output ( W/Lb )
2 or 4 Cycle
the displacement of the recommended engine by the W/ci rating
to find the amount of power required of the electric motor
example; a .40 plain bearing engine will make .40ci x 1250W/ci
= 500W of power. Consider
that many .40 size trainer models weigh 6Lb, the power loading
works out to 83W/Lb.
Once the required power is known we can look at the
motor and battery. Look
at the motor specifications for power rating.
Choosing a motor with a power rating equal to the
requirement is safe. Erring
on the safe side one would choose a motor capable of slightly
more than is required. The
motor must be able to handle the required power and the torque
to turn a propeller appropriate for the model.
A high-speed model will work best with a small prop at
high RPM, while a slower model will work better with a larger
prop at lower RPM. Power
is a product of RPM and torque.
For a given amount of power one can have a lot of
torque and low RPM, or high RPM and low torque.
Getting a lot of both requires more power.
RPM and torque is related to the flight speed of the
model. Take a
look at the list of Himax motors.
Find the ones that meet the power requirements.
Now look again at the prop size.
Choose a motor that uses a prop size suitable for the
that the smaller props work best with models designed to fly
fast. On test
flights it is best to try several props in the suggested range
which draw an acceptable amount of current to see what works
difference of an inch in diameter or a couple inches in pitch
can change the way a model flies drastically.
If prop size doesn’t narrow the selection to one
motor consider gearboxes or the simplicity of an outrunner
motor direct drive. Also
consider the voltage required to see what kind of battery is
battery must be able to support the current requirement of the
motor with the chosen prop.
Check the current rating of the battery to make sure it
will not be overworked. The
ESC must also be sized to handle the voltage and current