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Choosing a Motor

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Choosing a Motor

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How to Choose a Brushless Motor for an Airplane

  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) 

Airplane Power ( HP ) Weight ( Lb ) Power Loading
( W/Lb )
Piper Cub 65 1220 40
B-17 4800 65000 55
Pitts Special 260 1626 120
Spitfire IV 1440 5000 215

The 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. 

Performance Power Loading ( W/Lb )
Mild ROG 50
Mild Aerobatics 75
Aggressive Aerobatics 100
3D or High Speed 125-150
Competition 300

There are many aircraft designed for internal combustion or glow engines.  We can also approximate the required power based on engine displacement.  A survey of available engines resulted in the chart below in terms of watt per cubic inch displacement(W/ci) 

Engine Type  Power Output ( W/Lb )
Sport 2 or 4 Cycle 1250
BB 2 Cycle 1500
High Perf. 2C/4C 1800
Racing, Ducted Fan 4000

Multiply the displacement of the recommended engine by the W/ci rating to find the amount of power required of the electric motor system.  For 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 model.  Assume 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 best.  A 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 needed.  The 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 requirements.

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