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Electric Flight Glossary

& discussion of electric flight terms


Part 3 - L to P

by Andrew Gibbs

There are many technical terms and words relating to model aircraft in general, and electric flight in particular. The aim of this detailed glossary is therefore to offer an accurate and detailed yet concise explanation for any of the terms the electric modeller is likely to come across. Let us know if a term you'd like to see covered is not discussed here!


White Diamond by Andrew Gibbs


LiPo/Li-Poly/Lithium battery
This is a modern type of battery and it is in almost universal use for flying models. The nominal voltage is 3.7 Volts per cell (Vpc). The absolute maximum safe battery voltage is 4.2 Volts per cell, and the minimum cell voltage before serious damage results is 3.0 Volts per cell, under no load conditions.

LiPo batteries are light in weight, are capable of delivering a high current and are able to store relatively large amounts of energy. As a result, the combination of LiPo batteries and brushless motors have transformed electric flight. However, the disadvantages of this battery type are that they are relatively fragile and can burst into flames if abused, damaged in a crash or over charged. LiPo batteries are an excellent source of power for model aircraft provided they are used properly.

LiPo batteries are examined in more detail in this Gibbs Guide.

Li-Ion/Lithium Ion
This is another type of lithium-based cell, but these are rarely used these days for electric power models. Maximum charged voltage is 4.1 Volts per cell. They have a hard case, and are therefore are physically more durable than LiPo cells. They are also more resistant to electrical abuse such as overcharging. However, they are heavier than LiPo types, and do not have such a high capacity for a given weight.

several lipo batteries

Lithium Polymer (LiPo) batteries have transformed electric flight in recent years.

lipo battery fire with flame

LiPo batteries can catch fire if overcharged, as seen here. (Photo courtesy of Simon Sheldon)


LVC (Low Voltage Cut - Off)
This is another name given to the PCO function. See the entry for 'Power Cut Off'.

The term mAh is an abbreviation for milli-Amp-hour, which is used in battery terminology as unit of measurement of battery capacity. 1,000 mAh equals 1 Amp hour. For example a fully charged 3,000 mAh battery can supply 3,000 mA (the same as 3 Amps) for one hour before it is exhausted.

Motor losses
A well designed electric motor can achieve an impressive efficiency. However, no electric motor is 100% efficient as there will inevitably be losses of energy within the motor. There are four causes of lost energy within an electric motor, which are termed copper loss, iron loss, air losses and friction losses. Each of these losses is detailed separately within this glossary.

Nickel Cadmium (Nicad) cells
Nicad cells have a nominal voltage of 1.2 Volts per cell. Compared to LiPo cells, nicads are heavy, and have low capacity for a given weight. However they are extremely durable. For many years nicads were the only practical cell for use with electric power aircraft and were in widespread use both as flight packs and for RC power. However, because of the toxicity of cadmium, nicad cells are no longer available. However, due to their durability there are still a substantial number of nicad packs in use in modeling applications, especially for RC system power supply. For reasons of safety, all batteries in safety critical applications such as for RC system power supply duty should be regularly inspected and tested before use.

nicad receiver battery

Nicad batteries are unfortunately no longer available for modeling use. This is a pity as they were more durable than hydrides, and had much to recommend them.

opto isolating ESC for model aircraft

Opto isolating ESCs cannot offer a BEC function. Models equipped with opto isolating ESCs therefore need a separate power supply for the RC system.


The abbreviation is sometimes found in modeling discussions. It simply stands for New Old Stock i.e. an unused, but old item.

Opto isolator
Electric motors and ESCs inevitable generate an amount of electrical noise. Because all the components of the power system and the RC system are linked, there is a tendency for this noise to be fed to the receiver, compromising its performance.

To combat this problem, some ESCs use an opto isolator chip. Although the ESC is still connected to the receiver, the opto isolator feature allows the receiver to be electrically isolated form the ESC. Instead, the communication between the receiver and ESC is accomplished optically, using pulses of light in the manner of a fibre optic cable. This feature greatly reduces the amount of electrical noise which reaches the receiver.

Because there is no direct electrical connection between the ESC and the receiver, the ESC cannot supply power to the RC using a BEC function. The RC system must therefore be supplied by a separate battery dedicated to RC system power.

brushless outrunner electric motor for model airplane

The brushless outrunner motor is very popular for electric power model aircraft.

small ESC with BEC and PCO for model airplane

All ESCs have a PCO function. This cuts power to the motor at an appropriate point to prevent the battery from becoming excessively discharged.


Outrunner Motors
The outrunner motor is a design of brushless motor in which the magnets are fixed to a rotating outer case. The windings are located at the centre of the motor, arranged around an inner (fixed) former. The magnets run outside of these windings, hence the name of this motor type. The propeller driver is attached to the rotating outer case.

Outrunners have a relatively large diameter, and this characteristic naturally gives them a high torque, making them suitable to drive relatively large diameter, efficient props. This is the principal reason they are the usual choice for the majority of electric RC sport and scale models, as well as indoor 3D models and park fliers.

You can find a lot more information about brushless motors in the guide Mastering Motors, available here

PCO (Power Cut - Off)
PCO is the abbreviation given to the Power Cut Off function of an ESC. The PCO detects when the battery voltage falls to a user-defined threshold, and initiates a reduction or shut down of power to the motor to prevent the battery from falling to an unacceptably low voltage. If this were permitted, the battery may become damaged and/or, when RC system power is derived from the flight battery via a BEC, control of the model could be lost.

Permanent Magnets
The permanent magnets used in brushed and brushless motors have varying degrees of magnetic strength and temperature resistance. Ferrite magnets are weakest, while Samarium Cobalt and Neodymium types are substantially stronger. Permanent magnets generally lose magnetic strength as they are heated, but recover as they cool down again. However, there is a temperature beyond which the magnet will suffer a permanent loss of its magnetism.

Partially de magnetized motors cause the motor to draw significantly more current and generate more heat accompanied by a reduced power output. For these reasons, care must be taken not to allow electric motors to become excessively hot in use. In all cases, it is worth remembering that the outside of the motor case will be a lot cooler than the magnets inside, so a good rule of thumb is to make sure that the outside of the motor does not exceed 80-100C. The temperature of motors and other components can easily be measured after flight by means of a simple, cheap infra red thermometer.

The pitch of a propeller is the forward distance the prop will move, if there is no resistance to movement. For example, a prop with a 6 inch pitch will theoretically move forward 6 inches in one revolution.

Pitch to diameter ratio (p/d ratio)
The pitch to diameter ratio of a propeller is simply the ratio of the prop’s pitch to its diameter. For example, to find the p/d ratio a prop with a 12 inch diameter and a 6 inch pitch (12 x 6) the calculation would be: p/d = pitch/diameter = 6/12 = 0.5. A 12 x 6 prop therefore has a pitch to diameter ratio of 0.5. Similarly, a 10 x 8 has a p/d ratio of 0.8, a 8 x 6 has a ratio of 0.75 and so on.

The p/d ratio is of important significance when matching a prop to a model. Fast models should have props with a high p/d ratio e.g. 10 x 8, while slow models are best off with a much lower ratio, for example a 10 x 5.

Pitch speed
The pitch speed of a propeller is the speed it would travel forwards if there was no resistance to its movement, in other words, if no model was attached to it. The pitch speed is found by multiplying the prop pitch by the rpm:

Pitch speed = Prop pitch x Prop RPM

Pitch speed gives a useful indication of how well suited a particular power system is to a particular airframe. The pitch speed will always be higher than the airspeed of the model.

You can find a lot more information about props in the guide More Than Motors, available here

Power (Watts)
Power is a measure of the rate at which work is done. Power can be either mechanical or electrical. Mechanical power is usually measured in horsepower. The definition of mechanical power is Power (hp) = Torque x RPM.

Electrical power is measured in Watts, or for larger amounts of power the kilowatt. A 0.40 capacity model engine typically develops about 1 hp, which is equivalent to 746W.

The power of an electric model aircraft is found by multiplying the voltage and the current:

Power (Watts) = Voltage x Current (Amps)

The propeller, or prop for short converts the rotational energy provided by the motor into thrust. The rotating prop unavoidably also imparts a slight swirling motion to the airflow. Larger props are more efficient than smaller ones, and small increases in propeller diameter can yield a relatively large increase in efficiency.

Prop driver
A prop driver is a device fitted to the output shaft of a motor which is designed to allow easy attachment of the propeller.

Prop drivers for outdoor models are usually of the collet type. These work well, giving a secure, solid fit. Occasionally a set screw type will be encountered, but these are best avoided. The prop driver must be correctly sized for the motor shaft diameter in question. Common motor shaft diameters are: 1.5mm, 2mm, 3mm, 3.17mm (1/8?), 3.2mm, 5mm and 6mm.

Prop saver
Low powered indoor models sometimes use a different type of prop adapter, called a prop saver. These consist of a metal adapter, designed to allow the prop to be held on with a rubber band. This arrangement, which is only suitable for very low power models, gives the prop an increased measure of crash resistance.

Pusher prop
A pusher prop is one which is located behind the motor driving it. The prop is said to push the model, rather than pulling it as in a tractor installation. In spite of this terminology, reality the propeller works in exactly the same way regardless of whether it is in a tractor or pusher configuration.


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