Propulsion System

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Overview

The UAV's propulsion system provides the necessary power to propel the aircraft for forward flight or hover. The main propulsion system types are electric systems and gas systems. Both systems can be used to drive propellers or ducted fans. Each system has its own advantages and disadvantages and is best suited for a specific UAV application.

Types

Electric Propulsion Systems

Electric propulsion systems store energy using a battery, and use electric motors to drive the propellers or other propulsion devices. Electric systems are easier to operate and produce less noise than gas systems. Multicopter UAVs use electric power systems almost exclusively for various reasons. One of the reasons is that an electric system's thrust can be controlled more precisely and responds faster to throttle input. This is crucial to provide the differential thrust control required by this type of UAV. Also electric systems are more reliable, minimizing the possibility of crash due to motor shutdown or failure. The main drawback of electric systems is their flight time. Although electric systems are more efficient than fuel systems, batteries can hold less energy per weight compared to fuel, and thus electric systems provide significantly less flight time. Also, batteries require significant recharging time when depleted, while fuel tanks can be refueled very quickly.

Fuel Propulsion Systems

Gas propulsion systems store energy in the form of fuel, and use an engine to drive the propellers or other propulsion devices. Various types of engines exist for UAVs, including two-stroke or four-stroke internal combustion engines running on nitro or gasoline fuels. In general, fuel systems are more difficult to setup, use and maintain than electric systems. They are also noisier, less reliable, and cannot be controlled as precisely as electric systems. However, they can provide significantly longer flight times, and can be quickly refueled. For these reasons, fuel power systems are often used in airplanes and helicopter UAVs that require longer flight times.

Selection

The following summarizes the main advantages and disadvantages of each propulsion system type.

UAV propulsion system type comparison

Components

Brushless Electric Motor

Brushless electric motors are the most common type of electric motor used in present-day UAVs. While brushed motors use brushes to mechanically switch the phase of the windings in order to keep the motor running, brushless motors require the use of an Electronic Speed Controller (ESC) board to do this task. However, they provide various advantages over traditional brushed motors including better thrust-to-weight ratios, higher efficiencies, increased reliability, higher torque per weight, and lower noise. More recently, brushless motors are also being used for camera gimbals to provide inexpensive and effective stabilization of camera footage.

Lithium Polymer Battery

Lithium polymer (also known as Li-Poly or Lipo) batteries are the most common type of battery used in present-day UAVs. They are commonly used as the main power source for the UAV's propulsion system, and/or as a secondary power source for electronic subsystems. Their high energy density and discharge rates make them ideal for use in flying vehicles. However, they can be easily damaged or even pose fire hazards if not used or maintained properly.

Internal Combustion Engine

An internal combustion engine produces power through the combustion of a fuel and air mixture in a combustion chamber. The motor's rotating shaft displaces a piston which compresses the fuel and air mixture. The mixture is then ignited, producing high-pressure gases which push back the piston, rotating the shaft. Popular types include the two and four-stroke engines, and popular fuel types include nitro and gasoline fuels. Although internal combustions engines are not as reliable as electric motors, they provide longer flight times and can be quickly refueled.

Propeller

Propellers are one of the devices used to produce thrust in an UAV. Like a fan, propellers spin a set of blades around an axis to generate a stream of air which in turn thrusts the propeller and aircraft forward. The propeller geometry is specified by its twist and chord distribution, as well as its airfoil distribution. Different geometries provide varying performance characteristics such as maximum thrust, efficiency, and maximum speed.

Ducted Fan

A ducted fan is a propulsion device composed of a specially designed propeller placed inside a cylindrical duct, which increases the efficiency of the propeller. Ducted fan units in UAVs are usually of smaller diameter than propeller units, and operate at much higher rotational speeds. The result is that ducted fan units generally produce less thrust than propellers and are heavier and are less efficient, but they allow for significantly higher cruise speeds. Electric motors are usually used to drive ducted fan units.

Setup

The following schematics show how the most common types of propulsion systems are set up in a UAV.

Electric Propulsion Systems

Single-Motor Electric Propulsion System

Electric propulsion system schematic

The main components in an UAV electric propulsion system are a battery, a brushless motor and the speed controller. The battery is connected to the speed controller, which is connected to the motor and receiver. The receiver receives the control signal from the transmitter in the ground, and sends the signal to the speed controller, which varies the speed of the motor. Different cable types are used for each connection. The connection from the battery to the speed controller is made by two wires, typically two thick black and red wires. Three cables connect the speed controller to the brushless motor. These cables can be connected in any order, and any two can be switched if the motor is rotating in the wrong direction. The speed controller connects to the receiver through a standard 'servo cable', which has three wires: ground, voltage and signal.


Multi-Motor Electric Propulsion System

Multi-motor electric propulsion system schematic

Multi-motor configurations like multicopters require more than one electric motor. The setup is similar to the single motor system, except that a power distribution board is used to connect multiple speed controllers to one battery. In the case of multicopter UAV's, the speed controllers are connected to a flight controller board which controls the motor speeds based on user input an stability algorithms.


Fuel Propulsion Systems

Internal Combustion Engine Propulsion System

Fuel propulsion system schematic

The main components of an internal combustion engine propulsion system for a UAV are the engine, fuel tank and throttle servo. One or more fuel tanks can be connected to the engine to provide fuel. The servo controls the throttle lever of the engine, and receives the control signal from the receiver. Since the receiver is not connected to any speed controller to provide power, a battery must be connected to the receiver to power it. The receiver provides power to the servo through the same signal cable. Since the battery only needs to power the receiver and servo, small batteries of around 1600 mah and 4 to 6 Volts are used.