Mechanical

Part 2 – Electric Motors

Aim

To gain a basic understanding of the principles of electric motors, specifically the types commonly used in small unmanned aircraft. Also, to gain a sound understanding of the considerations and concerns present when working with electric motors and the effects they can have on other on-board systems.

Objectives

At the end of this briefing you will be able to:

  • Discuss the BASIC principles of operation of an electric motor, of the types commonly used in small unmanned electric aircraft
  • Understand Voltage, Current and Resistance
  • Describe how to reverse the direction of rotation of the electric motors discussed
  • List and describe the considerations and concerns when working with electric motors and their associated equipment
  • Nominate the implications and possible effects  on other equipment on-board the aircraft when an electric motor is used

Electricity

Describing Electricity

Voltage: The potential difference in charge between two points in an electrical field (also called electromotive force).

Current: The rate at which electric charge flows past a point in a circuit. In other words, current is the rate of flow of electric charge.

Electrical impedance: The measure of the opposition that a circuit presents to a current when a voltage is applied. 

Resistance: Once of the forms of impedance.

The Water Analogy

Consider a water tank at height above the ground. At the bottom of this tank there is a hose.

The pressure at the end of the hose represents voltage.

The water in the tank represents charge. The more water in the tank, the higher the charge, the more pressure is measured at the end of the hose.

Resistance is similar to the size of the hose. A big hose means lots of water flows (LOW resistance). A small hose means only a little water flows (HIGH resistance).

Think of this tank as a battery! We store energy, and then release it. If we drain some of the water out of our tank the pressure created at the end of the hose will go down. This represents decreasing voltage.

The Formula

Voltage = Current x Resistance
Or: V = IR

A simple electric circuit.

If  the battery is at 1.5V, and the lamp draws 0.5A, the RESISTANCE is:

V=IR
Then,
R=V/I
Therefore,
R = 1.5/0.5 = 3 Ohms (Ω)


Electric Motors

How Does an Electric Motor Work?

The answer is a circuit and magnets!

The magnetic field of a flowing current.

The voltage across the battery will cause a current (“I”) to flow through the wire.

Current flowing through the wire will cause a magnetic field to be created around the wire.

The Permanent Magnet DC Motor

The heart of any motor! A Direct Current (DC) motor uses a current flowing in one direction to cause the motor to spin.

When a current is passed through a coil of wire it becomes an “electromagnet”!

The magnetic field around an electromagnet.

Using a simple wire will work, but not very well. Placing a (ferrite based) core in the coil concentrates and focuses the magnetic field.

This is very predictable, and the “Right Hand Rule” can be used to determine the “North pole” and the “South pole”.

Magnetic Attraction & Repulsion

Magnetic attraction.
Magnetic repulsion.

A Simple Motor (Magnetic Field)

A constant magnetic field to start as the basis of the motor.
A direct current to create a second magnetic field.
Putting the two together makes movement (the “Elastic Band Theory”)!

To get the DC motor to keep spinning (and not “elastic band”), then we need to switch the direction of the current!

This is usually done with brushes and a commutator.

Adding brushes & commutator (to the bit that spins).
A full DC motor that will keep rotating!

Three-Phase Motors

What is a Three-Phase Motor?

What if we don’t want to use brushes and a commutator (as these will wear out eventually)?

A three-phase motor uses alternating current in a specific manner to avoid needing brushes and a commutator!

The Basic Principles

A basic triangular sine wave (it’s a start!).
Creating a properly referenced three-phase sine wave.
A properly referenced three-phase sine wave.

Putting the Phases Together

The “red” phase.
The “yellow” phase.
The “blue” phase.

Making it Spin – Time Interval 1

The input signal at time interval 1.
The motor phases at timer interval 1.

Making it Spin – Time Interval 2

The input signal at time interval 2.
The motor phases at timer interval 2.

Making it Spin – Time Interval 3

The input signal at time interval 3.
The motor phases at timer interval 3.

Reversing the Spin Direction

To reverse any 3 phase motor swap any two phases!

Making it Spin in Reverse – Time Interval 1

The reverse input signal at time interval 1.
The reverse motor phases at timer interval 1.

Making it Spin in Reverse – Time Interval 2

The reverse input signal at time interval 2.
The reverse motor phases at timer interval 2.

Making it Spin in Reverse – Time Interval 3

The reverse input signal at time interval 3.
The reverse motor phases at timer interval 3.

Three-Phase Motors for Small Unmanned Aircraft

Brushless Electric Motors

Brushless electric motors come in several different physical configurations.

Different brushless electric motors.

In the inrunner configuration, the permanent magnets are part of the rotor. Three stator windings surround the rotor.

In the outrunner configuration the stator coils form the centre (core) of the motor, while the permanent magnets spin within an overhanging rotor which surrounds the core.

Common Terms – The “kV Rating”

The term “kV” – as generally used by hobbyists – refers to the so-called rpm constant of a motor.

Expressed in the most simple terms, this alludes to the number of revolutions per minute that the motor will develop when 1V (one Volt) is applied to the motor with no load attached.

A motor with a kV rating of 4600, and a 12V supply, 4600 x 12 = 55,200 RPM. This is the maximum RPM the motor can achieve under no load.

What does this mean?

  • A motor with a higher kV will have more top end speed, but not as much acceleration/torque
  • A motor with a lower kV will not be as fast, but will accelerate faster.
  • The KV figure allows a comparison of similar motors!

Common Terms – Motor Turns

Motor Turns is the same for either brushed motors or brushless motors. The wordturns” stands for the number of turns of wire around each of the motor’s rotor poles.

  • The higher the number of wirings/turns means less top speed, but higher acceleration/torque.
  • The lower the number of turns equals higher top end speed and lower torque/acceleration.

For example, a motor with a turn rating of 5.5 will have less acceleration/torque but higher top speed than a motor with a 12 turn rating.

Common Terms – Current Rating (Amps)

The max current rating is the maximum amount of current that a motor is able to handle safely. This current is measured in Amps. The continuous current rating of a motor is the Amps that a motor can handle safely over a long period of time.

It is a great idea to find an ESC that has a current rating that is higher than your motor by at least 20%. It will be a good safety cushion.

The estimated current rating of a motor is usually on the factory specs sheet, however other factors affect the actual current that a motor will draw. Such things typically include the kV rating of the motor, the battery voltage, how heavy the aircraft is, prop size. The harder a motor needs to work to reach it’s top speed, the higher the current will be.

Common Terms – Watts

Watts are the power rating of the Motor.

The simple math here is Amps x Volts = Watts.

The motor should have a watt rating on its specification sheet, e.g. “180W”. This is the amount of power that the motor should produce safely. Running anything over this rating could damage the motor, especially over an extended period.

Common Terms – Motor Efficiency

The efficiency of a motor is generally a function of the quality of the motor. A 70% efficient motor produces 70% power and 30% heat. A 85% efficient motor produces 85% power and 15% heat.

If the battery is supplying the Motor/ESC combination with 180 watts, an 85% efficient system will produce 153 watts (85%) power, with 27watts of heat. (27 Watts of heat is capable of melting solder).

A Typical Brushless Motor

A Turnigy SK3 1340kV Brushless Motor

Motor Specifications:

  • Turns: 24T
  • Voltage: 2~3S LiPo (7.4~11.1V, max: 12.6V)
  • RPM/V: 1340kv
  • Internal resistance: 0.052 Ohm
  • Max Loading: 28A
  • Max Power: 375W
  • Shaft Dia: 4.0mm
  • Bolt holes: 25mm
  • Bolt thread: M3
  • Weight: 76g
  • Motor Plug: 3.5mm Bullet Connector

Summary

You should now be able to:

  • Discuss the basic principles of operation of an electric motor, of the types commonly used in small unmanned electric aircraft.
  • Describe how to reverse the direction of rotation of the electric motors discussed.
  • List and describe the considerations and concerns when working with electric motors and their associated equipment.
  • Nominate the implications and possible effects  on other equipment on-board the aircraft when an electric motor is used.

Part 1 – Propellers

Aim

To gain a basic understanding of the principles of propeller aerodynamics, and how they are used to create thrust for unmanned aircraft.

Objectives

At the end of this briefing you will be able to:

  • Describe the naming convention for propellers
  • Discuss the meaning of the pitch of a propeller
  • Discuss propeller pitch and performance

Propeller Design

Propellers in Simple Terms

The aircraft propeller consists of two or more blades and a central hub to which the blades are attached.

Each blade of an aircraft propeller is essentially a rotating wing.

As a result of their construction, the propeller blades are like aerofoils and produce forces that create the thrust to pull, or push, the aircraft through the air.

The engine furnishes the power needed to rotate the propeller blades through the air at high speeds, and the propeller transforms the rotary power of the engine into forward thrust.

Propellers for Unmanned Aircraft

Examples of different sizes of propellers for unmanned aircraft.

Pitch

Pitch is the displacement a propeller makes in a complete spin of 360° degrees.

Example of how propeller pitch effects the translational motion.

This means that if we have a propeller of 40” pitch it will advance 40 inches for every complete spin as long as this is made in a solid surface; in a liquid environment, the propeller will obviously slide with less displacement.

The pitch concept is not exclusive to propellers, other mechanical devices like screws also use it. For instance, a screw with 10 mm of pitch will advance 10 mm for every complete turn of the screwdriver.

Propeller Blade Angle

Relative wind speed affecting the angle of attack of a propeller blade.

Fixed Pitch

Fixed-pitch and ground-adjustable propellers are designed for best efficiency at one rotation and forward speed.

They are designed for a given aircraft and engine combination. Since the efficiency of any machine is the ratio of the useful power output to the actual power input, propeller efficiency is the ratio of thrust horsepower to brake horsepower. Propeller efficiency varies from 50 to 87 percent, depending on how much the propeller “slips.”

Propeller slip is the difference between the geometric pitch of the propeller and its effective pitch.

Geometric pitch is the theoretical distance a propeller should advance in one revolution; effective pitch is the distance it actually advances.

The Twist on a Propeller

The reason a propeller is “twisted” is that the outer parts of the propeller blades, like all things that turn about a central point, travel faster than the portions near the hub.

Propeller blade twist.

If the blades had the same geometric pitch throughout their lengths, portions near the hub could have negative AOAs while the propeller tips would be stalled at cruise speed.

Propeller Pitch and Efficiency

The pitch of the propeller is generally chosen to provide the speed characteristic of the aircraft for the purpose required.

Increasing the blade pitch increases the blade drag, and decreasing the blade pitch decreases the blade drag.

A larger (coarser) blade angle, for a given RPM, will adsorb more power and require more torque to turn it at the requested RPM.

Usually 1° to 4° provides the most efficient lift/drag ratio, but in flight the propeller AOA of a fixed-pitch propeller varies — normally from 0° to 15°. This variation is caused by changes in the relative airstream, which in turn results from changes in aircraft speed. Thus, propeller AOA is the product of two motions: propeller rotation about its axis and its forward motion.

Propeller Designation

Propellers are designated by two numbers:

  • Diameter
  • Pitch

A propeller designated as a 12-6 propeller is therefore:

  • 12″ in diameter
  • 6″ of pitch.

…where pitch is the distance a propeller will move forward in one revolution in a perfect fluid (which air is not).

Theoretically a 6″ pitch will move forward 6″ with each complete (360°) revolution of the propeller.

How Pitch Affects Propulsion

The properties of a propeller with high pitch:

  • High speed flight
  • Poor Acceleration
  • Poor Climb
  • Can be difficult to slow down for landing

The properties of a propeller with low pitch:

  • Low speed flight
  • Good Acceleration
  • Good Climb
  • Finer speed control throughout throttle range – particularly at low throttle settings

Pitch in Simple Terms

An easy way grasp the concept of propeller pitch is to draw a parallel to the gearing in a car.

Low pitch propellers = low gear in your car.

It will get you up hills well but will not take you any where fast.

High pitch propellers = Beginning your drive in fifth gear.

It will take forever to accelerate to speed but the plane is cruising when it gets there.


Propeller Performance

Propeller Balance

An out of balance propeller can be the cause of a lot of problems.  Some of these problems manifest as:

  • Prevents the engine from developing full power.
  • Causes excessive vibration through the airframe.
  • Causes excessive vibration through on-board electronics, leading to premature failure.
  • Causes fuel foaming which can cause the engine to run ‘lean’.
    • The result is the engine loosing performance and power, to stall, or just not run smoothly.

This is all amplified in a smaller aircraft.

Trimming Balance

Before you attempt to balance a propeller, be sure to clean it.

Most propellers are close to being in balance when purchased, so they should only need a small amount of work to bring them into perfect balance. 

If the propeller is severely out of balance – return it because too much material would have to be removed which would significantly change the shape of the blade.

Propeller Balancing Device

If one blade is heavier than the other, then the usual method to bring the propeller into balance is to remove material from the heavy blade using sandpaper.

Trimming Heavy Propellers

Do not trim the tip of the heavy blade!

Although the blade may balance statically, it will not be balanced when it starts to spin, because of unequal mass distribution.  Material is generally removed from the face (front) of the propeller or from the back of the propeller.

Generally all that is required is to sand the face a little.

Propeller Tracking

Occasionally you may encounter a propeller that does not track properly.  Either the mounting hole is off-centre or the hub is not square to the plane of rotation. 

In either case, if the propeller is noticeably out of track you should not use it.

It is easy to see if the propeller is tracking correctly.  Stand back for safety and look at the propeller from the side and from the rear.

From the side, both tips should be clearly visible in the same line.  If you see two lines, then the hub is not square to the plane of rotation.

The recommended procedure is to return the propeller – it is defective, and may require too much modification to ‘repair’.


Selecting Motors & Propellers

Propeller Selection

Beware the “hobby mentality

The propeller should be chosen to match the aircraft — not the engine. An appropriate engine should then be chosen! 

Consider this. Mounting a racing propeller to a scale WWI aircraft will severely limit the model as an early warbird has so much airframe drag that the propeller will never be able to deliver it’s full potential, with the result that the aircraft will be a sluggish flyer at best.

Additionally, using too ‘slow’ a propeller – one with low pitch – on a model intended to go fast may prevent the aircraft from gaining enough speed to fly at all. A basic mistake is finding a propeller that works great – with a certain engine in a certain aircraft – and then imposing that propeller on that engine regardless of the aircraft!

Number of Blades

With ‘model’ sized aircraft the most efficient propellers are two bladed.  Because the diameter of our propellers tend to be small, multiple blade propellers disturb the air that the trailing blade is entering tending to make them less efficient.

Generally, with smaller aircraft, for best overall performance, it is recommended to utilise 2-blade propellers.

Matching Propeller and Motor

Propeller-Motor Thrust Output

The Reality of Motor and Propeller Selection

Test & Verification Procedure (Actual procedure – V-TOL Aerospace):

  1. The propeller was fitted to the motor, which was mounted on the scales to provide down thrust, pushing against the scales.
  2. The current meter was setup in line with the battery and Electronic Speed Controller to measure current drawn by the motor.
  3. The servo checker was setup to control the motor through the ESC.
  4. A camera was mounted such that it could see the current meter and the display on the scales.
  5. For safety the video display and servo checker were operated from behind a safety barrier.

Test Results

ThrottleVoltsAmpsThrust (g)
μs%9×4.59×69×4.59×69×4.59×6
1000012.5712.570000
11001012.5512.550.30.36560
12002012.5212.491.61.7165150
13003012.4712.423.13.7285290
14004012.2712.216.27.6480500
15005012.1512.0620.312.5680705
16006011.9711.9116.119.6900910
17007011.7811.7322.825.811401085
18008011.5211.4429.434.513301240
19009011.3911.0638.139.515251345
200010011.1311.0138.944.315501390
Collected thrust data for differently sized propellers
ThrustVoltsAmpsWatts
(g)8×69×69×4.58×69×69×4.58×69×69×4.5
10012.412.512.51.61.10.919.8413.7511.25
20012.412.412.53.42.22.142.1627.2826.25
30012.312.412.45.53.83.567.6547.1243.4
40012.212.412.38.75.54.9106.1468.260.27
50012.112.312.312.77.86.7153.6795.9482.41
6001212.212.216.89.98.7201.6120.78106.14
70011.812.112.1221310.7259.6157.3129.47
80011.7121226.215.713.4306.54188.4160.8
90011.711.911.934.118.415.5398.97218.96184.45
100011.611.711.739.223.418.1454.72273.78211.77
110011.611.627.622.6320.16262.16
120011.411.633.324.5379.62284.2
130011.311.437.828.9427.14329.46
140011.211.444.232.4495.04369.36
150011.238.1426.72
Collected power draw data for different thrust values

Given the test data, and noting that cruise speed for the condor aircraft is attained at 8A/100W with a 9 x 4.5 propeller, it can be seen that the most efficient propeller to produce the required amount of thrust is the 9 x 4.5 propeller.


Safety

Hazards of Propellers

Safety must always be first and foremost in your thoughts when handling unmanned aircraft, or components from unmanned aircraft.

  • Propellers spin at relatively high speed.
  • Propellers are made from hard material.

This is why should we pay special attention to the propeller when completing an inspection of an aircraft!

What happens when things go wrong?

Example of a propeller that was broken while spinning.
A broken propeller that was found after an incident during an indoor test.

All that stopped the blade segment from passing all the way through the partition was the fabric on the other side!


Summary

You should now be able to:

  • Describe the naming convention for propellers.
  • Discuss the meaning of the pitch of a propeller.
  • Discuss propeller pitch and performance.