AVI30316_MechanicalElectrical

Part 4 – High-Performance Batteries

Aim

To gain a basic understanding of the requirements and use of high-performance batteries commonly used to power small unmanned aircraft.

Objectives

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

  • Discuss the characteristics of a lithium polymer cells.
  • Consider the vulnerabilities and dangers associated with lithium polymer batteries.
  • Explore the measures we can employ to maximise service life.
  • Discuss battery selection methodologies explore ‘safe’ disposal of depleted batteries.

Lithium Polymer Batteries

General Construction

DO NOT ATTEMPT TO DISASSEMBLE A BATTERY!

Lithium Polymer (Li-Po) batteries are a type of high-performance (high current output) power storage devices. This comes at a cost, as they have a much lower power density (i.e. lower capacity, low Ah/mAh) compared to batteries of a similar size.

A typical construction for a lithium polymer battery

Typical Cell

Think of a battery cell as a “ideal” cell in series with a resistance.

A battery broken down into it’s internal components.

Internal Resistance & Voltage Drop

Voltage drop of the internals of a battery.

Effect of Internal Resistance – Charging

Internal resistance during charging.

Result: Cell appears to be more fully charged than it actually is.

Effect of Internal Resistance – Discharging

Internal resistance during discharging.

Result: Cell appears more discharged than it actually is!

External Connections

Battery connectors.
Internal view of the balance connector wiring.

Internal Construction

Inside a Li-Po “foil pouch” cell you will find a long piece of very thin plastic film – the polymer!

Laminated onto the polymer are the thin lithium carbon coated aluminium & copper anode & cathode electrodes. These are laminated in an alternating pattern on the front and back side of the polymer separator film.

Everything will be saturated with a greasy solvent based organic electrolyte.

An “un-rolled” lithium polymer battery cell.

The long internal polymer film (which is over 7 feet long in the case of a 5000 mAh cell), is folded accordion style back and forth upon itself. The entire folded material is then heat sealed into the foil pouch, along with the gelled electrolyte.

As a matter of interest, the gelled electrolyte has a very sweet solvent smell, not dissimilar to nail polish remover, or acetone.

Charge & Discharge Cycle Chart

Typical charge and discharge cycle charts for Li-Po batteries at room temperature (20°C). These particular charts are based on Sanyo data.

Lithium polymer discharge cycle chart.

The cell voltage is reasonably constant out to about the 20% capacity state, and then falls off precipitously, similar to NiCad and NiMH cells.

Lithium-Based Battery Charge Regime

Lithium-based battery charge regime.

Battery Management

Service Life as a User

Fact: We are battery users, not battery manufacturers.

This means that we can only use the battery in accordance with manufactures directions!

From the user perspective, battery life can usually only be extended by preventing or reducing the cause of unwanted parasitic chemical effects which occur in the cells.

Shelf life & Cycle Life

Fact: Battery performance deteriorates over time regardless of whether the battery is used or not.

Shelf Life refers to how long a battery can remain on the shelf – in other words not being used – before it is no longer serviceable.

Cycle Life refers to the number of charge/discharge cycles a battery can be subjected to before it is no longer serviceable.

Cycle Life

The effects of voltage and temperature on cell failures tend to be immediately apparent, but their effect on cycle life is less obvious. We have seen above that excursions outside of the recommended operating window can cause irreversible capacity loss in the cells.

The cumulative effect of these digressions is like having a progressively debilitating disease which affects the life time of the cell. In the worst case, this can cause sudden death if you overstep the mark.

At about 15 ºC cycle life will be progressively reduced by working at lower temperatures. Operating slightly above 50 ºC also reduces cycle life but by 70 ºC the threat is thermal runaway.

The battery thermal management system must be designed keep the cell operating within its sweet spot at all times to avoid premature wear out of the cells.

Beware: The cycle life quoted in manufacturers’ specification sheets normally assumes operating at room temperature.

Li-Po Battery Service Life

Fact: Batteries have a finite service life.

This is due to the occurrence of unwanted chemical or physical changes to, or the loss of the active materials from which they were manufactured.

(Otherwise they would last for ever!)

Some Li-Po batteries have been rated by the company for 300+ cycles, but those numbers are usually in a ‘perfect’ setting. It is reasonable to expect around 200-250 cycles from any good/decent Li-Po manufacturer.

Battery life greatly depends on how the battery is used and stored. There are well documented cases of over 300 cycles with little drop off in measured performance (voltage under load, IR, etc, although capacity will tend to drop some with time).

Inappropriate care can make that number just about as low as you want. At the very least it will accelerate the capacity loss.

Limit the amount of discharge, store properly and routinely, etc.

Managing Battery Stock

Batteries – like all other aircraft components – are a controlled item. Batteries are a critical component, and come under the controlled maintenance program.

As a minimum there must be:

  • Battery pack numbering and identification:
    • Serial Number
    • Voltage and Capacity
    • Maximum Charge Rate
  • A Record or Log for each battery:
    • The Record or Log must be accessible
    • The Record or Log must be useable
    • The Record or Log must be sensible

Other useful information that could be recorded (where appropriate):

  • Record (Log) Usage Information:
    • Date / Time of Day / Flight Duration
    • Start capacity – end capacity = consumption in mAh
  • Record (Log) Charge Information:
    • Start time and finish time
    • Charge rate
    • End Capacity
  • Add operational information to Record (Log):
    • Ambient temperature
    • Maximum altitude (calculate operating temperature)
  • Installation considerations:
    • Vibration and Shock Absorption
    • Access
    • Identification
    • Cooling
    • Mounting
  • Storage considerations:
    • Charge as per manufacturers instructions
    • Identify and isolate from other equipment as necessary

Alphanumeric Designation

The alphanumeric designation for battery cells – as used in radio control applications – refers to both the number of cells in the battery pack, and the way in which those cells are interconnected.

Series (S): Denotes batteries connected in series. For example, 3S denotes a three-cell battery wired in series, thus multiplying the voltage to three times that of a single cell.

Parallel (P): Denotes batteries connected in parallel. For example, 2P would be two batteries connected in parallel to give twice the current capacity, but the voltage of one cell only.

Thus, a 3S2P battery contains 6 cells, and has three times the voltage and twice the capacity of any individual cell.

All cells should be identical if they are to be manufactured into a single “battery pack“.


Specific Notes on Managing Lithium Polymer Batteries

Main Points of Li-Po Batteries

Lithium ion polymer batteries, or lithium polymer batteries (abbreviated “Li-Po“) are rechargeable batteries which have technologically evolved from lithium ion batteries. Li-Po batteries contain a dry polymer suspended in an electrolytic gel (no metal-based conductive elements).

Li-Po Batteries are extremely volatile. The chemical compounds within the battery are flammable, and if not cared for correctly the battery can catch fire, or even explode.

Because of the volatile nature of Li-Po batteries, chargers specifically designed for use with Li-Po batteries must be used. Failure to do so incurs the very real risk of explosion and/or fire.

Battery output leads must never be allowed to short together, as immediate damage to the Li-Po battery will result.

Li-Po batteries must never be allowed to discharge below a certain point. It is therefore critical that low voltage cut-out devices are employed to protect the battery.

A battery that has been involved in a severe crash should be isolated for a reasonable amount of time (a few hours). It should never be immediately loaded into a vehicle, or transported from site until it is confirmed safe. This is because of the elevated risk of fire.

Li-Po in Comparison to Lithium-Ion

Lithium-Ion Batteries began their development in 1912, but did not become popular until they were adopted by Sony in 1991.

Lithium Ion Batteries have high energy-densities and cost less than lithium-polymer batteries.

Lithium-Ion batteries:

  • Do not require priming when first used
  • Have a low self-discharge
  • Suffer from aging – even when not in use
  • Usually come in a rigid plastic case
  • Have a nominal voltage of 3.7V
  • Don’t like near-freezing temperatures

Charged initially with a constant current, has a gradually increasing voltage. When the voltage limit per cell is reached, then the charger switches to a constant voltage with a gradually decreasing current flow.

Safe Transport

There have been very few cases of batteries suddenly exploding when they are not being used, abused or charged (i.e. during transport and storage).

Lithium batteries are commonly air-freighted protected by a few layers of bubble wrap and small versions are carried around in mobile phones. Should it be required to ship a battery pack, care must be taken when packing to ensure that it cannot be physically damaged.

It has been reported that some Li-Po battery fires have been caused when a dog was attracted to the smell of a lithium battery – and bit it.

Charging & Safety

The majority of lithium battery fires occur during charging, therefore charging should only take place where a fire cannot spread. Fire safety must always be a prime concern when working with Li-Po batteries.

Additionally it is most strongly recommended that Li-Po batteries are not charged inside a vehicle, and in particular a moving vehicle.

Charging in an purpose built and approved fire/explosion resistant bag is recommended. Alternatively a heat-resistant ceramic container with a loose fitting lid as flames, smoke and gas are released should the battery “vent”. Metal containers can be used, but ensure the charging wires cannot be cut or shorted.

Keep batteries separated so that a fire cannot damage other batteries. The charging container should be kept away from anything flammable.

Battery Charging & Charge Balancing

Many Li-Po batteries, particularly larger packs, come with a second, smaller, multi-wire plug – which is to be used for balance charging.

Balance charging simply ensures that all individual cells within the battery pack are at the same voltage. If a battery is not balanced, some cells may be overcharged, others may be over-discharged. In either case the life of the battery pack will be compromised.

Lithium batteries are not automatically balanced by applying a small “overcharge” in the same way that nickel-cadmium or lead-acid batteries may be balanced.

Note: Not all balancers are compatible with all chargers – some research into compatible types will be necessary in order to avoid battery damage.

For example, some balancers are only able to balance a small amount of amps, which, depending on the Li-Po cell, may not be enough to achieve a balanced state. Some of the ‘newer’ Li-Po chargers are able to individually charge each cell using the balancer plug, thus eliminating the need for a separate balancer, and at the same time ensuring sufficient power to complete the balancing process.

Storage Tips & Techniques

Extracting the longest service life out of a battery packs is of prime concern. One of the key factors contributing to the service life is correct storage.

The greater portion of the service life of many battery packs is spent in storage. The conditions that a battery pack is exposed to during storage directly impact on the achievable service life from the battery pack.

Additionally a unique characteristic of Li-Po batteries is that their life span is dependent on aging from time of manufacture, not just a number of charge/discharge cycles. An older battery will not perform as well as a new one, due solely to its age.

This limitation is not widely publicized, and consequently is not well known. This is because as Li-Po battery ages, it’s internal resistance increases.

Under load the effect of internal resistance is to cause the battery terminal voltage to drop, which in turn reduces the maximum current that the battery can provide to the load. To add to this phenomena – as Li-Po batteries age, usable capacity is lost.

Cell Storage – Voltage

A fully charged Li-Po cell will produce a terminal voltage of approximately 4.2 volts. Li-Po’s are different from other battery chemistries as they should never be stored fully charged. In fact, LI-Po batteries should be stored approximately “half full”, or at 50 State of Charge (SoC).

Many of the newer Li-Po battery balance chargers include a “Storage Mode”, which charges the pack to the proper reduced voltage state for storage purposes.

Some commercial chargers charge cells to 3.85Vdc in Storage Mode.

Storing battery packs at the proper voltage level is the simplest thing you can do to lengthen their usable life span (assuming of course proper application).

‘Storage’ should not only be considered as a long term (e.g. “over the winter” situation). Even if, for example, you only fly on weekends, these battery packs are technically in storage all week – week after week – during the entire flying season. Those cumulative hours can add up slowly degrading the battery packs.

Cell Storage – Temperature

Li-Po batteries produce energy via a chemical reaction that occurs inside sealed foil envelopes. The output power is produced by a chemical reaction. The aging/degrading process is also in reality a chemical reaction.

A chemical reaction doubles its speed for every ten degrees increase of ambient temperature.

It is for this reason that Li-Po batteries do not perform as well in cold weather. Cold “slows down” the chemical reaction process – something we need to be aware of when anticipating aircraft performance.

Reducing storage temperature slows the chemical reaction of the aging/degrading process, however there is a limit as to how cold is acceptable. Li-Po batteries should not be for example frozen solid. Laboratory testing has determined that the typical household refrigerator (0 to +5 degrees C) is the perfect storage place.

Li-Po battery packs should be placed in plastic zip top storage bags prior to placing them in the refrigerator for storage. When removed from the refrigerator prior to use, leave the batteries in the zip top storage bag to prevent any atmospheric moisture from condensing on them as they warm. After the batteries have attained room temperature, that may be used as normal.

Storage & Battery Degradation

Storage Temperature40% Charge100% Charge
0 °C (32 °F)2% loss after 1 year6% loss after 1 year
25 °C (77 °F)4% loss after 1 year20% loss after 1 year
40 °C (104 °F)15% loss after 1 year35% loss after 1 year
60 °C (140 °F)25% loss after 1 year40% loss after 3 months
Permanent capacity loss versus storage conditions (source: www.batteryuniversity.com)

Over Discharging

Li-Po batteries are intolerant of over discharging, and tend to die if discharged below approximately 2.5 V.

In operation, controller circuitry should prevent the cell voltage from dropping below 3.0 V.

Cell temperature should never exceed 90 °C in order to prevent the internal separator polymers from melting and allowing plate shorting through physical contact.

Batteries with discharge rates of 20C or 25C are commonly available on the commercial market. In battery discharge terminology, each “C” is a discharge current equivalent to the value of the energy capacity of the cell (it is not the abbreviation for the Celsius degree unit).

In the case of a 1,200 mAh rating, 1C is equal to a discharge current of 1,200 mA, or 1.2 amps. A 10C cell can deliver a continuous current to a load of 10 x 1.2 A = 12 A during its discharge cycle.

Safe Disposal of Failed Li-PO Batteries

FOLLOW THE MANUFACTURERS DIRECTIONS FOR SAFE DISPOSAL!

Unlike others, lithium-polymer batteries are environmentally friendly.

It must be remembered that should the outer case of a Li-Po battery be compromised, the lithium inside is highly volatile and will react violently with water.

For safety reasons, it is recommended that Li-Po cells be fully discharged before disposal.

If the battery is physically damaged, discharge is not recommended. Li-Po batteries must also be cool before proceeding with the disposal instructions.

Safe Discharging

For Li-Po packs rated at 7.4V and 11.1V, connect a 150 ohm resistor with a power rating of 2 watts to the pack’s positive and negative terminals to safely discharge the battery pack.

Connecting the battery pack to an Electronic Speed Controller/motor system, and allowing the motor to run indefinitely until no power remains to further cause the system to function is not an preferred method of discharging the battery pack.

Discharge the battery pack until its voltage reaches 1.0V per cell or lower. For resistive load type discharges, discharge the battery for up to 24 hours.

Safe Disposal

After discharge, submerse the battery into bucket or tub of salt water. This container should have a lid, but it should not need to be air-tight. Prepare a plastic container (do not use metal) of cold water. And mix in 1/2 cup of salt per gallon of water. Drop the battery into the salt water. Allow the battery to remain in the tub of salt water for at least 2 weeks.

Remove the Li-Po battery from the salt water, wrap it in newspaper or paper towels and place it in the normal trash.

When neutralised these batteries are “reported” to be landfill safe.

Note: Some cells might react to any attempt to discharge them by bloating more. Should this occur, or should any doubt arise as to the stability of the cells, the cells should immediately be placed into a saltwater bath. It may take several days (or weeks) for a fully charged cell to deactivate in saltwater, however it will eventually deactivate.


Li-Po Battery Safety Summary

  • Keep Li-Po batteries in a cool environment
    • Do not leave Li-Po batteries in direct sunlight. Elevated battery temperatures increase the likelihood of the battery chemicals reacting in a hazardous manner.
  • Do not overcharge your battery
    • Excessive charging will cause the battery pack to heat up, expand, or explode.
  • Handle Li-Po batteries carefully
    • Try to prevent holes being punctured into the battery. The chemicals inside the battery may leak out. These chemicals may become volatile in contact with air.
  • Never leave Li-Po battery unattended while charging
    • Ensure the correct charge settings for the battery are being used.
    • The charging area should be away from any flammable material.
    • If smoke appears, or there are signs of the battery expanding, or “puffing out”, disconnect the battery charger from the wall socket, eliminating any further flow of energy into the battery.
  • Use Li-Po specific chargers only
    • EXTREMELY IMPORTANT: Using a NiCd or NiMh charger will create an unwanted reaction within the battery that may result in a fire. Remember to only use Li-Po specific battery chargers!
  • Never leave Li-Po batteries plugged to an Electronic Speed Controller (ESC)
    • Li-Po batteries are always ageing, even when they’re not connected. The chemicals inside the Li-Po battery are extremely volatile.
    • This also puts a heavy reliance on the system failsafes, which are the only thing from stopping a motor powering up if the battery is connected.
  • Never arc the positive and negative terminals
    • A positive-negative short creates a continuous flow of uncontrolled energy through the battery, with nowhere for the energy to be released.
  • Check the battery after each run period
    • Check for signs of puffing, for severed or cut wires, for dents and/or damage, and the overall condition of the battery.
    • If the battery is starting to puff out, swell, or looks like it has been punctured, do not continue to use it.
    • Charging a damaged battery MUST NOT BE ATTEMPTED!
  • Chemical reactions of Li-Po battery are not instantaneous
    • Be careful if you see a battery starting to smoke without fire. Although it has not yet reached the reaction of a fire, it is highly likely that it will reach the stage of combustion, and burst into flames.
    • Remember, if you see smoke, UNPLUG THE CHARGER FROM THE WALL SOCKET AND MOVE THE BATTERY TO A SAFE LOCATION.
    • Wait at least 10-15minutes before moving it again just to be safe.
  • Never leave Li-Po batteries in your car
    • This is for the same reason why you should store your Li-Po Batteries in a cool environment.
    • Heat = Reaction. Reaction = Expansion. Expansion = Cause for burning! BE AWARE!
  • Do not leave batteries unattended with children
    • Children do not know how to handle a battery safely. Be extremely careful and limit access to batteries.
    • There are no age restrictions with respect to the use of batteries, however common sense is required.
  • The “C” rating – and what it means
    • A 3S 11.1 Li-Po battery with 5000mAh have 1C = 5 ampere.
    • If the battery is rated for 25C continuous use that means it can output 125 ampere, or almost 1400 watt.

Part 3 – Pulse Width Modulation

Aim

To gain a basic understanding of the principles of pulse width modulation, which is commonly used as motor and servo control signals in small unmanned aircraft.

Objectives

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

  • Discuss the basic principles of operation for pulse width modulation.
  • An understanding on how pulse width modulated signals are generated.

Pulse Width Modulation

What is Pulse Width Modulation?

Pulse Width Modulation (PWM) is a means of generating an electrical pulse of a specific width (i.e. duration) can be altered.

PWM signals with different duty cycles.

PWM is a process mainly used for getting an analog signal using a digital source.

H-Bridge Inverters – Creating AC from DC

Single-phase H-bridge (voltage source) inverter topology:

Switching rules:

  • Either A+ or A- is closed, but never at the same time*
  • Either B+ or B- is closed, but never at the same time*

Corresponding values of Va and Vb:

  • A+ closed ⇒ Va = Vdc
  • A- closed ⇒ Va = 0
  • B+ closed ⇒ Vb = Vdc
  • B- closed ⇒ Vb = 0

* Same time closing would cause a short circuit from Vdc to ground (i.e. a “shoot-through“). To avoid shoot-through when using real switches (i.e. there are turn-on and turn-off delays) a dead-time or blanking time is implemented.

H-Bridge Inverters – Square Wave Modulation

How a H-Bridge creates a PWM signal.

H-Bridge Inverters – Square Wave Operation

The different outputs of a H-Bridge inverter.

The Vab = 0 time is not required but can be used to reduce the rms value of Vload.

Lagging Current

Many loads have lagging current – consider an inductor!

There must be a provision for voltage and current to have opposite signs with respect to each other.

Examples of voltage and current having opposite signs.

H-Bridge Inverters – Current Flow

Load current can always flow, regardless of switching state:

Example of current flowing left to right through the load.
Example of current flowing left to right through the load in a different direction.

H-Bridge Inverters – Where the Current is Goes

Example of load consuming power.
Example of load generating power.

H-Bridge Inverters – Circuit Isolation

The four firing circuits do not have the same ground reference. Thus, the firing circuits require isolation.

Example of why ground isolation is needed in a H-Bridge inverter.

Signal Generation

Pulse Width Modulation through Harmonics

Harmonics with square wave modulation.

Sine Waves with Digital Signals

Question: How can a sinusoidal (or other) input signal be amplified with low distortion?

Answer: The switching can be controlled in a smart way so that the FFT of Vload has a strong fundamental component, plus high-frequency switching harmonics that can be easily filtered out and “thrown into the trash”

Sine wave constructed from digital signals.

Unipolar Pulse Width Modulation

Vcont is the input signal we want to amplify at the output of the inverter. Vcont is usually a sinewave, but it can also be a music signal.

Rules for creating a unipolary PWM signal.

Vtri is a triangle wave whose frequency is at least 30 times greater than Vcont.
Ratio ma = peak of control signal divided by peak of triangle wave.

Unipolar PWM using H-Bridge Inverters

A unipolar PWM signal created using a H-Bridge Inverter.
Deconstructing the signals of a unipolar PWM signal.
The idealised waveform to create unipolar PWM signal.

Amplification of Unipolar PWM

Ratio ma = peak of control signal divided by peak of triangle wave.
Ratio mf = frequency of triangle wave divided by frequency of control signal.

Load voltage with ma= 0.5 (i.e. in the linear region)
Load voltage with ma = 1.5 (i.e. over-modulation)

Voltage Ratios in Single-Phase Inverters

Variation of RMS value of no-load fundamental inverter output voltage (V1rms ) with ma. For single-phase inverters ma also equals the ratio between the peak output voltage and the input Vdc voltage.

The linear, over-modulated, and saturated modulation zones.

Load Frequency Components

RMS magnitudes of load voltage frequency components with respect to Vdc / √2 for ftri >> fcont.

H-Bridge Inverter Performance Analysis

100Hz Signal as Input, Inverter Output
FFT of 100Hz Inverter Output
Inverter Performance with Music Input

Summary

  • Very efficient
  • Distortion higher than linear amplifier, but a linear amplifier has, at best, 50% efficiency
  • Perfectly suited for motor drives where voltage and frequency control are needed
  • Well suited for bass music amplification, such as automotive applications, or where high power is more important than a little loss in quality

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.