FPV: Drone Pilots Guide

Aircraft Knowledge

Multirotor Components

R/C Equipment

The R/C equipment comprises of two key components: the transmitter (Tx) held and operated by the pilot, and the receiver (Rx) which is mounted in the quadcopter. The signal is emitted from the antenna mounted on the top of the transmitter. As with most types of antenna, this dipole (two element) antenna emits signal from the sides – and has a null zone along it axis. (Think of a doughnut slid over your antenna.) So never point your antenna at the model.

Most R/C equipment nowadays transmits on the 2.4 GHz frequency. (Some sets are on 5.8 GHz and a few are on UHF frequencies.)  Frequency is directly related to wavelength – and 2.4 GHz is in the microwave waveband with a wavelength of around 12.5cm – which is why the antennas are short. (Old style R/C equipment from the end of the last century was generally on 35 MHz and used telescopic antennas that were about 1m long (and these were 1/8 of the actual wavelength)).

R/C equipment took a huge leap forward in reliability in the early 2000s. Firstly the signal went from analogue to digital – which allowed error checking circuitry to be incorporated within receivers. The next major improvement in reliability was from the introduction of robust frequency ‘locking’ systems. With the old R/C equipment you needed to fit a crystal into your receiver and a matched one into your transmitter to lock the two onto a set frequency. But there were only a limited number of frequencies available – and if another modeller used a crystal set that was on the same frequency as yours then it would block your signal and the model would crash. Also if random interference from outside sources occurred on that frequency, the model would crash.  The new systems use ‘spread spectrum’ technology – either with ‘frequency hopping’ or ‘direct sequence’ technologies.  Frequency hopping involves the transmitter and receiver randomly swapping around the frequency band every millisecond or so, and then ‘agreeing’ and updating a schedule of such meetings. Each time they meet, data is exchanged. But if interference disrupts one meeting they simply move to the next as agreed. With direct sequence spread spectrum technology the transmitter selects a number of clear channels when first turned on – generally two, but with more advanced technology this can be ten. It then sequentially hops across these, sending tiny amounts of data to the receiver which is bound to it and therefore also following the same hopping sequence.

A final new development has been diversity. Some receivers are equipped with two antennas so that these can be mounted in different orientations, doubling the chance of clear reception of the incoming signal.

These new technologies mean that the link to the model is very robust.  But you should be aware that microwave radio signals are very poor at penetrating buildings, human bodies, water or trees – so you need a direct ‘line-of-sight’ between the Tx antenna and the receiver (Rx) antenna on the model.

The R/C transmitter has two primary control sticks and generally some switches and buttons for the ancillary functions. The primary control sticks can be moved in any direction to give proportional control – meaning that the further they are moved, the more response is demanded. There are two basic transmitter arrangements, called Mode 1 and Mode 2.

Mode 1 transmitters

Here the left stick controls pitch when moved forward and backward, and yaw when moved left and right. The right stick controls the throttle when moved forward and backward, and roll when moved to the left and right.

Mode 2 transmitters

Here the left stick controls throttle when moved forward and backward, and yaw when moved left and right. The right stick controls pitch when moved forward and backward, and Roll when moved to the left and right.  Most commercially available quadcopters are supplied with Mode 2 transmitters, and this is the way most multicopter pilots fly.

With most transmitters, swapping the mode is either a reasonably simple job involving swapping a spring and ratchet – and with the latest generation the task is simply clicking on a tick box in the software.

A brushless motor is effectively a brushed motor engineered in reverse. A brushed motor consists of a shaft fitted with electromagnetic coils (i.e. an armature), spinning inside a stationary case fitted with permanent magnets.  The brushless motor has the permanent magnets fitted to the rotating components and the electromagnetic coils fitted to the stationary part. This means that there is no difficulty in getting electrical power to the electromagnets. The result is a smaller, lighter, more efficient, more powerful motor with greater longevity.

There are no maintenance points with brushless motors – except that it is possible to bend the main shaft in a prop strike accident. This will result in vibration, which cause difficulties for the IMU as well as leading to parts dropping off the quadcopter.  It is impossible to straighten the motor main shaft, and swapping the motor is the cost-effective remedy. Motors are specified in terms of their physical dimensions and the revolutions per volt that they produce.  This is known as the kv rating, with ‘k’ being the number of thousand rpm per volt.  So a motor with a kv rating of 1200 will spin at 1200rpm when powered by one volt.  (This figure is the unloaded value – the actual rpm will be reduced when the motor has a propeller attached.)   If you were building your own quad from scratch there is a science to selecting the motors, propellers and ESCs such that they all match and deliver the power required.

The other point about brushless motors is that they are effectively AC, and the ESC therefore connects with three wires to the motor.  If you connect these incorrectly, the motor spins in the opposite direction. If that is the case, you simply swap any two wires and the rotation direction is reversed.

Propellers

The latest generation of pre-assembled quads are fitted with self-tightening propellers that cannot be installed upside down or on the wrong motors. This makes life very simple.  But you may well encounter quads with traditional bolt-on propellers so it is important you understand them.

The first key thing to note with the propellers is that a quad has two clockwise propellers and two anti-clockwise propellers. (If all four rotors span the same way, the quad body would also spin.)   Great care has to be taken to ensure the correct propellers are fitted to each motor; the pattern is simply that every second propeller spins the opposite way to its neighbour. But you need to be certain that the anti-clockwise propellers are fitted to the anti-clockwise motors.

The second key point is that propellers have a top and a bottom – and fitting them upside down will result in significantly reduced thrust from that motor. Looked at from the end, a propeller blade is shaped like a wing, with a curved top surface and a flat (or slightly under-cambered) bottom surface.  They need to sit on the motors with the curved surface facing the sky.

Propellers come in different diameters and pitches (which is the angle the blade is set relative to the hub). And they are made of different materials: wood. Plastic, carbon reinforced etc.   You should never mix and match: all four props must be of identical material, size, pitch etc – but two c/w and two ccw.  Be aware also if tempted to experiment with propeller sizing that, if you increase the propeller size or pitch then it will create more thrust, and at the same time the motor will pull more current from the battery. It is very easy to get to the situation where the current demands exceed the rated abilities of the motor or the ESC or the cables – with disastrous results.

Most propellers need balancing so that vibration is minimised.  Balancing a propeller requires the use of a special propeller balancing frame. Generally any imbalance in weight is in the hub. Once the propeller is centralised on the spindle it is aligned vertically, and then let go to see if it tends to fall one way. If it does, then the hub is heavier on that side. Then it is held horizontally and released. If it falls in any direction then the hub is heavier at that side. Generally drilling small holes into (but not through) the hub with a 1mm drill, in the heavy location, will produce a balanced blade which stays at whichever position it is released in.  Often the process requires several repetitions, drilling a small hole, checking the balance, drilling another small hole – until good balance is achieved. It is time well spent.

Batteries

All quadcopters are powered with Lithium Polymer (LiPo) batteries. These use the same technology as the batteries used in mobile phones, tablets, laptops etc.   Any LiPo battery is made up of a number of cells: the most common being the three cell.  Whilst the technology used in lead acid and torch batteries generally produces 1.2volts per cell, with Lithium Polymer the nominal voltage is 3.7 volts per cell.  So by arranging these in series, a 3 cell pack produces a nominal voltage of 11.1v.

The voltage determines how brightly your bulb will burn, or how fast your motor will rotate. Besides voltage, the next most important measure of interest with a battery is its capacity – which is how long it will power your bulb – or motor – for.  This is recorded as a mAh figure, meaning milliamps / hours. So a 2200mAh battery will supply 2200 mA for one hour.  A 5000mAh battery will therefore provide power for considerably longer than a 2200mAh battery.

The final key measure with LiPo batteries is the ‘C’ rating. This denotes the highest rate of discharge that this battery can sustain without damage.  So if you have a 2200mAh battery that has a ‘20 C’ rating, then you could discharge it at 20 x 2200 = 44,000mA = 44 amps. And a 2200mAh battery with a ‘30 C’ rating could be discharged at 30 x 2200 = 66,000mA = 66 amps.  So the battery needs to be chosen to match the current draw of the quad motor/propeller set up – if the quad pulls 50 amps when being manoeuvred then, in this theoretical example, you would need a 30C rated battery – as even a 25C rated battery would be overworked.  Unsurprisingly, batteries with a higher ‘C’ rating are more expensive. So buying batteries with high ‘C’ rating unnecessarily is not a sensible option. (With Ready to Fly (RTF) quadcopters the manufacturer will have done all these measurements for you!)

Battery Charging

There have been cases of LiPo batteries bursting into flames whilst being charged. Invariably this is due to the battery having been damaged previously, say in a crash, or due to the use of an incorrect charger.  LiPo batteries must be charged with a dedicated LiPo charger – correctly set up for the number of cells you are charging and the charge current appropriate for the battery pack. (Most quality chargers will not allow you to connect say a two cell (7.4v) 1000mAh pack and try to charge it as 4 cell pack (14.8v) or at 5 amps – but really it is up to you to understand the chargers and the batteries and use them intelligently.)

A fully-charged LiPo cell will have a voltage of 4.2v per cell – so 12.6v for a 3 cell battery.  Never over-discharge a LiPo as this does irreversible damage to the batteries chemistry. 3.0v is the absolute lower limit, but you should try and stay clear of that. 3.3 volts per cell when the battery is loaded (i.e. powering something) and 3.7 volts per cell with the battery ‘off-load’ are recommended lower limits.  (Most ESCs have a low voltage cut-off that will do this for you. Similarly the DJI Naza has a low battery warning and cut-off system – but be aware that if you enable this the model will autoland when and where the critical voltage is reached: more than one Phantom has been lost in the sea this way.)

Fireproof LiPo charging bags are available. The battery is placed inside the bag whilst charging so that any fire is contained.

Charging Tips for LiPo Batteries

  •   Always use a dedicated R/C LiPo balance charger. (This ensures that all the cells within the battery are brought up to the same level – and avoids the danger of one cell becoming weaker whilst the other cells are over-charged.)
  •         Double check that you have set the charger correctly for the pack being charged – this includes the cell count as well as the current settings.
  •         Ensure that the charging leads are connected correctly.
  •         Always charge LiPo batteries on a fire-proof surface.
  •         Never charge batteries near flammable products.
  •         Always remove the battery from the quad before charging.
  •         Allow the battery to cool down after use before charging it.
  •         Never leave a charging lithium polymer battery pack unattended.
  •         Do not charge inside a car.  If using a car charger, run the leads outside the car. Do not charge whilst driving!
  •         Do not store batteries inside a car. (Heat can damage the battery – and if it goes up in flames you will lose your car.)
  •         Do not charge a lithium polymer battery pack at a rate over 1C – unless it is a designated ‘fast-charge’ pack. In that case, do not exceed the maximum charge rate specified.
  •         Never charge a lithium polymer battery pack that has been punctured or damaged or has swollen.

Never let the positive and negative battery leads touch!

Handling & Storage tips for LiPo Batteries

  •         Keep LiPo battery packs out of reach of children.
  •         Do not put battery packs in pockets or bags where they can short circuit.
  •         Do not transport or store LiPos with sharp or metallic objects.
  •         Do not store your LiPo pack in extreme temperatures below 0C or above 50C.
  •         Store your LiPo pack in a non-flammable container. A LiPo Sack or metal container is best.
  •         For long- battery life, store LiPos in a cool place at about a 50% charged state (3.85 v per cell).

 

Global Positioning System (GPS)

A GPS receiver works by locking onto a number of the approximately thirty special satellites orbiting the earth. By timing the signals from these it is able to triangulate its position to an extraordinary degree of accuracy.  The DJI unit will generally hold the quad within a one metre square box.

You can use this website to see how many GPS satellites are above the horizon in your location at any particular time: http://satpredictor.navcomtech.com/   The more satellites your GPS has locked on to, the greater the accuracy of your ‘fix’.  But with thirty satellites in orbit, you are only likely to have fifteen on your side of the globe at any one time, and some of those may be too low on the horizon for your receiver to capture.  Generally about ten will be in view, and most GPS units will have a good fix once six or more have been acquired.

Mulitrotor flight modes

ATTI mode

‘Atti’ is short for ‘Attitude’. In this mode the quadcopter brain is acting to level the machine. So after any input from the pilot, once the control sticks are moved back to neutral the Flight Controller will bring the quad back to level.  The important point with atti mode is that the Flight controller is not trying to fix the quad’s position over the ground – just its attitude.  So if a wind is blowing, the quad will be carried along by the wind. And if you were flying quickly in a certain direction and then released the stick to neutral, the quad will skid in the original direction a little way.

GPS mode (Sometimes known as ‘Position hold’)

GPS mode is like Atti mode, but with the addition of the GPS position locking, both vertically and horizontally.  In GPS mode, if a wind is blowing, the quad will adjust itself to maintain a constant position.  And if you were flying quickly in a certain direction and then released the stick to neutral, the quad will hold that position in space.  (For filming, atti mode gives the smoother flight path as the quad is not ‘fighting’ to get back to a particular position all the time.)

Manual mode

For aerobatics. The flight controller does not attempt to restore the quad to straight and level flight. So if you put it nose down then it will stay nose down, even when you release the controls.  The quad has to be flown every single second.  But this does allow you to perform flips and rolls.

Failsafe mode

R/C aircraft have been fitted with a failsafe mode for many years, but the failsafe mode was simply that if the model lost R/C signal then the throttle would close and the crash would happen very soon and very close by.  With the GPS equipped quads there is usually a failsafe programme, triggered either by you positioning a switch or by the quad losing the R/C link, which will then fly the quad back to its starting point where it auto-lands.  This ‘Return to Home’ (RTH) feature is a very impressive trick – but you need to understand this function fully.  Firstly, if you have not waited and stored the home point before the take-off, then the quad will almost certainly try to fly to its last stored home point. If the quad is new, then that position is probably in China. So always wait for home point to be stored before launching!

Secondly, the quad will fly directly back to its home point. On DJI machines it will do this at a height of 20 metres.  If there is a building 40 metres tall in the way, then it will crash into it.  And thirdly, if you haven’t enabled the failsafe option, then it cannot operate when you want to use it. ‘Enabling’ generally means going into the Software Assistant and ticking one of the failsafe modes.  And then going into your R/C transmitter set-up and setting the R/C failsafe so that, on loss of signal, the receiver moves all the stick functions to the centre – especially the throttle.  This means that the throttle will be at 50%. (It is impossible for the Flight Controller to fly the quad home if you have set the throttle to zero.)

Intelligent Orientation Control (IOC)

The DJI Naza flight controller has two modes of ‘Intelligent Orientation Control’ which can be selected with a transmitter switch.  ‘Course Lock’ and ‘Home Lock’. Their operation is clearly detailed in the DJI Manual.  The key point to be aware of is that, if you do not check the switch positions carefully before every take-off, then at some stage you will take-off with one of these modes accidentally activated, and once the trigger requirements are met your aircraft will suddenly no longer respond normally to your commands. This is completely disconcerting and could easily result in a lost or crashed model. (If this ever happens to you, the trick is to release the control sticks and engage GPS mode. The aircraft will stop and hover where it is, and you can collect your wits and identify the problem.)

UAS Ground Control Systems and Autonomous Operations

There are various systems for programming way-points into quads using laptops and iPads etc.  You must remember that to be legal you must have continuous unaided visual contact with the machine so that you can ‘monitor its flight path in relation to other aircraft, persons, vehicles, vessels and structures for the purpose of avoiding collisions’ – which does imply that you have to have the ability to control the aircraft in an emergency so that the collision is avoided.

  1. a)                 SUA limitations

The effects of temperature and altitude on your drone’s performance were discussed in the Meteorology chapter, along with causes of turbulence. Other factors that will affect the performance of your drone are increases in payload – and the distribution of that payload.

Whilst flight controllers can cope with a payload that is not located on the aircraft’s centre of gravity, this usually results in lurching take-offs, as the weight pulls the quad over and then the flight controller corrects this motion.  So it is always best to try to arrange to position any payloads such that the aircraft’s centre of gravity is on its centre line.

There is a finite limit to the payload that any quad can lift – and it is quite easily reached.  If the quad has a ready-to-fly weight of 1kg, then adding 300g of cameras and gimbals etc could easily see it struggle to lift off.  In this case, fitting slightly bigger propellers (9” instead of 8”) or increasing the battery voltage (e.g. by swapping from a 3 cell to a 4 cell) would be a fix – but this would involve checking that the current draw was still within the safe limits of the ESCs, motors and battery.

  1. b)                 SUA maintenance

A fundamental part of owning and operating a quadcopter is maintaining it.  Whilst many modern gadgets can be operated successfully without ever reading the manual, this is certainly not true of quadcopters.  The popular DJI range are typical and are detailed below:

  1. i)     Manuals. The DJI manuals are downloaded from their website. http://www.dji.com/

Choose the ‘All Products’ tab and then select your precise product. Then select ‘Downloads’.  The Manual will be available in a choice of languages, along with a Quick Start guide and any manuals for related items such as the battery and charger.

  1. ii)   DJI Software Assistant. The Software Assistant is the programme you install on your computer that interfaces with the quad and allows you to adjust settings, select or de-select options, check the quad status and recalibrate the IMU etc. The ‘Software Assistant’ for your product is downloaded from the same page as the manual, as described above. There are many versions of the Software Assistant, specific for different DJI products – so make sure you are using the correct one.  If you progress and own several DJI quads then you may well need to have several different versions of the Software Assistant downloaded.

iii) DJI Firmware Updates. Firmware updates are issued fairly frequently – especially for the newer products. You should always take advantage of these.  When you connect the quad to the Software Assistant you will be alerted to any firmware updates available, and simply need to click to accept these.

  1. iv)  There are many forums in existence where fellow quad flyers share experiences and advice. These can be great sources of advice. But really you should use these as a back-up to the manual.

See the next section here: First Person View