Learning to fly a Multirotor from Scratch (Theory and Practice)

[teslahed]

Learning to fly a Multirotor from Scratch
Theory and practice

How do they fly?

Multirotor aircraft are similar to conventional helicopters in many ways but they are not the same and it';s well worth learning the basic theory that governs their flight before jumping in and trying to fly one yourself. So before you turn on a radio controller I will go over the basics in order to give you a general overview of how they fly and why they differ from more conventional aircraft of the sort that most people are likely to be at least somewhat familiar with.

In the simplest terms they are no different to any other helicopter in that the upwardly thrusting propellers produce more lift than the aircraft has weight in order to cause it to leave the ground. Modern brushless electric motors have excellent power to weight ratios and are very responsive and the latest lithium polymer (LiPo) batteries we use are much better in terms of energy storage density than the previous generation of Nickel Metal Hydride (NiMh) batteries and other older technologies. In part this explains the rapid development of these kinds of aircraft in recent years and also explains why they can now be used to carry useful payloads.

There are some obvious differences between multirotor aircraft and more conventional helicopters. Firstly as the name suggests multirotor aircraft have more than one main set of rotors. Secondly they have no tail rotor. The simplest most basic type which I will use to explain the principles that apply to all other multirotor aircraft is the humble quadcopter.

The Quadcopter (an introduction)

The quadcopter is the most basic form of multirotor aircraft and as the name suggests it has four main motors and sets of rotor blades. These four main rotor blades work in conjunction with each other to produce lift, control and stability and it makes sense to explain and understand these three points before having a go yourself and certainly before flying anything larg or complicated. Small quadcopters fly using the same principles as larger multirotor aircraft without the risks associated with heavier and more powerful and expensive aircraft which is why I suggest using smaller aircraft when getting started.

You have control!
(in theory)

The controls for the aircraft are thrust, pitch, roll and yaw, similar to any other simple helicopter. Understanding what these four controls do to the aircraft how to use them and why is vital if you want to be a pilot.

If you are flying using a ';mode 2'; radio controller then the thrust stick will be on the left on the up and down axis. The yaw stick is also on the left on the side to side axis. The pitch stick is on the right on the up and down axis and the roll stick is also on the right on the side to side axis.

There are many other ';modes'; of radio controller layout that it';s possible to use when learning to fly. Some modes are more popular than others and this varies from place to place. I recommend mode 2 because it makes the most sense to me and seems to be the most popular in the UK. It';s what I';m used to and seems to work very well. You can use any mode that makes sense to you. The mode of the radio controller just means which stick axis on the radio controller maps to which axis of rotation or thrust on the aircraft itself.

http://www.rc-airplane-world.com/rc-transmitter-modes.html

Thrust is probably the simplest and most obvious of the controls. As you move the thrust stick on the controller up from it';s default base position all four motors will spin up at increasing speeds producing more upwardly directed thrust until the total is greater than the aircraft';s weight. This is the point at which the aircraft will leave the ground. A well tuned multirotor aircraft should be set so that it flies at around the half stick position on the controller';s thrust stick so that you have power in reserve in order to manoeuvre and climb and get yourself out of trouble when needed. This stick is unsprung so once you';ve got it into the right position you don';t need to maintain pressure to hold a certain level of thrust and keep the aircraft in the air.

If you look at any multirotor aircraft you will notice that unlike a fixed wing aircraft there are no forward or rearward pointing motors. All the thrust is directed upwards as you take off which means that unless you do something clever the only way is up. Useful if you want to fly an expensive electrically powered hot air balloon but otherwise not that impressive or practical.

Just the same as a more conventional helicopter the way we get around this is through thrust vectoring â€" diverting some of the moving air backwards as well as downwards in order to thrust the aircraft in the equal and opposite forwards and upwards directions (Newton';s third law of motion for the physicists amongst you). The way we vector the thrust is to pitch (or tilt) the entire aircraft forwards while simultaneously increasing the total amount of thrust generated by the motors and props to prevent the multirotor from losing altitude now that a component of the total thrust is no longer acting in the downwards direction.

Pitch is handled differently to thrust in that not all of the motors are behaving in the same way. In order to fly forwards the multirotor aircraft will need to pitch forwards and to do this it will spin up the back two motors relative to the front two motors to produce a turning force (torque) about the centre resulting in the pitching rotational movement. Once a sufficient angle is achieved the motors will equalise and the angle will be held allowing for forward flight. The greater the angle held the larger the forwards component of thrust and the faster the aircraft will accelerate and move through the air.

On a mode 2 radio controller pitch is managed on the right hand stick â€" push the pitch stick forwards to pitch forwards and back to pitch back. This stick is sprung so you will need to maintain pressure to hold a forward pitch angle. If you release the stick it will spring back to the centre position and the aircraft should follow suit.

Roll (or tilt sideways) is similar to pitch in that in order to roll left the aircraft will spin up it';s two right motors relative to the two left motors in order to create the turning force about the centre. Once a sufficient roll angle is achieved the motors will equalise again and this angle will be held thrusting the entire aircraft sideways through the air.

On a mode 2 radio controller roll is managed again on the right hand stick. Push it left to roll left and right to roll right as you';d expect. This axis is also sprung in the exact same way as the pitch axis.

You will need to learn to carefully coordinate both pitch and roll if they are on the same stick as they are on a mode 2 radio controller. At first you may find that you accidentally move one axis slightly whilst deliberately moving the other. You will quickly learn to manipulate the sticks to move only the axis you want to use but this is definitely a case of practice makes perfect.

Yaw is different to both pitch and roll. One of the most obvious differences between a multirotor aircraft and a conventional helicopter is that there isn';t a separate vertical tail rotor on a multirotor. Helicopters require a vertical tail rotor in order to prevent them from spinning about their centre in opposition to the torque produced by the single set of main horizontal rotor blades but multirotors are designed with an even number of motors and props spinning in opposition to each other so that they can create lift without any net torque. In terms of a quadcopter there will be a 2 clockwise and 2 anti-clockwise motors and props to produce this effect. When the pilot wishes to yaw left or right the matched pairs of clockwise and anti-clockwise motors and props will spin up and down in opposition to each other in order to adjust this balanced torque to produce a net turning force without altering the total amount of lift. This means a well balanced multirotor aircraft can yaw without climbing, pitching or rolling. It also means that a well trained pilot can mix thrust, pitch, roll and yaw simultaneously to manoeuvre their aircraft as they see fit.

On a mode 2 controller yaw is managed via the same left hand stick as thrust. You may find it tricky at first to yaw without accidentally powering up and down, or power up and down without accidentally yawing. Again practice is key here which is why I recommend starting with smaller and safer aircraft. The yaw axis on the left stick is sprung the same as the pitch and the roll axes on the right stick even though the throttle axis is unsprung. This means that if you want to maintain a rate of rotation you will need to hold the stick away from its centre position similar to pitch and roll. It also means that if you keep a delicate hold of the left stick you shouldn';t find it impossible to move the looser unsprung throttle axis without affecting the stiffer sprung yaw axis.

You may notice that yaw behaves somewhat differently to pitch and roll. When you release either the pitch or the roll axes on the radio controller stick the aircraft will spring back to centre the same as the stick does. When you release the yaw stick the aircraft will maintain it';s new yaw position and will not move or spring back to it';s previous orientation.

Stability

Fortunately for the budding quadcopter pilot there';s more going on than just control. Namely stability. Stability means that your multirotor aircraft is fitted with a variety of 3 axis sensors (gyroscopes and accelerometers) that can detect both rotation and acceleration on the X and Y and Z (roll, pitch and yaw) axes. As a pilot you needn';t worry too much about the technical details but you will still benefit from the technology involved and it';s still worth gaining a basic understanding of what';s going on. What this means in real terms is that as well as you having direct control over thrust, pitch, roll and yaw, the flight controller on your aircraft is using the sensors it has access to to minutely alter these same variables hundreds of times per second to keep the aircraft pointing the right way up by self stabilising, and moving as it should given the pilots control inputs. This process is completely transparent to the end user on a well tuned multirotor aircraft. The result is that when you take off your aircraft will auto-level to keep itself pointing the right way up until you use your controls to point it in a different direction and then once you start telling the aircraft to do something like hold a given angle to start moving forwards you wont need to constantly adjust the controls just to keep the aircraft behaving well and flying sensibly. The quadcopter will feel ';locked in'; to the directions you give it and shouldn';t exhibit any nasty or unpredictable flying characteristics. Without this kind of electronically generated stability it would be impossible to fly a small multirotor aircraft and it is in part the recent explosion in cheap sensors driven by the mobile phone industry that';s made multirotor aircraft practical; the sensors really are the same as those used by your phone to flip the display when you turn the screen (amongst other things).

This self levelling ability generated by the flight controller working with the sensors is what allows the aircraft to spring back to level when you release the right hand stick and makes basic flight easier, safer and more predictable. Especially when starting out.

Bringing it all together
(Flight training practice)

To start with you may be happy simply using thrust to get your aircraft off the ground then pitch and roll to keep it hovering on the spot by fine tuning things and then shuffling it backwards and forwards about. If you mix pitch and roll together you can get diagonal movements and cover a lot of ground. There is no reason you can';t fly the quadcopter forwards, backwards, left and right using just the right hand stick without ever touching yaw to rotate the aircraft and it makes sense to practice this to start with whilst balancing the height by adjusting the thrust using just the up and down stick on the left.

This is definitely the easiest way to move the aircraft about a small space and I would recommend starting this way. Keep the angles from horizontal low in order to prevent the aircraft from running away from you as speed can quickly build up otherwise. The controls can feel quite sensitive when you are not used to them so keep stick movements small and precise to make life easier.

If you lose control of the aircraft and it starts picking up speed and flying away from you the best thing to do is to lower the throttle in a controlled fashion and allow the aircraft to hit the ground. This might seem wrong but is almost certainly better than letting the out of control aircraft continue to gain height and distance until you lose it completely. You can usually repair a downed aircraft, especially if you';ve picked a good flying spot with nice soft grass to crash in. If the aircraft flies away you may never see it again or worse, hit something or someone off in the distance.

Once you';ve mastered the basics of shuffling your quadcopter around a small space you';ll be ready to progress to the next stage. It';s very important to get used to rotating the aircraft using yaw alongside roll, pitch and thrust and this is where things can start to get a little tricky. Yaw will let you turn your aircraft to face the direction you are travelling in and I would recommend you learn to do this next because it';s an important skill to master if you want to progress from the very basics up to the level of a genuinely competent multirotor pilot. Yawing does introduce some new problems however;

Yaw Anti-Clockwise

While you are standing behind your aircraft and using Pitch and Roll along with Thrust to control the thing you may have noticed that your perspective is the same as the aircraft';s. This makes controlling the aircraft very simple as you don';t have to worry about thinking too hard in terms of what the aircraft is doing relative to what you are doing.

As soon as you turn the aircraft this changes. Your perspective is no longer the same as it';s perspective. If you Yaw the aircraft 90 degrees anti-clockwise to the left then Roll right now becomes forwards, Pitch forwards becomes left, Roll left becomes backwards and Pitch backwards becomes right. This is from your perspective. From the aircraft';s perspective nothing has changed.

Fortunately up is still up and yaw clockwise / anticlockwise still has the same effects as well, so you don';t have to worry about those ones.

But it gets worse!

Nose in flying

As you continue to use Yaw to rotate your little quadcopter around by another 90 degrees you';ll end up with an aircraft that';s ';nose in';. This means that it';s front (nose) is now pointing directly towards you; the pilot.
This is the point that separates the real pilots from the amateurs.

As before with a 90 degree anti-clockwise turn to the left your perspective is now very different to the aircraft';s. In fact all the controls with the exception of yaw and thrust are now reversed. This means that Pitch forwards will tip the quadcopter backwards towards you, Roll left will tip the aircraft to your right, Pitch backwards will tip the quadcopter forwards away from you and Roll right will tip the aircraft to your left. Yawing clockwise will still turn the aircraft in the clockwise direction and vice versa and thrusting upwards will still cause the aircraft to climb.

I cannot stress how important learning to deal with this issue is when learning to fly multirotor aircraft. At first this is likely to cause you major brain ache and any number of crashes. You will need to practice flying a small aircraft until you';ve mastered this issue as you really don';t want to get confused when flying anything larger and more more expensive or fragile.

The trick here (other than practice) is to learn to think from your aircraft';s perspective at all times. If you can avoid losing track of where your aircraft is pointing and can learn to always imagine yourself as (or in) the aircraft then after some practice you will find that things just ';click'; at which point controlling your multirotor regardless of it';s orientation relative to yourself will seem natural. The reason why there are coloured LEDs on many multirotor quads is to aid with this kind of orientation issue so try to pay attention to the LEDs and use these to help as much as possible.

One technique that some people find makes hovering on the spot ';nose in'; easier is to imagine that you are balancing a stick in the palm of your hand. If you are balancing a stick and it starts to move left then you move your hand left to compensate and if the stick starts to move right you move your hand right to compensate. This analogy isn';t helpful for everyone but some people find it makes a real difference in terms of visualising what they need to do with the right hand stick on their radio controller whilst attempting to hover their multirotor aircraft with it facing them.

There are a few exercises we can do to get to this point where this seems natural a little quicker.

Basic Circles

Learning to fly your aircraft in a simple circle is one of the easier and more useful exercises to start with and I would encourage you to get the hang of this as soon as possible. You can start by using yaw to rotate the aircraft on the spot. You';ll notice than in the course of turning the aircraft it will naturally end up pointing nose in for at least some of the flight. If you keep your aircraft yawing at a slow and steady rate you probably wont find this too difficult because the aircraft will go into and come out of the nose in configuration without things getting too overwhelming at any one point.

If you want a real challenge try pausing the rotation whilst the aircraft is facing you and see if you can keep it hovering nicely on the spot like this. If you can feel calm and relaxed and even move the aircraft about in a controlled fashion whilst it';s facing you then you are doing very well.

Slow Forwards Circles

Once you';ve started to get the hang of turning on the spot you can mix this up with a little forwards flight. Use your Pitch control to add in a little forward motion. Not too much â€" we';re not racing here â€" just enough to get the feel of things. When you are happy that your aircraft is moving forward in a slow and steady fashion try slowly yawing it in order to bring it around in a nice sedate circle. Again you';ll find that your aircraft naturally goes into and out of the nose in configuration without requiring too much effort on your part.

It';s well worth practising this until it feels easy and natural ideally in both directions so that you don';t end up learning to only turn one way. Most people are either right or left handed and it';s a good idea to try to avoid learning to only turn in your preferred direction and best to get into good habits early unless you want to have to relearn things later down the line.

Faster Circles

Once you start to get the hang of this it';s only natural that you';ll want to try going a bit faster. As you increase the speed by increasing the forward pitch angle you will start to run into problems. As you turn your aircraft will naturally carry on going in the direction it was previously heading in due to momentum (Newton';s first law of motion for the physicists). The faster you are going the more your aircraft will tend to side slip when you turn and this can cause problems if you want to keep things under control especially if you are flying in a restricted space.

The solution to this problem is to mix roll in with yaw and pitch. If you bank the aircraft into the turn you';ll be using some sideways thrust to counteract this tendency to side slip. If it helps you can think of it as being like riding a motorcycle round a corner or if you prefer you can visualise it as being like one of those 1920';s era racetracks with the banked turns â€" whatever works best for you. The faster you want to turn the more you will need to use roll to compensate and this will also affect the amount of thrust you need to use to maintain altitude. The important point is that you will need to learn to intuitively mix all 4 of your controls in order to turn and at first this can seem quite daunting.

If you choose not to bank the aircraft to counteract the effects of momentum then the only thing that will be slowing you down is drag. I have not specifically mentioned drag yet but it';s well worth taking into consideration. Drag is the reason you have to keep the multirotor tipped forward by a certain amount if you want to maintain forward flight. You cannot simply pitch forwards to accelerate then level out or you will start to slow down again. You will encounter this this for yourselves as you gain flight experience. Different sizes, shapes and weights of multirotor aircraft will experience different amounts of momentum and drag and these variables will have to be managed and balanced against each other by the pilot. The faster your aircraft is going and the heavier your aircraft is the longer it will take drag to bring it to a stop and the more important banking to compensate becomes.

Clockwise circles (turning to the right)

If you want to do a clockwise circle whilst moving forwards with your quadcopter you will need to coordinate all 4 controls as I will now explain.

Firstly you will need to pitch forwards (using the right hand if using mode 2) to get your aircraft moving forwards and then maintain a suitable pitch angle by holding the stick in position.

Secondly you will need to increase the thrust (on the left stick if using mode 2) in order to make up for the reduced amount of downwards thrust now that you have redirected some of it backwards. The thrust stick is unsprung so does not need constant pressure to maintain a certain level.

Thirdly you will need to start yawing to the right (again on the left stick if using mode 2), in order to get the aircraft turning clockwise to the right. This stick is also sprung and will need to be held to maintain the rate of rotation.

Fourthly you will need to roll to the right (using the right stick if using mode 2) to bank into the turn in order to prevent the multirotor from slipping sideways to keep the turn nice and tight. This stick will again need to be held in position for as long as you require the aircraft to bank sideways.

Coordinating all 4 controls in this fashion can seem like an advanced version of patting your head whilst rubbing your belly at first but practice is key here. You will want to try and try again until it starts to feel natural. It really helps if you understand what you are doing and why which is why I';ve tried to explain in so much detail exactly what';s going on but at the same time I can guarantee you that most pilots won';t be consciously thinking about all these variables when they make a banked turn. They just do it based on experience and muscle memory and an intuitive understanding of what they want to do more than a conscious decision making process in terms of how much to move each stick. This is the reason why I recommend starting with small and cheap training quads â€" you need to rack up stick time and you need to make mistakes in a safe and controlled environment with aircraft that wont break or damage things when you crash. It';s the best way to learn the skills to the point that it feels natural.

Once you';ve gotten the hang of this you';ll find you can alter the speed of turns by yawing more quickly, banking (rolling) more heavily and pitching forwards more or less to vary the forward speed as you go into the manoeuvre.

Anti-clockwise Circles

I don';t think I need to explain in as much detail how to do an anti-clockwise circle. As long as you can understand the process you shouldn';t need to follow a checklist of exactly what each stick does in order to pull off a manoeuvre. Just think about what you were doing before and reverse the yaw and roll elements if you want to turn in the other direction. I would encourage everyone to practice turning in both directions an equal amount so as not end up only able to circle effectively one way as that would be very limiting.

Figures of Eight

Once you can effectively circle in both directions the next step is to combine these manoeuvres into one smooth operation. This is called the figure of eight and it';s as good a test of basic piloting skill as anything. Once you';ve mastered nose in flying, circles, and the figure of eight there isn';t much else you';ll need to learn to call yourself a competent multirotor pilot.

In order to do a figure of eight you need to be able to circle in both directions and then swap from one to the other. The tricky part is the transition from one direction to the other which you can do either whilst travelling towards yourself or away from yourself during the circling manoeuvre.

Learning both types of figure of eight is a good plan if you don';t want to limit yourself in the future so I strongly recommend everyone gives this a go now until familiar with both directions.

Free Flight

Once you can do circles in both directions, hover nose in, do figures of eight of both types and generally fly without too much mental effort, I';d suggestion you spend some time engaged in free flight practice. Place a number of obstacles to fly around and through, practice landing on small tables and generally flying about.

Don';t worry too much about crashing as this will limit the chances you take and speed at which you progress. Flying small, cheap and low weight aircraft when you are getting started will really help you learn the basics because you';ll be able to make mistakes in a way you couldn';t if flying something expensive.

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Flight Modes

Multirotor aircraft rely as much on the electronics as they do on the aerodynamics to keep themselves in the air. It really is a synergy between these two aspects as well as the pilot themselves that allows them to fly the way they do. I';ve already mentioned both the accelerometers and the gyroscopes and how they affect the way the quadcopter flies but it';s worth going into a bit more detail about the various different options available both on the hubsan X4 training quads and also on more sophisticated aircraft.

Auto-Stabilise (Auto-level)

Most simple multirotors fly in auto-stabilise flight mode (AKA auto-level). As the name suggests this flight mode allows the quadcopter to keep itself pointing the right way up using both the accelerometers and the gyroscopes. Because accelerometers can detect the downward pull of gravity they are able to tell the flight controller which way is up which allows the aircraft to automatically adjust it';s alignment with the horizon to keep things on the level. This makes flying easier for learner pilots and at the same time it';s the go-to flight mode for professionals flying larger multirotor aircraft when they are not interested in showing off or doing stunts and just want the best level of control with the greatest possible amount of safety and least risk of unexpectedly crashing.

So it makes a lot of sense for new fliers to use a basic ';auto-level'; flight mode when learning to fly.

If you take your hands off the right hand stick on the radio controller the aircraft will naturally right itself and this makes your job as a pilot much easier, especially when starting out. You can think of this as being the electronic equivalent of dihedral on a fixed wing aircraft if that helps.

What this means in terms of flying style is that in order to keep your little quadcopter flying forwards you need to keep the pitch stick on your radio controller pushed forwards by an amount that is proportional to the amount you want the aircraft itself to pitch forwards.

If you get confused with orientation or anything else whilst flying you can simple release the pitch and roll stick and let it spring back to centre. The aircraft will then follow the stick and re-level itself.

This does not remove control from the pilot as it will not make decisions for you. All it does is lower pilot workload â€" similar to dihedral on a fixed wing aircraft. A well tuned quadcopter can seem quite easy to fly with auto-level turned on once you';ve had some practice and learned the basics for yourself but it';s no substitute for skill or control and it will not stop you getting into trouble or crashing the thing if you don';t keep your concentration.

You will notice that auto-stabilise flight mode does not control thrust or yaw, it only affects pitch and roll. The easiest way to crash an auto-stabilising quadcopter is to stop paying attention to thrust levels as you need to manually adjust this when you pitch and roll the aircraft to make up for the loss of upwardly directed thrust. If you don';t you';ll very quickly lose control and crash.
The other way to crash an auto-stabilising multirotor aircraft is to forget about momentum. It may be true that the aircraft will self level if you take your hands off the right hand stick but if you discount momentum what you';ll find is that your quadcopter carries on flying in the direction it was previously moving in for quite some time. If you are flying in an enclosed space or near any kind of obstacle then you may well collide with something before your aircraft comes to a stop. If you are flying outside then you may lose sight of your aircraft before it slows to a stop. You';ll need to use reverse banking to cancel out this momentum in a similar fashion to using roll to bank round turns if you want to keep things under control and this is where pilot skill really comes into play.

You will find that the multirotor aircraft gets blown about in the wind and you will need to make pitch and roll adjustments to keep it in place.

If you lose orientation whilst flying in self-stabilising mode then you may still crash if you think you are counteracting momentum and accidentally add to it instead. It';s very easy to end up with ';run away quadcopter'; syndrome.. Knowing which direction to bank in to start and stop any manoeuvre is a vital skill that you will need to learn to do quickly and intuitively without thinking too hard or for too long in order to be a good well controlled pilot. This means maintaining visual orientation with the aircraft and keeping track of the front at all times.

Different flight controllers and different multirotor aircraft can be set to respond differently in auto-level mode. Many will have a maximum bank angle which may commonly be anything from 30 degrees to 45 degrees or more. Lesser maximum angles will improve stability at the expense of slowing the horizontal acceleration and deceleration of the aircraft as you will always be closer to level but less able to divert thrust away from the vertical when required to do so. Once you are flying your own aircraft you will want to play with this variable to adjust things to your personal preferences and the requirements of your operating environment.

Acro mode (gyros only)

Acrobatic (acro) flight mode (or gyros only) is where things really get fun. If you turn off the accelerometers then the multirotor aircraft no longer knows which way is up and this becomes fully the pilots responsibility. I don';t recommend this flight mode for complete beginners but it';s worth giving it a go once you';ve mastered basic auto-level flight mode.

The important difference between acro flight mode and self-stabilising is that in acro mode the quadcopter will no longer re-level itself when you take your hands off the sticks, instead it will maintain it';s current angle with the horizon. If you pitch forwards for forwards flight you only need to push the appropriate stick on the controller forwards when you want the aircraft to rotate forwards. If you take your hands off the sticks they will spring back to centre but the aircraft will maintain it';s pitch angle thanks to the gyroscopes alone without any input from you or the radio controller. The pitch stick now controls angular velocity (the further from centre you push it the quicker the aircraft rotates) rather than directly affecting the angle of the aircraft with the horizon. This makes smooth manoeuvres easier and more natural but it also means that you are left with full responsibility in terms of re-levelling the aircraft when you want it to level out. If you want to slow the quadcopter down more quickly than drag alone will do the job then you will have to pitch backwards to counteract the momentum you';ve built up and then when you want to stop you';ll need to pitch forwards again to move the quadcoper back to the level position to prevent it from starting to accelerate backwards.

This increases pilot workload a lot and will make everything seem much harder, especially at first. Pilots that want to play around with aerobatics and stunt flying or demonstrate full and total control of the aircraft love this flight mode and it';s great for showing off as well as developing your manual piloting skills to the highest possible level. At the same time if you are flying commercially you';ll not need to use this flight mode when piloting something that weighs several kilograms or more that may be carrying an expensive payload. It';s just not worth the risk for most people and offers no real advantages in terms of safety, performance or reliability when flying the kinds of sensors we will be looking at later.

However; if you do have your heart set on doing a loop the loop this is the best flight mode to be using!

Altitude Hold Mode

Altitude hold mode builds on Auto-stabilise / Auto-level mode by allowing the multirotor aircraft to automatically maintain altitude using data from a barometric sensor that measures changes in air pressure associate with height.

This means that you no longer have direct control over thrust instead this is managed by the flight controller to maintain the altitude of the aircraft. Pitch, roll and yaw are managed in the same way as in auto-stabilise mode so it';s just thrust that';s different.

You can still direct the aircraft to climb or fall. The way that this works is that there is a dead zone in the centre of the thrust stick that corresponds to altitude hold â€" usually between 40% and 60%. As most multirotor aircraft are balanced to hover at the 50% stick position when flying in auto-stabilise and acro flight modes this should feel natural after a little practice. If you want to ascend or descend you will need to move the thrust stick further than you are used to in order to get it out of the dead zone and when you want to re-enable altitude hold you will need to move the stick back to near the centre position.

This can further reduce pilot work load over auto-stabilise because it means that you can put the thrust stick into the centre position and then fly around without having to constantly adjust the levels of thrust to keep the aircraft at the same altitude.

It does present a few problems in that the barometer is prone to interference. Rapid changes in air pressure caused by environmental conditions such as extreme weather that are external to the aircraft can throw the barometer readings off and cause the aircraft to rise or fall unexpectedly.
If the barometer is not well insulated from the effects of the slipstream in fast forward flight this can also throw the readings off but this should be managed by careful consideration when constructing the multirotor aircraft and so shouldn';t affect the pilot.

Some flight controllers lack barometer sensors and this includes the small Hubsan X4s that many people start training on. I would not recommend people learn to fly using an altitude hold mode as you need to be able to fly in stabilise mode and manage the thrust levels yourself in order to be safe and controlled pilot because of the fact that altitude-hold mode can fail for all the reasons already described. It';s also often the case that a practiced pilot can manage altitude more accurately in auto-level mode than the aircraft would manage in altitude hold mode. Some of the better flight controllers will manage 20cm accuracy in altitude hold mode but others may drift upwards and downwards by a meter or more. If you can fly in self-stabilising mode then you can fly in altitude hold mode because it';s generally easier.

If altitude hold mode starts to fail then switching back to self stabilising may save the aircraft as long as you are capable of flying using this flight mode.

GPS Position Hold (Loiter)

Once you start flying larger more sophisticated aircraft with better equipped flight controllers and  sensors you';ll have access to more flight modes. As soon as you add GPS into the mix you can start to do some really clever automated stuff.

The most basic GPS flight mode is position hold or loiter. This flight mode uses data from both a GPS receiver and also a magnetometer to maintain heading and location at the same time as using all the same sensors as altitude hold mode to maintain it';s angle relative to the horizon and it';s altitude too. Effectively it';s a mix of everything altitude-hold mode does with some new stuff thrown in.

Most people will already be familiar with the capabilities of GPS receivers as well as some of their limitations. With a good signal from a sufficiently high number of GPS satellites it';s possible to get very accurate positional data and the better flight controllers (such as the Pixhawk or DJI Naza) will mix this with information from the accelerometers and barometer to cancel out as much movement as possible. This means that it';s possible to get a well configured multirotor aircraft holding position to the nearest meter cubed or less, even in windy conditions.

If you want to ';nail it to the sky'; then GPS position hold is the best flight mode for the job. Once properly set-up it enables you to leave the multirotor aircraft in a fixed position in the sky with no input from the pilot at all. This can be very useful when filming if you want the aircraft to observe a fixed point on the ground without moving and can be useful in many other situations as well.

As well as holding position GPS position hold mode also allows the pilot to control the aircraft';s position in a similar fashion to the way that altitude hold allows the pilot to control altitude. If the pilot moves the right hand pitch and roll stick then the aircraft will bank and then change position in the sky as a result. If the pilot moves the thrust stick up or down beyond the central dead zone then the aircraft will ascend or descend. When the pilot releases the right hand pitch and roll stick the aircraft will auto-level and start holding it';s new position and if the pilot centralises the thrust stick the aircraft will start holding altitude again after gaining or losing height. Yaw remains unaffected and the pilot can rotate their aircraft as they see fit. This allows the pilot to shuffle their aircraft about the sky moving from one fixed position to another but it will tend to slow everything down and make everything more predictable.

It does not allow the pilot to take their concentration off the aircraft as there are many situations where GPS position hold can malfunction or fail and the pilot needs to be ready to take control back at any time by switching to one of the none-GPS controlled flight modes. Stabilise mode is probably safest unless the pilot is sure that the barometer is not affected by whatever is affecting the GPS and / or magnetometer.

Sometimes due to heavy cloud cover, electromagnetic anomalies, deliberate GPS jamming by the authorities or malicious third parties, hardware failure or other issues, GPS performance can be affected or even completely destroyed. The GPS will report the number of satellites it is getting information from as well as the quality of the GPS lock and if this falls below a set value then it is unwise to rely on GPS flight modes and can even be dangerous to do so.

Anyone that learns to only fly using the simple GPS flight modes will find everything much easier until the day something goes wrong, then they won';t have the manual skills to fall back on at which point things can go very bad very fast. So even though it might be tempting to learn to fly using a GPS equipped quadcopter I wouldn';t recommend it.

GPS Return to Home (or Return to Launch)

GPS return to home (aka return to launch) is a very useful feature that most GPS equipped flight controllers offer. When your multirotor aircraft takes off it saves the GPS coordinates and when you engage the GPS return to home flight mode the aircraft will compare it';s current position with the saved launching position and attempt to fly back to where it came from.

It will make use of the barometer to maintain altitude as it does this as well as stabilise flight mode to keep itself near level and flying safely. It will also use the magnetometer to point itself in the right direction.

This means that whilst this flight mode can be very useful as a way of reducing pilot work load at the end of a flying session in order to bring the aircraft back it cannot be totally relied upon as a ';get out of jail free card'; because it can fail for all the same reasons as GPS position hold and altitude hold flight modes. It';s not unknown for people to engage return to home mode and have the aircraft fly off in the opposite direction never to be seen again. In a worst case scenario this could be extremely dangerous or at least very expensive.

If the aircraft is well configured, has a good quality working GPS lock and isn';t experiencing magnetometer interference then return to home can be expected to work very well.

Most flight controllers allow the pilot to configure this flight mode to make the aircraft climb to a preset height before it attempts to return to it';s launch position. This means that it';s possible to ensure the aircraft will clear obstacles like trees before it starts moving back to it';s launch position. If this isn';t set properly then return to home can be a liability as the aircraft will blindly fly into anything in between itself and the pilot. It also means that if the pilot is flying underneath an obstacle then activating GPS return to home mode could be a very bad idea.

Many people with well tested and reliable GPS return to home systems will use them as a form of failsafe. In the event that the aircraft loses all reception from the radio controller the multirotor can be configured to automatically fly back to the pilot before landing at their feet. Despite the fact that GPS can be less than perfectly reliable, on a well configured multirotor it will probably be better than never seeing the thing again. So in an emergency using return to home in an attempt to recover the aircraft can make a lot of sense.

[quadfather]

Chris, this is a fantastic write-up IMO.  You';re right, it is a long read which may put off some people, but those with a keen interest will read it.  It';s well structured and has a good flow from basic to advanced modes and topics.

My only suggestion would be to refer to throttle and pitch sticks, instead of left and right sticks.  Not everyone flies mode 2.  Maybe a small paragraph about different controllers and identifying which is which by the spring action of the sticks.  It only needs a few words.

Apart from that minor niggle a good read.  Perhaps you could save it as a PDF and attach it to the post.  People might want to read it offline.

[teslahed]

Thanks Tim. I think you are right about the mode two / sticks thing. I will try and correct that.

[guest325]

Nice well explained article; should prove to be useful to newbies (and some of the more experienced)!

[kilby]

It may be useful to include a link to Joshuia Bardwells tutorial playlist for some of the flight basics as a picture (or video) as a visual example is sometimes worth a thousand words.

For the most part he uses the demo version of Freerider (at least until he gets to the Acro section)

https://www.youtube.com/watch?v=391D5dX7LKg&list=PLwoDb7WF6c8kjYXam4m3msvRbkORU41GY

[nub]

quite a bit of work gone into that Tesla a very nice write up indeed.

[teslahed]

I wonder where my article went? It appeared to have been deleted from this thread. I have just reconstituted it from my saved version.

[teslahed]

.

[ched]

It appears there is an issue that seems to be about html entities (things like apostrophes and quotes being saves as their equivalent html codes)  and them being displayed. Gaz is working on a fix. It appears that the empty posts are in the database just when the forum software goes to display them it wont because of the html entities.

Nice guide by the way, good to get it back to top of the forum now and again.

[Gaza07]

I';m not sure its the apostrophes now because other posts new and old have them and are displayed its a very strange problem, Its best not to edit these missing posts because the actual post is till there in the database but if you edit it and re-save it blank then you will have replaced what was there with a blank entry,
I you edit it and put the original post back then that will be fine, I';m trying to work out how to fix this issue but currently have no fix,

Ched99uk can you make a post with some apostrophes and then see if they are displayed 

[ched]

I tried earlier but due to the restore the post disappeared  ;D
So here goes again.
So start with an apostrophe ', then try double quotes ", and lastly an ampersand &.
Just adding a bit of text to the end of post.

[Icefever]

@ Teslahed....Many thanks for this very comprehenive write-up,  as a complete newbie it's a great read,  I'll more than likey re-read it a few more times before I'm done building my quad.