It is something that can seem like magic. When you are flying a drone you are perfectly aware that there is a huge amount of physics, calculations, software, and engineering that goes into making your flight super enjoyable. However, for some people just operating the drone is not good enough. I know that I am one of the people that needs to know exactly what is happening in my drone so that I am a better pilot and understand how to resolve issues if they pop up. In this article I’m going to go over all of the ways that a drone changes direction whether that is up, down, left, right, forwards, backwards, and yaw left and yaw right.
A drone changes direction through changing the rates at which each of its propellers rotates. While a drone is hovering, adjacent propellers are rotating in opposite directions to keep the drone stable. By changing the rate of each of the motors we can manipulate the drone through the air in all three dimensions.
For the drone pilot there is very little thought that goes into what is happening and they can simply move the joysticks to control the drone. However, the drone is doing so much more work because it needs to calculate its own acceleration, momentum, height, and altitude whilst also combating any environmental conditions like updraughts and horizontal winds.
The physics of keeping a drone in the air is actually very complicated.
The physics of keeping a drone in the air
At the heart of the drone is the flight controller which sends the motors information through an electronic speed control unit about which way they should be rotating and also how fast. The flight controller will also look at the GPS data, the acceleration and direction that the drone is pointing in before sending the information to the motor so that it remains in a stable hover at a fixed GPS location.
Each one of the motors spins a propeller. The propellers are used for propulsion and controlling the movement of the drone. These propellers at like the fins of a fan which pushes air towards the ground. As the air is pushed towards the ground the drone is able to suck itself into the air. There is a low pressure that is created at the leading and top edge of the propeller so the drone literally sucks itself into the air.
If you want to hover the drone has to displace as much air as it weighs. Displacing more air will cause the drone to climb and if the propellers spin a little bit slower and displace less air the drone will descend.
The propeller direction of the drone is very important as each of the drone motors needs to spin in the opposite direction to the motors that it is next to. This cancels out any angular momentum and so the drone remains in one place facing in one direction.
Drone propeller direction
My DJI Mavic air has four motors and propellers. The way that each of the propellers spins is determined by the motor. And you need to make sure that the propellers are inserted in the proper motors otherwise the drone would simply spin out of control.
It is easy to work out which way the motor should be spinning by looking at the leading edge of the propeller as it spins in the direction of the front edge of the propeller.
For most drones the drone propeller direction for each motor is as follows:
- Front Left – Clockwise motor (CW)
- Front Right – Counter Clockwise motor (CCW)
- Back Left – Counter Clockwise motor (CCW)
- Back Right – Clockwise motor (CW)
I have annotated this on the photo of my DJI Mavic below:
All of the movements of the drone are controlled by spinning the propellers in a variety of different ways. To control the movements some propellers are slowed down whilst others are sped up. Sometimes this happens altogether (as in the case of climbing or going up) and sometimes they happen at different times on different sides of the aircraft (as in the case of forward and backward movement).
In the next section I’m going to go over all of the main movements of a drone and illustrate exactly how each of the main movements are achieved.
The main movements of a drone
flying a drone involves from a pilot’s perspective simply moving the joystick through their range of motion. That simple joystick action translates into quite a complicated movement from the drone. The left joystick of the drone is responsible for climbing as well as yaw (rotating clockwise or anticlockwise in a fixed location).
The left-hand joystick
The left-hand joystick of the controller makes the drone climb and descend and also turn clockwise and anticlockwise. Essentially, it is all of the actions of the drone where it stays in one GPS location.
Up / climb
When you push up on the left-hand joystick of a drone controller all of the drone propellers increase in their speed or revolutions per minute. The revolutions per minute increases which generates more upwards thrust by displacing more air and forcing it towards the floor.
All of the propellers increase in rotational speed.
The propellers have to be very careful to accelerate at exactly the same rate otherwise another direction will be created in its movement. It takes a lot for a drone to spin each of the propellers at precisely the right number of revolutions per minute to climb and not move from its fixed GPS location.
Down / descend
For a drone to decrease in height the exact opposite of the climb happens. That is, each one of the drone propellers decreases in the number of revolutions per minute. This essentially, allows gravity to be a greater force than the thrust generated by the propellers.
As a drone is descending to stop it needs to slightly increase the number of revolutions per minute to increase the thrust so that it equals the pull of gravity towards the earth. In Toy Story Buzz light year is told that he is not flying here simply falling with style. This is exactly what drones are doing when they are descending. They are simply allowing gravity to pull them towards the earth which is a fancy and controlled form of falling.
Yaw is when the drone turns clockwise or anticlockwise while it remains in one GPS location. That is, that the drone is not moving forwards or backwards. This movement requires quite a complicated calculation on behalf of the drone’s software.
When the drone is turning clockwise or anticlockwise it needs to control the propellers which are diagonal to each other. If the right diagonal propellers rotate faster they generate a force due to the increased angular momentum allowing the drone to rotate to the left. If the left diagonal propellers rotate faster they generate the opposite movement.
So, anticlockwise or clockwise movement of the drone is caused by the propellers diagonally opposite each other spinning faster or slower.
The right-hand joystick
The right-hand joystick of the drone controller is what I have jokingly called the fun joystick. This is because this allows you to zoom your drone all over the place. The great thing about a drone is that not only can it move forward or backwards but you can also move from side to side.
The orientation of the propellers around the drone allow you full range of movement in the horizontal plane. By combining different joystick combinations you are also able to yaw and move at the same time. The combination of different movements is all still done by the minute control the software has over the revolutions per minute each motor is spinning.
All of the movements of the right-hand joystick create an increase or deep crease in the rotational speed of the propellers which are next to each other. In the sections below we will talk about what each one of the movements correlates with.
If the drone wants to move to the left to correspond with moving the joystick to the left the right-hand side propellers spin faster than the left-hand side propellers.
This causes the left-hand side of the drone to dip slightly whilst the right-hand side of the drone raises slightly. The overall thrust of the rotors remains the same as that enables the drone to stay at a fixed altitude but the slight difference between the left and right hand side of the drone rotors causes the drone to drift to the left.
Movement of the drone to the right by moving the joystick to the right hand side causes the left-hand side propellers to increase in terms of the number of revolutions per minute the motor is spinning and the right-hand side of the drone propellers reduce ever so slightly.
As with the case of moving left this causes the right-hand side of the drone to dip slightly whilst raising the left-hand edge of the drone. The overall thrust of the motors remains the same as this enables a drone to maintain a fixed altitude.
To move a drone forward, the front edge of the drone needs to dip slightly whilst the back of the drone raises.
This is achieved by lowering the revolutions per minute of the front to motors and raising the back to motors. This difference in propeller spinning causes the drone to move forwards.
To move the drone backwards the back edge of the drone needs to dip slightly whilst the front edge of the drone raises slightly. You will notice that this is the exact opposite to the forward motion.
For each of the movements of the right joystick you will be able to observe which way the drone tilting to achieve the movement. Sometimes, particularly in high wind environments, and moving very rapidly the drone will not hold its vertical position very well.
I notice that when I move my drone backwards very quickly at a relatively high altitude that the drone would simply drop in height. So the calculations aren’t perfect but they can easily be fine tuned by providing a manual trim whilst flying.
A simple diagram for all of the drone movements
Here is a simple diagram for each of the drone’s movements which demonstrates how each of the rotors moves as it is performing each one of the movements discussed above. That is, up, down, clockwise, anticlockwise, forward, backwards, left, and right.
In the above diagram the propellers which spin faster during each of the movements is highlighted with a red plus. The left-hand joystick is shown on the left-hand side whereas the right hand joystick movements are shown on the right-hand side. Please note that if the drone is maintaining a constant height the other propellers spin at a slightly reduced rate as not to change the overall thrust that the drone is producing.
All of these combinations are enough to move the drone completely throughout 3D space.
Other things the drone is doing.
A drone is also doing a load of fancy calculations to keep itself in a stable flying pattern. The drone relies on a whole range of different sensors which work together to feed information to the microcontroller board which then is able to keep the drone in a stable position.
Some drones are not able to auto hover or maintain a stable altitude and that is because they lack some of the components that is required to make those calculations.
The range of sensors that are in a drone includes:
- GPS – this obtains the coordinates of the drone by pinpointing its location relative to a number of satellites that are in geo-stationery orbit above the earth.
- Barometer – a barometer detects the pressure and allows the drone to indirectly compute its height. The higher the drone the lower the air pressure.
- Magnetometer – this detects the earth’s magnetic field and is able to calculate the drones orientation relative to the earth’s magnetic field. This is essentially the drone’s compass. It is also one of the things that need to be regularly calibrated especially if you have travelled a fair distance to fly your drone.
- Accelerometer– it measures the acceleration of the drone but it is mainly used to know the direction in which gravity is pulling the drone.
- The gyroscope – a gyroscope provides the angular velocity of the drone and is used to compute its orientation in a 3D environment.
These sensors are constantly feeding information to the control unit which is used to determine the rotational speed of the propellers. Drones use a PID controller to work out how to move the drone through a 3D space.
PID stands for Proportional, Integral, Derivative and can be tuned on a range of drone software. Don’t be put off by the naming of these gains they are really just a fancy way of saying the following:
- P looks at the present error – if the current setting is far away from the set point the P setting will push to keep it close to the set point. The further away it is the harder it will push.
- I is the knowledge gained from past errors – this looks at past errors (caused by continuous external forces) and will counteract them.
- D is a prediction of future errors – as P starts pushing the value close to the set point the D value will stop you from massively overshooting.
The Effect Of Each PID Parameter
As you start playing with your drone’s PID values you’ll notice that each value affects the drone in different ways. Let’s take a look at each value in a little more value.
The P value (also known as the gain value – is one of the most important aspects of regulating the flight of a drone.
The value determines how hard the drone should work to correct itself to achieve the desired flight path (controlled by the controller transmission). It can be both too high or too low:
Too high and the drone will oscillate. This is symptomatic of aggressive overcorrection by the drone and you’ll see high-frequency oscillations.
Too low and the copter will feel like it is slow to respond and you may even hear the motors spool up slowly.
To find a good p-value you should gradually increase the P value until the drone starts to oscillate then set this value to 50%.
I gain is the setting that determines how hard the drone should respond to external forces – like holding it’s position in wind or due to an off center of mass ( quite easily caused by upgrading components of your drone.
Too high and the drone will feel unresponsive
Too low and you’ll find that you have to manually correct your drones flight pattern.
You want to leave the I value as low as possible without have to correct the flight pattern manually.
The D gain is like a shock absorber for the P value.
If the D value is too low the drone will not react rapidly enough.
If the D value to too high the drone will oscillate with rapid small amplitude oscillations. It can also decrease the quads response and cause the drone to feel sluggish.
Increase the D gain until the drone starts to oscillate will small rapid oscillations. Reduce to 50% of this value.
The PID controller is responsible for keeping your drone nice and stable. It also provides smooth changes of direction and if you are building your own drone it is so important to get this feature set right so that your drone is a pleasure to fly and does not do anything at the ends of joystick movements.
Can a drone be programmed?
There are many drones that can be programmed by using what they call “waypoints”. Waypoints are essentially GPS locations in the sky which cause the drone to automatically fly through these specific pre-defined points which have a longitude and latitude and an altitude which the drone must pass through.
There are many waypoints and automatic drone pass that can be downloaded online and sideload it into your drone flying software.
Drones are quite often programmed for research purposes and if you want to know about the best programmable drones check out my in-depth article about the best programmable drones research [full guide] – click here.
Drones fly and change direction through changing the revolutions per minute on each of its four propellers and motors. This results in each of the four motors generating a different amount of thrust and therefore moving the drone through the sky or manipulating it at a fixed location as in the case of yaw (using angular momentum).
The great thing is that a lot of this software and the calculations are all automated which means that your flying experience is not dependent on you being able to understand how each of these movements are created but rather it is as simple as moving a joystick and enjoying the flying process.
By getting to the end of this article you are well ahead of many of the drone pilots who do not understand how to a drone changes direction. Now that you know all there is to know about each one of the movements you are able to control your drone better and anticipate and troubleshoot any problems which may occur during your flight.