How to: Quadcopter
It seemed only right after the WatchDrone project I finished a few weeks ago to go in-depth into how drones actually work. I’ve split the topics into two parts to prevent a massively long post. Part 1 (this post) will go into the electronics and hardware on board a drone that actually make it fly. Part 2 will cover the video systems used to control drones beyond LOS (Line of Sight) flying.
Quick disclaimer: you’ll need some basic electronics/engineering knowledge to fully understand some parts of this article.
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Frame
There is a range of different types of multirotors, designed for specific purposes. They can be classified by the number of motors (and rotors) they have. Octocopters have eight motors, and are mainly used for carrying large payloads, such as cine-cameras. Their high motor count means they can lift greater weights, and have redundancy in case a motor or speed controller fails mid-flight. This gives them a higher mass, and therefore they are less maneuverable. Hexacopters have six motors, and are used in a similar way to octocopters but with reduced payload capacity (and a lower price tag).
Quadcopters (the most common type of multirotor) can carry high-quality cameras for aerial photography, such as DJI’s consumer offerings, but can also be configured for racing. Their lower mass due to their lower motor count makes them more maneuverable. Tricopters are somewhat of a niche, as they need a complex tail rotor linkage for yaw (rotational) control, which makes them easier to break and harder to fix, as well as reduced thrust compared to quadcopters. However, because their yaw movement is controlled by tilting the rear motor and not simply changing the RPM of opposing motors, they are very ‘swoopy’ in flight, which makes them really fun to fly recreationally (my first build was a tricopter).
Each type of multirotor has different sub configurations. Octocopters can have either the standard layout of 8 arms, with one rotor per arm, or the X8 format, with four arms and one motor on top and one below the end of each arm. Similarly, hexacopters can also use six arms, or have a Y6 layout, with a frame similar to a tricopter’s but with motors on each side of the ends of the arms. In these layouts, vertically corresponding motors must be contra-rotating (spinning in opposite directions) so the net movement of motors on each arm is cancelled out to 0. If they spun in the same direction, there would be a huge amount of torque produced, impacting the flight performance. These double-motor layouts face tuning issues due to prop wash, as the upper rotor forces choppy, turbulent air into the propellor below, creating oscillation. This means the on-board controller must not only keep the multirotor level but also dampen the vibrations caused by the upper propellers. For this reason neither the X8 nor the Y6 layouts are commonly used.
Quadcopters also have different layouts: X and H. Pure X frames are extremely maneuverable, as all their mass is concentrated in one point. H frames offer the most room to attach equipment but become harder to fly fast due to their distribution of mass and their greater surface area. + frames are simply a rotated variation on the X layout. Most modern drones use a combination of the two, in a ‘stretched-X’ or Deadcat configuration.
Tricopters can also be broken down into two categories. Y frames are the most common due to their maneuverability. T frames are rarely used but prevent propellers from appearing in the shot of a forwards-facing camera (with a narrower FOV than the angle formed by the tangents of the circumferences of the propellers).
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Multirotor frames are made most commonly out of carbon-fibre sheet. Racing drones often have a single bottom plate made from >= 4mm sheet, meaning they are extremely rigid and can withstand high-energy impacts. Racing drones have dedicated mounts that rotate the pilot’s FPV camera to keep it level even when the drone is tilted forward, and for an action camera to record the flight. Larger platforms have replaceable arms, which are easier and cheaper to replace compared to a one-piece baseplate, but add extra weight due to their mounting hardware. Some frames are designed to fold to make them easier to transport, but this is a feature omitted in most racing drones as it also adds weight. The size of a multirotor’s frame is measured as its diameter in mm. The recommended propellor size (in inches) is also used as a classification for racing drones. For instance, a 210mm racing drone with 5 inch propellers can be classed as a 5 inch build.
Power
On to the power source. Drones use Lithium-Polymer batteries (LiPo) due to their ability to discharge at very high rates (measured by their C rating), resulting in improved throttle response and a higher power output from the motors. LiPo batteries can be categorised also by voltage. Each LiPo cell is 3.7V. A 1 cell LiPo (1S) is therefore 3.7V, a 2S 7.4V, a 3S 11.1V, a 4S 14.8V and so on. LiPo packs’ capacities are measured in mAh, with the typical racing drone using a 1200-1600 mAh pack, giving ~6 mins of slow flight and ~3-4 mins high-speed flight. To simplify the wiring of the multirotor, a power distribution board is used to provide power to each component. However, most modern racing drones use flight controllers with integrated power distribution boards, saving weight and further simplifying the wiring setup.
The wires used to connect the battery to the power distribution board are measured in AWG, which varies based on the voltage of the pack and its C rating. The wires used on multirotors are typically coated in silicone (not silicon), as it is more durable and has a higher melting point than PVC insulation. This means wires with smaller diameters can be used, reducing weight. This is possible as although smaller diameter wire has higher resistance, increasing the heat produced at high currents, the silicone insulation will not melt/burn like PVC insulation would with the same heat applied, meaning that silicone wires can handle more current.
These wires are split by a pair of connectors: one male and one female. There are a range of different connectors that can be used by hobbyists, which are categorized based on the amount of current that they can pass through. The ones used most by RC enthusiasts are JST, EC3, XT30, XT60 and Deans. All LiPos with two or more cells also have a balance lead, used by the LiPo's charger to change the voltage of individual cells.
These wires are split by a pair of connectors: one male and one female. There are a range of different connectors that can be used by hobbyists, which are categorized based on the amount of current that they can pass through. The ones used most by RC enthusiasts are JST, EC3, XT30, XT60 and Deans. All LiPos with two or more cells also have a balance lead, used by the LiPo's charger to change the voltage of individual cells.
Computation
The flight controller is the main ‘computer’ on board the multirotor. It receives inputs from the pilot and outputs these commands in a format that can be used by other components to change individual motor speed. At a minimum, it will contain orientation and acceleration sensors (gyroscope and accelerometer) so that it is able to check that the commands have had the desired effect.
Flight controllers are increasingly being designed for specific purposes. Most frames have mounting points for a flight controller that is proportional in size to the frame. It’s usually a good idea to use rubber standoffs for isolation, as they reduce the vibrations that the FC receives, making the multirotor fly more smoothly. The standard sizes for flight controllers are as follows: 45x45mm, 30x30mm, 20x20mm and 16x16mm. As an example, a 210mm frame will be designed to work with a 30x30mm flight controller, as it fits the frame and does not sacrifice on any features a pilot flying a 5 inch quadcopter might want, eg PDB, OSD etc.
Flight controllers can carry out many functions. On larger aerial photography builds, the flight controller would likely be connected to a GPS module, so that the multirotor would be able to return to its take-off position in case of a loss in communication with the pilot’s radio. For most builders, the most interesting and relevant features that a flight controller could include are: PDB, ESCs, OSD, VTX, RX and data logger.
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| A 5th generation Naze 32 flight controller |
Over time, the main processors inside flight controllers have become faster and more advanced. They are classified as the F series of chips to simplify things: F1, F3, F4 and F7, with a higher number indicating a greater speed of the chip and more ports that can be used. F4 boards are currently the best for most users due to their stability, simplicity and speed. For a more in-depth look into flight controllers, check out Oscar Liang’s article.
Power delivery
However, the flight controller cannot directly interface with the motors of the multirotor (unless it has integrated ESCs). The ESCs (Electronic Speed Controller) of a drone take the flight controller’s output and directly change the speed of the motor they are connected to via PWM (Pulse Width Modulation). If the flight controller needs to speed up motor 2, it outputs the signal to the ESC connected to the M2 output of the board. The ESC then can directly control M2’s speed.
The key differentiation factors among ESCs are in their input voltage range and their continuous/burst current output. ESCs are rated to work with specific voltages, and will burn out if they are given too high of a voltage (LiPo cell count). Their maximum continuous current output is the highest current they can supply to the motor for an indefinite amount of time (given in Amps). Their burst current output is the highest amount of current they can possibly give to the motor, which will only be for a brief amount of time before the ESC burns out.
ESCs are now using the DShot protocol, which means they receive their inputs in digital form from the flight controller. This results in faster response time, error checking between the two boards and more precise throttle control. DShot is measured in the number of bits that can be transferred between the FC and ESC in one second. For instance, a DShot600 ESC can receive 600,000 bits/second.
Next up, the motors. There are two types of electric motor for multirotors: brushed and brushless. Brushed motors are cheaper but produce much less thrust, and will ultimately fail in a set period of time due to their internal design, which uses ‘brushes’ to transfer current to the motor’s magnets. These brushes wear as a result of the friction produced when they spin. Brushed motors are always found on low-end micros (>120mm) designed for beginners due to their easy-to-manage (read: poor) performance and their low price.
Brushed
The naming scheme for direct-drive brushed motors follows the format: core-diameter, length-kKV (a gear drive would add an extra xT on the end, where x is the number of teeth on the gear). As an example, the CL-0820-18 from Micro Motor Warehouse is coreless, 8mm in diameter, 20mm in length and is rated at 18,000KV. The KV rating is very useful when selecting a motor for a build. KV refers to the RPM of the motor if 1V is applied to it with no load (no propellor). For the CL-0820-18, this means that when using a 1S LiPo (3.7V), the motors will be able to spin at a maximum rate of 66,600RPM.
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Brushless
Brushless motors are more expensive than brushed motors but are superior in every other way. Brushless motors do not have a limited lifespan, as they have no brushes to wear out. As a result, they are almost silent (without load), have a faster response time, can be electronically controlled for more precision and are much more powerful. Their speed and torque are controlled by pulses in the current delivered from the ESCs (PWM). This is in contrast to brushed motors, which simply rely on changes in voltage to change their speed.
Brushless motors come in two forms: inrunners and outrunners. Inrunners typically have much higher KVs than outrunners but comparatively low torque. The rotational element of the motor is sealed inside of a ‘can’, which makes them ideal for use on RC cars in conjunction with a gearbox. Inrunners are also used for EDFs (Electric Ducted Fans), which make up for the lack of propellor pitch in their number of blades, making the load suited for high-RPM motors. EDFs are found on electric RC jets, and can make a great sound even though they don’t run on kerosene. Outrunner motors are used in all hobby-grade multirotors, and have been the catalyst for the growth and advancement of the hobby, along with improvements in battery and camera technology.
For a real-world example of a brushless outrunner, let’s take a look at the F40III 2306 2750KV from T-Motor. T-Motor has been making motors for quite some time at this point, and has some solid options for multirotor builders. F40III is the model, the third iteration of the F40 lineup. 2306 refers to the stator of the motor. The stator is the part of the motor that doesn’t spin, which contains the wire coils. Specifics about a motor's coils are largely irrelevant to the average buyer, as they are usually fairly similar from motor to motor, although some high-end models use silver instead of copper. Keep in mind that the thickness of the wire used and the number of windings in the coils directly affect the thrust the motor is able to produce, but as such will increase the current drawn by the motor. The first two digits represent the stator’s diameter, and the second two the stator’s height. 2306 means the stator will be 23mm in diameter, and 6mm in height. The strength of the magnets and the air-gap between both the stator and magnets help to increase the power of the motor.
The same rules for KV and brushed motors also apply for brushless motors. A higher KV means more thrust will be produced, but more current will be drawn from the ESCs and battery. Therefore, it is recommended to use high KV motors, like this variant of the F40III, with lighter-weight, lesser-pitched, and fewer-bladed propellers to prevent ESC burn-outs and ruined batteries. Lower KV motors will be less damaging for the drone’s power system overall.
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It’s also important to consider the efficiency of the motor. The efficiency is measured in g/W, and is different depending on the propellor used. Motor weight, although unimportant for larger aerial photography builds, is important for smaller multirotors as it will affect the motor’s ability to change speed, impacting the multirotor’s maneuverability. miniquadtestbench.com does excellent tests on most new brushless motors for mini racing quadcopters. Here’s the page for the F40III, where I got all of the information on the motor. As you can see, there is an incredibly useful interactive chart which shows the efficiency, thrust and amps drawn by the motor for a range of 5 inch propellers. There is also a full data set with throttle positions and testing conditions for this motor at this link. If you navigate to the Data Explorer feature of the site, you can compare every motor with each aspect of its tested performance. This site is a very powerful tool when researching motors for a mini-quad build.
Propellers
Propellers are another key aspect of building a multirotor. For racing drones, propeller size has a knock-on effect on the entire build, changing the frame size and subsequently the class that it can be raced in, as well as the motors and ESCs used. Propellers also have their own naming scheme: diameter X pitch X number of blades OR diameter, pitch X number of blades. 5x4x3 (or 5040x3) propellers will therefore be 5 inches in diameter, have a pitch of 4 inches and will have three blades.
The diameter of a propellor directly affects how much thrust can be produced, as a larger diameter will result in a greater surface area, and subsequently will produce more thrust but also draw more current. Since micro multirotors are physically limited on the prop diameter that they can use, they make up for the loss in thrust from the propellers with higher KV motors, eg the CL-8520-18 Dark Edition motors we looked at previously.
The pitch of a propellor is the theoretical distance the propellor would move through a solid material in one rotation. A greater pitch increases the thrust produced, but also the current drawn by the motor. The number of blades the propellor has also determines the weight and surface area of the propellor, meaning that an increase in the number of blades will also directly result in more thrust.
The material of a propellor is also a factor that should be understood. For a larger build, where crashing is less likely to be caused by the nature of the flying it does, carbon fibre propellers are often the better choice as they are stiffer and will not flex compared to plastic propellers. Due to this, they are very dangerous because they will cut straight through anything like a blade, instead of flexing or breaking off. In addition, their high price means that they are cost-prohibitive for miniquad pilots, who will break their propellers on a very regular basis.
Control
Furthermore, it is unlikely for multiple 2.4GHz systems to cause interference with one another, unlike when using older systems. This can be done in multiple ways. The most common way is to spread the transmission across a range of frequencies. This is achieved by switching the chosen frequency incredibly quickly within a set band or range. Because the rate at which they shift is so fast, it appears as if they are transmitting across the entire band, and even if a competing system comes into the same band, it will not interfere to the same degree as on an older setup.
Transmitting
The term ‘transmitters’ refers not only to the radio module, but also the control system used by the pilot to fly them. Model aircraft have two sticks, connected to 2 axis potentiometers via a gimbal. These gimbals can be swapped out, and some pilots upgrade them for improved stick-feel. Other important aspects of the radio are the module (containing the actual RF transmitter) and antennas, both of which I’ll cover in more detail in Part 2 of this post.
A transmitter’s overall quality can often be ascertained from the number of channels it has. A 4 channel TX (transmitter) is the bare minimum needed to operate a drone. This gives you throttle, yaw, pitch and roll control. On multirotors, that’s often all you need. However, if you have a complex scale aircraft with flaps, lights, landing gear and a motorised canopy, the extra switches (channels) of a 10 channel radio will be useful to control the model. Because adding extra channels increases the complexity of the radio, cheaper radios tend to have fewer channels. That’s why better quality radios, in terms of gimbals and overall build quality, are usually the ones with most channels. That’s not to say a lesser-channel-count radio isn’t good, as there are exceptions like the Turnigy Evolution (4CH) which is certainly not of poor quality, but in general it’s a surefire way to tell how good a radio is. The receiver must match both the protocol and number of channels of the radio for the most functionality. You can still use an FrSky D4R-II (4CH receiver) with an FrSky Taranis X9D+ (9CH transmitter), but you’ll only be able to use four of the nine channels available from the radio.
Something to watch out for when buying a transmitter is the ‘mode’ it comes set in. The mode of a transmitter determines which axis of each stick the four basic control channels (throttle, yaw, pitch and roll) correspond to. For instance, a Mode 2 radio has the throttle and yaw on the left stick, with pitch and roll on the right. Mode 4 swaps the yaw and roll control and is used by some miniquad pilots but is nowhere near as widespread or popular in the multirotor industry as Mode 2. Whilst it is usually possible to change the mode of a radio after buying it, you’ll have to open the radio up and do some work on it.
Receiving
When it comes to receivers, you can either use a universal protocol or a proprietary one. PPM, PWM and PCM are supported by most flight controllers but are mostly inferior to newer proprietary ones. As an example, SBUS, compatible with FrSky and Futaba radios, is a digital protocol (unlike PPM, PWM and PCM), meaning that it only needs to use three wires to connect to the flight controller. This not only saves a large amount of weight on smaller builds but also means that even micro receivers such as the FrSky XSR are able to use up to 16CH. Furthermore, compared to PPM, SBUS has lower latency, making the drone more responsive to the pilot’s control.
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Another aspect of a radio system to consider is the number of receiver offerings available. One reason to consider the options from FrSky and Spektrum (the most popular systems for drone building) is because of the wide variety of receivers that are made for each. Having a large range of options means there will be a receiver that suits your build’s needs in terms of FC interface, number of channels, weight and price, for anything from micro brushed quads up to large AP octocopters.
So that’s Part 1 complete! Hopefully you found this post informative. Keep an eye out for Part 2, where I’ll cover FPV and go more in-depth with RF transmitting and receiving.
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