Now that you’ve chosen or built a frame, the next step is choosing the right powerplant. Since most of the current drones are electric, we will focus on generating purely electric propulsion through brushless DC motors. The structure of the power plant includes motors, rotors (propellers, abbr. props), ESC and battery.
Which motors you use in your build will determine the maximum load the drone can lift, as well as how long it can stay in flight. The power plant must necessarily consist of motors of the same brand and model, this approach will ensure its balanced operation. At the same time, it is worth noting that even absolutely identical (Brand/Model) motors can have a slight difference in speed, which is subsequently equalized by the flight controller.
brushed vs brushless
In brushed motors, the rotor with the winding rotates inside the stator, on which the magnets are fixed rigidly. In brushless motors, everything is reversed; the winding is attached rigidly to the inside of the stator, and the magnets are mounted on the shaft and rotate. In most cases, you will only consider brushless DC motors (BC). Motors of this type are widely used in the ham radio industry to assemble products ranging from helicopters and airplanes to drive systems in cars and boats.
Pancake brushless motors are larger in diameter, flatter and generally have higher torque and lower KV (details below). Small UAVs (usually palm-sized) often use small commutator motors due to the lower price and simple two-wire controller. Even though brushless motors come in different sizes and have different characteristics, choosing a smaller size does not mean that it will be cheaper.
Inrunner vs Outrunner
There are several types of brushless DC motors:
- Inrunner — inner rotor. The winding is fixed on the stator, the magnets are mounted on the rotor shaft which rotates (usually used on RC boats, helicopters and cars due to high KV).
- Outrunner – outer rotor. The magnets are fixed to the stator, which rotates around a fixed winding. The lower part of the motor is fixed. (as a rule, motors of this type have more torque).
- Hybrid Outrunner — technically it is an “Outrunner”, but implemented in an “Inrunner” package. This approach made it possible to combine in one type the torque of the “Outrunner” and the absence of external rotating elements as in the motors of the “Inrunner” type.
KV rating — max. the number of revolutions that a motor can develop without losing power at a given voltage. For most multi-rotor UAVs, a low KV value (eg, 500 to 1000) is desirable as this helps to ensure stability. While for acrobatic flight a KV value between 1000 and 1500 will be relevant, in tandem with smaller diameter rotors (propellers). Let’s say the KV value for a particular motor is 650 rpm, then at 11.1V the motor will rotate at a speed of: 11.1 × 650 = 7215 rpm, and if you use the motor at a lower voltage (say 7.4V) , then the rotational speed will be: 7.4 × 650 = 4810 rpm. It is important to note, however, that using a lower voltage generally means that the current consumption will be higher (Power = Current × Voltage).
Some manufacturers of brushless motors may indicate in the specification information about the maximum possible thrust (Thrust) generated by the motor, coupled with the recommended main rotor. The unit of thrust is usually kilogram (Kg/Kg), pound (Lbs) or Newton (N). For example, if you are building a quadcopter and you know the thrust value of a single motor = up to 0.5kg in combination with an 11-inch main rotor, then four of these motors will be able to lift at maximum thrust: 0.5kg × 4 = 2kg. Accordingly, if the total weight of your quadcopter is a little less than 2kg, then with such a power plant it will take off only at maximum speed (max. thrust). In this case, it will be relevant to either choose a more powerful “motor + main rotor” combination, which will provide greater traction, or reduce the total weight of the drone. At max. thrust of the power plant = 2kg, the weight of the drone should be no more than half of this value (1kg, including the weight of the motors themselves). A similar calculation can be made for any configuration. Let’s assume that the weight of the hexacopter (including the frame, motors, electronics, accessories, etc.) is ‑2.5kg. So each engine for such an assembly should provide (2.5kg ÷ 6 motors) × 2 = 0.83kg of thrust (or more). Now you know how to calculate the optimal thrust of motors based on the total weight, but before making a decision, we suggest that you familiarize yourself with the sections below.
- Connectors: DC commutator motors have two connectors “+” and “-”. Swapping wires in places changes the direction of rotation of the motor.
- Connectors: Brushless DC motors have three connectors. To learn how to connect them, as well as how to change the direction of rotation, refer to the “ESC” section below.
- Windings: windings affect the KV of the motors. If you need the lowest possible KV but prioritize torque, Pancake brushless DC motors are your best bet.
- Mounting: most manufacturers have a common mounting scheme for BC DC motors, which allows frame companies not to resort to making so-called adapters. The template is usually metric, with two holes spaced 16mm apart, and two more holes spaced 19mm apart (at a 90° angle to the first).
- Thread: The mounting thread used to mount the brushless motor to the frame may vary. Common metric screw sizes are M1, M2 and M3, imperial sizes can be 2–56 and 4–40.
2. Main screws (Propellers)
The rotors (propellers, abbr. props) for multi-rotor UAVs originate from the propellers of radio-controlled aircraft. Many will ask: why not use helicopter blades? Although this has already been done, imagine the size of a hexacopter with helicopter blades. It is also worth noting that the helicopter system requires a change in the pitch of the blades, and this significantly complicates the design.
You may also ask why not use a turbojet, turbofan, turboprop, etc.? Of course, they are incredibly good at providing a lot of thrust, but they also require a lot of energy. If the primary purpose of the drone is to move very quickly, rather than hovering in tight spaces, one of the above motors might be a good option.
Blades and diameter
The rotors of most multi-rotor UAVs have two or three blades. Propellers with two blades are most widely used. Don’t assume that adding more blades will automatically increase thrust; each blade works in the flow disturbed by the previous blade, reducing the efficiency of the propeller. A small diameter rotor has less inertia and therefore it is easier to accelerate and decelerate, which is important for acrobatic flight.
Pitch/Angle of Attack/Efficiency/Thrust
The thrust generated by the main rotor depends on the density of the air, the number of revolutions of the propeller, its diameter, the shape and area of the blades, and also on its pitch. The efficiency of a propeller is related to the angle of attack, which is defined as the blade pitch minus the helix angle (the angle between the resulting relative speed and the direction of rotation of the blade). Efficiency itself is the ratio of output power to input power. Most well-designed propellers are over 80% efficient. The angle of attack is affected by relative speed, so the propeller will have different efficiency at different motor speeds. Efficiency is also greatly affected by the leading edge of the main rotor blade, and it is very important that it be as smooth as possible. While a variable pitch design would be best, the added complexity required over the inherent simplicity of multi-rotor means that a variable pitch propeller is almost never used.
The rotors are designed for clockwise (CW) or counterclockwise (CCW) rotation. The direction of rotation is indicated by the slope of the blade (look at the propeller from the end). If the right edge of the blade is higher — CCW, if the left edge is CW. If your drone is designed with inverted motors (as is the case with the Vtail, Y6, X8 configurations), be sure to change the direction of rotation of the rotors so that the thrust is directed downwards. The front side of the main rotor should always be facing the sky. The documentation that comes with the flight controller usually contains information about the direction of rotation of each propeller, for each multi-motor configuration supported by the controller.
The material(s) used to make the rotors (propellers) can have a moderate effect on flight performance, but safety should be a top priority, especially if you are new and inexperienced.
- Plastic (ABS/Nylon, etc.) is the most popular choice when it comes to multi-engine UAVs. This is largely due to low cost, decent flight performance and exemplary durability. Usually in the event of a crash, at least one propeller is broken, and as you master the drone and learn to fly, you will always have a lot of broken props. The rigidity and impact resistance of a plastic propeller can be improved with carbon fiber reinforcement, this approach max. efficient and not so expensive compared to a propeller fully executed and carbon fiber.
- Fibre-reinforced polymer (carbon fiber, carbon fiber reinforced nylon, etc.) — is “advanced” technology in many respects. Carbon fiber parts are still not very easy to manufacture, and therefore you pay more for them than for a regular plastic screw with the same parameters. A propeller made from carbon fiber is harder to break or bend, and therefore, if it crashes, it will do more damage to anything it comes into contact with. At the same time, carbon propellers tend to be well made, stiffer (providing minimal loss in efficiency), rarely need balancing, and are lighter in weight than any other material. Such propellers are recommended to be considered only after the user’s piloting level becomes comfortable.
- Wood — a rarely used material for the production of rotors of multi-rotor UAVs, since their manufacture requires machining, which subsequently makes wooden propellers more expensive than plastic ones. At the same time, the tree is quite strong and never bends. Note that wooden propellers are still used on radio-controlled aircraft.
Folding props have a center section that connects to two pivoting blades. When the hub (which is connected to the output shaft of the motor) rotates, centrifugal forces act on the blades, pushing them outward and essentially making the propeller “stiff”, with the same effect as a classic non-folding propeller. Due to low demand and the large number of parts required, folding propellers are less common. The main advantage of folding props is their compactness, and in combination with a folding frame, the transport dimensions of the drone can be significantly smaller than the flight dimensions. The accompanying advantage of the folding mechanism is the absence of the need, in case of a crash, to change the entire screw, it will be enough to replace only the damaged blade.
Like UAVs, rotors can have a wide range of sizes. Thus, there are a number of “standard” motor shaft diameters in this industry. As a result, rotors are often supplied with a small set of adapter rings (they look like washers with holes of different diameters in the center) that are installed in the central mounting hole of the prop, in case the diameter of the main rotor hole turned out to be larger than the diameter of the shaft of the motor used. Since not all manufacturers supply props with a set of such adapter rings, it is recommended to compare the diameter of the bore of the purchased props with the diameters of your motor shaft in advance.
The screw can be fixed on the motor based on which of the mounting methods your motor supports. If the motor shaft does not imply any mounting options (threaded connection, various mounting devices, etc.), then special adapters are used, such as propsavers and collet clamps.
- propsaver — is a sleeve with symmetrically arranged side holes into which screws are screwed. The bushing is put on the motor shaft and fixed with side screws. A propeller is installed on top of the sleeve, which, in turn, is fixed with a rubber ring that comes with the sleeve (usually there are several of them in the kit). Due to their unreliability, but at the same time, fast installation, they are best suited for short-term test flights during the drone assembly process.
- Collet clamp – compared to the propsaver, it is a more balanced and reliable adapter. The collet clamp consists of a split cone-shaped sleeve with a threaded connection (collet), clamping sleeve, washer and spinner nut. First, a collet is put on the motor shaft, then a clamping sleeve, then the main screw (propeller) with a washer goes, and the cook-nut closes the clamp design.
Brushless motors with an external rotor (like “Outrunner”), as a rule, in its upper part, have several threaded holes designed for installing various adapters and mounts. An equally popular option for mounting the propeller on the motor shaft is a self-tightening nut. The shaft of such a motor has a thread at the end, the direction of which is opposite to the direction of rotation of the rotor. This approach eliminates spontaneous unscrewing of the fixing nut, ensuring the safe and reliable operation of the drone.
Rotor protection — designed to exclude direct contact of the UAV power plant with an oncoming object, thereby maintaining its integrity and performance, and also to prevent injury from rapidly rotating propellers as a result of a collision with people and animals. The propeller guard is attached to the main frame. Depending on the version, it can either partially overlap the working area of the power plant, or completely (ring protection). Screw protection is most often used on small (toy) UAVs. The use of protection elements in the assembly also brings a number of compromises, including:
- May cause excessive vibration.
- It usually doesn’t take a lot of hits.
- Can reduce thrust if too many mounting posts are placed under the propeller.
Unsatisfactory balancing occurs with most inexpensive propellers. To make sure of this, you don’t have to go far, just insert a pencil into the central mounting hole of the screw (as a rule, in case of imbalance, one side will be heavier than the other). Therefore, it is strongly recommended to balance your props before installing them on the motors. An unbalanced propeller will cause excessive vibrations, which in turn will negatively affect the operation of the flight controller (manifested in the drone’s incorrect behavior in flight), not to mention increased noise, increased wear of the power plant elements and deterioration in the quality of shooting of the suspended camera.
A propeller can be balanced in a number of ways, but if you’re building a drone from scratch, you should definitely have an inexpensive propeller balancer in your arsenal of tools that makes it easy and simple to determine the weight imbalance in the propeller. To equalize the weight, you can either sand the heaviest part of the prop (evenly grind the central part of the blade, and in no case cut off part of the propeller), you can also balance by sticking a piece of adhesive tape (thin) on a lighter blade (add pieces evenly up to those until a balance is reached). Please note that the farther from the center you make a balancing upgrade (grinding or adding tape) to the propeller, the greater the effect based on the torque principle will be.
ESC (English Electronic Speed Controller; Russian electronic speed controller) — allows the flight controller to control the speed and direction of rotation of the motor. With the correct voltage, the ESC should be able to handle max. the current that the motor can consume, as well as limit the current passing through the phase during switching. Most hobby drone ESCs only allow the motor to rotate in one direction, but with the right firmware, they can work in both directions.
Initially, the ESC can be confusing, because there are several wires / pins / connectors available from two sides to connect it (ESC can come with or without soldered connectors).
- Power supply: two thick wires (usually black and red) are provided to supply power from the distribution board/wiring harness to which power comes directly from the main battery of the aircraft.
- 3 connectors: Three connectors are available on the opposite side of the controller, designed to connect to the three bullet connectors (usually included with the motors) on the brushless motor. The use of connectors when connecting the ESC allows, if necessary (in case of failure), to quickly change the controller without using a soldering iron. It happens that the bullet-shaped connectors that come with the motor do not match the connectors on the regulator, in this case, just replace with the right ones. Which of the three is plus and which is minus? The landmark is simple, the incoming positive wire from the battery goes into positive on ESC, similarly with a minus.
- 3‑pin R/C servo connector with thin wires: through which the processing of the signal coming from the receiver is carried out, of which one wire is signal (transmission of a gas signal to the ESC or input), the second “minus” (or ground), and a positive wire (not used if there is no built-in BEC; with built-in BEC is 5V power output, which can later be used to power on-board electronics).
At the time of the birth of aircraft modeling, an internal combustion engine was used as a power plant, and on-board electronics were powered by a small battery. With the advent of electric traction and regulators (ESC), the latter began to include the so-called battery elimination circuit — BEC (in English Battery Eliminator Circuit; or on-board power converter; as a rule, provides an additional source of current with a voltage of 5V at a current strength of 1A, or above). In other words, this is a voltage converter used in the LiPo assembly to voltage for powering the on-board electronics of the drone.
When assembling a multi-rotor, you need to connect all the ESCs to the flight controller, but only one BEC is required, otherwise there may be problems with supplying power to the same lines. Since there is usually no way to disable BEC on an ESC, your best bet is to remove the red (+) wire and wrap it with duct tape for all but one of the ESCs. It is also important to leave the black wire (ground) for the common ground.
Not all ESCs on the market are equally good for multi-rotor applications. It is important to understand that before the advent of multi-engine UAVs, brushless motors were used primarily as a power plant for radio-controlled cars, airplanes and helicopters. Most of them do not require fast response times or updates. SimonK or BLHeli built-in ESCs are able to respond very quickly to incoming changes, which generally makes the difference between a stable flight or a crash.
Since each ESC is powered by the main battery, the main battery connector must somehow be divided into four ESCs. This is done using a power distribution board or a power distribution harness. This board (or cable) splits the positive and negative terminals of the main battery into four. It’s important to note that the types of connectors used on the battery, ESC, and backplane may not match, so it’s best to choose a “standard” connector whenever possible (eg Deans), which is used everywhere. Many inexpensive boards may require soldering, in this case the user decides which particular connector to use in the assembly. The simplest power distributor might include two input terminal blocks, or solder all the positive connections together and then all the negative connections together…
Batteries used in drones are now exclusively Lithium Polymer (LiPo), with some of the compositions being quite exotic, such as Lithium-Manganese or other lithium variants. Lead acid is simply not suitable, and NiMh/NiCd is still too heavy for its capacity and often fails to provide the required high discharge rates. LiPo offers high performance and discharge rate at a light weight. The disadvantages are their relatively high cost and constant safety problems (fire hazard).
In practice, you will only need one battery for your UAV. The voltage of this battery must match the motors you choose. Almost all batteries in use today are lithium-based and contain several cells (cans) of 3.7V each, where 3.7V = 1S (i.e. one cell battery; 2S two cell, etc.). Therefore, a battery labeled 4S will likely have a nominal value of 4 x 3.7V = 14.8V. Also the number of cans will help you determine which charger to use. Note that a high-capacity single-cell battery may physically look like a low-capacity multi-cell battery.
Battery capacity is measured in ampere-hours (Ah). Small size batteries can be as low as 0.1Ah (100mAh), medium sized UAV batteries can range from 2–3Ah (2000mAh — 3000mAh). The higher the capacity, the longer the flight time, and, accordingly, the heavier the battery. The flight time of a conventional UAV can be in the range of 10–20 minutes, which may seem short, but you must understand that the UAV constantly fights gravity during the flight, and unlike an airplane, it does not have surfaces (wings) that provide assistance in the form of optimum lift.
The discharge rate from a lithium battery is measured in “C”, where 1C is the capacity of the battery (usually in amp-hours, unless you’re considering a palm-sized drone). Most LiPo batteries have a discharge rate of at least 5C (five times the capacity), but since most motors used in multi-rotor UAVs draw high current, the battery must be able to discharge at an incredibly high current, which is typically about 30A or more.
LiPo batteries are not entirely safe as they contain pressurized hydrogen gas and tend to burn and/or explode when something goes wrong. Thus, if you have any doubts about the performance of the battery, in no case, do not connect it to the drone or even to the charger — consider it “decommissioned” and dispose of it properly. The tell-tale signs that something is wrong with the battery are dents or bulges (i.e. gas leaks). When charging a LiPo battery, it is best to use a LiPo safe box. Battery storage is also best done in these boxes. In the event of a crash, the first thing you need to do is unplug and check the battery. A boxed battery can add weight, but will really help protect the battery in a crash. Some manufacturers sell batteries with and without a hard case.
Most LiPo batteries have two connectors: one is designed to be used as the main “discharge” wires capable of handling high current, and the other, usually smaller and shorter, is the charging connector (usually a white JST connector), in which one pin corresponds to grounding, and the rest, the number of battery cans. You connect it to the charger, through which the charging (and balancing) of each cell of the battery is carried out. The charger must be sure to report when charging is complete, and be aware of the safety concerns associated with lithium polymer batteries. After the end of the charging process, it is best to immediately disconnect the battery from the charger.
The battery is the heaviest element of the drone, so it should be installed at the center dead center to ensure the same load on the motors. The battery does not require any special mounting (especially self-tapping screws, which can damage the LiPo and cause a fire), so some of the mounting methods used today include Velcro straps, rubber, plastic compartments, and others. The most common battery mounting option is to hang the battery under the frame using a Velcro strap.