How to Choose Motor and Propeller for Quadcopter
This tutorial is geared towards helping you get a basic understanding of the multirotor motors and propellers. These instructions and advices will help you select the requisite motors and propellers for your quadcopter or any other multirotor for that matter. You can pay heed to a general set of instructions. In addition, we will also be underlining a few important facts that you need to keep in mind when looking out for the ultimate combination of motor and propellers that provides you with the most effective of results.
Wondering where to start from? Decide on motor size first!
At the outset, you need to answer a couple of questions:
- What is the total weight of your quadcopter that you plan to have?
- What is the size of your quadcopter’s structure overall?
As far as the total weight of the quadcopter is concerned, you can only assume it at the initial stages since you have not yet developed it. The total weight will of course include everything ranging from the frame, FC, PDB, cables, motors, ESCs, battery, payload (HD camera, gimbal etc.) and other components. Keep in mind that the total weight at the outset is always going to be a conservative approximation. You always can go back and update your total weight when you decide finally on the final components that you are going to use in your quadcopter.
The maximum size of the frame helps you determine the optimum propeller size. These two important pieces of information will provide you the sufficient idea regarding the thrust needed by the motors to lift the quadcopter in the air by using propellers of restricted sizes.
2:1 Thrust to Weight Ratio
A rule of thumb is that the dronie should be able to at least provide two times the amount of thrust than the weight of the quadcopter. Remember that this is the minimum magnitude of thrust required by a drone so that it becomes easier to manoeuvre while hovering over a certain location. If the motors are not able to provide the requisite amount of thrust, then you might come across difficulties in controlling your device. With a thrust that is not comparable to the weight of the drone, your device might not even possess enough power to take off.
To explain these things a bit more, let’s consider this example. If we have a quadcopter with an overall weight of about 1 Kg, the total thrust that the motors should be able to produce at maximum throttle should be 2 Kg at the minimum or say 500 g per motor in case of a quadcopter.
In case of developing drones for racing purposes, the thrust to weight ratio is likely to go further up depending upon your requirement. It is not common for racing quadcopters to portray thrust to weight ratio of 8:1 or even 10:1. The quadcopter will certainly be a lot more faster and quicker; accelerating swifter and cutting corners a lot better. However, bear in mind that a high thrust to weight ratio would make it more difficult for the pilot to control the device as a little throttle will be enough to make the quadcopter go around in circles.
If you are looking to transform your multirotor into a slow moving platform for aerial videography and photography, we recommend you to go for a thrust to weight ratio of 3:1 or 4:1. Such a configuration will let you exercise precise control over your device and also attach more payload in the time to come. The additional payload can be in the form of bulkier cameras or additional batteries if you are striving for enhanced flight duration.
Motor Size and KV
Brushless motors in a common remote controlled multirotor can be identified by a 4-digit number – AABB. “AA” corresponds to the stator width while the “BB” represents the stator height. The wider and taller the motor is, the more torque it has the capacity to produce.
KV is another crucial factor. It is the theoretical increment in rotor’s revolutions per minute (RPM) when the voltage would increase by 1 Volt without any load. For example, when 2300 KV motors are powered by a 3S LiPo (12.6 V) battery, the motor tends to revolve at 28980 RPM. Keep in mind that we have used some serios estimations just for the sake of elaboration.
However, as you attach a propeller to the motor, the RPM will definitely go down because of the increased resistance. The motors which have higher KV produce more RPM and hence rotate the propeller faster. However, motors with lower KV generate lower torque. This is precisely the reason why dronies prefer to use heavier propellers with motors having lower values of KV and lighter propellers with low KV motors. You can always modify the KV value of a motor by increasing or decreasing the number of copper wire winding in the motor.
You need to arrive at the right balance in terms of RPM and torque when choosing a motor and propeller for your drone. When you attach bulkier propellers with motors with high values of KV, the motor will attempt to rotate the propeller fast resulting in drawing more current and producing more heat in the process. This can inevitably result in the motor being overheated and the copper coils in the motor getting short circuited.
N and P
You might have come across a notation such as “12N14P”. The digit before the alphabet N represents the number of electromagnets in stator while the one before alphabet P portrays the number of permanent magnets in the motor.
Majority of the motors that are available in the market boast the 12N14P configuration. Some of the motors with lower values of KV come with greater number of electromagnets and permanent magnets that helps generate more torque. Such motors are generally on the higher end of the price scale. This is good information to have but not a compulsory information when looking to buy motors for your next quadcopter.
Frame Size => Propeller Size => Motor Size and KV
On most of the occasions, just by getting to know the size of the quadcopter frame, we can determine the type of motor that will suitable for our project. This is due to the fact that a smaller frame size would mean smaller propeller sizes and hence smaller motors and lower values of KV. Nevertheless, we suggest you to have a look at the motor thrust data to ensure that the current drawn does not exceed the safety rating of the motor when attaching the propellers that you want for your quadcopter.
The table below has been tabulated to give you a simple idea and has been compiled based on the assumption that you are using 4S LiPo batteries. There are a lot of people who would be using motors with higher or lower KV motors than the ones provided in this table. Frame size refers to the wheelbase which is the diagonal distance between motors.
Voltage and Current Draw
You must keep in mind that the voltage has a great influence on the type of motor and propeller that you choose for your quadcopter. When you apply higher voltage to your motor, it will surely attempt to spin a lot faster and hence will draw more current from the battery. Again, it is very important to check out the thrust data for your motors.
Reading Motor Specifications
Motors come with specifications provided by manufacturers at the time of their development. You should look out for the motor’s power, thrust, RPM etc. in these specifications. Below is the example of 18-11 2000kv micro brushless outrunner (10g).
Comparing between Motors
Once you have finalized the size and KV values of your motors, you still will have plenty to of options to choose from. Here are few things that you need to consider when choosing the motor that best suits your objective.
- Current Draw
- Weight – Moment of Inertia
The final decision rests on your own predilection; it basically depends on the way you want your quadcopter to perform. You get faster speeds by instilling higher thrusts. Simultaneously, you need to ensure that the device is efficient enough; i.e., it does not consume power more than that supported by the hardware installed in it.
Also, keep in mind that your preference for motor and propellers is likely to have an impact on the batteries you use. If your motor has the ability to draw optimum current from the battery at 100% throttle, the battery must be equipped to support a discharge rate in accordance with the required power. Moreover, you also need to ensure that the batteries do not get overheated and get discharged way too quickly. You can view the C rating in this regard.
Another aspect that most of the dronies do not pay too much attention to is the motor weight. This is an important factor when it comes to racing drones. In addition to having an influence over the total weight of the quadcopter and the weight to thrust ratio, it also affects the moment of inertia of your device. For example, when your quadcopter is performing those aerial stunts such as rolls and flips, your quadcopter will require some time to gain the requisite angular acceleration and travel down to the target spot. With heavier motors, the quadcopter is likely to take more time to do so and hence will give you an impression as if it were not as responsive as you would prefer it to be.
Actually, the whole game is about bringing that much needed balance or an area where you want your quad to perform better at the expense of the other one!
More Tips on Motor Efficiency
A multirotor remote controlled aircraft performs more efficiently when it is as light as possible. This article portrays how to obtain the ideal balance between opting LiPo batteries for your multirotor device. When talking about overall power efficiency, battery and weight are not the only parameters that have a say in this complex equation.
When opting the right motors, we also need to look at the motor efficiency in addition to the motor KV and thrust. It is more or less similar to cycling. In cycling, when you are going at a lower gear, you can move faster, but you will have to pedal faster as well while facing more resistance. While scaling rocky terrain which gets steeper by the moment, pedalling will get harder and harder by every passing moment. There will come a point in time when you will get slower and slower despite the fact that you are pedalling very hard. This is a point where you will lose in terms of efficiency.
The same process is relevant to the brushless motors. The higher the efficiency, the better it will be for your quadcopter. A motor that is 70% efficient implies that it will generate 70% power and dissipate 30% energy in the form of heat. A motor, on the other hand, that is 90% efficient will produce 90% power and only 10% energy will be lost in the shape of heat.
If you are using a less efficient motor, it will result in more heat loss and hence reduce flight timing. Moreover, you won’t be getting sufficient thrust on full throttle. More significantly, since the motor is inefficient, you won’t get the requisite response from your device. The motors will reach the target revolutions per minute at a longer interval of time which will negatively affect steadiness and responsiveness.
Other Manufacturers Don’t Tell You
- Best Propellers
By response, we mean the ability of a motor to change its revolutions per minute. The easiest way to gauge this is to determine how long it takes for a motor to attain its optimum RPM from zero RPM. This in turn influences the responsiveness of the motors.
If you are able to explore the below mentioned data somewhere else as well, you will certainly be able to recognize the required motor that suits your quadcopter.
Features of Motors that Might Interest You
- Solid/Hollow Shaft
- Type of Magnets (N52, N54)
- Arc magnets
- Smaller air gaps
- Soldering Tabs on motor
- ESC Integration
- Cooling Design
Once you buy your new motors, you should firstly balance them out. This might not be an essential practice but we recommend it to be a good one indeed. We perform this exercise on larger motors such as 2212 or others. We do not think it really important when it comes to mini quad motors such 2206 or smaller.
You can choose your motors from a number of brands. Here are a few options for 250 mini quad motors.
CW and CCW Motors
Propeller shaft thread rotation is the basic difference between CW and CCW motors. The actual aim is to use a couple of CW motors and CCW motors on a quadcopter, so that when the motors spin, all of the four propeller nuts get fixed in their designated slots. It really does not matter which one you should pick as they are almost similar to each other and the only difference lies in the propeller shaft thread. But we recommend you to get all motors of the same threads lest you may confused with all the different propeller nuts.
Propellers are responsible for producing the requisite thrust by spinning and stirring the air surrounding them. The more air is stirred in the surroundings, the more thrust is generated.
The Basics – Size/Pitch
Every quadcopter comes fitted with a couple of clockwise and anti-clockwise propellers. Propellers come in different sizes and pitch. For example, 9×4.7 (sometimes referred to as 9047) propellers are 9 inches in length and 4.7 inches in pitch. 5x3x3 (referred to as 5030×3) means three blade 5 inch propeller with a pitch of 3 inches.
The length of a propeller is measured from one tip to another and is also commonly known as the diameter. This is because when you spin a propeller, a circle is formed and the diameter is the length of the propeller.
Pitch is also sometimes called as Pitch Length which can be said to be the travel distance of a single propeller’s rotation. As a rule of thumb, if you increase the propeller diameter or pitch, they are likely to draw more current from the batteries due to the motion of more air in the surroundings. Remember that we have assumed the RPM to be constant in this entire scheme of things. Keep in mind that with more current drawn from the battery, it becomes hard for the propellers to spin. The bottomline is that if you use larger propellers or greater pitch length, you will get greater drone speeds but at the cost of more power comsumption.
Effects of Prop Size and Pitch
You need to strike the right balance when choosing the propeller size, length and pitch. Generally, a propeller that has a lower pitch will spin faster at a higher RPM. The motors will not need to provide more energy for the propellers to spin and hence will draw lesser current. If you want to perform incredible aerial stunts, you ought to opt for lower pitch propellers to furnish higher acceleration; thereby consuming lesser power from the system. This will also have a positive impact on the aircraft’s steadiness.
A propeller with a higher pitch, on the other hand, will move greater amount of air, which in turn can disrupt the UAV’s stability and produce propeller wash. More thrust will be generated at the cost of more current drawn from the batteries; giving your faster speed.
A propeller which is smaller in size will be easier to stop and speed up. On the contrary, a larger propeller will take longer amount of time to change its revolutions per minute due to its inertia.
Propellers consist of a number of materials including plastic, carbon fibre, wood etc. Unique capabilities and characteristics can be achieved from each type of materials such as carbon fibre and wooden propellers are a bit hard and provide smooth flight. Propellers made of plastic, on the other hand, are more durable and reliable. You can read this post for more details.
Another important parameter that has a crucial role to play in the performance of your quadcopter is the shape of the propellers. The most prominent difference among propellers of different shapes and sizes constitutes their tips: pointy nose, bull nose (Bull Nose) and Hybrid bullnose (HBN). Propellers with the same size and pitch are most efficient with pointy tips and produce lesser thrust. Bullnose propellers generate more thrust but simultaneously draw more current from the batteries. This is primarily because of their larger surface area as opposed to those with tiny tips. A hybrid bullnose propeller is a blend of the two types of propellers discussed above.
Number of Blades
Dual blade propellers are the most commonly used propellers out there. Drone racers and frestyle dronies take a fancy to tri-blade propellers. Quad-blade and hex-blade-propellers are also available in the market. As a matter of fact, the more blades a propellers sports, the more surface area it will boast and hence will produce more thrust. However, in such a case, the propeller will draw more current from the batteries and will result in lesser efficiency.
Testing is only for Reference
It is crucial that you go through motor thrust tests and determine which propellers work well with motors that you plan to opt for your quadcopter. A propeller performs differently in combination with two motors of different types.
Even the propellers which are of the same size and pitch, performance tends to be different for different materials and different manufacturers. You can have a look at the comparison of 5030 propellers produced by different manufacturers.
However, these tests are performed in static environment. Hence, they do not represent the actual performance of motors/propellers in real time situations in which weather conditions and other factors play an important role. One of the most significant differences between static tests and real time flights is motor thrust which tends to be generally 5% to 10% lesser in real time scenarios than in test conditions.
Your quadcopter can attain higher levels of efficiency with a well balanced blend of appropriate motor and propeller. It will also enhance your battery output but will also allow you to exercise greater control over your device. We hope that you will have benefitted by this post in choosing the accurate mix of motors and propellers for your next quadcopter. Let us know if you have any ideas or questions in store for us in the comments section below.