If until recently choosing the engine for the drone was child's play, today the scenario has completely changed, how to choose the right engine with such a large market?

There are new types of motors and choosing is complicated. In this guide we explain in depth how to choose the right motor for your Drone, the features to keep an eye on and other factors that can influence the performance and efficiency of a motor. This will help you choose the optimal and effective motor for building your racing drone.[/vc_column_text][vc_column_text]

Engine types:

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HOW TO CHOOSE AN OPTIMAL ENGINE WHERE TO START?

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Frame size

First of all, you need to know the size of your frame . This will determine what propellers and motors you will be able to use in your quadcopter.

Total weight of the quad

You can estimate the total weight of your quadcopter once you have selected all the components. This should include everything you plan to take on board: frame, FC, PDB, RX, VTX, antenna, motors, propellers, ESC, LiPo battery, additional payload like GoPro, etc. Add 10-20 grams to compensate for wires, buzzers, zip ties, etc. This is also a good practice.

You probably won't get exactly the gram, and it's better to overestimate the weight and have extra power than to underestimate and not be able to fly.

Thrust requirement

Once you have calculated the weight of the quadcopter and the dimensions of the frame, you can now calculate approximately how much thrust the motor and propeller combination will need to provide to lift the aircraft.

Thrust to weight ratio

A general rule of thumb is that you should be able to provide at least double the thrust compared to the total weight of the quad. Remember that this is the bare minimum to ensure that you have a stable helicopter that is easy to control. If the thrust provided by the motors is too small, the helicopter will not respond well to the controls, it may even have difficulty taking off.

For example if we have a quadcopter that weighs 1Kg, the total thrust generated by the motors at 100% throttle should be at least 2Kg, or 500g per motor (for a quadcopter). Of course, it's always nice to have more thrust available than necessary...

To fly faster like drone racing, you should expect the power to weight ratio to be much higher than this. It is not uncommon to see someone build a mini quad that can reach a thrust ratio of 10:1 or even 13:1. In general, for a drone racing, we recommend having at least 5:1.

With a higher thrust to weight ratio, a quadcopter will have better agility and acceleration. When the thrust to weight ratio is too high, however, the quad can become very difficult to control. A small touch of the throttle would be enough to "push the quad into orbit like a rocket". :D Of course, this also depends on the pilot's skill.

Even if you simply plan to fly a slow aerial photography platform, you should aim for somewhere between 3:1 and 4:1. This not only gives you better control, but also offers space for additional payload in the future. For example, heavier cameras or larger batteries for extended flight time. But if you are into racing, then there is no limit :D Go as high as you feel comfortable![/vc_column_text][vc_column_text]

ENGINE DIMENSIONS

[/vc_column_text][vc_row_inner content_placement="middle"][vc_column_inner width="1/2"][vc_column_text]The size of brushless RC motors is usually indicated by a 4-digit number: AABB. "AA" is the stator width (or stator diameter) while "BB" is the stator height , both are measured in mm (millimeter).

What is brushless motor stator? - A stator is the stationary part of the motor, it has "poles", which are wound with copper wires (windings). The "poles" are made of many layers of thin sheet metal that is laminated together with a very thin insulating layer in between.

• Taller stator = more power at higher RPMs
• Wider stator = more torque at low rpm

Increasing the width or height of a motor will increase both the size of the permanent magnet and the electromagnetic stator coils. The main difference is that when you increase the height of the stator, the size of the permanent magnet increases more than the coil size and when you increase the width of the stator, the size of the electromagnetic coil increases more than the permanent magnet.[/vc_column_text][/vc_column_inner][vc_column_inner width="1/2"][vc_column_text] [/vc_column_text][/vc_column_inner][/vc_row_inner][vc_column_text]

KV

“KV” is the velocity constant and commonly translates to “thousands of revolutions per minute per volt.”

This is an important parameter of brushless motors, which indicates the theoretical increase in RPM (revolutions per minute) when the voltage increases (without load, i.e. the propeller). For example, when powering a 2300KV motor with a 3S LiPo battery (12.6V), it will spin at approximately 28980 RPM without props (2300 x 12.6). Usually this is just a rounding estimate specified by the manufacturer.

Once a propeller is mounted on the motor, the RPM drops dramatically due to air resistance. Higher KV motors would try to spin the propeller faster, and it could draw more current. This is why we tend to see larger props paired with lower KV motors, while smaller, lighter props are more suitable for higher KV motors.

The KV value can be determined by the number of windings of copper wire on the stator. Generally, the more winds the lower the KV of the motor, while the less winds the higher the KV.

The magnetic strength of magnets can also affect the KV value, stronger magnets will increase the KV.

By pairing a high KV motor with a propeller that is too large, the motor will try to spin as fast as it would with a smaller prop, but this will require more torque. As it tries to produce the required torque, it will draw more current and subsequently generate too much heat. This will eventually lead to overheating and burning out the motor. This is because as the motor overheats, the coating on the coil will begin to melt and cause electrical shorts in the motor.

Generally speaking, the heavier dial usually pairs with medium-low KV motors, the lighter dial usually uses high KV motors.

Driving torque

Some say that lower KV motors have higher torque and higher KV motors have lower torque . While this is possible, it is not entirely true. KV has almost nothing to do with torque, but is mostly about the current and voltage limits of the motor.

As explained, higher KV motors have shorter windings and therefore lower resistance. This lowers the maximum rated voltage and increases the current draw for the motor and propeller combined, and not much else.

The couple is mainly defined by

• Stator size: the greater the torque
• Materials: the type of magnets, the quality of the copper windings
• Motor construction: things like air gap, number of poles etc.

All things being equal, two identical motors should theoretically have the same torque even if one motor has a different KV. Lowering the KV just means you need a higher voltage to get it to the same RPM (power). It's a bit more complicated than that of course, but this is a reasonable approximation.

Voltage sag with high KV motors kills torque. In theory they would have the same amount of torque, but they can't in reality.

The couple is a double-edged sword.

Higher torque motors allow for quicker RPM changes and quicker response times, you’ll get less oscillation and will give you instant, snappy response. But higher torque motors also feel sharper and more robotic, while lower torque motors generally feel smoother and softer. The choice depends on your flying style and personal preference, and higher torque is not always better.

Many pilots today are experiencing oscillation problems more often than ever, and the problem can be traced back to modern high-torque, high-power motors. These are so powerful that they can amplify the output and create an oscillation feedback loop, which is very difficult to eliminate. Soft mounting the flight controller may be enough to solve it, but we should try to eliminate it at the source and stay away from extremely powerful motors.

N and P Numbers - Poles and Magnets

You may have seen specifications like "12N14P" printed on a motor case. The number before the N indicates the number of electromagnets in the stator, or poles, and the number before the P indicates the number of permanent magnets in the bell housing.

Different sized motors have different numbers of poles, 22XX and 23XX motors generally have 12 poles and 14 magnets.

The number of poles determines the spacing between the poles if you have fewer poles, you can put more iron content in the stator, so you get more power out of the motor. But with a higher number of poles, the magnetic field is spread out more evenly, and therefore you have a smoother motor because you have more precise control over the rotation of the bell.

• More poles = smoother
• Less poles = More powerful

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HOW TO DECIDE THE ENGINE SIZE?

You can find the component sizes to use in this order: Frame Size => Prop Size => Motor Size

Knowing the frame size, we can estimate what size motor we should use. The frame size limits the size of the props and each size of propeller requires a different engine RPM to generate thrust efficiently, this is where the KV motor comes in.

You also need to make sure that your motors produce enough torque to turn your choice of propeller, this is where your stator size comes in. Generally a larger stator size and higher KV means more current draw.

This table below is a general guideline, it is not a hard-set rule, you may also see people using slightly higher or lower KV motors than this table suggests.

It is assumed that you are powering the quad with 4P LiPo batteries and that the frame size refers to the wheelbase (i.e. diagonal motor to motor spacing).

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Voltage and current consumption

It is important to understand that voltage has a big impact on your choice of motor and propeller. Your motor will try to spin faster with a higher voltage, so it will draw a higher current. Make sure you are aware of how much thrust your motors produce and how much current they will draw.

Once you know the current combination of motor and propellers, you are now ready to choose ESC for your drone.

Engine Specifications

Once you have decided on the size of the motor , you will probably still have many options to choose from. To choose the best motor for your application, you can consider the following factors:

• Push
• Efficiency and current absorption
• Weight

The decision here really depends on your application, flying style, and how you want your aircraft to perform.

Push

Thrust is probably the first thing people look at when choosing an engine.

Higher boost gives you faster acceleration, but you also need to be aware of current and efficiency. Don't abuse your batteries with an amperage hungry motor/prop combo.

If your quad draws a lot of current at full throttle, the maximum discharge rate of the battery must be able to keep up. The battery must also have enough capacity to provide acceptable flight time.

While thrust is an important consideration when selecting an engine, it is not the only thing to consider.

Engine weight

The weight of a motor is often overlooked, which can be a very important factor for stunt and racing drones.

Because the motors are mounted at the four corners of the frame, they have a strong influence on the responsiveness of your quad. Heavier motors increase the angular momentum of inertia of your quad, the motors have to work harder to change attitude.

In practice, when your quad does flips and flips, it takes time to gather angular acceleration , move to the desired position, and stop. Heavier motors will take longer to regain that angular velocity , and will also take longer to slow down.

Efficiency and current absorption

Engine efficiency is typically calculated by dividing thrust by power at 100% throttle, measured in grams per watt (g/w). The higher this number, the more efficient the engine.

It is important to consider efficiency across the entire throttle range, not just the top end. Some motors may be efficient at lower RPM, but may lose efficiency as they draw increasingly higher current as they approach their limits.

Another good way to evaluate efficiency is to use "grams per amp" (thrust/current).

Generally, the more thrust generated , the more current drawn to produce that thrust, so high-thrust, low-current motors are preferred. Inefficient motors generate too little thrust or draw too much current.

Each engine responds differently to different propellers, carefully choosing the propeller is the key to balancing thrust and efficiency.

Interesting data that manufacturers don't tell

Many properties of the quadcopter engine are not mentioned by the manufacturers and can only be found through more technical tests.

• torque
• Response time
• Temperature
• Vibrations and balance

Driving torque

Torque is the force that turns the propeller, it determines how quickly an engine can increase and decrease RPM. In other words, how easy it is for the engine to move the mass of the rotor, the prop, and most importantly, the air.

Torque greatly affects the performance of your quad, especially how precise and responsive it feels in flight. A motor with high torque will provide a snappier response, due to the quicker RPM change. You may also experience less washout with more torque.

High torque also means that it can carry heavy objects (at the cost of drawing more current). If a low torque motor is driving a propeller that is too heavy for it (i.e. over-engineered), the motor will not be able to produce enough force to spin it at the desired RPM, resulting in poor efficiency and overheating.

One drawback of high-torque motors, however, is oscillation. High-torque motors are capable of changing RPM so rapidly that they actually amplify the error (in the PID loop), causing oscillations that are difficult to eliminate even with PID regulation, particularly in the yaw axis.

Response time

Motor response time also depends on torque, motors with high torque often have a faster response time . A simple way to measure response time is to see how long it takes a motor to reach maximum RPM from 0.

Response time will be largely influenced by the weight and pitch of your propeller choice. Remember that atmospheric conditions can also have an effect. At high altitudes , for example , the air is thicker, which means there are more air molecules that the propeller must physically move to produce thrust . At high altitudes, your propellers will spin faster and react more quickly to throttle changes, but overall thrust will be reduced, because there are fewer air molecules to interact with.

Temperature

Temperature affects brushless motors because the magnets used in our motors have a weaker magnetic field when operating at high temperatures, they also demagnetize more quickly when the motor gets too hot , which affects their lifespan.

Over-driving the motors and using full throttle will cause the motors to overheat . This will degrade the performance of the motor and magnets over time, so motor designs that aid in cooling often equate to longer life. This is, of course, provided you don't destroy it in a crash first!

Vibration

Vibrations caused by engines can have a number of unpleasant side effects on the performance of your quad.

If an engine has poor balance or build quality , it can cause vibrations that can affect the PID controller. Since the vibration frequency changes at different levels, this can make tuning the quad very difficult.

A motor that is suffering from vibration will also produce more electrical noise than one that is running smoothly. This electrical noise can affect the gyro sensor, further degrading flight performance and will also degrade the quality of your FPV video if you are powering your FPV system from the same battery as your motors and ESCs.

Many have successfully fitted soft motors , and soft flight controllers to reduce vibration, with some really positive results.

Remember that damaged, bent, and unbalanced propellers can also cause problematic vibrations. [/vc_column_text][vc_column_text]

CONCLUSION

Motors are an essential part of our quad so when choosing them remember to pay particular attention as the wrong motor could have a significant impact on the performance and flight characteristics.

Remember It's all about balance if the engines are carefully selected, With the right combination of engine and propeller, the quad should be efficient, have normal fuel consumption (hence correct flight times).