Guides Quadcopters/Multirotors

Choosing motors and propellers for multi-rotors

Written by V Kadamatt

If you want to build a multirotor or if you are looking to upgrade or revamp your existing quadcopter or multirotor, you definitely want to be choosing motors and propellers that are right for your craft.

With a myriad of available options with different configurations and sizes of both propellers and motors, this can be a confusing and daunting task. This post is written in hopes of shedding some light on this matter.

We will cover some basic technical details you will need to understand first and then put together that information into a more actionable one.

If you want more understanding on multirotors, check out our article on how multirotors work.

 

Brushless motor basics

Choosing motors for multirotors: Motors

Having a basic idea about brushless motors and the ability to understand different configurations and specifications will go a long way  if you want to tweak and build your multirotors.

Though the specifics behind the inner workings of these motors are beyond the scope of this post, we will cover enough for you to be choosing motors and propellers that are right for your raft.

KV rating

The first thing you need to understand is the KV rating. Here, it stands for the number of rotations the motor makes per minute for every volt applied to it. Higher the KV, the faster it rotates per volt.

Most motors come with the KV rating printed on it. If you are unsure, you can use a tachometer to get a rough idea of the RPM (Divide the RPM on the tachometer with the number of volts the battery you connect to has)

Generally speaking, high KV ratings are found on smaller motors and these are used to drive smaller propellers. These motors tend to be thinner. Larger motors with lower KV drive larger props. These motors tend to be wide and short.

The choice should be on whether you want speed and acrobatics or the ability to carry heavy loads and fly high (typically for aerial photography). For speed, you typically want a high KV – small prop setup. For carrying heavier loads, you want larger propellers and motors capable of such propellers.

Voltage, Power (Watts) and Thrust

The voltage specification on your motor determines how much voltage can be applied to it. This generally comes as a function of propeller size. As the amount of voltage applied on the motor increases, it is recommended that the prop size be reduced. Most manufacturers will have a recommended propeller size.

Power (Watts) is Voltage x Current. Generally, higher the Watt rating of the motor, higher the power of the motor itself. This is maximum potential horsepower your motor can produce, without being damaged.

Thrust is roughly the measure of how much the motor is capable of producing a propulsive force. Most manufacturers will print the thrust rating of motors on the motor itself. If you are unsure, you can use an online calculator. It is still recommended to contact your manufacturer to make sure.

So that’s it for motors. Hopefully by now you have an idea about the KV, voltage and thrust ratings of motors and what it means. We will be using this information very soon.

 

Propeller dynamics

Choosing propellers for multirotors: Hobby Ace propeller

Understanding propellers and knowing how to choose the right ones is important as it typically goes hand in hand with the choice you make with the motors. While you can use this information to decide what propeller to choose, never go overboard and always stay within the prop size ranges recommended for the motor.

Propeller diameter and pitch

When you go to purchase propellers, the specifications on it are defined in terms of diameter and pitch (D x P). For example, a 9 x 4 propeller has a diameter of 9 and a pitch of 4.

The diameter is the end to end length (in inches) of the propeller. It determines how much surface area of the prop is in contact with air around it at all times. Note that slight variations in propeller sizes can drastically change the responsiveness and efficiency of your craft.

Larger props swinging at lower RPMs are paired with lower KV rated motors and are more efficient relative to smaller props paired with higher KV rated motors.

Larger props are used in scenarios where there are heavy loads (like a camera) attached to the craft and are ideal for aerial photography and videography. Smaller props are ideal for speedy crafts ready to take on acrobatic maneuverers.

The ‘pitch’ of a propeller can be defined as the distance (in inches) travelled per revolution. Higher the pitch of a prop, the speedier your craft gets. This comes with a price – higher pitched props produce turbulence and are not as energy efficient.

Lower pitched propellers, like the longer ones are ideal for flight efficiency, hence used for aerial photography and videography while higher pitched propellers are picked for speed and acrobatics.

When picking the right propeller, it is important to consider both the diameter and pitch. Finding the balance between both is one of the key factors to building an amazing craft.

Material used

Propellers are built with different materials including plastic, carbon fibre and wood. The material that the prop is built with can make a difference in flight times and efficiency. Generally, carbon fibre props tend to be lighter and can add to flight times.

As a general rule of thumb, quality matters more than the specific material used and can make a world of difference in your flying experience including reduced vibrations and increased responsiveness and stability.

Rotation

It is very important to purchase both clockwise and counter clockwise propellers while building or tweaking your craft. Both should be purchased in the same amount. It would be simply impossible for your quadcopter to hover and fly around if all of the props rotate in the same direction.

 

Choosing motors and propellers: Putting it together

Choosing motors and propellers

Now that we have shed some light into the basics of motors and propellers, it is time to put that knowledge to use to build or tweak our craft.

First, we need to consider the weight our craft. While measuring the weight, it is important to consider everything including any cameras or loads that maybe attached to it. Next, we have to take into consideration the thrust to weight ratio of the multirotor. This is the most important element.

We want our craft to be able to hover at about half (50%) throttle. This is to ensure that we have ample headroom for speeding and gaining altitude when necessary. To get this, it is generally recommended to aim for a thrust to weight ratio of 2:1.

However, we should aim for a little bit more than that to account for any inefficiencies with the motors and/or propellers. Hence, a 2.5:1 thrust to weight ratio would be ideal. For example, if our craft weighs 500 grams, we should then aim for a total thrust of 1250 grams.

Note that when we measure the thrust, it is the total thrust. Hence, while picking motors, we divide the total thrust by the amount of motors. Hence, for quadcopters, we divide the thrust by 4. In our previous example, thrust per motor would be 1250/4 or about 312 grams. For tricopters, we divide total thrust required by 3 and so on.

After you have picked your motor, it now boils down to picking the right propeller to go along with it. As mentioned before, stick to using propellers near what is recommended for the motor for any particular cell count.

We have already covered the basic things that you need to know about propellers. Remember that larger propellers with lower pitch ratings are more efficient than smaller propellers with higher pitch ratings. Higher KV ratings on the motor isn’t everything for every scenario. Remember – balance is key and you may need to play around a little bit before you find the perfect combination for your specific purposes.

Other important things to consider –

  • Make sure your ESCs can handle the motors

It is important the measure the current draw of the motors to make sure that the ESCs can support it. Never load the ESCs with motors that draw more than the current rating of the ESC.

Suppose our motors have a current draw rate of 10 Amps, it is important to make sure that the ESCs have a current rating of at least + 30% to allow for some headroom in case of unforeseen circumstances. In this example, we will be picking at least 13A ESCs

 

Conclusion

With the right motor-propeller combination, you should be able experience a stable, responsive flight. Not only that, you would be able to measure better flight times even if your craft carries objects like a camera.

Striking the right balance comes from trial and error. With experience, you will get better at it. If you are just starting out and are nervous, you may want to check out other people’s multirotor builds and what component combinations have worked for them and emulate that.

With a little bit of internet research, you will come across tons of build guides – both videos and websites. It will be easier to see what to expect if you follow them. This will simultaneously reduce the risk of messing up while giving you the experience of tweaking and building your craft. You can slowly progress to move on to your own builds as you gain confidence!

Choosing motors and propellers for multi-rotors was last modified: July 1st, 2016 by V Kadamatt

About the author

V Kadamatt

Artificial Intelligence , UAV and RC enthusiast. Software professional, part time philosopher and star gazer.
I also love physics, mathematics, economics, psychology, fantasy, Sci-Fi and futurology.

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