This tutorial aims to help you choose well performed motor and propellers for your quadcopter. There are some general rules you can follow and consideration to bear in mind when choosing the optimal and effective motor and propeller combination.
Where to Start? Decide on Motor Size First
To begin with, you need to answer these 2 questions:
- What’s your quadcopter’s planned total weight?
- What’s the size of your frame?
The total weight of the quadcopter doesn’t have to be exact, as you haven’t built it yet. It should include everything: frame, FC, PDB, wires, motors, ESC’s, battery, payload (such as HD camera and gimbals), etc. Remember this is just an estimation so you can always go back and refine the number when you have better idea which parts you are going to use.
By knowing the size of the frame, you should know what the maximum size propellers that can be used.
By knowing these 2 things, you can now calculate roughly how much thrust the motors need to deliver to life it in the air, by using propellers of the maximum size or smaller.
2:1 Thrust to Weight Ratio
A general rule is that you should AT LEAST be able to provide twice as much thrust than the weight of the quad. This is the bare minimum to ensure you have a stable copter that is easy to control at “hovering” style flying :) If the thrust provided by the motors are too little, the quad would not respond well to your control, it might even have difficulties to take off.
For example if we had a quadcopter that weights 1kg, the total thrust generated by the motors at 100% throttle should be at least 2kg, or 500g per motor (for a quadcopter).
For fast flying like drone racing, you should expect to go much higher than this, it’s not uncommon to see someone build a mini quad that can achieve 8:1, even 10:1 thrust to weight ratio. The quad can perform much more agile and dynamic, and accelerate much faster and corner better. One advantage thought when you have excessively high thrust to weight ratio is the quad would become very hard to control, because a little increase in throttle is enough to “shoot the quad in to orbit”. :)
But if you just plan to fly the multirotor as a slow aerial photography platform, you might want to go somewhere near 3:1 or 4:1. It also gives you room to add extra payload in the future (like heavier cameras, or maybe extra batteries to extend flight time)
What is Motor Size and KV
Brushless motors in RC is normally indicated by a 4-digit number – AABB. AA being the Stator width, and BB being the stator height.
KV is another important parameter, it’s the increase of motor RPM when voltage go up 1 volt without load. Higher KV motor would attempt to spin the propeller faster, but lower KV motor normally generates higher torque. That’s why we tend to use larger props on low KV motors, and smaller props on high KV motors.
If by pairing high KV motors with excessively large propellers, the motor will attempt to spin it very fast like they would do with smaller props, and therefore drawing too much current and generating too much heat. Eventually it could burn out the motor due to overheat and shorts in motor coils.
Frame Size => Prop Size => Motor Size and KV
Most of the times by knowing frame size, we can guess right away what sort of motor we should use. This is not super power, simply just because frame size limits props size, and prop size limits motor size and KV. But anyway, always check motor thrust test data to verify the current draw does not exceed safety rating with the propellers you want to run.
This table is only a simplified example to give you some ideas, you might see people using slightly higher or lower KV motors than this table suggests. Frame size is referring to wheelbase (aka diagonal motor to motor distance).
|Frame Size||Prop Size||Motor Size||KV|
|150mm or smaller||3″ or smaller||1306 or smaller||3000KV or higher|
|450mm||8″, 9″, 10″||2212|
Reading Motor Spec
When selecting motors, there usually are specification that comes with the motor either provided by the seller or manufacturer. You should be able to find information about the power, thrust, rpm etc. This is an example of the 18-11 2000kv Micro Brushless Outrunner (10g) on Hobbyking.com. Google it if you are not sure what each name represents in the data sheet, they are really basic and I am not going to go over them here.
I would be so lost if I was just starting, There are so many choices, I wouldn’t know where to start. So another way of narrowing down the range motors would be checking out other people’s builds, to see what motors they use. After seeing a few builds that are similar to what you are trying to build, it should have given you a pretty good idea what size motors you would need.
Comparing between motors
After you have decided on the size and KV of the motors you want to get, you might already have a handful of motors to choose from. To pick the best motor for your application, you should consider the following factors:
- Current Draw – battery
- Weight – Moment of Inertia
So the decision here really depends on your preference, how you want your aircraft to perform.
Higher thrust gives you faster multirotor, but you also need to look at efficiency making sure it’s not using a ridiculous of power that might shorten your flight time significantly. Also current draw of the motors on high throttle can affect your choice of battery too, when they draw large amount of current at full throttle, your battery’s max discharge rate has to be able to keep up in order to provide the power, as well as they don’t overheat and go buff (C rating).
Motor weight is more important in racing drones. It doesn’t only affect the total weight of your quadcopter and thrust to weight ratio, it also affects the moment of inertia of your quad. When your quad is rolling or pitching, it takes sometime to pick up angular acceleration and eventually move to the desired position. If your motors are heavy, it will take longer to do this, and therefore feels less responsive.
It’s all about balance really, or which area you want your copter to excel at.
More tips on Motor Efficiency
A multicopter is more efficient when it’s lighter, so you need to pick a good battery that has good capacity but light weight. This post shows you how to find the perfect balance when choosing LiPo batteries for your multicopter.
Battery and weight is not the only factor we need to consider when it comes to Efficiency, there is also motor efficiency. When choosing motors, apart from motor KV and thrust, we also need to look at Watt’s and efficiency.
It’s like cycling, cycle at lower gear enable you to go faster but it’s harder to peddle. If you are going uphill, and it’s getting steeper, you will eventually come to a point where peddling becomes too heavy and you are getting slower although you are peddling very hard. This is where you lose efficiency.
The same applies to brushless motor, the higher efficiency the better. A 70% efficient motor produces 70% power and 30% heat. A 90% efficient motor produces 90% power and 10% heat.
With less efficient motors not only are you wasting a lot of power (and flight time), you also get smaller thrust on full throttle. Most importantly, because the motor runs so inefficiently, the response time suffers. It will take the motors more time to change RPM and this will have negative influence on stability.
Other Factors Manufacturers Don’t Tell You
- Temperature (explain more in future update)
- Best props (explain more in future update)
- Vibration (explain more in future update)
Response – how fast the motor can change RPM. Easiest way to find out about this is by measuring how long it takes for a motor to go from 0 RPM to full RPM. This obviously will have an effect on how responsive the motors are.
If you can find the following data somewhere it can help you even more to identify the best motor for your choice.
Features of Motors that Might Interest You
(explain more in future update)
- Solid/Hollow shaft
- Type of Magnets (N52, N54)
- Arc Magnets
- Smaller air gaps
- Soldering tabs on motor
- ESC integration
- Cooling design
After you received your motors, the first thing you should do is to balance it. Although it’s not always necessary, it’s good practice. I personally only do this on large motors though like 2212 or bigger. For mini quad motors I normally skip this step. (2206 or smaller)
There are so many options, brands you can choose from, it will cause you headache :D For example these are the options for 250 mini quad motors.
A quadcopter uses two clockwise(CW) and two counter-clockwise(CCW) propellers. Propellers are classified by length and pitch. For example 9×4.7 propellers are 9 inch long and has a pitch of 4.7.
Generally, increased propeller pitch and length will draw more current. Also the pitch can be defined as the travel distance of one single prop rotation. In a nutshell, higher pitch means slower rotation, but will increase your vehicle speed which also use more power.
When deciding on length and pitch, you need to find a good balance. Generally a prop with low pitch numbers can generate more torque. The motors don’t need to work as hard so it pulls less current with this type of prop. If you want to do acrobatics, you will need torque propellers which provide more acceleration and it puts less pressure on the power system. Lower pitch propellers will also improve stability.
A higher pitch propeller moves greater amount of air, which could create turbulence and cause the aircraft to wobble during hovering. If you notice this with your quadcopter, try to choosing a lower pitched propeller.
When it comes to the length, propeller efficiency is closely related to the contact area of a prop with air, so a small increase in prop length will increase the propeller efficiency. (pretty much like swimmers with larger hands and feet can swim faster, but also more tiring for them)
A smaller prop is easier to stop or speed up while a larger prop takes longer to change speeds (inertia of movement). Smaller prop also means it draws less current, that is why hexacoptors and octocopters tend to use smaller props than quadcopter of similar size.
For larger quadcopters that carry payloads, large propellers and low-kv motors tend to work better. These have more rotational momentum, and will more easily maintain your aircraft’s stability.
There are a few different types of propellers, such as plastic, carbon fibre etc, check out this post for more info.
Some tips on how to choose Motor and Propeller
There are two ways I usually do.
- Observe and Research – go on to youtube and search people’s flying video, not only you can see the result of their quadcopters, you can also check out their setup, what motor and propeller they are using. It’s essential to learn from people’s experience because it’s free :-)
- Understand the maths and Experiment – if you have a great mathematical mind and have spare budget, and could not find any information about the motor and propeller combination you want to use, you could experiment with different setups! Although it requires more money and more time.
Even the for props of the same size and pitch, when they are made of different material and by different manufacturers, the performance also tend to be very different. Here is an example where I compare the 5030 props of two different brands.
With a well balanced motor and propeller combination, your quadcopter should achieve great efficiency, not only improve battery life time, but also allows great user control experience. Hope this post has helped you select the best motor and propellers, and don’t hesitate to share your ideas or questions with us.:-)