You won’t believe how much maths and aerodynamics are involved when the engineers are designing the propellers and motors. Therefore you know how difficult it is to choose the best motor and propeller combination for your quadcopter for best performance.
Fortunately this is not a new subject, and there are so many people shared their experience about this. Also there are some general rules you can follow to choose the optimal and effective motor and propeller combination.
As part of the quadcopter tutorial I will share some information for beginners about how to choose motor and propeller for quadcopter, which also applies to other types of multicopter.
Here is a guide about the types prop adapters for your motors, and the pros and cons you might be interested.
Some basic concepts when selecting Motors
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.
To choose a motor we first need to how much weight you are planning to take, and then to work out the thrust required to lift the quadcopter. A general rule is that you should be able to provide twice as much thrust than the weight of the quad. If the thrust provided by the motors are too little, the quad will not respond well to your control, even has difficulties to take off. But if the thrust is too much, the quadcopter might become too agile and hard to control.
A rule of thumb is Required Thrust per motor = ( Weight x 2 ) / 4
For example if we have a quadcopter witch the flying weight might be around 1 KG. Using the equation above, we now know we’re looking for a total thrust of 2 KG and 500g per motor. Of course you will have to guess the final weight of your quadcopter, when estimating, don’t forget to add the weight of the motors and propellers which vary. If you are thinking about FPV or aerophotography, you will need to add the weight of the camera as well.
Although you can choose the motors for the weight you want to carry, it’s always a good idea to carry as little weight as possible. Lightness is very important to all aircraft because any excess weight could reduce your battery life and maneuverability.
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 cycle 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 (thus flight time), you also get smaller thrust on full throttle. Most important, because the motor runs so inefficient, the response time suffers. It will take the motors more time to change rpm and this will influence the stability of your quad, and the copter will be less stable.
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.
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.
Some basic concepts when selecting Propellers
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.:-)