Multirotors need propellers to convert rotary energy from the motor into lineal thrust. This article covers the most basic aspects and tries to give some guidelines to choose an appropriate propeller for a multirotor.
Choosing a propeller usually comes paired with choosing a motor. Its is a reiterative process where one has to check if the combination works considering mainly the weight and intended application of the multirotor.
Propeller characteristics
Diameter
Measures the diameter of the propeller. Propellers with a bigger diameter are intended to work at slow revolutions that their smaller counterparts. Also bigger propellers have more inertia, thus smaller ones allow for faster movement (acrobatics).Pitch
The pitch is related to the inclination of the blades, but does not measure it. Pitch measures the distance the propeller would move forward in rotation in a perfect medium. The following image illustrates this. Propellers with more pitch travel faster that those with less pitch at the same RPM, but require more power and behave less "smooth".
Material
Propellers can be made out of different materials. For multicopter the most common is plastic, but sometimes carbon-fiber is used, which is very light and durable thus decreases the inertia of the propeller.Shape
Multicopters use most frequently propellers with two blades. Some multicopters use propellers three blades, which allow for higher thrust in exchange of efficiency.Rule of thumb
A very popular rule of thumb for multirotors says that the sum of the maximum thrust generated by all propellers should be twice the weight of the complete vehicle.
That said, for a 1.2Kg quadcopter which has 4 rotors, every motor-propeller pair has to be able to generate 600g of thrust ((1.2Kg / 4 )* 2).
With a 2:1 relation the multicopter should have enough power to quickly adjust to the PID while not being to wasteful. A higher ratio, like 2.5:1 or 3:1 allows for acrobatic maneuvers or extra lift power. Less than 1.6:1 will have problems getting airborne.
Knowing how much thrust a motor-propeller combination yields is no easy task, but some tips are presented along this article.
Another way to estimate what propeller and motor are suitable for a given multirotor is to consider what others are using. A Internet search for multirotors with similar characteristics, such as weight, battery type or intended application should give a good idea of what to mount.
Propeller power
Calculating
the generated thrust is no easy task and requires some fluid-dynamics
knowledge. Luckily there are free simulators to our disposal (they will
be discussed later).
For now the thrust of a propeller can be considered dependant on the required power when spinning. This power depends on the propeller size and on the rotation speed. The following formula by Bob Boucher gives an estimation for it:
For now the thrust of a propeller can be considered dependant on the required power when spinning. This power depends on the propeller size and on the rotation speed. The following formula by Bob Boucher gives an estimation for it:
Power = Kp * D ^ 4 * P * RPM ^ 3
- Power is the required power in watts.
- Kp is a constant intended to normalize the efficiency of propellers and is dependant on the propeller brand.
- D is the diameter. Expressed in feet.
- P is the pitch. Expressed feet.
- RPM is the rotation speed. Expressed as multiple of 1000 revolutions per minute.
Simulation
A
quick way to obtain an estimated idea of the performance of a
propeller-motor combination is to simulate. There are a few web-based
simulators which allow for this.
Here are some of them:
Here are some of them:
There are also non-web-based simulators, for example Drive Calculator by Christian Persson.
Calibrate a propeller
Newly
bought propellers need to be calibrated. Because of the casting process
involved, one blade can be slightly heavier than the other. This will
create turbulences when spinning, specially at higher RPM.
This can be accomplished quite easily. A quick Internet search should yield several results. In a near future a quick how-to will be published on YALNEB.
This can be accomplished quite easily. A quick Internet search should yield several results. In a near future a quick how-to will be published on YALNEB.
I would like to thank you for the efforts you have made in writing this article. I am hoping the same best work from you in the future as well.
ReplyDeleteYou are very welcome indeed. Do not hesitate to link to or replicate this post on your aeromodelling homepage, but please do not forget to cite me as the source :) Have fun!
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