Note: This is the second part of a series of posts on How Quadcopters Work. For the introduction to this series, click here.
Propellers – or just props – appear to be the simplest part of a quadcopter.
Usually just a skinny piece of plastic or carbon fiber, props often get overlooked.
But they are vitally important. And they aren’t very well understood. Worse yet, if you pick the wrong prop, your quad could be too loud or fly poorly or pull too much current and blow out a motor.
In this post, I plan to cover the basics of what propellers are, how they work and how they are specified.
Thrust is the reason propellers exist. For an airplane, a propeller produces thrust to move the airplane forward. For a quadcopter, the 4 propellers work together to produce thrust that lifts the quad off the ground. By changing the amount of thrust of each prop, a quadcopter can move forward, backwards, side to side and rotate.
Thrust is often measured in grams. What this means is that for your quadcopter to hover, you need 1 gram of thrust for every gram that your quadcopter weighs. In order to do anything beyond hovering, you will need more than 1 gram of thrust for every gram that your quadcopter weighs.
The first thing most people notice about propellers is their size. The distance from tip to tip of a prop is called the diameter of the prop. The amount of thrust a prop can produce is directly related to its diameter.
What most people call pitch is actually the pitch length.
Pitch length has to do with how far a propeller will move a quadcopter under ideal conditions during one revolution.
So if your props have a 4″ pitch will move the quadcopter 4″ forward for every revolution they turn.
Speed and Torque
The speed of the of a prop is usually measured in rotations/revolutions per minute (RPM). Speed is directly related to thrust.
Increase the speed, increase the thrust.
Decrease the speed, decrease the thrust.
One problem that increasing the speed of a prop creates is drag. So as the speed increases, you not only get more thrust but the prop also gets more difficult to turn. More difficult to turn means more torque is required from your motor.
CW vs CCW
Props can be made to be spun clockwise or counter-clockwise. I will get into this more in a later post when I talk about flight dynamics, but for now it is probably enough for you to know that 2 props on a quadcopter have to turn clockwise and 2 have to turn counter-clockwise.
When you have a prop off-center like they are on a quadcopter, it will not only produce thrust but will also cause the quadcopter to rotate about its center. Using 2 CW and 2 CCW props allows us to cancel out that rotation.
How are props specified?
When you are buying props, they will usually be specified by their diameter and their pitch. Different vendors and manufacturers have different ways of doing this but the most common tend to follow one of these two patterns:
For example, if a prop has a diameter of 5″ and a pitch of 3″, you might see that specified as either 5×3. Or you might see it specified as 5030.
If a prop has a diameter of 6″ and a pitch of 4.5″, the first way of specifying it would be 6×4.5. The second way would be 6045.
So, those are the basics of propellers. The main take-away is that props are responsible for producing the thrust that allows quadcopters to lift and maneuver. By picking different combinations of prop diameter and pitch, you can match the amount of thrust to the weight of your quad.
The next post in this series will look at the basics of motors. In it I’ll look more at speed and torque and how it is we get the props to rotate.
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Have any questions about propellers that I didn’t answer. Leave a comment below and we can discuss it.