Selecting Props for Your Multirotor

by | May 2, 2014 | design, theory | 0 comments

I have decided to spend the next week or so writing some posts covering basic design decisions for choosing multirotor components. Multirotors have many different parts, each of which is available in a number of different models. Choosing the correct mix of components will greatly effect the price and performance of your multirotor. My hope is that this and the next few posts will help you choose the best mix of parts for your own multirotor build.

To kick off this series of component-choosing tutorials, let’s discuss choosing props.

Selecting Props

Props are classified by two numbers, length and pitch. These two measurements describe the shape of a prop, which affects how a prop performs and how well suited a prop is to particular flying styles, like acrobatic sports flying, or slow, stable flying for aerial videography.

When shopping for quadcopter props the length and pitch are usually expressed in one of two ways. As an example, let’s use a prop with a length of ten inches and a a pitch of 4.5. You will see this combination written as either 10x4.5 or 1045.

Now let’s discuss what length and pitch mean and how length and pitch affect a prop’s performance. First let’s address the easier-to-understand measurement, a prop’s length. The length of a prop is measured from one tip of the prop to the other tip. A prop’s length affects its performance in two main ways. First, since a prop’s efficiency, in terms of the lift it produces per unit of energy with which it is supplied, is closely related to the contact area between the prop and the air. Therefore, a longer prop is more efficient. However, a longer prop also draws more current from the power system; it is like how a swimmer with large hands can move more water per stroke, but that swimmer also uses more energy to move those large hands through the water. Second, shorter props have less inertia, so it takes less energy to make shorter props accelerate and decelerate than it does for longer props. This can be a good or bad thing. For acrobatic sports flying, this leads to better agility in the air. For aerial videography though, you want longer props since they make the quadcopter more stable in the air.

Now for pitch. A prop’s pitch is basically the amount of rotation between the blades. A prop with a greater pitch has more twist between the blades. For multirotors, you always want a low-pitch (4.5 to 4.7) prop for three reasons. First, a low pitch prop generates more torque. Second, since low-pitch props have less friction with the air, because they cut through the air at a lower angle, low-pitch props draw less current. It is a bit like using a low gear on your bicycle, it takes less energy on your part (where you are like the motor) to move the pedals while, at the same time, you are able to generate more torque. The end result of all this is that with low-pitch props, the multirotor can accelerate faster.

The last reason we want to use low-pitch props is that they improve the multirotor’s stability. High-pitch props move more air per revolution. This may seem like a good thing, but in fact, this effect can cause turbulence which can make the quadcopter shake during flight.

So at the end of the day, when choosing props, the decision comes down to the length of the prop. Shorter props draw less current than longer props and they are better for acrobatic sport flying. However, shorter props offer less stability than longer ones. Longer props then are better for aerial videography since they offer better stability.