I think most people which like to build a quadcopter have a problem is “how to choose propellers “. In fact, propellers are the same important to motors and lipo batteries. As we know, most people usually choose brushless motors and famous lipo batteries(like Gens Ace & Tattu); but for the propellers, they may know less about them, so this article will tell you the knowledge of the propellers and teach you how to choose propellers.
The Basics – Size/Pitch
On every quadcopter, there are two CW (clockwise) and two CCW (counter-clockwise) propellers. There are propellers of different length and pitch. For example 9×4.7 (sometimes 9047) propellers are 9 inch long and has a pitch of 4.7 inch. 5x3x3 (sometimes 5030×3) means 3-blade 5″ propeller that has a pitch of 3 inch.
The Size of a propeller is measured from tip to tip, and it’s sometimes called diameter, as when you spin up a prop you get a circle and the diameter is the size of the prop.
Pitch is sometimes called pitch length too, which can be defined as the travel distance of one single prop rotation.
Generally, either increased propeller pitch or diameter will lead to higher current draw, because more air is moved (assuming RPM is the same), and it gets harder to spin. In a nutshell, larger propeller or higher pitch length will increase your vehicle speed but also use more power.
Effects of Prop Size and Pitch
When deciding on prop size, length and pitch, you need to find a good balance.
Generally a prop with lower pitch numbers can spin faster (higher RPM), the motors don’t need to work as hard to spin it so it pulls less current. If you want to do acrobatics, you will need lower pitch propellers which provide higher 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 more prop wash. It generates more thrust in the expense of higher current draw, but giving you higher top speed.
A smaller prop is easier to stop and speed up while a larger prop takes longer to change RPM due to inertia.
Propellers can be made of different materials, plastic, carbon fibre, wood etc. Each type of material gives unique features, for example carbon fibre and wooden props are stiff and best known for their smooth performance, certain plastic compound are very durable… check out this post for more info.
The shape of the propeller also plays a big part in performance. The most distinctive difference would probably be the tip of the props: pointy nose, bull nose (BN) and Hybrid bullnose (HBN). Basically for propellers that are the same size and pitch, pointy tip props are the most efficient however pulls less thrust. Bullnose props can pull higher in thrust number, however they also draw a lot more current. This is due to the fact that they have a larger surface area than than the pointy tip props. And HBN props is in between the two.
Number of Blades
The most common propellers are probably 2 blades (or dual-blade). The next up is Tri-blade which is also very popular among racers and freestylers. You can sometimes even see quad-blade and even hex-blade. Basically, the more blade a prop has, the more surface area and thus can generate more thrust. However that also leads to higher current draw and lower efficiency.
Testing is Only for Reference
It’s important to check out motor thrust tests, and see which props work better with the motors you plan to run. Same propeller can perform very differently on 2 different motors.
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.
However these bench tests are done in static environment, and therefore it doesn’t represent the motor/propellers true performance in the wild – where air are moving and there are many other factors that can have some effect on it. One of the most noticeable difference between static bench test and real flight is motor thrust, which is normally 5%-10% less in real flight than in static bench test.
With a well 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 propellers.