This article explains the basics of propellers (props) for quadcopters. We go over the concepts of pitch, shape, difference in the number of blades and how these affect thrust, efficiency and performance.
Knowing The Basics – Size of Propellers
Propellers exist in different length and pitch. These are the first 2 terms you must know.
The length of a propeller is the diameter of a disc the prop makes when it’s spinning. Pitch can be defined as the travel distance of one single prop rotation. it’s sometimes called pitch length too. The larger the prop (either increasing diameter, or pitch or both), the harder it’s going to spin.
Effects of Prop Length and Pitch
Propellers generate thrust by spinning and moving air. The more air it moves, the more thrust it generates.
By increasing either prop length or pitch, it gives more thrust and leads to higher current draw. Effectively it increases the surface area and drag, so more air can be moved and more power is required to spin it (assuming RPM is constant in this case). In a nutshell, larger propeller or higher pitch length will increase your aircraft’s speed but also use more power.
Generally speaking, a prop with smaller diameter or pitch can spin faster (higher RPM), because the motor doesn’t need to work as hard to spin it so it pulls less current. They tend to run smoother and feels more responsive to the sticks. The faster change of RPM due to less inertia helps stability of the quadcopter.
A higher pitch propeller moves greater amount of air, which might create turbulence and cause more prop wash. Props spin slower and you might find the lack of “snappy” feeling. But it generates more thrust and higher top speed in the expense of higher current draw.
Surface Area and Shape
The more surface area, the more air the propeller can move and thus generate more thrust. But the downside is higher current draw, more drag and drop in power efficiency.
The shape of a propeller plays a big role in performance because it’s closely related to surface area. The most distinctive difference would probably be the tip of the props: pointy nose, bull nose (BN) and Hybrid bullnose (HBN).
HBN has more surface area than pointy nose, while BN has even more surface area than the HBN. Pointy tip props are the most efficient of the three, however it pulls the least thrust. The other two props can give more power in the cost of more current draw.
The shape of the tip also changes how the prop interacts with airflow when spinning, generally I found props with smooth/round edges perform more efficiently than blunt/square tip props. I have to say this is probably a personal experience, so please experiment with different styles to find your preference.
Fun fact: You can create a bullnose prop by cutting the tip of a pointy nose prop :) For example, you can get a 5045BN by removing the tip of a 6045 normal prop with a pair of scissors.
Number of Blades
For mini quad pilots, tri-blade propellers are equally popular as the two blades, they are commonly used in both racing and free-style flying. Some people prefer triblades because it has more grip in the air. Basically, by adding more blade it’s effectively adding more surface area, and therefore it generates more thrust in the expense of higher current draw and more drag.
More blades props actually exist, quad-blade and even hex-blade! The quad blade are said to be great in indoor tracks, and perform well at cornering. But the increases in surface area makes them really inefficient, and much lower in RPM. As for the hexblade? I wouldn’t even bother, I think they make it as a joke :) simply too inefficient and draws too much amps.
Two Different Prop Directions
On every quadcopter, there are two CW (clockwise) and two CCW (counter-clockwise) propellers. CW and CCW props are supposed to rotate at the opposite direction to generate thrust, as well as having opposing yaw motion that cancel each other out in flight.
So when you buy props, make sure they come in pair, or grab both CW and CCW props if they are sold separately. I actually like the idea of buying props in only one direction, because I tend to break more CCW props for whatever reason. :) (related to being right handed?)
How to Read Spec
There are 2 types of format that manufacturers use.
L x P x B or LLPP x B
L- length, P – pitch, B – number of blades.
For example 6×4.5 (also known as 6045) propellers are 6 inch long and has a pitch of 4.5 inch. Another example, 5x4x3 (sometimes 5040×3) is a 3-blade 5″ propeller that has a pitch of 4 inch.
BN indicates Bullnose props.
Material and Weight
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 really stiff and best known for their smooth performance. While certain plastic compound are extremely durable. Check out this post for more info. In fact, majority of the pilots in this hobby are using “druable plastic” props. They last much much longer and could save you tons of money in the long run.
Also note that lighter props tend to perform better than heavier ones for mini quad in my experience, because there is less moment of inertia. RPM changes faster so the multirotor would feel more responsive in theory.
What To Use on Mini Quad?
It depends on the size of your quad and your motors, mini quads normally use 4, 5 and 6 inch props. Roughly:
- 150-180mm – 2700KV+ motor running 4″ props
- 210mm – 2300KV-2700KV motor running 5″ props
- 250mm – 1900KV-2300KV motor running 6″ props
To pick the right pitch, shape and number of blades, really depends on what sort of flying you want to do also.
Testing is Only Reference
It’s important to look up motor thrust tests data, and see which props work best with the motor you plan to use. Same propeller can perform very differently on 2 different motors. Even for props of the same size, they can perform very differently on different material and design. 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 real life – where air are dynamic and there are many other factors that can have some effect in 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. Also people like to talk about “smoothness” of a motor and propeller, which is not measurable quantitatively, and more of a pilot’s feel.
As long as the motor/prop combo meets your amp draw requirement, you should try out as many props as you can to find the one that “feel” right for what you do. Hope this post has helped you select the best motor and propellers, and don’t hesitate to share your ideas or questions with us. :-)