Quadcopter Propeller Basics for Drone Pilots

Quadcopter Propeller Introduction

The purpose of your quadcopter propellers is to generate thrust and torque to keep your drone flying, and to maneuver.

The upward thrust force generated by the propellers is usually measured in pounds or grams. To keep your drone flying at a hover, the upward thrust needs to equal the weight of your drone. The thrust to weight ratio TWR (thrust divided by weight), indicates how much thrust your drone generates relative to its weight. A good rule of thumb is to design the TWR to be at least a value of two.

Typically, quadcopter propellers produce more thrust the faster they spin. They are also influenced by the flight dynamics of your quadcopter. Some propellers produce much more thrust when the drone is stationary, as opposed to when it is flying. Other props perform much better at higher speeds.

Torque is generated when the propellers accelerate up or down. This force is responsible for the ability of the drone to rotate on the yaw axis. Torque is an effect of Newton’s third law, where every action has an equal and opposite reaction. As the propeller rotates, and pushes through the air, the air pushes back and causes a counter rotation on the body of the drone. This is why all of the propellers on a multirotor drone do NOT rotate in the same direction (see Figure 1). The counter rotation effect of all propellers cancels out, and you have no rotation. By changing the relative RPMs of the motors, you can cause a yaw rotation of the drone.

 

Your drone has two different types of propellers and motors. One set is designed to spin clockwise, while the other is designed to spin counter clockwise. Be sure to identify which ones are which.

Quadcopter Propeller Pitch

The quadcopter propeller pitch is a measurement of how far that a propeller will move through the air for every single rotation of the motor/propeller. This is only a theoretical approximation since real world factors such as prop material, air density, and efficiency can influence this distance. However, the pitch is a good measurement that will help you better understand your propeller performance.

The higher the pitch value, the faster your plane will go.

To better understand the effect of propeller pitch, imagine two wood screws as shown in Figure 2. The screw on the left has a coarse thread (higher pitch), and the screw on the right has a finer thread (lower pitch). If you were to take a screwdriver, and screwed both into a piece of wood by making sure you turn the screwdriver at the same speed for both screws, you would notice that the screw with the coarser thread (higher pitch), would sink into the wood more.

This is the same effect as a propeller cutting through the air.

In the figure, the two arrow lines show the position of the propeller tips. You can see that the high pitch propeller covers the same distance travelled with only half the rotation of the propeller. With both motors/propellers spinning at the same RPM, the higher pitch propeller will travel further in the same amount of time. In other words, the plane with the higher pitch propeller will fly faster.

Changing the propeller pitch will also change the dynamic response of your quadcopter.

To produce the required thrust, a quadcopter propeller needs to be designed to be within a specific range of pitch angles. An almost flat propeller will be just as bad as a steep pitched propeller, that is it will not generate any lift.

The flatter the blade, the easier it is for the motor to rotate the blade through the air. Toy grade or low-end quadcopters are equipped with low power motors. These motors need to spin faster to create lift, but higher RPMs are easier to generate with a smaller motor than torque.

Quadcopter Propeller Size

The size of the propeller is the distance from tip to tip. Longer propellers can generate more thrust at the same speed, but requires more torque from the motor to turn the prop. A larger propeller size does not mean that you will be able to fly faster. That is mostly determined by the propeller pitch.

The surface area of the propeller also determines how much thrust it can generate. The higher surface area allows more air to be moved, thereby generating more thrust. This also comes at the cost of more power draw from the motor.

The length of your quadcopter frame determines the maximum size of propeller that you can use. This can easily be determined by taking the smallest of the length or width of your drone frame, and dividing by 2. Then you need to make this slightly smaller to provide rotation clearance between adjacent propellers.

For a square frame configuration (see Figure 3), the maximum propeller size is X/2.
For a rectangular frame configuration (see Figure 4), you take the smallest of the length or width dimension, in this case it is obviously Y, and divide by two, that is Y/2.

Overlapping the propellers is not recommended since the downward airflow of one quadcopter propeller will reduce the thrust efficiency of the propeller below. There is also a bit of side to side airflow due to tip vortices, so you want to allow for some spacing between the propellers to avoid this effect.

Number of Blades

Two bladed propellers are more efficient at producing thrust as opposed to three or four bladed propellers (see Figure 5) so long as the tips do not reach supersonic speeds. For the purposes of quadcopter propellers, this is not a problem.

Adding blades increases the amount of thrust that is generated, but at the cost of efficiency.

You will see many drones that have 5-inch triple blade propellers. The aerodynamic environment of drones negates the inefficiencies of tri-blades in the 5-inch size. Tri blades also have a higher torque generation which makes the yaw axis of the quadcopter much more responsive.

There are 4, 5, and even 6 blade designs, but these have diminishing returns due to inefficiency.

Propeller Material

Propellers are usually made from a plastic compound, though carbon fiber propellers are also available. The type of material used to make the quadcopter propeller affect its stiffness and durability.

Most hobby drone pilots are using durable plastic propellers that are slightly flexible and bend when your drone crashes. This allows them to last much longer, and save you money in the long run. These propellers have a disadvantage of causing vibration as they rotate, which can affect the flight performance of your drone.

Carbon fiber propellers have a number of advantages, and disadvantages. They produce less vibration due to their stiffness, and sound quieter when flying. They are much lighter, and are stronger than plastic propellers. However, they are expensive, and the inflexibility means the motor bearings will take the brunt of the impact force on crashes.

Propeller Weight

For better flight performance, you want to go with lighter propellers as opposed to heavier ones. Lighter props have less moment of inertia, which means the motor needs to apply less torque to generate the same RPM. This also results in faster RPM changes which leads to better drone flight response.
The weight distribution greatly affects the amount of vibration that the propeller will generate. A perfectly balanced prop will produce next to no vibration, while an unbalanced prop will affect the flight performance, and will sound terrible.

Bull-Nose Propellers

The more surface area that a propeller has, the more air it can push thereby creating more thrust. The downside is higher current draw, increased drag, and reduced power efficiency. Bull nose propellers are more commonly being used on modern quadcopters (see Figure 6).

Bull-nose quadcopter propellers have a greater surface area, and create more thrust. Tapered propellers create less tip vortices, but sacrifices thrust as the propeller gets thinner. These are better suited for slow moving quadcopters where stability is more important for applications such as aerial photography.

The added surface area of the bull-nose propeller means that you are adding weight, especially further away from the hub. This increases the moment of inertia of the propeller, and requires the motor to apply more torque, drawing more power from the battery.

Bull-nose propellers are commonly used on small racing frames to gain a higher thrust at a smaller size.

Quadcopter Propeller Sizing

When selecting propeller sizing you need to understand the format that manufacturers use. There are two common formats that are used: LxPxB or LLPPxB.

L represents the length, P is the pitch, and B is the number of blades. You may also see a BN designation that stands for bull-nose.

You may also see an R or a C after the sizing numbers. The R stands for reversed, which is the rotation direction of the propeller. An R propeller needs to be mounted on a drone motor that rotates clockwise. A propeller with a C designation is to be mounted on a motor that rotates counter-clockwise.

Folding Propellers

There are many drones that are designed to highly portable so you can easily take them anywhere with you. To accomplish this, foldable propellers are included in the design (see Figures 7 and 8).

At the high RPMs needed to generate lift to fly your drone, the outward force due to the motor rotation is sufficient to keep the propeller in its proper position. So, when you take off and jam the throttle on high, you can be sure your drone will fly. The foldable propellers are designed to maintain balance, so you do not lose any thrust generation efficiency.

There is a slight advantage that a foldable propeller has that a fixed prop doesn’t. On a fixed prop, when you push the throttle to high, all the motor torque is transferred to the propellers. With a foldable propeller, there is a bit of an absorbing effect that helps to cushion the impact on your drone. This will quickly balance out and the full torque will be applied to the propeller. The cushioning effect is beneficial to reduce shock impact on the camera.

Propeller Damage

If your quadcopter has crashed, be sure to take a careful look at the propellers for any damage. A few scrapes on the propeller are ok, but flying with nicked propellers will make it nosier, and increase vibration. This will have the effect of shortening the life of the motors, since the bearings will rotate as if they are unbalanced.

The quickest way to check for any damage is to run your fingers along the leading edge of the propeller. If you feel any major nicks, then it’s time to replace the propeller.

Be sure to keep a few extra propellers on hand if they need to be replaced. Most quadcopters that you buy come with extra propellers.

Remember, that there are two different directions that the propeller can rotate (see previous section). Be sure that you are mounting the correct propeller on the right motor.

Propeller Thrust and Speed

To know how much thrust a given propeller will produce, you must examine motor thrust test data which most manufacturers provide. The same propeller on two different motors can have vastly different performance due to variances in design, torque, power, and RPM.

However, these bench tests are done in a static environment, and do not reflect the actual environment when your quadcopter is flying. Typically, you can expect to see 5 to 10% less thrust in real flight as compared to the bench tests.

Be sure to take this into account when you are sizing your propeller and drone motor.

Quadcopter Propeller Summary

Hopefully this article has given you a better understanding of how the propeller specifications affect the flight performance of your drone. For the amateur drone pilot, you need to understand the sizing specifications so you can order the proper replacement quadcopter propeller.

If you are building your own drone, then you will have a better idea of the options available to you as you select the propeller for your quadcopter.

References

RC Airplane World

Carbon Fiber Props