DRONE PROPELLERS

We have discussed the evolution of drones and different types of drones in the previous session. Have you ever wondered how a drone flies? So in this session, I will try to explain to you the simple physics and tell you in detail about drone propellers.

PROPELLERS: 

Propellers are the necessary part of the drone and maintain a key role in the flight of a drone. They are also the most commonly damaged and replaced components. So take time to choose the right propeller for your drone. Propeller size and design play an important role in getting thrust.  So the purpose of the study is to identify the best propeller design for a specific size of a drone.

DRONE THRUST:

Drone thrust is an amount of upward force generated when a drone is at full throttle. With that, you can determine how much a drone can lift. The drone thrust must be higher than the drone weight for the drone to take-off and fly.

To explain to you in detail about how a drone flies lets consider a quadcopter as a reference.

HOW A QUADCOPTER WORKS AND FLIES:

Quadcopters are very easy to fly in any direction. The quadcopter has 4 rotors which are individually connected to 4 different motors. Where two diagonal rotors rotate in a clockwise direction and the other two rotors rotate in an anticlockwise direction as shown in fig.1. These two opposite rotations balance out and keep the drone steady. If all the rotors spin in the same direction it would in net torque causing the complete drone to rotate.

DIRECTION OF PROPELLERS:

For a drone to fly, a force must be created which is equal to or exceed the weight of a drone. Now quadcopter uses motor speed and propeller direction to control the gravitational force against quadcopter.
The spinning of drone propellers blades pushes the air down. As per newton’s third law which states that every action has the equal and opposite reaction. Therefore, as the rotor pushes air downward, the air pushes the rotor upwards. Faster the rotor spin greater the lift.

 A quadcopter can do 3 things in the vertical plane.

1. Hover still: To hover still, the net thrust of 4 rotors must be exactly equal to gravitational force.
   
2. Vertical Ascend: It can be achieved by increasing the net thrust to more than the net forces in a downward direction.
   
3. Vertical Descend: It can be achieved by decreasing the net thrust to less than the net forces in a downward direction.
   

PRINCIPLE AXIS OF FLIGHT:

Pitch, Roll, and yaw are 3 different axes of flight.

• Pitch (lateral axis): This is the movement of quadcopter either forward or backwards.
  
• Yaw (vertical axis): This is the rotation of the head of the quadcopter either to the left or right.
  
• Roll (longitudinal axis): In this quadcopter fly sideways, either to left or right. In this movement, the head of the drone remains in the same direction.
  

If you want your drone to fly in forwarding direction increase the RPM(rotation rate) of rotor 3 and 4 and decrease the RPM of rotor 1 and 2. But the thrust on rotors will be equal to the weight so that the drone stays at the same vertical level. Since one rotor moves in a clockwise direction and other moves in the anti-clockwise direction those motors still produce zero angular momentum.

Yaw refers to the movement of the drone in a clockwise or anti-clockwise direction concerning the center axis. If there is no torque on drone motors, then the total angular momentum of the drone is zero. To understand the angular momentum of a quadcopter, assign each motor with an angular momentum value as +5,-5,+5 and -5 which is equal to zero. Now decrease the angular velocity of motor 1 to change angular momentum from +5 to +3. Now the net angular momentum changes to -2. So the drone rotates in a clockwise direction. Decreasing the spin of motor 1 causes a decrease in the thrust of motor 1. So the net upward force is not equal to the gravitational pull. So the drone will be unstable it changes its vertical position concerning net upward force. To rotate drones without any instabilities, decrease the spin of motor 1 and 3 and increase the spin of motor 2 and 4. So that the net angular momentum will remain zero.

 To roll drone toward left without changing its head position, increase the thrust on the motor on the right and decrease the thrust on the motor on the left. To rotate the drone without any imbalances decrease the spin of motor 1 and 3 and increase the spin of motor 2 and 4. As there is no torque on a drone the total angular momentum will be zero.

FORCES THAT AFFECT DRONE

• Fi is the thrust produced by the four propeller perpendicular to the plane of rotation of propellers.
  
• Wi is the angular velocity of propellers.
  
• Mi is the reaction moment about the Z-axis.
  
• My and Mx are the moment along X-axis and Y-axis.
  
• L is the distance between two diagonal motors.
  

FACTORS CONSIDERED FOR EFFICIENT PROPELLER:

Propeller design efficiency is obtained by using the power output it produces. The power output for a fan is how quickly the fan can accelerate the surrounding airflow. It can be calculated from equation

Propeller design efficiency factor:

SIZE OF PROPELLERS:

The first thing we consider when we are looking for a propeller for our drone is size. Larger propeller sweeps through more air and has high thrust and better grip but it consumes more power and responds slower to the inputs from the motor and covers a large area.

Small propeller sweep through less air, consume less power, have high thrust, respond quickly to the inputs from the motor.

BLADE CONFIGURATION:

Blade configuration refers to the no of blades on propellers. Increasing the no of blades is a substitute for increasing the size where space is constrained. The most efficient propellers are one blade propellers but due to its imbalance, they are not in use. Most of the multirotor drone propellers have two blades. Due to fluid dynamics and complicated physics increase in the no of blades is not as efficient as an increase in size. But it does produce more thrust for slightly more power.

MATERIAL:

The propeller was manufactured in fiberglass reinforced plastics which are very stiff and keeps the most efficient shape regardless of how fast it spins. But when they hit to the ground these propeller breaks and drones no longer fly. But recent propeller is made up of polycarbonate. It is a type of plastic with good stiffness but it will bend rather than break when hit to the ground. There are other materials used in propeller design but polycarbonate is the best.

 

PROPELLER PITCH:

It is defined as how far that a propeller will move through the air for a single rotation of the propeller.
High pitch propeller has high thrust and top-end speed, but more low-end torque. They respond slowly to the input, consume more power and used where the multirotor is moving quickly.
Low pitch propeller has low thrust and top-end speed, but less low-end torque. They respond quickly to the input, consume less power and are efficient at low speeds.

 

CONCLUSION:

 Hopefully, this article cleared all your doubts about how drone fly and provided a little more insight into what features to look for to find the best prop for your drone.

Read related topics:
Introduction to drones