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Quadcopter blade rotation and lift : How and why

Written by V Kadamatt

In this post we shall address some of the very basics of the physics of how quadcopter blade rotation works and why it works that way. Aerodynamics and aviation is a complex subject in and of itself, and the very many subtleties and mathematics of it goes beyond the scope of this post. For a more detailed understanding on multirotors, check out our article on how multirotors work.

In this article, we shall try to explain all this in simple language and smaller scope so that you at least have an idea of what is going on and why!

Quadcopter blade rotation and motor direction : The How

quadcopter blade rotation : The How


To have a balanced quadcopter, the propeller rotation has to be toward the quadcopter’s main body at all times. To achieve this, a CCW motor has to be placed at the front right, a CW motor at the front left, a CW motor at the back left and a CCW motor at the back right (as shown in the figure above).

This means that motors that rotate in the same direction will be placed at the opposite ends of the quadcopter.

Make sure that the placement of propellers on the motors are proper. You want to place a CCW propeller on a CCW motors.

If your propeller doesn’t come with directions, it is easy to figure out what direction your propeller is supposed to be spinning by finding out which side the leading edge of the propeller is. This is easy – the leading edge is the ‘sharp’ or ‘protruding’ edge of the propeller.

The Why

Flight Basics

First we will address why we need propellers at all and how it generates lift. To those of you who are unfamiliar with Bernoulli’s principle, Wikipedia definition states – “for an inviscid flow of a non-conducting fluid, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid’s potential energy”.

In simple terms, a fast moving fluid has lower pressure than a stationary fluid. What does this mean for aircrafts? Well, don’t mistake ‘fluid’ for liquid.  Air is gas and essentially a fluid.

A popular misconception is that due to the curvature of the wing at the top,  the air at the top of the wing has to “catch up” with the air at the bottom. So, the speed of the air increases at the top in relation to the bottom. This causes a difference in pressure, and this difference in pressure through Bernoulli’s principle, is what creates the lift. However, this has been debunked.

A wing is constructed in such a way that the air at the bottom is slowed down and deflected down the wing, making it higher pressure than the atmospheric pressure (Pd > Pa, where Pd is the pressure at the bottom of the wing and Pa is the atmospheric pressure) (see figure below).

WIND PRESSURE

 

By the Coanda Effect, the air above the wing is guided along the curved surface of the wing. Remember that this is only possible if the atmospheric pressure is greater than the pressure at the surface of the wing, making it ‘stick’ along the surface. Meaning, the pressure at the wing is lower than the atmospheric pressure at the top (Pu < Pa, where Pu is the pressure right at the the top of the wing).

Combining both, we get (Pu < Pd). This means that the pressure at the top of the wing is less than the pressure at the bottom of the wing.

Since the air sticks to the surface at the top, the air is deflected down when it reaches the back of the wing. This deflection downward also causes the air at the bottom to deflect downward. This turning of the angle of the wind flow downward causes the “push” or “lift” upwards.

The wind can be turned in two main ways : Increase the speed of the wind around the wing, by speeding up the plane or increase the angle of attack by tilting the plane upwards, which turns the wind down at a sharper angle. Flaps can also be used but that is a topic for another day!

Hence, the following are responsible for the lift in a wing:

  1. Newton’s third law (every action has an equal and opposite reaction) – generates a lift in a wing at the bottom, since the mass of air is pushed down and back (lift and drag).
  2. Bernoulli’s explanation is incomplete, but it is right that the pressure difference between the air at the top and at the bottom due to the Coanda Effect generates a lift towards the lower pressure (top).

A propeller uses the same principles to keep these forces in action.

However, it keeps spinning to produce a push backward in order to move forward (Newton’s third law in action). In a helicopters and multirotors, this also means a push downwards to move upwards due to its placement. Essentially there are two explanations:

  1. Mass of air is pushed downwards to generate lift.
  2. Difference in air pressure at the top and bottom of the props generates lift. This is why ducting a propeller can improve efficiency by limiting “air leak” due to centrifugal force that is lost as vortices. In simple terms, it stops the higher pressure air at the bottom of the propeller from freely moving to the top and uses this energy instead, to contribute to lift.

Placement of motors and propellers in a quadcopter

Essentially when propellers rotate, applying the rotational analog of Newton’s third law of motion (Every action has an equal and opposite reaction), it generates a torque effect on the quadcopter’s body in the opposite direction.

Hence, if all motors rotate in the same direction, then the quadcopter will keep rotating (or yaw) in that direction. The cause and effect is an important function to understand with quadcopter blade rotation.

In order to counteract this torque effect, we need an equal amount of motors that spin in the opposite direction.

Helicopters work in a similar way. There is a main propeller at the top that generates the lift and another one at the tail end that acts as a counter-acting agent to the torque effect the main lifts propeller generates. You must have seen a helicopter’s tail end getting gunned in action movies (or even real life, if your life is adventurous!), causing the helicopter to spin out of control before it crashes down eventually.

Hence with this setup in place, to yaw (or rotate) in a certain direction, the quadcopter blade rotation must work in a way that the speed of two opposite motors that rotate in the same direction of the intended yaw is increased relative to the other two motors. For example, increasing the speed of two counter-clockwise motors will yaw the quadcopter to the counter-clockwise direction and vice versa.

Conclusion

That’s all! Hope this was informative enough and that you enjoyed the post. If you are a beginner looking to purchase a quadcopter, check out our buying guide for beginners! For quadcopters with more fancier features, check out this guide.

If you have any further questions, comments or suggestions, please feel free to drop them below in the comments section!

 

Quadcopter blade rotation and lift : How and why was last modified: March 20th, 2017 by V Kadamatt

About the author

V Kadamatt

Artificial Intelligence , UAV and RC enthusiast. Software professional, part time philosopher and star gazer.
I also love physics, mathematics, economics, psychology, fantasy, Sci-Fi and futurology.

2 Comments

  • this is interesting but do drone props even use Bernoulli’s ? I am guessing they simply push a mass of air downwards and not use air pressure

    • Hi Nicky, thank you for your comment! First of all, the original, most popular Bernoulli’s explanation (the equal time argument) is a widespread misconception and the article has been updated (reference added) to reflect this.
      I am no expert in aerodynamics, but I do think that the difference in air pressure plays a role. Ducting a propeller for example can improve flight efficiency by limiting energy loss that is caused by higher pressure air at the bottom being allowed to freely move to the lower pressure air, creating vortices. What ducting does is, it creates a “wall” that prevents this free movement and the saved energy is instead used for lift. Having a curved lip on the duct further increases it’s efficiency which can be explained by the pressure difference too.

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