Quadcopter Physics
By: Abhirami Bhargan
Tags: Flying Robot, Drones, Aviation, Robotics, Unmanned Aircraft Systems

The future belong to the flying robots, The Unmanned Ariel Vehicles(UAV) as they can also be termed. The true potential of drones are yet to be uncovered! This is a perfect blog for you to retouch the basics of quadcopters as we will cover the crazy science of drone flight here.

All of these drones and our flying machines are a part of family of rotating wing aircrafts called rotorcrafts. Let’s go ahead and see the type of drones.

Well, we better focus on quadcopters. To put it formally, a quadcopter is an aircraft equipped with four independently controlled rotors mounted on a rigid frame. A quadcopter has two possible configurations “+” and “X”. X-configuration is a more acrobatic configuration. well, but it’s more basically a personal choice.

Now we are ready with basic understanding of a quadcopter, let us check out how the hovering works!

The main forces acting on a quadcopter is reflected in the figure:

· Lift force- Vertical force generated by the propeller.

· Gravity force- The force which keep you on ground.

· Thrust- Power generated by the motors that push and move the plane.

· Drag force- The force that opposes the motion of the quadcopter.

How do drones fly?

Each arm of the quadcopter has a motor attached to a propeller. Propeller blades is the paramount of a drone. Each cross-section of the propeller generates a lift force when air flows over the blade. Yes, the air foil principle. Spinning blades push air down. Of course, all forces come in pairs, which means that as the rotor pushes down on the air, the air pushes up on the rotor. This is the basic idea behind lift.

Time for take off! Increase the rotor speed of all four motors so that lift force overcomes weight of the drone. The drone will now take off ,when the required height is reached reduce the rotor speed until lift force exactly balances the drone weight. Now you achieved levitation.

why not rotate all the propellers in same direction?

Answer is simple! Newtons third law. If all propellers spin in the same direction drone body would have spun in the opposite direction. Wait, now why is this the case? If all the rotors were to spin in the same direction, it would result in a net Torque causing the complete Quad to rotate. By spinning the motors in opposite direction we are making the net reaction torque on the quadcopter zero. Accordingly the diagonally opposite rotors rotate in the same direction.

Do the position of counter rotating matter?

Hovering is achieved by accelerating each motor until they produce a force one fourth that of gravity and as long as we have two counter rotating motors the torque from spinning the propellers will balance out and the drone will not spin. It doesn’t actually matter where you put the counter rotating motors as long as we have two of them spinning in one direction and two in the other. well, the quadcopter developers settled on a configuration with opposing motors spinning in same direction and yes, there must be a reason for this! And there is, This design is to prevent unwanted yaw when moving. You can’t make the drone yaw right without also accidentally rolling and/or pitching and/or climbing. Not very useful! (we will check out these terms shortly.)This has also to do with the mathematics of drone flight. Let’s not get into such messy details.

Basic movements of a Quadcopter

A quadcopter has 6 degrees of freedom. 3 Rotational and 3 Translational. Translational motion include up/down(vertical), left/right (lateral)and forward/backward(longitudinal). Pitch(Y-axis), Yaw(Z-axis) and Roll(X-axis) are the three angular motions.

Maneuvering of a quadcopter is pretty basic. Any motion can be achieved by varying the speed of the propellers in certain sequence. Give your brain a quick warm-up figuring out the logic behind these sequences.

Well, you are now ready to get out in the world of flying robots!