Basics of multirotors

The most popular designs are probably quadrocopters. There are also other types of configuration:

Popular multirotor designs:
Bicopter/Twincopter
Tricopter (Y3, T3)
Quadcopter (X4, Y4, V-Tail, A-Tail)
Hexacopter (Y6 and flat))
Octocopter (X8 and flat)

Multirotors can have various frame designs. It’s not a rule that a quadrocopter has an “X” shaped frame or “H” shaped frame. Octocopter can be for example built in a version with 8 booms with motors or It can build with 4 booms with two motos one above another at the end of each boom.

All multirotors, however, share like VTOL flight characteristics. Most of them have fixed pitch propeller connected directly to the motor. The most common multirotor use is probably aerial filming/and photography and drone racing.

How a multirotor flies?

Let’s explain on a quadrocopter. Two propellers rotate clockwise and two propellers rotate counterclockwise. To fly forward you need to generate more thrust on the back of the multirotor so the back rises and drone goes forward.  To fly backward you need to generate more thrust on the nose of the multirotor so the nose rises and drone goes backward.  To fly sideways you need to generate more thrust on left or right side. To yaw left/right you need to simultaneously add rpm motors rotating in one direction while giving less rpm to all the motors rotating in another direction, this way the quadrocopter generate more torque so it turns and quadrocopter stays level at the same altitude. A quadrocopter is equipped and controlled in flight by a special controller which act’s as a computer bridge between your RC transmitter and all the motors. We bascially tell and tell the quadrocopter to behave in air like we want it to fly. and the  multirotor controller equipped with various sensors stabilizes the multirotor in the air by constantly changing rpm of the motors many times per second to maintain level flight

 

Safety

Some drones are equipped with an emergency parachute which reduces the damage during impact.

What happens during motor fail?

Quadrocopter after losing one engine falls from the sky like a spinning rock. In designs like hexacopter or octocopters, there is, however, a possibility to continue flight even after losing its engine. There are however some algorithms which allow even a quadrocopter to fly even after losing two of its engines by completely changing it’s flying characteristics but’s that not a solution for a heavy quadrocopter with camera gimbal underneath.

 

POWER and BATTERY

Most multirotors are powered by an electric brushless motor. Bigger battery = more weight = more energy required to fly = more power consumption = quicker battery drain.  It just has all to be calculated and there are some optimum levels for each machine for it’s designed task.

 

QUADROCOPTER VS HELICOPTER

Quadrocopter benefits of mechanical simplicity. Basically, you can just attach four motors to some booms sticks and let the electronic stabilization take care of correcting flight dynamics.  There is, however, a design limit with a quadrocopter with fixed pitch.  For a bigger camera you can design a quadrocopter with fixed pitch propellers but when you need to lift a human then you have to think about more factors in design.  You can’t just make a giant quadrocopter with giant fixed pitch props because they would have too big inertia and it would be impossible to fly by just controlling the motor rpm because the change of the propeller speed would take too long to make it the way to control the flight. You can of course design a bigger multirotor with fixed pitch but you have to use more motors with more props (or quadrocopter with a collective pitch)

Helicopters’ varying-pitch propeller mechanism is a more efficient solution, because it takes more energy to slow down / speed up propellers than keeping the motor and propeller at a constant speed. Basically, to be more controllable and stable, the more frequent the motors change speed, and therefore the less efficient it will get. Finally The larger your rotor disk is, the less energy wasted in moving the mass of air. This is why long flight time quads tend to use large propellers but again, they are less agile.