Eli5 why do drones have four blades and helicopters only have one?

Question

Eli5 why do drones have four blades and helicopters only have one?

In: 2

Answer 1

Small rotors can change speed rapidly.

A drone can be controlled by changing each rotor speed independently. This is very simple to do, the rotor is just a fan in front of the motor, the computer changes the motor power, and you control the craft with a decent response speed. This would work with any powerplant, except slow response powerplant such as turbines. Electric, or piston engine would do the job.

Big rotors have massive inertia, don’t change speed easily. So, when you scale it up, drone or not, quad rotor or single rotor, you can’t control the craft by rotor speed changes. What is done then is to use a mechanical system to allow some actuators to change each blade angle. The rotor rotates always the same speed, the blades angles determine the thrust and direction of thrust. In this condition, any powerplant is good for the job, including rockets or pulse jets at blade tip, turbine engines connected to the rotor by a gearbox, piston and electric engines. What you must have is a way to change the blade pitch to achieve fast response to the flight controls. Rotor speed may or may not vary, but won’t let you control the craft quick enough. So in case you have variable rotor speed, that’s just there for efficiency purposes, not control.

Consequently, on big rotorcraft, it can be easier and cheaper to use one single big rotor with individual blade pitch control, compared to many small rotors with individual blade pitch control. As the more rotors you add the more things get expensive and complicated, more parts means also more points of failure.

This is the great scheme of things. Small: multi rotor and control by rotor speed. Big: as few as possible rotors with pitch controls.

In the middle, there you can find bizzarre things that are a mish-mash of the two. The middle is crafts around 100-500kg. In this area none of the classic configurations is beneficial, so you can find drones with tilting rotors, pitch change, or single rotors controlled in speed (the latter flies like a drunk monkey, the bigger the drunker).

It’s square cube law, same law that makes impossible to fly a parrot scaled down to 1 cm long or fly a bee scaled up to one meter big. Mass increase by 8 times every time you double the dimensions, so things that move decenti well in small scale may become incredibly clumsy incredibly quickly when scaled up.