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Comment by TylerE | Hacker News Reader

Drones are usually copters so they can do vertical takeoffs and hover.

They're quadcopters rather than single mainrotor + tailrotor because it greatly simplifies the mechanics.

Single rotors need a cyclic mechanism to vary the blade pitch depending on where the blade is in the rotation and the control inputs. This is VERY complicated with a lot of moving parts that have to maintained and adjusted. (See https://upload.wikimedia.org/wikipedia/commons/thumb/4/4f/Si... for one example)

Quads (and hexes, etc) can have full control authority via RPM only, using a simple injection molded constant pitch prop.

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To underscore the relative simplicity, you can make a quadrotor with just 4 moving parts. Each electric motor has only one moving part, the rotor, and the prop is attached directly to it without even any gears.

Just noting cheaper quads often use geared props to compensate for their under-powered motors.

I used to fly gas RC helicopters and the joke was that RC helis are collections of small expensive parts desperately trying to get away from each other.

Quadcopters take simple hardware and control it elegantly with complex software. The beauty is that you only need to write the software once. All other tech requires more complicated hardware.

You have to admire the balancing effect of the torque on quad-rotors. Two CW and CCW rotors cancelling out the yaw (or introducing it, when required).

Well, full-ish control authority. They have to angle themselves in a certain direction in order to accelerate in that direction. Though perhaps that isn't so different from a helicopter.

whats the benefit of a single rotor then and why arent we seeing quad rotor configurations for combat or transport helicopters?

The main reason is that multirotors are a much more recent solution, and they started as small unmanned vehicles because the regulation is much simpler (the requirements for airworthiness certification for multirotors are not standardized yet). But I believe that we will see more and more applications for multirotors in the future. As a concept, they are safer than helicopters, with much simpler mechanics.

Single rotors are easier to fly, and more efficient. Also fewer things that can fail.

Basically with a quad if you lose any of the 4 engines you're going down.

This is much less of an issue with an unmanned aircraft.

With a single rotor if you lose any of them you're going down too :)

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