Rotary drones: Why 4, 6, 8+ rotors?

Factual Questions
1

It seems that most of the consumer drones available now are quadcopters, though 6- and 8-rotored drones are also quite readily available.

What are the advantages of increasing the number of rotors on a craft? There’s more redundancy, of course, with the possibility of staying airborne if one or more rotors fail. Anything beyond that?

And at what cost, though? It seems like more rotors would require more power (or is that more a factor of total mass being moved, i.e. 4 big rotors vs 8 tiny ones?) so less battery life. Does the # of rotors affect efficiency, maybe interfering with one another in terms of airflow?

2

The main one is torque. On a quadrotor, two props spin in one direction while the other two spin opposite, giving a net zero torque.

There are odd-number rotor designs but they all have to overcome the torque problem. The obvious example here is the standard helicopter, which has one main rotor and then a smaller one at right angles to cancel the torque.

Generally, you want fewer rotors, because they become more efficient as the diameter increases. A fewer number of larger rotors are thus more efficient than a large number of tiny rotors.

Edit: I see I read your post slightly wrong (reading the title, I thought you were asking why even numbers of rotors). The efficiency point stands, though.

Small rotors do have a response time advantage, too. It’s faster to spin up two small rotors than one big one, so you get better maneuverability.

3

Smaller rotors with less inertia also makes it easier for the autopilot to keep the drone flying level.

4

I would imagine safety is also a point. The single rotary blade blades on large model helicopters are fully capable of killing people and have.

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5

Mechanical simplicity is one thing. Conventional helicopters with a single main rotor have a complex mechanism to be able to tilt the blades differently at various points in the cycle for steering. It’s hard to scale down that mechanism into a cheap-to-make drone.

Quadcopters (and copters with more blades), on the other hand, have simple fixed-position rotors and are steered by varying the speed of the rotors relative to each other. That mechanism is a lot cheaper to make, but requires a more involved control system which was impractical before computers stopped being the “expensive part” of the system.

Given that you’re steering by adjusting rotor speeds, more rotors means more degrees of freedom, which means finer control over steering.

6

Incidentally, a 1-propeller drone with fixed pitch propeller is possible (but looks silly).

7

And there was a TED talk a few years ago where they took a standard quadcopter and cut off two of its rotors and the control system adapted.

Still, it’s a lot easier, even for the computer, to manage with at least four rotors intact. :slight_smile: