why aren’t all helicopters quadcopters?

Question

So – clearly quadcopters are more stable (see all the drones), so why aren’t actual helicopters all quad copters?

In: 5

Answer 1

Drones are not built that way because the are more stable. The design is because it is a cheaper way to build them if you power them with batteries.

Quadcopters drones work because they have electric motors that quickly can control the rotational speed of a fixed rotor.

Regular helicopters have one or more internal combustion engines usually gas turbines. They do not change the rotational speed fast. Helicopters usually operate them at a constant rotation speed for the flight

So the control system of a helicopter is a https://en.wikipedia.org/wiki/Swashplate_(aeronautics) that controls the angle of attack of all blades. There are both changes for them all if you want more or less lift to go up and down. It also has directional control so you then have a higher angle of attack on now side to provide more lift there and to move around. To that add a tail rotor.

A quadcopter also needs a structural part that keeps the rotors apart and the weight compared to strange do not scale in a way that is advantageous for a large helicopter. The square cube law is in effect

there are helicopters with multiple rotors that provide lif like a https://en.wikipedia.org/wiki/Boeing_Vertol_CH-46_Sea_Knight , https://en.wikipedia.org/wiki/Bell_Boeing_V-22_Osprey etc. They have a mechanical connection between the rotors so they rotate at the same. A V-22 has one engine just below each rotor, they are still connected because you do not what a single engine failure to result in a crash.

The reason they have multiple rotors is to provide enough lift for a CH-46 and similarly designed. A single-rotor would be very large. A V-22 have two so you can fly like an airplane with them tilted forward and the lift is provided by the wing. A single-rotor ontop is cheaper for most helicopter requirements.

For drones, it is ok if a single engine failure results in a crash but that would not be ok for a crewed helicopter.

So what is a cheap and good design for a small electrically powered front that does not carry humans is not a cheap or good design for an internal combustion engine power helicopter with humans onboard.