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 stabilized by changing each rotor rpm. Tiny, lightweight rotors that are spun faster or slower by a computer controlled electric motor for each one.

Helicopters are bigger and sudden rpm changes is not a good way to control it.

1 It takes too much to spool up or down due to rotor mass.

2 engines are turbine type, which won’t spool up or down easily. For that scale of things, electric power plant is too heavy and batteries are more than too heavy at that scale. Fuel burning and turbine is the only practical powerplant there.

So helicopters use fix speed rotors, controlled by changing each blade’s angle of attack, by cyclic and collective control. This give the pilot instant control on the thing.

At this point a single, or two counter rotating rotors is the best way to keep it simple.

You have to transmit the power from engines to rotors, and for safety purposes you need all rotors and all engine to be mechanically connected, so a single engine failure won’t stop a single rotor. All rotors are always spinning the same speed and engine shortages are spread equally on all rotors. Quad rotor would require so many shafts and gearboxes, and each will subtract power due to gears and bearings creating some friction. Plus adding multiple points of failures, of the lethal type.