So – clearly quadcopters are more stable (see all the drones), so why aren’t actual helicopters all quad copters?
In: 5
1) Helicopters have variable pitch. That’s how change direction, speed up, and basically fly. Drone quadcopters don’t. Variable pitch allows the helicopter to autorotate in the event of an engine failure, instead of crash like a drone.
2) Four rotors isn’t four times the redundancy, it’s four times the chance for failure. Quadcopters don’t fly well on 3 rotors.
3) The ‘heavy lifting’ is done by the outer parts of the rotor, because they’re moving faster than the inner part of the rotor. This favors one (CH-53) or two (CH-47) large rotors over four smaller rotors.
4) Likewise, to compensate for the smaller rotor diameter, quadcopter blades spin faster. This has the advantage of helping with retreating blade stall (the retreating blade is also going forward into the relative wind as the aircraft goes faster, until it’s not going fast enough to work). But it also means the advancing blade is going faster and is limited by the speed of sound (blade tips going supersonic causes a host of problems). And the blade has to be stronger (and hence heavier) to compensate for the higher forces that come with higher RPM.
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