As some of the comments have mentioned, one of the major issues in using the rotor speed to adjust for lift instead of the collective is purely the speed of the response, but this is not the only one. Some of the issues that you will run into include
- A large rotor blade will have a relatively large amount of inertia, which means that it takes a while to speed up or slow down the blade, and this is going to be prohibitive for safe flight
- The rotor itself is attached to the engine via a clutch, which allows the rotor to spin freely if the engine quits. This is a requirement for auto-rotation, but it also means that you can't use the engine speed directly to slow the blade down, you'd have to include some sort of additional brake to slow it down if you want to maintain the ability to auto-rotate
- Both piston and turbine engines, unlike an electric motor, have a significantly narrower/smaller range of RPMs that they can function at, with different amounts of power at different RPMs. The engines themselves are also limited in how quickly they can speed up and slow down, which means that even if you could speed up or slow down the rotors themselves rapidly, the engine might not be able to function at the required speed.
- As the rotor size increases, so does the speed that the tip of the rotor is traveling at. If the tip of the rotor starts generating shockwaves because it has exceeded the speed of sound, you'll have significantly reduced lift and a large increase in sound volume. The tip of a small rotor isn't going to get anywhere close to super sonic, so this isn't a problem.
- The tail rotor itself is mechanically connected to the main rotor, and will need to create different amounts of lift to control the yaw of the helicopter. Since you're probably not going to want to descend or ascend every time you turn left or right, you'll still need a variable pitch for the tail rotor. I already have to adjust the pitch of the tail rotor at various times, and it would add additional complexity to the piloting of the aircraft.
- The rotor of a real helicopter is moving a much larger volume of air, and the pilot already has to manage the issues caused by it. For example, if I descend too quickly, I can descend into my own rotor downwash, which will not provide lift. If the wind is blowing in the correct direction, my rotor downwash can be blown into my tail rotor, which will cause a loss of tail rotor effectiveness. Having multiple rotors increases the complexity (and likelihood) of this occurring.
I'm sure there's a number of issues I haven't even thought of that would be difficult or impossible to overcome, but this is just the start. Most of these issues don't exist on a small drone because:
- It has electric motors that have a much wider torque curve, and a much faster response time, so they can adjust the speed of the rotors quickly.
- The rotors themselves have very little inertia, and their speed can be adjusted quickly.
- There are multiple rotors, and no tail rotor, so you don't have the mechanical complexity associated with them.
- Because of the significantly smaller amount of air that is being moved by a smaller aircraft, you have significantly less aerodynamic complexity. I don't think you're likely to encounter any issues with settling into your own turbulence, for example.
- You can't auto-rotate. In a drone, it doesn't matter as much, since there's no one sitting in it, but as a helicopter pilot, I very much want the ability to autorotate.
This is certainly not a complete list of issues, but are just a small subset of issues that I can think of off the top of my head.