[syntaxing]

Because smaller prop means you need to spin faster to hit the same airflow. And efficiency usually drops by x^2 as a function of rotation velocity. Also, at high speed you have vacuum issues near the tip of the blade. Therefore, having four small rotors can easily be up to a magnitude less efficient than a helicopter.

[t0mas88]

Efficiency drops by "propeller" speed and then falls off a cliff when the prop tips go super sonic. So you really can't just make them bigger and spin faster.

[namibj]

If you have scimitar blades, this isn't an issue up to ~Mach 1.5 if the airspeed remains well subsonic.

[dtgriscom]

The comment was "quadcopters don’t scale up too well"; you're explaining the reverse.

[tweedledee]

They are explaining how quadcopters don't scale very well. You may be misunderstanding the explanation. The surface area covered by the rotor is much less for quadcopters and it is difficult to make up for that. Plus small rotors are less efficient. One upside for quadcopters is simplicity but that advantage diminishes with scale.

[tweedledee]

For completeness I should add; Quadrotor configurations rely on varying prop speeds to control pitch, yaw, and roll. The larger the props the more difficult it is to vary the prop speed in a timely manner. Electric motors are more weight efficient at high speeds but larger props need to be slower. You could add a gearbox but either way you are gaining weight and complexity. Plus parts start to get really expensive as you scale up and get out of hobbyists range.

[spookthesunset]

And to put an even finer point on it, it isn’t just control authority but keeping the damn thing even stable. The only reason those things stay aloft is the controller senses minute changes to the orientation (called attitude) of the craft and adjusts the motor speeds in tiny amounts to fight those changes. Without that quick feedback loop the craft would flop all around and crash.

It’s one of the reasons you don’t see gas powered quads. Gas engines just cannot react fast enough and precise enough to correct subtle changes in attitude.