I have often wondered if larger, shrouded, multi-bladed rotors that can spin at a much lower speed can work. Perhaps uneven blade spacing — or even articulated blades with tunable lead-lag hinges to modulate blade spacing — to spread the noise over a greater frequency range.

Complex problem for sure.

> tunable lead-lag hinges

See, drones traded moving parts for the annoying whiny motors(but fixed in a rigid frame). If we are going to use moving parts anyway, there's helicopters.

First-hand experience: helicopters are wildly more difficult to fly.

Perhaps the lead-lag hinges can be elastomeric and one could purchase props with different fixed hinge stiffnesses for different amounts of noise dampening or frequency spread but perhaps at the cost of prop-speed change response? There'd probably have to be some compensatory mechanism in the flight- or motor-control software that'd be set for a given prop's hinge elasticity.

Of course, I am just idly speculating here on whether there's any benefit at all to any of my blather.

Lots can be done, but at the expense of fuel efficiency.

Which is much less of a problem when you can charge the drones from a carrier.

Our drones need a blimp mode : ) :)

A lighter than air mothership makes a lot of sense. You could have the delivery drones glide to the location, fire up the propellers for the controlled package dropoff, then fly back.

Kids with airsoft guns will have a field day

Blimps aren't normally brought down by a few bullet holes. The pressure inside is close to atmospheric pressure, so helium leaks out very slowly.