Maybe this can be efficient if the drone had a fixed wing so it can glide. Or we can make the "structure" really really tall -- perhaps the delivery plane can launch drones that basically paraglide into the city below, and the drones can fly back to a collection center once they've delivered the payload.

Alternatively, you could use a ground-based slingshot to vertically propel gliders, an approach that is already used in the military and for some long distance UAV testing. However, the wing-based UAV vs. rotor-based UAV physics are very different. Landing an elevated glider with a clear path to its target is one thing, maneuvering between trees and power lines in a complex environment without rotors is quite another.

Then you need to look at a 'convertible' UAV with both features... undoubtedly this raises the cost, reduces the carrying capacity, and is already being pursued.

Finally, you could zoom out and re-evaluate the business case. Why not use a network of local dispatch points (handover points to ground-based UAVs or vans) to reduce the distance to the consumer and with the smaller geographic scope negotiate private airspace access on locally agreeable terms. I believe this is the future for general consumption (food, regular deliveries within small package weight/dimension limits).

Wouldn't the cost of elevating the loaded drones to the rooftop of that tall structure negate any monetary gains? Wouldn't the cost be exactly that of simply using grounded vehicles that elevate themselves?

(edited for clarity)

In summary: Rain the deliveries onto the neighborhood with a railgun.

In real terms, I'm doubtful that the majority of people's goods would withstand that kind of rapid acceleration. Consumer electronics frequently break from hitting the floor, a rail gun would probably shred them. You could accelerate more slowly, but then you need more runway, and you might end up building a skyscraper anyways to get a delivery radius that's more than half a mile.

flight is appealing over ground based drones as one delivery truck or base station could concurrently service multiple blocks with lower risk of someone abducting or damaging the drone. Although in all honesty I'm not sure if weight, range, carrying capacity, or speed would work out in a compelling form factor.

I saw a mockup video of this once. I think it was for UPS, but I might be remembering that part wrong. The upshot was that it's not useful in cities, but could be used to make exurban and rural deliveries much more efficient because the delivery truck doesn't have to trundle down a bunch of long driveways.

Take NYC for example: Delivery people on e-bikes are still harassed incessantly by the NYPD and community boards by people being scared of the possibility of being hit by them.

Factor in our paranoia about terrorism and a single accidentally dropped package would likely result in counter terrorism and bomb squads being called in.

I couldn’t imagine any proposal for giant flying robots with multi-pound payloads hovering overhead be taken seriously by even the most progressive leaders.

In hard to reach / rural areas, I could see some benefits from drone technology though. Drone chainsaws, anyone?[0]

[0] https://nypost.com/video/watch-this-helicopter-saw-blaze-thr...

However, swarms of little drones flying in the sky of cities or private properties seems incredibly irritating, however efficiently they improve deliveries.

A better way to think about it is as a shared data structure. It is a 3d vector space with some attributes that help predict the future. Each drone has a different version of this data structure in memory. The data structure has an initial state at take-off and is updated using radio communication and sensor data.

If the data structure is perfectly synchronised then planning is easy. There is plenty of sky and simple rules can be applied. But it never will be perfectly synchronised. in that sense it is like a globally distributed database.

The data structure may have been edited by another drone and the change not communicated. You may have lost communication and not even realised. Local conditions may have reduced GPS quality giving you false location information.

So perhaps an interesting metaphor is concurrency control. Do we lock airspace or can we have transactions? Can we partition or shard the airspace? When is consistent state required?

Additionally they can usually fly higher which further reduces the noise pollution.