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Comment by ChrisMarshallNY | Hacker News Reader

Almost looks like a ground-effect vehicle. Doesn’t go too high; which may not really be an issue. Just high enough, so tarmac isn’t necessary.

I seem to recall a discussion on HN, where it was explained that quadcopters don’t scale up too well. I don’t remember the reasoning.

The one thing that concerns me about commodity-level flying vehicles, is the way people drive the ones that are stuck on the ground. I would want autonomous vehicles to be devloped and refined, before flying ones.

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Quadrotors use rpm-based control, which is simple and effective, but doesn't scale as the rotor gets bigger and the moment of inertia increases. You could switch to pitch-based control, but then you lose the simplicity, which is a big part of the appeal. The other issue is that the efficiency of any propulsion system is proportional to the disk area. Four smaller propellers have the advantage of zero net torque on the airframe, but carry a substantial penalty in terms of efficiency, which cuts into performance.

Fixing these issues inevitably leads you to a helicopter.

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.

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

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.

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.

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.

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.

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

Quadcopters scale up fine, in fact as other commenters have mentioned they get more efficient as they get larger.

However, quadcopters are mainly useful because they are simple and cheap. Once you add the amount of redundancy necessary to safely carry a human (I am not convinced they have hit that bar here) it's no longer cheap, and it makes sense to spend a bit more on a helicopter, which is much more efficient.

I’m not sure what reasons there would be for quadcopters not scaling up well. Bigger rotors should be more efficient like with any other aircraft. Perhaps there are scaling issues with quickly changing the speed of larger rotors to maintain flight control? That could be resolved by using variable-pitch rotors, although at that point you’ve lost the key advantage of multirotors (which is their mechanical simplicity) and you’re probably better off with just one even bigger rotor with cyclic pitch control (i.e. a helicopter).

helicopters also have one major advantage for manned applications: autorotation. there's at least a chance to survive loss of power in a helicopter. a quad with no power just falls.

Autorotation is a great thing to have when your motor fails, but presumably multirotors would at least make up some reliability by having several motors and presumably being able to safety land when one motor fails.

This machine includes a fast-deploying ballistic chute.

I'm kind of surprised they use only 4. There are drones with 6 and 8 rotors, which can still fly if 1 fails, which they use in the film industry to fly heavier, more expensive cameras. There are also ways to get quads to fly on less than 4 rotors if one becomes damaged, although as a human I'm not sure I'd want to be in one doing it. Here's a Ted Talk with a live demo from 2013: https://www.youtube.com/watch?v=w2itwFJCgFQ

I think its really 8 though, just pancaked on top of eachother?

The site says this can fly with one motor out.

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