Except that there are many examples now of human scale 4, 6, and 8 rotor craft. So perhaps the litmus test is 'efficient enough'. And the article did point out the need for improved batteries.

As energy density in batteries gets closer to kerosene the 'mechanically simpler' aspect of these crafts flips the value equation in their favor.

Relevant: "Why haven't quadcopters been scaled up yet?"[1]

The gist is, scaling up multi-rotor systems isn't as simple as making things bigger - the size adds many issues that the small models don't face (models having nearly impossible performance characteristics compared to full scale versions). Also, there's safety issues regarding controlling such as system when one or more rotors/motors fail.

It's not to say it cannot be done, because obviously it can. It's a question of "why?", when there's clearly better, more simple and efficient ways to get around.

Also, battery power density (and weight) has a long way to go before it will match Kerosene, or even regular AvGas - "Could an electric engine provide the same performance as jet engines on current aircraft?"[2].

[1] https://aviation.stackexchange.com/questions/3300/why-havent...

[2] https://aviation.stackexchange.com/questions/26910/could-an-...

> It's not to say it cannot be done, because obviously it can. It's a question of "why?", when there's clearly better, more simple and efficient ways to get around.

I observe that in spite of the 'simple and efficent' ways to get around there is a tremendous amount of capital being invested to find new transportation modalities. Perhaps one or more of those investments will surprise you.

There are two things that a full-sized electric multi-rotor vehicle must contend with to be practical:

1) Battery power density must have a tremendous breakthrough, in both terms of weight and stored capacity.

2) Electric motors capable of producing the energies required to produce the thrust necessary to propel a full-sized vehicle must also have a tremendous breakthrough in size, weight, and heat dissipation (especially if we're discussing vehicles capable of typical cruise altitudes over long distances).

It would be far more simple and efficient to make a full-sized Kerosene, AvGas, or any other petroleum-based fuel multi-rotor vehicle... but then you must also overcome the unnecessary challenges multi-rotor systems introduce at this scale. Which... leads back to today's average helicopter and airplane designs.

Like I said, it's not impossible... it's just not practical for these reasons. If someone wants to do it, they can... obviously... but it's not the most effective way.

> Perhaps one or more of those investments will surprise you

Indeed, people often invest in some very surprising ideas... sometimes even without really understanding what they are getting into...[1]

[1] https://aviation.stackexchange.com/questions/33541/are-fixed...

The motors are there. Batteries not. A vehicle carrying multiple persons would have a gas turbine and a high speed generator. The electricity would be used to run distributed direct drive thrusters. The key here is to bypass the complex, heavy, expensive, maintenance intensive and unreliable gearbox. Also no shafts are needed. Power can be varied instantly.

> Also no shafts are needed. Power can be varied instantly

You need to be able to decouple from the motor/engine for safety. Gearboxes provide this ability. One cannot autorotate if a motor has seized and locked the rotor in position, or has been damaged and pegs to full RPM. Also gearboxes can vary torque and RPM without having to adjust the engine/motor RPM/Power.

Also, varying power instantly isn't necessarily a desirable trait in a full-scale aircraft. You have rotors/propellers traveling close to, or exceeding the speed of sound. Instant acceleration would put a lot of forces never before seen on these components.

Usually in aircraft, the RPM of the rotor/propeller remains fairly constant, or within a small bounds. Typically you increase power to the engine/motor, and the gearbox keeps the rotor/propeller RPM's constant.

> unreliable gearbox

They're a lot more reliable than you seem willing to give credit for. Even in your run-of-the-mill car, the gearbox usually fails well after many other major components, especially with regular maintenance (where it may never fail over the lifetime of the vehicle). Aircraft are subject to very stringent maintenance and inspection schedules.

> The motors are there

Seems they aren't quite.[1]

[1] https://aviation.stackexchange.com/questions/26910/could-an-...

If you have fixed pitch, you can't really autorotate anyway. Safety would be provided by redundancy.

With collective pitch, it becomes interesting. Does the motor help or hinder autorotation?

In a hybrid system, you anyway decouple the propeller and turbine with the electric system.

Your car gearbox operates at much lower speeds (because it's not attached to a turbine) and has a lot less weight constraints. Helicopter powertrains require a lot of maintenance.

The stack exchange link talks about jetliners while we're talking about copters.

> If you have fixed pitch, you can't really autorotate anyway

Helicopters are not fixed pitch... your run-of-the-mill hobby multi-rotor is, but that's due to the reasons mentioned above in my previous posts (eg. simplicity at this small scale).

> Safety would be provided by redundancy

Unless you suffer a total electric failure. Or even just a failure on one side of the aircraft (or even just one motor, in a quad setup). Being able to mechanically work these systems is fairly important, since it's less likely both mechanical and electrical systems fail simultaneously.

> In a hybrid system, you anyway decouple the propeller and turbine with the electric system

Unless your electric system is the failure.

> Your car gearbox operates at much lower speeds (because it's not attached to a turbine)

Even some model multi-rotors use gearboxes. You have a motor, spinning either slower or faster than the desired RPM for the rotors... a gearbox solves that difference, and provides a mechanical means of fully decoupling from the motor should the need arise.

> Helicopter powertrains require a lot of maintenance

This is true, but this is not really an issue... and nobody really is looking to "solve" this non-problem. Aircraft require a lot of maintenance and inspections.

And all I'm saying is that you are looking backwards to guess what will happen in front of you. Historically when people have done that, they have been surprised by what came next.

You're absolutely right... but it's important we put faith into feasible ideas instead of chasing down fun, but terribly unproductive and/or unsafe ideas... and also obey the laws of physics.

Differentiating between the two is sometimes difficult, especially when people, generally, want to "believe" in a fantastical vision of tomorrow.

> 2) Electric motors capable of producing the energies required to produce the thrust necessary to propel a full-sized vehicle must also have a tremendous breakthrough in size, weight, and heat dissipation (especially if we're discussing vehicles capable of typical cruise altitudes over long distances).

This one had me wondering, especially since one of the reasons that model quadcopters even exist is because of breakthroughs in brushless motors which have good power to weight ratios for things that fly. So trying to compare a state of the art helicopter engine and a state of the art brushless motor lead me down this path ...

It is notoriously difficult to get details out of Tesla but according to this: https://chargedevs.com/newswire/elon-musk-cooling-not-power-... the Model S electric motor is 362 bhp at a weight of 70 lbs (5.2 HP/lb), and according to this: https://www.geaviation.com/sites/default/files/datasheet-T70... the engine used in the AH-64 Apache is 1662 shp at a weight of 458 lbs (3.62 HP/lb).

The point about cooling is well taken (Elon mentions it as a challenge) however in both model airplanes and quadcopters the motor typically is mount directly under the rotor so that not only does the prop wash provide the lift for the craft, it provides a lot of cooling for the motor driving it.

Again, not saying that you're wrong or that I'm right, I'm just saying that things are changing rapidly, people are investing in that change, and that is a recipe for surprising results.

Just a note on your comparison - aviation engines/motors are not directly comparable to land vehicle engines/motors. They have different expected duty cycles[1][2] and thus, are designed quite differently.

I believe the engines in an Apache are jet turbines, which may also not be directly comparable, but that's only a guess. They are also "war machines", and likely are built to tolerate abuses Tesla cannot imagine and do not design for (my assumption here is that is what adds quite a bit of weight to the Apache engines).

All this is to say the obvious, one could not simply take a Tesla electric motor and install it in an aircraft, expecting good things.

Things may eventually change, and ultimately you may be right - I just don't want to put that much faith, yet, into this idea.

[1] https://aviation.stackexchange.com/questions/11527/what-are-...

[2] https://aviation.stackexchange.com/a/34361/2294

It's hard to see how to overcome the rotor inertia issue in larger craft. So I'd expect human-sized quads would just use collective pitch control. Here's a hobby quad that has taken this approach: https://hobbyking.com/en_us/assault-reaper-500-collective-pi...

I beg to differ: https://www.youtube.com/watch?v=OazFiIhwAEs

Well played! (Though I would argue that the parent is still technically correct about multirotors / quadcopters being very inefficient compared to other flight methods)

That said, I am really curious at actual lifetime costs of a system like this, and if it makes up for lack of efficiency by being simpler mechanically (leading to lighter weight, less maintenance costs, etc.) I think thats going to be the big thing that determines if this is a viable commuting option vs. rich person's toy.