The caveats being that batteries, air traffic control, and regulations have to change so in short, "I've your 25 today, perhaps your grand children will see a benefit here." :-(. Somehow I think even the hyperloop is nearer term than this technology, and its still out there.
Less efficient, but much simpler mechanically.
As energy density in batteries gets closer to kerosene the 'mechanically simpler' aspect of these crafts flips the value equation in their favor.
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-...
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.
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...
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-...
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.
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.
Differentiating between the two is sometimes difficult, especially when people, generally, want to "believe" in a fantastical vision of tomorrow.
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.
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-...
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.
Let's say you have one 2 MW rotor disk. Rotors are tip speed limited. Let's make it have an angular velocity a. Torque would need to be 2 MW / a.
Having four 0.5 MW disks would have same tip speed but half the span so double the angular velocity and thus each would need torque of only 0.5 MW / (2 x a). Torque dropped to one eighth, and the motor became lighter!
Now, at the one end you have a traditional helicopter with one to a few main rotors, high torque, cyclic and collective pitch in each (because torque is slow to change).
At the other end you have a huge amount of small thrusters that are directly connected to their own motor and thrust is varied by electric means.
In between you have things like intermediate number of thrusters / rotors having maybe a simple gear and/or collective pitch control.
You can check any hobby shop or Alibaba to verify the above.
Probably related to some physics of magnetic field vs current. Would need to dig deeper into that dome day...
In a car, it's a bit similar thing. With a combustion engine it optimizes to a single engine with fixed + variable ratio and shafts to the wheels and things like differentials. Lots of places to have oil, to maintain, to break, being heavy too. Tesla has 1-2 electric motors with a fixed gear ratio of about 10. Some concept vehicles have a direct drive motor near each wheel or even inside the wheel.
The electric motor has different characteristics than a turbine or a piston engine. Hence it can affect the whole machine design.
Small multi-rotors have to spin faster to compensate for their reduced efficiency, leading to the characteristic drone sound.
Computer cooling guys know this too... bigger slower fans move more air and do it with less noise.