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-...
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...