=aircraft
There are now
many startups and some automobile companies working on electric
VTOL aircraft for short-range human transport. That's certainly something
you can do; the problem is, that ends up being worse than a conventional
helicopter in almost every way.
Some executives and investors saw
quadcopter drones and wondered if you can use the same system to transport
people. The answer was, you can, but the range is quite short. The proposed
solution was transition from vertical takeoff to horizontal flight, and
that's a relatively unexplored type of aircraft so the proposed designs vary
widely.
You could also reach the same approach from the other
direction. Horizontal-takeoff electric aircraft exist, but their range is
much shorter than conventional aircraft. Because range is short, they would
be used for short trips, which makes VTOL necessary, which makes range even
shorter, but there's still a market for that.
The most successful VTOL aircraft
that transitions to horizontal flight is the
V-22 Osprey.
The V-22 has
0.426 kW of power per kg of takeoff weight. Gas turbines can produce 10
kW/kg, so their weight is acceptable - in fact, I think the gears in the
V-22 weigh more than the engines.
Electric motors are typically 3
kW/kg or less, and there's a tradeoff between power-weight and efficiency.
That's still OK; the bigger problem is batteries. Li-ion batteries are
typically specified for 1C discharge rates, meaning their full capacity is
discharged in 1 hour; 2C means discharge in 0.5 hours. Supposing (200 Wh/kg)
capacity and 1C discharge, that's 0.2 kW/kg.
Most helicopters have
lower power-weight ratios, typically ~0.2 kW/kg, but they can't transition
to forward flight; there are tradeoffs involved, and the V-22 wasn't
designed by complete fools.
How, then, do quadcopters fly? The answer
is, they use batteries with higher discharge rates. But of course, there's a
reason Li-ion batteries are typically used at 1C. If you want 3C batteries,
capacity goes down a bit and cost goes up a bit, but the biggest sacrifice
is cycle life: most quadcopter batteries are only good for 100 to 200
cycles.
Let's suppose you spend $200/kWh
on high-discharge-rate batteries, and they last 200 cycles, then that's
$1/kWh just for batteries. Let's say $1.10 including the electricity.
If you compare that to a 40% efficient gas turbine, the batteries are
more expensive than buying fuel. That's not even considering the reduced
payload fraction with electric aircraft.
How expensive is that,
overall? Let's say 1 passenger and their accomodations takes 200 kg, and that
payload requires 60 kWh of batteries. If a trip is a full discharge - which
it wouldn't be, for safety reasons - that would be $66 per trip per person,
just for batteries and electricity, not including the labor of actually
swapping the batteries.
That might be acceptable for
short-range transport in eg NYC, but aircraft also have other costs.
Normally, aircraft manufacturing and maintenance is very expensive. That's
not a problem for small quadcopters because they're mechanically simple and
occasional crashes are acceptable. VTOL with transition to horizontal flight
means mechanical complexity. Carrying passengers and being large enough to
destroy a house means crashes aren't acceptable. So, manufacturing and
maintenance would be expensive.
It's true that batteries and electric
motors are cheaper than gas turbines and turbofans. Turbofans are perhaps
$1/W. Batteries are perhaps $0.10/W. High-performance electric motors might
be another $0.10/W, and of course there's a tradeoff between power-weight
ratio, cost, and efficiency. Then you need gears and propellers. The overall
cost is still lower than a turbofan, but the lower payload ratio ends up
making net purchase cost per payload higher than a turbofan. Of course, good
turbofans are also large: you generally want at least 1 MW gas turbines for
good performance, with the minimum scale set largely by the need for cooling
channels inside the blades of the first turbine set after the combustion
chamber.
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Now, let's look at some specific proposed designs.
Hyundai
Here's a video
showing a mockup of their aircraft.
There are 2 basic
approaches to vertical lift for such aircraft:
1) tilting rotors with
variable pitch
2) 2-blade fixed vertical-lift rotors that stop after
transition to horizontal flight, pointed forwards for reduced drag
You can see that Hyundai's design uses both: 4 tilting rotors with variable
pitch, and 4 pairs of fixed-pitch vertical rotors. The tilting rotors use 5
blades, which I seriously doubt is a better choice than 3. The fixed rotors
come in pairs, on both top and bottom; usually that's used for
counter-rotating rotors, but Hyundai has them rotate in the same direction
to effectively get a 4-blade rotor that can be stopped in a low-drag
position. I suspect that counter-rotation is still better.
Why would
you mix rotor types like that? More lift is needed than thrust after
transition, so tiltrotors have excess thrust capacity. This design uses
enough tiltrotors for forward thrust, then adds fixed rotors for enough
vertical lift for VTOL.
Wisk
Here's their aircraft design. You can see they have 6 tilting
rotors and 6 fixed rotors. That's a lot of rotors, more than Hyundai decided
on. When you increase the number of rotors and decrease their size, what are
the effects?
advantages:
- more redundancy
- lower torque ->
lighter gearing
disadvantages:
- more complexity -> higher cost
- more complexity -> higher chance of a failure
- smaller propeller
blades -> lower
Re -> lower efficiency
- smaller motors ->
lower motor efficiency
- higher % of airflow blocked by the airframe
-> lower lift efficiency
Wisk wanted to cover the whole wing area with propellers in front, which is called distributed propulsion. There are some aerodynamic advantages to that, but I think they're outweighed by the drag of the fixed propellers at the rear.
Lilium
Here's their aircraft design. You can
see that Lilium uses ducted fans, all of which tilt. The general rule with
ducted fans is that, while the duct improves fan performance, it's not worth
the weight and drag unless you're close to the speed of sound or reducing
noise is very important. Most modern aircraft use turbofans, which are
ducted, but there are a couple other considerations there: using a large
propeller instead would require heavy gears, and the shroud provides
containment so that if blades come off they don't go into the passenger
compartment.
That configuration can
effectively give very high coefficients of lift, so maybe it's reasonable
for a STOL aircraft, but for actual VTOL it just doesn't seem like a good
idea.
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In conclusion, all of those designs seem worse than making a miniature V-22 with electric motors instead of turbines (and less payload and range) and I say that as someone who has a lot of problems with the design of the V-22.
update: "Conceptual Design of Tiltrotor Aircraft for Urban Air Mobility" is a 2022 NASA paper that validates my views here, showing tiltrotors having lower costs (than aircraft with separate lift and propulsion systems) for both fueled and electric aircraft, and electric aircraft having higher per-trip costs than fueled ones. It also finds that transverse helicopters are better than quadrotors, as I'd expect.