Ever played with a quadcopter? They aren’t remotely close to silent. Electric airplanes are much larger.
You don’t own a box fan or a window fan? Or ever heard the AC without the compressor kicked in? How about hearing the wind through the pines? Now if the blades are going just over freewheel in the wind speed, there could be an issue. Same as any other plane though.
Actually, that aspect of battery-powered planes is probably a net disadvantage. Changes to the center of mass driven by fuel consumption are reasonably well handled by pumps to move fuel between tanks, maintaining the center of mass.
But one major advantage of consumable fuel (such as jet fuel) is that you don’t have to carry the mass of a full tank all the way to your destination. In fact, it’s common for long-haul aircraft to cruise at a low-ish (and inefficient) altitude until they’ve burned enough fuel to climb to higher, more efficient altitudes for the balance of the trip.
Keep in mind that a fully-fueled 737 Max carries about 26% of its maximum takeoff weight (MTOW) in fuel, so it’s often about 20% lighter when it lands after a long trip.
The airframe for an electric plane will have to be stronger (and heavier) than for one that routinely lands “light” because it’s landing weight will be much closer to its MTOW, accelerating fatigue and leaving less margin on hard-but-fairly-routine landings. So the airframe gets reinforced, which makes it heavier, which reduces range, which requires more batteries, which make the plane heavier, etc.
In rocketry, aircraft design, auto racing and most other energy-management pursuits, light weight is its own reward.
I’d be delighted if electric planes became practical for anything but the shortest trips. But it’s going to take a huge leap forward in battery density, and there’s no improvement on the horizon that’s even close to big enough.
It seems more likely to me that two alternatives are more likely: either props driven by the electricity from fuel cells (as MD2000 suggests) or IC turbine engines powered by hydrogen.
Don’t forget the 3rd option : hybrid VTOLs where the electric props are for vertical takeoff. Like this one and this one. (I know the Cora presently uses an electrically driven rear prop but this is likely to make the prototype simpler - Sebastian Thrun has said that decent range requires it burn fuel)
Actually, hybrids make no sense to me, mostly because aircraft are not cars. That is, cars have highly variable speed, especially in traffic. They’re constantly accelerating and decelerating, so hybrids are able to store decelerative energy and use it to power part of the next acceleration.
Aircraft have large power requirements for takeoff, but they generally don’t get to recoup the potential energy they’ve stored as altitude until it’s time to land, at which point they’re about to connect to a power source. In the meantime, they’ve carried a heavy battery for the entire trip, which costs energy.
I take your earlier point about hovering being easier with electric props, but:
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most planes don’t need to hover, and a vertical takeoff generally takes more power than a rolling takeoff, and
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if you really needed to hover and decided that electric motors were the best way to achieve that, you’d power the props from your IC engine’s alternator, not from a heavy battery you have to lug along for the whole flight.
(The latter point is a bit of a trend in aircraft design; the 787 dispensed with bleed air from the turbines for cabin pressurization and instead uses electric pumps for that purpose. 787 engines also have giant alternators to handle the increased electrical load).
In general, if it makes sense to burn fuel at cruise, it makes sense to burn it on takeoff, too.
Minor nitpick, though not necessarily one aimed at SamuelA: an aircraft with electric props powered by an IC engine’s alternator is no more a “hybrid” than a diesel locomotive or, for that matter, my IC-engined Subaru.
Diesel locomotives turn the cranksjaft’s kinetic energy into electricity, which then drives the wheels, but in most cases there’s no significant storage capacity for that electricity.
And my Subaru uses its alternator to power several fans at the radiator and for the HVAC system…an electric fan is an electric fan, regardless of whether it’s cooling my radiator or lifting an aircraft.
Those converted plane that Harbour Air will fly are not going to be the most efficient electric airplanes. The X-57 (link in my first post) that NASA is experimenting with should lead to a much better craft.
One efficiency is that since the electric motor is much lighter than a turboprop or jet, they moved them out to the end of the wing. That eliminates the wingtip vortex that is a drag on planes. Another is the wing is much narrower than regular planes, so they save weight there as well as reduce drag during cruise flight. They have the additional 12 smaller props, but those only run some of the time, mostly at takeoff/landing. They induce greater lift in the narrow wing. During level flight, they’ll be shut off and the blades folded back to reduce drag. AFAIK, the plane has not flown yet, but they’re shooting for a 500% increase in efficiency.
Wrong, wrong, and wrong. The reason why you need batteries is because in order to hover in a stable manner, using inexpensive electric motors attached to inexpensive propeller, you need large surges of power. This is why you can hover with a quadcopter that is so cheap, and why it’s possible for a computer to control it with a very simple algorithm. While you can reduce the size of the batteries you need with a rear mounted, very powerful turbine engine, where during takeoff you feather the prop and run it soley as an alternator, you still need batteries as a buffer.
This is exactly why a hybrid car uses that battery - it has a similar coupling problem, though for slightly different reasons. My Prius does use the battery to both supply large surges of power and to recharge, taking power out of the drivetrain.
For aircraft batteries, you would design them for this purpose. You’d use very thin, very high current batteries that can charge and discharge rapidly. Capacitors could work for this purpose even better, the problem is they are presently just too heavy.
But I do sort of agree with you - refined VTOL designs, the battery would not weigh very much.
For aircraft on missions where a VTOL is not needed (long haul bulk passenger and cargo flights), I expect we will keep doing it using airliners that resemble the ones that we have today, just with further tweaks towards greater safety and fuel efficiency.
Addressed above. It’s exactly like a hybrid car because you are missing the fact that you need to control both attitude and altitude at the same time.
Say the port side is tipping over. Without batteries or capacitors, all you can do is decrease power to the starboard side rotors. The reason is that there is a long time lag before you can speed up the jet engine driving an alternator. So if you have no energy storage, this power decrease will cause an altitude loss, and this “dip” could cause your aircraft to strike terrain and take damage or worse.
Similar cases occur if there’s a sudden downdraft while the aircraft is hovering, etc. The batteries are needed to supply more power whenever the jet engine isn’t supplying enough total energy, or to take energy away, increasing efficiency when the jet engine speed has surged too high momentarily. (you could use resistors and for safety reasons might have some backup resistors)
You also could use the batteries to make a safe landing in the event of a sudden complete engine failure, another issue with this type of VTOL is that there is very little angular momentum in the props. They most likely cannot autorotate down. There *can’t *be much angular momentum, or you wouldn’t have stable control.
This need for reserve power might drive the battery sizes larger…
A helicopter works a completely different way. With a proper helicopter, the rotor mass is a buffer for angular momentum. So the pilot changes the pitch of the blades, increasing or decreasing lift in the immediate term, with the rotor serving as the energy storage buffer until the engines catch up.
The reason we are abandoning this is because it’s very expensive to make the mechanism of a helicopter, while high power speed controlled electric motors and batteries have gotten cheap, and because this new way offers much more precise control with much less control lag from a state error to a new state. There are a lot of axes coupled together in a helicopter, such that you cannot necessarily fix all of the errors at the same time, while the independent degrees of control of a quadcopter allow for a much more straightforward control algorithm.
If they are making 30 minute flights on water they are going to pound the hell out of the air frame. I don’t know what the TBO would be on inspections for such an accelerated program of stress. Also, 750 hp and similar torque numbers off idle will twist the hell out of motor mounts. I hope they do their engineering homework and pad the numbers on the side of caution.
On paper the instant hp sounds like a slice of sea-plane heaven but I’m cautious of the engineering behind it.
Given the weight difference between engine and motor they will have to extend the nose quite a bit.
I was thinking along the same lines. If we are talking about small aircraft, like a Beaver, then the problem with using batteries with an equivalent weight to fuel is that you remove flexibility to exchange fuel for payload. It’s unusual for a small aircraft to be able to take a full load of passengers, bags, AND fuel, so if you are effectively at a weight equivalent to fully fuelled then you are restricted on your ability to carry payload.
When it comes to large aircraft then, as mentioned above, you need to be able to land at the same weight as you had when you took off. You can’t factor in fuel burn to let you take-off from a long runway and land at a relatively short runway.
Of course, sometime in the future those disadvantages will be outweighed by the advantage of not being reliant on fossil fuels, but we aren’t there yet.
How feasible is it to engineer ground-based power stations that can direct a tight beam of microwave energy to solar panel-like receivers under the fuselage and wings to power the electric motors remotely?
About as feasible as Fusion power…
A scenario like that, at best, requires a very large array of antennae to receive the power. The proposals to beam power from orbit suggested farms of antennas a mile on a side, and then there’s spillage to the sides where it may not be completely healthy to linger for long times. By the time your aircraft is trailing a Mylar sheet of antennas the size of a football field, it’s going to have too much drag.
I would think so, and a longer nose opens up cleaning up the airframe aerodynamically. Look at the difference between a radial engined Otter and a Turbo Otter.
Specs here:
Link
I really have a hard time making the same argument about converting a turboprop, though. They are pretty reliable and have a great power to weight ratio. I suppose the instant torque of an electric vs the spool time of a turbine might be one consideration though. As noted above, none of this makes any sense until you can get batteries down on mass and up, up , up on power density. That will likely come with time, though.
RP A smartly designed battery pack that was modular could solve the whole payload vs range issue, but I would have zero clue as to how you could do that efficiently.
Another issue we haven’t discussed is certification. Certifying a new airframe takes a lot of money and time. Modifying an existing one less so, which pitches the argument more toward a proven design like the ones HA currently flies and less on an experimental model like the X-57.
Umm, R^2. The aircraft would be thousands of kilometers closer to the ground beaming stations. Also it would be ok for some of the beam to travel around the aircraft and be wasted, it would still potentially collect enough energy to fly. Obviously the passenger compartment would be a faraday cage like the door of a microwave oven, and obviously there is nothing for the beam that misses the aircraft to impinge on.
The question is : what kind of energy density can you get, and how many ground stations would you need for a practical point to point route?
Honestly…on the face of it, it doesn’t sound totally terrible.
The aircraft could in principle fly with just onboard supercapacitors and maybe backup airframe parachutes. Or, better, the backup batteries would be 1 time use lithium-air batteries - high energy density, good for an emergency landing, but lightweight.
Also, note that if the aircraft are VTOLs that launch from tall building, the rectenna would obviously be also on that building. So at the point the aircraft needs maximum power for takeoff or landing, it’s basically at point blank, just a few meters from the antenna.
It certainly has an appropriately futuristic theme to it.
Honestly, the more I think about it, the more it sounds totally feasible. Remember, phased array microwave power transmitters are essentially what’s in a modern wifi router, minus the power transmission part. The SpaceX wireless internet satellites will use a similar type of antenna to communicate with the ground stations. So the actual microwave beaming is far more practical than it would have been 40 years go.
This type of aircraft would essentially travel a point to point commuter route through a metroplex, and there would be periodic antenna on the ground beaming it power. So it flies from antenna to antenna riding the beam.
The advantages of this are:
a. Less air pollution (none if the energy source is clean)
b. Less noise
c. No burning through the onboard batteries every flight
d. No battery recharge delay between flights
e. Less risk of things like battery fires. yes, there would be backup batteries, but it’s cycling batteries that puts wear on them that can lead to failure
f. Lighter weight and more payload
g. Faster flight speeds
And the disadvantages are pretty obvious. Some would be a matter of scale (enough of these aircraft already flying a route, and the cost of adding beaming stations and switching the aircraft fleet over to beam riders is actually a net savings over time)
A big one, though, would be reflected emissions hurting people on the ground. I don’t know enough about this to say if this would be a big risk or not. Obviously this is a really powerful microwave beam, on the order of megawatts during VTOL maneuvers. I don’t know if the beam could reflect off the plane, or side lobes from the antenna could hurt people on the ground or tenants in the same building, etc. Not to mention, aren’t people with pacemakers one encounter with a leaking microwave oven from death?
Another thing overlooked about hybrid cars - the electric motor assists during acceleration, so the combustion engine does not need to be as powerful I had a Camry hybrid (BTW, ran for 10 years before trading in, still ran good, no problems). The engine was 1600cc, about the same size as my 1991 Honda Civic. For acceleration, it got an assist from electric. A normal sedan that size comes with 2400cc to 3L.
The same might apply for aircraft. For the extra 30% power needed for takeoff, use an electric assist inline with the engine power; for cruising at nominally 70% power, use the engine only. Or, use 60% (slower cruise) and recharge the batteries en route in preparation for the next take-off or any emergency go-arounds at the destination.
(funny story about one of my flight instructors and her husband - neither were slim individuals. They had an old 70hp Luscome on floats (this was around 40 years ago). One hot summer day they were zipping up and down the river repeatedly - but with their combined weight and hot air they simply could not get the plane off the water.
Do they really need a 45 minute reserve when they are flying over water and can land anywhere? I imagine most of their flights are across Georgian Bay to Victoria or down Puget Sound to Seattle.
I once spent an hour watching the seaplane operations in Vancouver Harbour. The most fascinating thing about was the difference between that and airports. In a tower-controlled airport, no vehicle can cross a runway or a taxiway without permission. There was no such control in the Harbour, with boats, even rowboats, crossing the taxiway constantly. It must be a nightmare for the guys in the tower. The person in charge is the harbour-master.
Most of their flights are around the Straits of Georgia (not Georgian Bay, that’s in Ontario) and thus over water. But they do have one from Vancouver to the west side of Vancouver Island where half of it is over land. It’s the long one to Tofino I mentioned upthread.
It’s not always a good idea to set down a float plane in any bit of water. The waves could be too high, for example. So the rules may not be different.