Electric cars are disconcertingly quiet to those of us used to the sound of the internal combustion engine. I know that we’re likely decades away from electricity-powered aircraft, if indeed we ever get there at all. But if we did, what would it sound like for the passengers inside?
For a propeller-driven aircraft, I imagine the sound difference will be like night and day, since the passengers won’t hear the sound of the combustion engine driving the plane. But they’d still hear the blades cutting through the air, and the sound of the wind passing over the craft, yes?
But in a jet-driven craft, I’m not sure there would be a difference at all. The sounds the passengers hear are the spinning turbines, the air passing through, and the air moving over the craft, yes? And not the combustion process that spins them in the first place? If they were spun by electricity, wouldn’t the spinning, the air moving out the other end, and the wind in general, make it still just about as loud as combustion-driven jet aircraft?
Interesting question. Almost no propeller driven passenger planes more than about 6 seats today have piston & cylinder internal combustion engines; they’re all turboprops.
So the combustion part of the energy production process is the same as in jets. In either case that’s lots of very high-speed rotating machinery that produces high frequency vibrations and high frequency noise in the cabin. That part of noise and vibration should be absent in a pure electric airliner. The combustion process also liberates lot of hot fast expanding air that has to be exhausted somewhere, a process that involves creating a lot of turbulence and noise that impinges on the plane and gets into the cabin.
Most of the noise and vibration of a current propeller-driven airliner is the from the props and from the air they beat up then bouncing off the fuselage & wings. There should be almost zero change in that from turbine to electric propulsion.
Lastly, as you say, there’s the general noise of the relative wind parting around the airplane, flowing sorta turbulently over the skin, and being left behind. That noise too should be unchanged between turbine and electric.
At a high level …
Nobody is talking seriously about purely electric jet engines. What IS being discussed is a hybrid engine where they take an ordinary turbine engine and add an electric motor as a part-time booster to augment the torque on the output shaft(s). That way they can size the engine smaller for more efficient cruise and make up the power shortfall with the electric boost for takeoffs, go-arounds, and engine failures.
People are talking seriously about purely electric light planes and small 9-passenger & cargo haulers. See
for an example. The electric motor might be powered by batteries or by a fuel cell consuming either conventional jet fuel = kerosene or hydrogen.
Another interesting idea for the small 10-30 passenger prop planes is distributed propulsion. This is using a large number of small electric motors driving small propellors all of which get their electricity from a central source or sources. See Electric Propulsion Technologies | NASA and
for more on this. The much smaller faster-turning props have the potential to make a much higher pitched vibration than do the large slow-turning props of existing turboprops. This may well give these sorts of prop planes much more of a traditional jet-like sound and vibration experience for the passengers.
In all, some noise sources will see little change, some will be reduced, and others eliminated. I’d expect a transition to noise that’s more heavily biased towards high pitched whines than low rumbly roars.
I suppose too, there is the option to otherwise configure propeller aircraft if the engine is not a huge hot chunk of metal - I’m thinking something like the Beech Starship. Put the props aft of the aircraft.
Is it even meaningful to imagine an “electric-powered jet engine”?
The fundamental definition of jet propulsion is that fuel is burned, and the resulting expanding exhaust itself flows out the back forcefully, causing an equal forward force in reaction, pushing the aircraft forward.
If you simply have an electric-powered turbine pushing air backward, how is that fundamentally different from a more conventional propeller?
Also, as I understand, the limiting factor of a propeller aircraft is the speed of the ends of the propeller blades hitting supersonic speeds - hence NASA’s clever props that are less diameter but you have a dozen blades?
Another issue is that props like wings have tip vortices - some of the air spills off the end. For a while, one trick you used to see on some prop aircraft was ducted fans, where the prop was inside a shroud to limit the power lost to the tips.
A modern turbofan gets the vast majority of its thrust from the cold unexpanded airflow from the fan. The thrust produced by the hot expanding flow out of the core is a bonus, not the main event. The real purpose of the fire is to generate torque on the shaft that turns the fan. The rest is just details.
In some sense the fan is just a propellor. But one with 20-30 blades and enclosed in a shroud that improves efficiency as @md-2000 commented above. And with fairly short broad blades and a high RPM compared to a conventional prop of 2, 4, or occasionally 6 blades.
Turning that fan with an electric motor is conceptually simple enough. Once we can make multi-tens-of-megawatt electric motors that can handle extreme heat, cold, and high altitude where the thin air means everything electrical wants to arc. Without weighing more than the plane they’re supposed to power.
My bet is the electric sorta-jet engine would end up with a propfan like “propulsor” as the argot now has it. See
for some info on what that might look like. Although the prototypes in that article are all completely non-electric.
Once we do have some electric motors to turn our props/fans/propfans, where do we get the electricity from? Aye, there’s the rub.
Choices are fuel cells, batteries, and turbine engines turning electrical generators. Or some hybrid system involving more than one of the above.
All topics of feverish research and eager startups all over the world right now.
The main difference is in cooling requirements. The benefits of a pusher design with a traditional internal combustion engine are often negated by the need to have large draggy scoops on the aircraft to cool the engine. An electric motor generates a lot less waste heat, and therefore you should technically be able to build a more efficient aircraft. Not hugely more efficient, but better.
I like the blown wing design, as it takes advantage of electric power to do something you can’t really do with traditiinal engines. For electric power to work in aircraft we need to find some big gains in efficiency. and this method has promise.
Going through my mental list of electrical heat warming…
I know that an issue with rail guns is that the magnets heat the projectile so much that the air around it catches fire and things start to get melty. In theory, you might be able to have a small trickle of fine, ferrous powder passing through a magnetic field and heating the air.
Tesla coils and other technology can create arcing through the air, heating it.
Infrared radiation - i.e. run electricity through a coil and bring it to a glow.\
Similar to item #1, use microwave rays to heat something that more readily heats up than air - e.g. water.
My feeling would be that if there’s a method it would probably involve a secondary medium - a “fuel” - like water or something that is easy to heat up.
Heat engines, in general, are a really bad idea, and I don’t know why humans love them so much. It’s almost always a better idea to not convert a bunch of your energy to heat, and just convert it directly into whatever it is you actually want. Not heating up the air as much is a benefit of electric engines, and you need all the benefits you can get, because batteries can’t store nearly as much energy as fossil-fuel tanks of the same size or mass.
Not arguing - what you say makes sense to me - but just to get more information out of you.
My understanding would be that a jet engine works - in essence - by creating a pressure differential by using heat to cause the air to expand in a very specific way, in a very particularly shaped object.
Maybe there’s some more efficient way to turn electricity into a propulsive force. But, likewise, it would probably be better if you could use kerosine to directly expand the air rather than creating waste heat, if you could…and yet that’s where we landed.
My feeling would be that “expanded air” and “heat” are pretty much synonymous. You basically need to escape jet propulsion or, I would think, you’re still going to need to create heat?
Compare that to today’s beastly mode of transport: the Boeing 777. Bangalore Aviation points out that a single GE90-115B engine puts out over 110,000 horsepower, or more than twice the design output of all the Titanic’s steam engines.
1 kW = 1.34hp. so to run a jet engine that size (about 12 feet in diameter) would require about 82MW of power. The problem as noted many places, is not the ability to create such an engine, it’s more how to produce the power to run it. I assume problems like arcing and cooling would be trivial if we can produce that much power - in a package light enough to get off the ground, let alone take passengers and cargo along for the ride.
For comparison, a Tesla Model 3 has about a 75kWh battery pack, and let’s pretend half its 2-ton weight is batteries. To fly with one of those engines as electric for an hour would require the equivalent of 1100 Tesla Model 3 battery packs, 1100 tons of batteries. For one hour’s flight, no reserve.
A 777 with two of those engines at take-off weighs 766,000lb or 383 tons. We’re not even accounting for what weight of wiring would carry 82MW. Even if we go with a simpler design with multiple smaller engines, embedded in wing, at the rear of the fuselage, etc. etc. the problem, as with cars, is still the power required, and a serious weight issue.
