[scottmsul]

Still seems a bit far from replacing jet engines? This article was going on about fuel emissions in the entire aviation industry, this is just an electric propeller. Anyone know how far we are from electric jets? Is such a thing even possible?

[aphextron]

>"Anyone know how far we are from electric jets? Is such a thing even possible?"

Electric will never replace long haul jets barring a revolutionary breakthrough in battery technology. But that's not the point. Much of commercial aviation takes place on distances of around 100 to 500 miles. That is, too far to drive but not quite far enough to take advantage of the efficiency of jet engines at high speed, high altitude cruise. For those applications electric propellor and ducted fan aircraft will be absolutely game changing. It will make these flights cheaper and safer by a factor of 10 at least. The vast majority of cost in a flight is maintenance of the engine. That all goes away with electric power. It will make commuter flights an affordable daily reality for normal people.

[boatswain]

Would a hybrid model be more effective? Using diesel generators to power electric motors?

[Gibbon1]

Hybrids are interesting because you can reach higher bypass ratio's because you can power n number of ducted fans off m engines.

Currently increasing fan sizes are getting problematic. That is the problem with the 737 MAX. The fan diameter of the new engines is too large to fit between the bottom of the wing and the ground. So it had to be mounted forward and up which fucked up the aerodynamics.

Two other advantages are, potentially maintaining thrust in an engine out situation. When you lose an engine not only do you lose half your power but the thrust is unbalanced. And faster throttle response. Turbine lag is a big issue with jetliners.

Note: The reasoning behind high bypass turbofans is thrust is proportional to delta v times the mass flow rate. Where power input is proportional to delta v squared. Bigger the fan the more efficient you are and the less fuel you use.

Diesel is kinda interesting. Manufactures are developing diesel engines for light aviation. I suppose you could have a hybrid diesel/electric aircraft.

[abakker]

I think "Electric Jet" is like saying "Electric Internal Combustion Engine" - A Jet is a type of engine, so an electric aircraft wouldn't be a jet.

I know people call Jet-powered aircraft "jets" but, that's really an abbreviation.

I think what you're asking though, is whether there are any electric propulsion systems with the same characteristics as jets - i.e. more thrust relative to speed. AFAIK, there are not, because a characteristic of chemical fuels is that increased compression and airflow can increase the efficiency of the reaction. Electric motors have a (relatively) fixed efficiency due to the battery.

[agurk]

The jet in jet-engine refers to the fact it uses jet propulsion - i.e. producing movement in one direction by ejecting a fluid in the opposite direction. This means that lots of things are actually jet engines - rocket engines and water jets being two common ones.

The propulsion of a 'jet engine' is normally through a gas turbine, which is a type of internal combustion engine, so you wouldn't get an electric gas turbine.

Replacing the gas turbine with an electric motor would in this case produce an electric jet engine.

[squilliam]

A modern jet engine is just a shrouded propeller that happens to be powered by a jet turbine. An electric ducted fan would be the battery-powered equivalent.

[aphextron]

>A modern jet engine is just a shrouded propeller that happens to be powered by a jet turbine.

No, it's not. A propellor creates thrust by slicing the air and creating lift just like a wing, but in a forward direction. A jet engine's fan blades create no thrust in themselves. They provide the compression needed for combustion to occur within the engine, and thrust is created by the expelling of hot exhaust gasses.

[agurk]

That is a description of a turbojet - the earliest type of aero jet engines[0]. GP is referring to modern turbofans which are defined by having a bybass - i.e. air that is compressed and then expelled rearwards without passing through the main turbine body[1].

This is done to trade the high speed of the jet exhaust for a larger amount of air moving more slowly. Making the speed of the jet plume closer to the true airspeed of the aircraft makes it more efficient.

[0] https://en.wikipedia.org/wiki/Turbojet

[1] https://en.wikipedia.org/wiki/Turbofan

[agurk]

And to be more detailed, in any jet turbine, the compressor blades will create power as the energy spent compressing the air will be recovered when it's exhausted out of the tailpipe. So it's not quite correct to say they will produce no thrust.

Looking at design of turbines, a lot of air is needed above the amount for combustion to help keep the post combustion chamber turbine from melting (or just weakening to the point of failure). Some of this will be the unburnt (nitrogen, etc) components of the ingested air but a lot of design goes into moving air around components to move heat away from them.

The less air needs to be compressed for the engine then the more efficiently it will run. This is often achieved by designing one that can run hotter. This is also one of the other ideas behind having the bypass on a turbofan, reducing the amount of air that is compressed.

[darksaints]

No, the exhaust is not the primary thrust generator, unless you're talking about military jets. The fans themselves are creating the majority of the propulsion. Modern high bypass designs have bypass ratios of ~10:1, meaning for every 1 lb thrust generated from the exhaust, 10 lbs comes from the fan.

https://en.wikipedia.org/wiki/Bypass_ratio

[catalogia]

Note that modern military jets have low bypass ratios, but not zero. They use low-bypass turbofan engines, not turbojets. Even cruise missiles use turbofans rather than turbojets. I think the only place you might see a turbojet flying today is in RC scale aircraft.

[Tuna-Fish]

Most of (>80%) the thrust of modern turbofan engines is directly produced by the fan, which mostly just pushed air past the engine, not into it.

[throwawaysea]

The bypass part of a turbofan does generate thrust.