A Little Gear That Could Reshape the Jet Engine • Hacker News Board

Re: A Little Gear That Could Reshape the Jet Engine

#21

Post by PhantomGremlin » Mon, Oct 19, 2015, 9:19 PM UTC

mhandley wrote: post #15

I wonder if instead of gearing a single large fan, you could just remove the fan from the turbine altogether, add a much larger alternator, and power several large electric external fans from each turbine? That would give you more fan area than you can get with a geared turbofan, optimal fan speed for efficiency, redundancy in case one of the fans suffered damage, allow cross-routing of power between turbines if one…

just remove the fan from the turbine altogether

I don't think you can. The large volume of airflow surrounding the turbine is what makes it a turbofan[1]. Without, it is a turbojet[2]. The airflow from the turbofan surrounds the hot exhaust gas and it is much quieter than the turbojet.

But, other than noise, the rest of your comment makes a lot of sense to me. But I know very little about this stuff.

[1] https://en.wikipedia.org/wiki/Turbofan [2] https://en.wikipedia.org/wiki/Turbojet

Re: A Little Gear That Could Reshape the Jet Engine

#22

Post by mschip » Mon, Oct 19, 2015, 9:21 PM UTC

blobert wrote: post #18

mhandley wrote: post #15

I wonder if instead of gearing a single large fan, you could just remove the fan from the turbine altogether, add a much larger alternator, and power several large electric external fans from each turbine? That would give you more fan area than you can get with a geared turbofan, optimal fan speed for efficiency, redundancy in case one of the fans suffered damage, allow cross-routing of power between turbines if one…

This idea would unfortunately negate one of the top benefits of a high-bypass turbofan. Noise reduction.

And noise reduction is actually a really important benefit of this engine as some airports have restrictions on noise.

Re: A Little Gear That Could Reshape the Jet Engine

#23

Post by mhandley » Mon, Oct 19, 2015, 9:33 PM UTC

blobert wrote: post #18

mhandley wrote: post #15

I wonder if instead of gearing a single large fan, you could just remove the fan from the turbine altogether, add a much larger alternator, and power several large electric external fans from each turbine? That would give you more fan area than you can get with a geared turbofan, optimal fan speed for efficiency, redundancy in case one of the fans suffered damage, allow cross-routing of power between turbines if one…

This idea would unfortunately negate one of the top benefits of a high-bypass turbofan. Noise reduction.

Yes, but why is a high bypass turbofan quieter? If I understood correctly, it's because most of the thrust comes from the large slow moving fan, so you don't need to blast nearly so much air through the turbine as a turbojet to get the same thrust.

The whole purpose of decoupling the fans is to give you an even higher bypass ratio - two or three large electric fans driven from one turbine can have a larger swept area than a single large geared turbofan, so can run slower for the same thrust, and hence be quieter.

Re: A Little Gear That Could Reshape the Jet Engine

#24

Post by rm445 » Mon, Oct 19, 2015, 9:37 PM UTC

dognotdog wrote: post #12

It'd be interesting to know what exactly they are doing in the gearbox, apart from the flexible mount to absorb excess torque. New materials? Surface treatments? Special lubrication?

This article has much more detail:

http://theflyingengineer.com/flightdeck/pw1100g-gtf/

The gearbox is described as the epicyclic type with the compressor driving the sun gear and the carrier of the five planet gears driving the fan. The article doesn't seem to identify any extra special sauce. It points out that geared turbofans have been used on an airliner in the past, and identifies some design trade-offs. "there may be a performance penalty, marked by slower cruise speeds for the same thrust setting, possibly slower max cruise speeds, slower green dot (best L/D speed), slightly shallower climbs, and possibly degraded single-engine performance. The single engine performance difference is expected to be the most prominent, with a possibly larger yaw, and a definitely reduced climb gradient, consequently lowering obstacle clearances."

My impression based on just reading that article is that, rather than a technological breakthrough enabling the geared approach, Pratt and Whitney have explored a rather neglected area of the design space, to arrive at a more efficient, lower noise engine with slightly worse performance in some other respects. And had to conquer numerous 'ordinary' engineering challenges to make it work.

Re: A Little Gear That Could Reshape the Jet Engine

#25

Post by phire » Mon, Oct 19, 2015, 9:50 PM UTC

mhandley wrote: post #15

I wonder if instead of gearing a single large fan, you could just remove the fan from the turbine altogether, add a much larger alternator, and power several large electric external fans from each turbine? That would give you more fan area than you can get with a geared turbofan, optimal fan speed for efficiency, redundancy in case one of the fans suffered damage, allow cross-routing of power between turbines if one…

Airbus are working on more or less this concept with the E-Thrust.

One Jet engine providing power and thrust and six electric fans along the wings for additional thrust. They have also added batteries, so the single jet engine doesn't need to be big enough to provide full take off power + redundancy. Currently jets require at least 2 engines, each large enough to complete the take off if the other fails. With this system you would need either the jet, or the batteries + fan to complete takeoff in the case of a failure in the other system.

http://www.airbusgroup.com/int/en/news-media/media~item=be67...

Re: A Little Gear That Could Reshape the Jet Engine

#26

Post by [deleted] » Mon, Oct 19, 2015, 9:50 PM UTC

mhandley wrote: post #15

I wonder if instead of gearing a single large fan, you could just remove the fan from the turbine altogether, add a much larger alternator, and power several large electric external fans from each turbine? That would give you more fan area than you can get with a geared turbofan, optimal fan speed for efficiency, redundancy in case one of the fans suffered damage, allow cross-routing of power between turbines if one…

[deleted]

Re: A Little Gear That Could Reshape the Jet Engine

#27

Post by porsupah » Mon, Oct 19, 2015, 10:20 PM UTC

phire wrote: post #25

mhandley wrote: post #15

I wonder if instead of gearing a single large fan, you could just remove the fan from the turbine altogether, add a much larger alternator, and power several large electric external fans from each turbine? That would give you more fan area than you can get with a geared turbofan, optimal fan speed for efficiency, redundancy in case one of the fans suffered damage, allow cross-routing of power between turbines if one…

Airbus are working on more or less this concept with the E-Thrust. One Jet engine providing power and thrust and six electric fans along the wings for additional thrust. They have also added batteries, so the single jet engine doesn't need to be big enough to provide full take off power + redundancy. Currently jets require at least 2 engines, each large enough to complete the take off if the other fails. With this sy…

Fascinating! Seeking out more information on E-Thrust led me to this article, which talks both about it, and where Airbus are now with electric aircraft:

http://eandt.theiet.org/magazine/2014/10/rise-of-electric-ai...

Re: A Little Gear That Could Reshape the Jet Engine

#28

Post by readams » Mon, Oct 19, 2015, 10:21 PM UTC

So basically this seems to be a extra-high-bypass turbofan achieved by gearing down the fan. But delayed hugely because giant megacorps move very very slowly.

Re: A Little Gear That Could Reshape the Jet Engine

#29

Post by HeyLaughingBoy » Mon, Oct 19, 2015, 10:24 PM UTC

GauntletWizard wrote: post #3

JSeymourATL wrote: post #2

“The Silicon Valley guys seem to have the attention span of 3-year-olds.”

It's a bit easier to be blasé about testing when your worst-case-scenario is that somebody misses a ride, vs when you kill ~150 people. The rapid cycle of innovation is a tradeoff that SV is able to make (and finds amazingly beneficial) because we're working with soft tools and have backups of our data. I'm certain that were human life so cheap, Pratt and Whitney would have a much shorter dev cycle, too.

It's also a very different mindset. I moved from a long career designing software for Medical Devices to desktop S/W and the hardest part for me is getting used to a much more relaxed approach to development process, documentation and testing.

It's weird when I realize I can just push something out the door and not have to wait for the reams of paperwork to be complete.

Re: A Little Gear That Could Reshape the Jet Engine

#30

Post by brwnll » Mon, Oct 19, 2015, 10:25 PM UTC

Could some knowledgable aircraft people answer these questions for me?

1. Can/are jet engines ever swapped out after a plane as been put into service? Or will these only ever be sold attached to new aircraft orders.

2. Any idea on what the engine costs? And what percentage of the overall airline cost is engine? I didn't see any references to costs of this engine vs existing models.