Question for aeronautical/engineering Oppo's

"Smallbear wants a modern Syclone, local Maple Leafs spammer" (smallbear94) 03/22/2018 at 11:45 • Filed to: None

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Unless all I’ve read is BS, one of the reasons propeller-driven aircraft fell by the wayside was because the propellers themselves were by necessity becoming bigger and bigger. As they did so they became less and less efficient due to the tips traveling close to the speed of sound. The added drag and turbulence meant performance increases would require exponentially more power out of the engine.

My question is this. The GE9X has an 11' fan. How is it that this issue seemingly doesn’t apply to large turbofans? They are after all in basic operation glorified ducted propellers.

ttyymmnn > Smallbear wants a modern Syclone, local Maple Leafs spammer 03/22/2018 at 11:47

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I’m going to wait for somebody who knows that the hell they are talking about, but I’m going to guess that the ducting plays a very large role in this. I wonder also just how fast that turbine disc is turning.

DC3 LS, will be perpetually replacing cars until the end of time > Smallbear wants a modern Syclone, local Maple Leafs spammer 03/22/2018 at 11:50

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I imagine it does, it’s just it’s a better option since it’s more efficient than an equivalent propeller. For example, to make a propeller have the same thrust as that 11' fan, it’d probably have to be bigger than 11'

Source- I have no knowledge or education about aerodynamics or engineering.

facw > ttyymmnn 03/22/2018 at 11:52

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Yeah, I am not well-versed here, but it seems to me you have at least a couple differences:

Many more blades should mean you don’t need to turn the fan as fast to get as much thrust. You couldn’t do this on a non-ducted plane as the air would mostly flow around the prop, but here the air is forced into the fan.

The fan does not provide the full thrust from the engine, so doesn’t have to spin as fast to produce the same thrust.

ttyymmnn > facw 03/22/2018 at 11:56

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It’s my understanding, and I may be talking out my ass here, that, at altitude, the turbine blades provide much of, though not all, the propulsion, which is why they are so efficient.

Damn. That was a lot of commas.

HammerheadFistpunch > Smallbear wants a modern Syclone, local Maple Leafs spammer 03/22/2018 at 11:58

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Look into UDF and why it was never put into practice. Hint: noise

Smallbear wants a modern Syclone, local Maple Leafs spammer > facw 03/22/2018 at 11:59

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I thought jets spun far faster than props though? Even if the fan turned much slower than the core it’s still moving pretty fast isn’t it?

Smallbear wants a modern Syclone, local Maple Leafs spammer > ttyymmnn 03/22/2018 at 12:00

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I mean, I know the core spins faster but the diameter is so small it doesn’t matter... but I’d have thought the fan was still moving at a good rate of knots

Smallbear wants a modern Syclone, local Maple Leafs spammer > HammerheadFistpunch 03/22/2018 at 12:02

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Interesting... So ttyymmnn’s guess about the ducting is right?

JawzX2, Boost Addict. 1.6t, 2.7tt, 4.2t > Smallbear wants a modern Syclone, local Maple Leafs spammer 03/22/2018 at 12:02

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https://aviation.stackexchange.com/questions/22316/why-do-turbofan-blade-tips-exceed-the-speed-of-sound-while-prop-tips-should-not

tl:dr

High-bypass turbofans are not functioning truly as ducted propellers, but mostly as compressors (air “pushers” as opposed to rotating airfoils). Also, the shroud prevents the formation of tip vortices (increasing efficiency at supersonic air velocities) and mitigates sonic artefacts.

Smallbear wants a modern Syclone, local Maple Leafs spammer > DC3 LS, will be perpetually replacing cars until the end of time 03/22/2018 at 12:03

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Well, yeah, but at the end of the day you have a really wide thing with tips that I’d imagine are close to supersonic.

HammerheadFistpunch > Smallbear wants a modern Syclone, local Maple Leafs spammer 03/22/2018 at 12:04

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I suspect it’s not the fan speed but the collapse of the wave, if you can prevent the wave collapse no drag and no noise

Smallbear wants a modern Syclone, local Maple Leafs spammer > JawzX2, Boost Addict. 1.6t, 2.7tt, 4.2t 03/22/2018 at 12:06

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Cool. ttyymmnn pretty much nailed it then.

Aremmes > Smallbear wants a modern Syclone, local Maple Leafs spammer 03/22/2018 at 12:06

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This got answered in Stack Exchange . Short story: the duct creates controlled flow conditions that do not exist with propellers, allowing the blades to reach supersonic air speeds without the same degree of penalty.

You can tell a Finn but you can't tell him much > Smallbear wants a modern Syclone, local Maple Leafs spammer 03/22/2018 at 12:07

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Ducting the fan does wonders to improve its efficiency and reduce tip noise. And while turbofans keep getting bigger and bigger they are starting to approach the size where they are getting supersonic flow across the fan blades. They have been working on geared fans for many years now and it seems somewhere that I read they may be nearing production readiness.

To put that 11' fan diameter into some perspective for you, the P-47 in your picture has a 12' diameter propeller and the Corsair had a 13'4" propeller.

ttyymmnn > Smallbear wants a modern Syclone, local Maple Leafs spammer 03/22/2018 at 12:11

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Smallbear wants a modern Syclone, local Maple Leafs spammer > HammerheadFistpunch 03/22/2018 at 12:12

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*eyes glaze*

...ok, I’m in over my head here...

Is that something similar to the concept of the oblique wing?

http://aero.stanford.edu/reports/vki_kroo_supersonics.pdf

Page 12. I remember reading about it in an old PM magazine, something about eliminating sharp peaks from the wave vs a conventional supersonic layout or some such thing. Like I said, over my head.

Highlander-Datsuns are Forever > Smallbear wants a modern Syclone, local Maple Leafs spammer 03/22/2018 at 12:13

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Q400 top speed 414 mph.

Smallbear wants a modern Syclone, local Maple Leafs spammer > You can tell a Finn but you can't tell him much 03/22/2018 at 12:13

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Based on the height of the mechanic I figured it had to be close.

JawzX2, Boost Addict. 1.6t, 2.7tt, 4.2t > Smallbear wants a modern Syclone, local Maple Leafs spammer 03/22/2018 at 12:14

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yup. pretty much.

Smallbear wants a modern Syclone, local Maple Leafs spammer > Aremmes 03/22/2018 at 12:14

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JawsX2 posted this. It’s really interesting.

For Sweden > Smallbear wants a modern Syclone, local Maple Leafs spammer 03/22/2018 at 12:15

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Think of the shroud as one big ground effect for the fan blades.

Smallbear wants a modern Syclone, local Maple Leafs spammer > Highlander-Datsuns are Forever 03/22/2018 at 12:15

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And the “Q” is for quiet. At least reportedly.

That thing has it’s own witchcraft going on.

For Sweden > Smallbear wants a modern Syclone, local Maple Leafs spammer 03/22/2018 at 12:16

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stanford

delet this

Aremmes > Smallbear wants a modern Syclone, local Maple Leafs spammer 03/22/2018 at 12:16

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Yeah, it wasn’t there when I posted.

Highlander-Datsuns are Forever > Smallbear wants a modern Syclone, local Maple Leafs spammer 03/22/2018 at 12:19

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I ride on these a lot, they are not as quite as a jet, but they are not bad either. They have great lift characteristics and get up to speed really quickly.

Smallbear wants a modern Syclone, local Maple Leafs spammer > For Sweden 03/22/2018 at 12:21

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OK, if I have this right, ground effect is basically when in addition to the normal pressure differential between fast air over the wing and slow air below, you actually have a bit of compression going on because the ground is so close.

So, the shroud is essentially helping to compress the air between it and the fan tips.

Higher pressure=higher speed of sound.

The fan is in effect raising the speed of sound so it doesn’t have to go supersonic.

...OK, is that what you’re saying, or am I way off?

Smallbear wants a modern Syclone, local Maple Leafs spammer > Highlander-Datsuns are Forever 03/22/2018 at 12:23

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To be fair, jets aren’t as quiet as billed either... at least not if you’re behind the wings...

For Sweden > Smallbear wants a modern Syclone, local Maple Leafs spammer 03/22/2018 at 12:23

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That’s part of it, but the main mechanism of the ground effect is that a vortex doesn’t have enough room to form with the ground in the way. It would be like dropping a wall into a traffic circle. The circular traffic would stop.

Spanfeller is a twat > Smallbear wants a modern Syclone, local Maple Leafs spammer 03/22/2018 at 12:32

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I’m not an engineer but I guess that a larger fan means more air can flow at a slower speed while also having more inertia. Also, the larger the fan the more air that can be pushed, as new jet engines tend to high bypass, and the higher the bypass the more efficient they are at cruise speed.

Also, being enclosed means taking about completely different aerodynamic properties.

Smallbear wants a modern Syclone, local Maple Leafs spammer > For Sweden 03/22/2018 at 12:38

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So:

- The shroud eliminates the drag from the vortex by limiting the space it has to form, and also by doing so you remove most of the turbulence and allow a more efficient flow of air.

- At the same time the shroud reduces the noise both by effectively raising the speed of sound and by providing a buffer to the outside.

Smallbear wants a modern Syclone, local Maple Leafs spammer > Spanfeller is a twat 03/22/2018 at 12:39

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Apparently. I didn’t realize how much of an impact the shroud actually makes.

For Sweden > Smallbear wants a modern Syclone, local Maple Leafs spammer 03/22/2018 at 12:40

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Yepperoni

Spanfeller is a twat > Smallbear wants a modern Syclone, local Maple Leafs spammer 03/22/2018 at 12:42

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Turbulent air can’t form if the space is too small!

To be honest, I want to see ram jets and scram jets become a reality but in terms of fuel efficiency these new GE engines are great and traveling faster than 900km/h isn’t really necessary...

Smallbear wants a modern Syclone, local Maple Leafs spammer > For Sweden 03/22/2018 at 12:43

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You have been most helpful.

Smallbear wants a modern Syclone, local Maple Leafs spammer > Spanfeller is a twat 03/22/2018 at 12:46

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https://oppositelock.kinja.com/1823989935

^ Nice to have people around who know what they’re talking about. :)

The concept of the scramjet intrigues me but I honestly just can’t see it being efficient enough for any practical use. Cool though.

3point8isgreat > Smallbear wants a modern Syclone, local Maple Leafs spammer 03/22/2018 at 13:11

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While there’s a lot of discussion going on here about the shroud’s effect, and that’s correct. But that’s not all there is to it.

Truth be told, the fan itself may not be going supersonic. Many of these engines have 2 or 3 different shaft speeds that correlate to different size blades. Think shafts inside of shafts to do this. By doing this you can have a very large fan spinning at a much lower RPM than your compressor sections.

Additionally the speed of sound increases with temperature. This mostly benefits the turbine side but could affect the later compression stages. You’ll see this a lot with power plant gas turbines. They all have to spin at 3600 RPM (3000 RPM in 50hz countries), but they’re sized to be as big as possible without having supersonic tips. (Fun fact, since 50hz turbines spin slower, they are often larger and more powerful than a 60hz turbine of a similar design.) So how do we make more powerful turbines? Make them run hotter so we can make the turbine blades bigger. 

coqui70 > Spanfeller is a twat 03/22/2018 at 23:22

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Try flying to Asia ... traveling MUCH faster than 900kph is something that needs to happen soon.

Spanfeller is a twat > coqui70 03/22/2018 at 23:50

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You can get half way across the world in less than a day; it’s insane if you think about it. While yes I do want to see transportation that cuts down on that time, I can understand why they focus on making airplanes flying at the current speed much less noisy, more human-friendly, and more fuel efficient.

coqui70 > Spanfeller is a twat 03/22/2018 at 23:53

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That perspective changes when you cross the Pacific once or twice a month and it’s coach ‘cause only fat cats get to fly first class.

Spanfeller is a twat > coqui70 03/22/2018 at 23:55

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Yah, I see your point. But capitalism dictates the outcome; if fuel is one day cheap enough that flying at a higher speed is possible, airlines will do it. The biggest challenge today is making sure prices can continue to drop, and to achieve that economy class seating has to be squished, and as a 188cm monster that does need to cross the Atlantic on 11hr flights four times a year... I get it.... it sucks.

coqui70 > Spanfeller is a twat 03/23/2018 at 00:00

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Lol ... I pray for upgrades.