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Fan Speed Question...

Mon Sep 19, 2005 10:33 pm

There's a post in civil av about the possibility of a 450 seat twin in the future, and people were theorizing that it would require engines with 160,000 pounds of thrust that would be significantly larger than the GE90s on the 777. My question is, why do they necessarily have to be larger? Can an engine the size of the GE90 be given advanced fan technology that woud allow the fanblades to spin at a much higher RPM? It seems to me that would produce more thrust just as well as making the engine larger, but I'm not exactly an engine expert. Is there an RPM limit on the fans in modern turbofans?

[Edited 2005-09-19 15:34:03]
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RE: Fan Speed Question...

Mon Sep 19, 2005 11:14 pm

Quoting Waterpolodan (Thread starter):
Is there an RPM limit on the fans in modern turbofans?

I won't get into the issue of loss of fan efficiency with tip speed (Hopefully some passing aerodynamist will put his oar in.) However, there are plenty reasons you wouldn't want to stay will the same sized engines. To get 60% more thrust, you'd generally have to run the engine 60% faster.

That means that any released blade will have 1.60^2 = 256% of the original energy, so you'll need a MUCH thicker containment casing. = WEIGHT and Cost

Also, the load produced in the root of the blade will be 256% of the original, so you need to thicken that up (all the way up the blade) = WEIGHT and Cost

So, your fan blade is heavier, and spinning faster. Even if the fan disc is man enough for the job, the stresses are >250% of the original , so your fan disc life will be a fraction of the originals. So, you need to refurbish the engine far more often.

To get the fan running faster, you need to run the Turbine faster, so that means more fuel, which mean MUCH higher turbine temperatures. And that means you'll be lucky to get the turbine blades lasting very long.

I could go on (Noise, Bird Strike, gyroscopic loads, oil temperatures, boiler pressure etc) but I think you get the picture.

So, what about an afterburner ?
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RE: Fan Speed Question...

Tue Sep 20, 2005 12:10 am

I'll take a stab at it, you don't want the velocity of the tip blade to exceed the speed of sound. It'd probably the same limitation on the max RPMs on helicopter rotor blades, mainly to prevent them from exceeding the speed of sound at the rotor tip.
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RE: Fan Speed Question...

Tue Sep 20, 2005 1:29 am

For all the reasons given above the engine would need to get larger, unless you get into new technology such as variable pitch fans or maybe look at a tandem counter-rotating fan design.

Tip speed is a major issue, but existing fans already run supersonic. Increasing fan speed would increase such tip losses.

The reason you'd need a 160,000 lb engine is the engine failure on takeoff case. Whatever the economics of a twin are in terms of maintenance costs, carrying all that extra thrust capability just for the remote chance of an engine failure seems very wasteful to me.

The 747-200 (with similar max capacity in high density config) needed 200,000 lbs thrust at takeoff, so the engine out thrust was 150,000 lbs. A twin engined version would have 300,000 lbs installed thrust for the same case.

A 160,000 lb thrust engine would be approx 25% larger physically than the current GE90. That is going to pose ground clearance issues and increases the chance of bird-strikes.

A four engine design has got to be more practical, not to mention beneficial in terms of wing bending relief.
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RE: Fan Speed Question...

Tue Sep 20, 2005 1:33 pm

A bigger engine in the same size cowl means less bypass ratio and unless you can make more power out of the same size engine then theres no chance of the same bypass ratio (a big deal for fuel effiency) on the same size cowl. Also though with the way of the engines that are used these days fan blades and fans in general are basically at the point that they cannot be inproved much more without a radical new design. The same goes for the engines the size to power is about as good as it gets the more power in a smaller engine generally speaking ... the less reliable and that is a big thing in Aviation today. So this is why the engines basically get bigger and bigger the a/c size gets bigger tho so why cannot the engines!

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RE: Fan Speed Question...

Tue Sep 20, 2005 10:48 pm

Quoting Jetlagged (Reply 3):
The 747-200 (with similar max capacity in high density config) needed 200,000 lbs thrust at takeoff

A modern airliner of the same capacity would be a lot lighter than the 742, with a commesurately lower installed thrust. The GE90-115B has generated 127,000lbf thrust, it is possible that a new version with a ~140,000lbf rating would not be incredibly larger.

I would say that such a large twin is not beyond the realm of possibility.

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RE: Fan Speed Question...

Tue Sep 20, 2005 10:57 pm

There is no reason that I'm aware of that an engine in the 160K thrust class couldn't be developed. It would be larger than the GE90 but I don't see that as any problem. The speculated 450 seat twin would be designed with sufficient ground clearance to accommodate the larger engines. There are actually efficiencies gained by creating larger engines. I can't quote a linkable source but I've heard from P&W engineers and others that one engine with the same inlet area as two smaller engines combined is much more efficient and has greater power. Another thing is that higher bypass ratios are easier to achieve in larger engines. There would be design and materials advances needed to make such an engine a reality but nothing on the order of what would be needed to create an 160K lb thrust engine the size of a GE90.

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RE: Fan Speed Question...

Wed Sep 21, 2005 6:49 am

Quoting Dl757md (Reply 6):
There are actually efficiencies gained by creating larger engines.

Tip clearance is a strong driver. For a given compressor or turbine tip clearance (radial gap between blade tip and case), the larger diameter engine is going to suffer proportionately lower tip loss effects. Adverse 3-D aerodynamic effects are also proportionately lower in the bigger machines.
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RE: Fan Speed Question...

Thu Sep 22, 2005 8:46 am

Quoting Waterpolodan (Thread starter):
possibility of a 450 seat twin in the future, and people were theorizing that it would require engines with 160,000 pounds of thrust

It all depends on how far you want to fly.

An A321 can move half that number of pax on two 30,000 lbs engines. So a twin taking 450 pax over a max 321 range would need no more than 60,000 lbs engines.

You could also just modify an A380 with two GE-90-115B and it would take 550 pax, not 7-8,000nm, but maybe 2-3,000nm.

A B777-200LR is supposed to take some 250+ pax over some 8,200nm. We can modify it with extra underfloor tanks, and it would take zero pax some 9,500nm instead. We could also make it twin deck, stuff in 450 pax, and probably fly them 4,000nm.

Very high powered engines are more for long range than for high number of pax. That's logical because a long range plane lifts a fuel weight which is many times heavier than the payload weight.

But anyway, would a 160,000 lbs engine be feasible? If it had to retain the 115B fan dimentions and rely on higher N1 speed, then the first thing to worry about would be that it would be too noisy to be certified for civil flight today. So it will definitely have to be larger.

Would such a twin be feasible? I doubt. I quad would do the same job on four 60,000 lbs engines. Or even less than that because it would have a much more favorable weight distribution on the wing. The twin would need a much stronger and heavier wing to support those two 160,000 lbs engines. Greater fan diameter would for ground clearance call for longer, stronger and heavier landing gear.

An A380 will take 550 pax in three classes far away on four times 70,000 lbs = 280,000 lbs installed thrust. On two times 160,000 = 320,000 it would never do the same job. It would need more like two times 200,000 lbs. Maybe more because structure weight mainly on wing and landing gear would have to grow considerably.
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