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DocLightning
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Swept Fans?

Sat Apr 28, 2012 5:15 am

The GE90-110/115 was the first engine to feature swept fan blades.


But the GP700 and Trent 900 followed



And then smaller engines like the GEnX and Trent 1000


And then even smaller with the PW1000 and GE's LEAP.


So what is the advantage? Why didn't they exist before? Why does GE's LEAP fan look so different from the others?
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
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Starlionblue
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RE: Swept Fans?

Sat Apr 28, 2012 5:21 am

Quoting DocLightning (Thread starter):
So what is the advantage?

The outer part of the blade moves faster than the inner. Fixed sweep means you have to compromise. Varying sweep means you can optimize between different spanwise parts of the blade.

Quoting DocLightning (Thread starter):
Why didn't they exist before?

If I had to guess: materials capable of doing it, and computer modeling of effects.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
tdscanuck
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RE: Swept Fans?

Sat Apr 28, 2012 9:54 am

Quoting DocLightning (Thread starter):
Why didn't they exist before?

They're hard to build and *really* hard to analyze. I don't think CFD advanced to the point that anyone was willing to trust the results enough until fairly recently.

Quoting DocLightning (Thread starter):
Why does GE's LEAP fan look so different from the others?

I'm not really following you here; I see differences in all of them. Which specific difference are you thinking of?

Tom.
 
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DocLightning
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RE: Swept Fans?

Sat Apr 28, 2012 3:51 pm

Quoting tdscanuck (Reply 2):
I'm not really following you here; I see differences in all of them. Which specific difference are you thinking of?

LEAP isn't swept until the very outer portion, then it sweeps back sharply and then forward a bit. The others have a forward sweep on the inner section with a gradual backward sweep halfway down the blade.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan
 
tdscanuck
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RE: Swept Fans?

Sat Apr 28, 2012 9:21 pm

Quoting DocLightning (Reply 3):
LEAP isn't swept until the very outer portion

LEAP-X is swept for the inner portion; it's just straight, rather than curved, so the sweep isn't as noticeable. The point where it flips from forward to backward sweep is also farther out, which accentuates the "no sweep" look. However, if you trace a radial from the centre of the spinner to the fan case, you can see the the blades are actually very swept right from the hub.

Tom.
 
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DocLightning
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RE: Swept Fans?

Sat Apr 28, 2012 10:16 pm

Quoting tdscanuck (Reply 4):
LEAP-X is swept for the inner portion; it's just straight, rather than curved, so the sweep isn't as noticeable. The point where it flips from forward to backward sweep is also farther out, which accentuates the "no sweep" look. However, if you trace a radial from the centre of the spinner to the fan case, you can see the the blades are actually very swept right from the hub.

Good point.

But why does that fan have such a different shape than the GTF or the bigger fans?

And what does sweep actually do? Yes, I know it makes it quieter and more efficient, but how?
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan
 
tdscanuck
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RE: Swept Fans?

Sun Apr 29, 2012 4:06 pm

Quoting DocLightning (Reply 5):
But why does that fan have such a different shape than the GTF or the bigger fans?

Not sure; I suspect CFM stumbled into some other branch of an optimization process.

Quoting DocLightning (Reply 5):
And what does sweep actually do? Yes, I know it makes it quieter and more efficient, but how?

Sweep helps lower the local Mach number (same reason you do it on wings). For a given chord, a swept airfoil "looks longer" to the airflow than an unswept one. This makes the wing look thinner to the airflow, so the pressure gradients aren't as big, so the speed changes aren't as big. Given that fans routinely run with local Mach numbers above 1, which creates noisy & lossy shockwaves, this helps reduce noise and losses.

Tom.
 
bellancacf
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RE: Swept Fans?

Sun Apr 29, 2012 4:51 pm

@tdscanuck: Thanks very much for all your patient, informative posts.

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