iowaman
Posts: 3878
Joined: Thu May 20, 2004 2:29 am

757 Engine Fan Blade Speed

Sat Dec 27, 2008 11:09 pm

I recently had a friend fly on a 752 from DTW-SEA and he commented that the loud distinctive roaring sound heard on takeoff with the 752's was possibly the fan blades exceeding the speed of sound. Is this true, or just sheer power of the PW2040's? I doubt they are actually breaking the sound barrier.

[Edited 2008-12-27 15:10:06]
 
Danfearn77
Posts: 555
Joined: Thu Jul 10, 2008 11:52 pm

RE: 757 Engine Fan Blade Speed

Sat Dec 27, 2008 11:14 pm

Good question. I would love to know. I seem to remember reading that the tip of the fan blade can indeed reach the speed of sound but i cannot remember where i read it or what aircraft it was regarding or even if it was true! So i look forward to knowing!
Eagles may soar high, but weasels dont get sucked into jet engines!
 
FlyASAGuy2005
Posts: 3965
Joined: Sun Sep 23, 2007 4:55 am

RE: 757 Engine Fan Blade Speed

Sat Dec 27, 2008 11:59 pm

http://www.airliners.net/aviation-forums/tech_ops/read.main/237871

Yes 

I brought the exact thing up in Tech-Ops a while back as the subject always interested me. As a matter of fact, I first got this explanation from DL TOC and I would figure they know what they are talking about.

A tad off topic but i'd like to bring up something I said in the other thread:

Quote:
Side note that kind of has nothing to do with this, but has anyone ever stood on the ramp or atleast near a CRJ and hear those engines scream (I hope this is a better example than wang crossfingers )when they are spooled up for taxi or on take-off? It's the best sounding engine during that time IMO...



[Edited 2008-12-27 16:03:45]
What gets measured gets done.
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: 757 Engine Fan Blade Speed

Sun Dec 28, 2008 1:15 am



Quoting Iowaman (Thread starter):
Is this true, or just sheer power of the PW2040's? I doubt they are actually breaking the sound barrier.

It's sort of a tricky question, since different parts of the blade are going different speeds and the airflow speed varies quite a bit within the fan. But there are definitely pockets of locally supersonic flow.

Tom.
 
MarkC
Posts: 238
Joined: Tue Apr 25, 2006 8:10 am

RE: 757 Engine Fan Blade Speed

Sun Dec 28, 2008 2:36 pm

2000's max out at nearly 4,600 rpm I believe. Whats the fan diameter....84 inches I think? Thats 264 inches circumference or 22 feet. So the tips move at 101,000 ft/min or 19 miles/min or 1,150 mph.

Thats just simple math. To find out the local mach number at the pressure and tempurature which exists at the tips at full speed would need someone with a much better understanding of aero than me. But, 1,150 is right.
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: 757 Engine Fan Blade Speed

Sun Dec 28, 2008 6:43 pm



Quoting MarkC (Reply 4):
To find out the local mach number at the pressure and tempurature which exists at the tips at full speed would need someone with a much better understanding of aero than me. But, 1,150 is right.

1150 mph is Mach 1.51 at standard sea level...even on the slow flow side of the blades, that's probably fully supersonic flow. It certainly would be on the fast side.

Tom.
 
Tristarsteve
Posts: 3364
Joined: Tue Nov 22, 2005 11:04 pm

RE: 757 Engine Fan Blade Speed

Sun Dec 28, 2008 7:08 pm



Quoting MarkC (Reply 4):
2000's max out at nearly 4,600 rpm I believe.

That sounds a bit fast to me. I would have said more like 3400 rpm.
Will look it up tomorrow.
 
MarkC
Posts: 238
Joined: Tue Apr 25, 2006 8:10 am

RE: 757 Engine Fan Blade Speed

Mon Dec 29, 2008 4:27 am

Its OK, Steve, you really don't have to look it up.

If I really knew, I would have said max allowable is 4,575 rpm corresponding to 100.5% N1. But, for the purposes of this forum, lets just say 4.575 might be a lucky guess.

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