Iowaman From United States of America, joined May 2004, 4098 posts, RR: 7 Posted (4 years 4 months 4 weeks 4 hours ago) and read 4463 times:
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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.
Danfearn77 From United Kingdom, joined Jul 2008, 1780 posts, RR: 9 Reply 1, posted (4 years 4 months 4 weeks 4 hours ago) and read 4436 times:
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 From United States of America, joined Sep 2007, 6520 posts, RR: 11 Reply 2, posted (4 years 4 months 4 weeks 3 hours ago) and read 4341 times:
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]
CAM2:"Lightning coming out of that one." CAM1: "What?"
Tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80 Reply 3, posted (4 years 4 months 4 weeks 2 hours ago) and read 4274 times:
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.
MarkC From United States of America, joined Apr 2006, 259 posts, RR: 0 Reply 4, posted (4 years 4 months 3 weeks 6 days 12 hours ago) and read 4145 times:
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 From Canada, joined Jan 2006, 12709 posts, RR: 80 Reply 5, posted (4 years 4 months 3 weeks 6 days 8 hours ago) and read 4094 times:
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.
MarkC From United States of America, joined Apr 2006, 259 posts, RR: 0 Reply 7, posted (4 years 4 months 3 weeks 5 days 22 hours ago) and read 4000 times:
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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