Propilot83 From United States of America, joined Jan 2001, 585 posts, RR: 0 Posted (4 years 4 months 2 weeks 5 days 3 hours ago) and read 4061 times:
I took a photo of a NASA Boeing 747-100's engine, and I noticed that I can see the engine blades seem to be very close to the interior cowling of the engine or near the metal surface surrounding the engine blades. Is this the case with all engine blades for every manufacturer? and if so, is it usual for the engine blades to be so close to the metal surface? I am asking this question, because when the engine is running at slow or fast speeds, is there any chance that the blades can scrape the metal cowling or metal surface? If it doesnt at its close proximity, WOW it must be an engineering marvel to have engine blades running at a high RPM only less than 0.5 inches from the metal surface or engine cowling!
Jetlife2 From United States of America, joined Jul 2006, 220 posts, RR: 25
Reply 4, posted (4 years 4 months 2 weeks 4 days 23 hours ago) and read 4001 times:
The clearance varies during operation even without ovalization. As the rotor and the case warm up and cool down they grow at different rates, tending to open up the clearance on an accel, and close it on a decel. In the HPT this is often controlled actively by modulating air on the outside of the case (Active Clearance Control or ACC).
Steady state running clearance is a direct influence on performance. Air over the tip is an aerodynamic loss since it is not going through the airfoil flowpath. So successive generations of designers have sought to optimize the clearance. It is not uncommon to have running clearances well under 0.5mm. (.020") . Cheers
Ex52tech From United States of America, joined Dec 2006, 559 posts, RR: 1
Reply 6, posted (4 years 4 months 1 week 3 days 7 hours ago) and read 3472 times:
When GE took the CF6-50 to "C" power, they installed a stiffener ring at the rear flange of the fan case. The stiffener ring helped to keep the case from flexing and becoming oval. They also went through several different types of fan blades, and closed up the tip clearance. The CF6-6 when it was uprated to "K" power had the stiffener ring added.
If you watch the inlet of some engines at night you will see sparks coming off of the fan blade tips as they drag along in the case's abradable material, blade stretch along with case flexing causes it.
"Saddest thing I ever witnessed....an airplane being scrapped"
MarkC From United States of America, joined Apr 2006, 259 posts, RR: 0
Reply 8, posted (4 years 4 months 1 week 2 days 23 hours ago) and read 3401 times:
The JT8D-200, 9D's, 2000's and 94" 4000's all use panels made of a kevlar impregnated resin as the "rubstrips" which are bonded to the fan case. They all have features (either grooves or holes) by which the depth of wear can be measured. Some engines have worse wear than others. The older 9D's got wiped out. 2000's and 4000's, not so bad. Also, and I have no idea why, the 9D's had panels of different thickness installed in a pattern around the case with the thinnest on the bottom.
The 100" and 112" 4000's have very little wear, and, instead of discrete panels, have the material applied directly to the case with a thin honeycomb material helping to retain the abradable. I actually do not know of a 112 that ever needed an abradable replacement due to wear, and those have been in service for almost 15 years.
747classic From Netherlands, joined Aug 2009, 1982 posts, RR: 14
Reply 10, posted (4 years 4 months 1 week 2 days 13 hours ago) and read 3262 times:
Quoting JetMech (Reply 2): The actual surface of the fan case adjacent to the tips of the fan blades is not metal. It is an abradable material which is soft enough for the tips of the blade to rub away
The P&W JT9D engine as installed on the 747 classic has an abradable seal around the fan blades, for optimizing the fan blade tip clearance.
When the engines are overhauled new seals are (sometimes) installed. During the first engine starts of new or overhauled engine sometimes the fan became stuck (zero N1 RPM indicating),
Action was/is : counter rotate the fan and try again.
If the abradable seal was worn to much, due turbulence, hard landings and reverse operation (vibration) a performance and/or SFC loss was noticed.
In extreme circumstances (high angle of attack and to much fan tip clearance) this could lead to a fan stall, a very dangerous situation. Suddenly the thrust delivered by the fan was reduced and then back again, resulting in a violent engine swing. Personally I encountered two fan stalls, when climbing out from ATH (relative low TOW, high angle of attack and in a turn) on both outboard engines of the 747-200 PH-BUG in the early 80's.
Lowering the angle of attack quickly cured the problem, as stated in the AOM (aircraft operation manual). However after landing we ordered an inspection and it was discovered that the fuse pins of engine strut No 4 were out of tolerance.
Both abradable seals of engines 1 & 4 were replaced.
[Edited 2009-10-29 13:30:16]
Operating a twin over the ocean, you're always one engine failure from a total emergency.
Baroque From Australia, joined Apr 2006, 15380 posts, RR: 60
Reply 11, posted (4 years 4 months 1 week 1 day 21 hours ago) and read 3144 times:
Quoting JetMech (Reply 2): The clearance would be somewhat less than 12.7mm, and would be closer to 2 - 3mm
You are a spoil sport JetM - next thing you will be wanting effecient jet engines.
For a basic read of what went into designing jet engines Frank Whittle's autobiog "JET" is recommended. Life was not meant to be easy in those days. Mind you I am not sure if the vagaries of nature or of the bosses of Rover were the biggest problems. And RR were not a bed of roses for Whittle either.
Baroque From Australia, joined Apr 2006, 15380 posts, RR: 60
Reply 13, posted (4 years 4 months 6 days 5 hours ago) and read 2842 times:
Quoting JetMech (Reply 12): I'd then have no trouble getting my dream job with RR
Alas, no longer driving the office pool vehicle I suppose. But yes, working on some of those design and production techniques must be fascinating - esp cooling of blades in the hotter end and the fan blades.
Quoting JetMech (Reply 12): I have read that Sir Frank was a very capable engineer.
I wonder if there are any Sir Franks these days who have an overview of the whole engine and how do they integrate the specialists. I guess someone knows, care to share?
MadDogJT8D From United States of America, joined Oct 2009, 393 posts, RR: 1
Reply 14, posted (4 years 4 months 5 days 5 hours ago) and read 2715 times:
I wasn't aware that the fan blades normally scraped the fan case until a flight last year. I was flying on a Delta 757 last year from JFK-FLL at night and was lucky enough to score an upgrade which put me in a window seat looking right into the fan blades of the engine (a PW 2037). I noticed that throughout the engine start-up and a few minutes into the taxi that there were very small sparks coming from the edges of the fan blades and it appeared that they were making contact with the fan case. I thought this was abnormal at first and thought of bringing it up to the FA's, but it did indeed stop once the engines got warmed up so I attributed it to normal engine operation. Was a little unnerving to see sparks coming from the engine though, made my heart skip a beat! Amazing how little things like that happen and most passengers have no idea!