Sponsor Message:
Aviation Technical / Operations Forum
My Starred Topics | Profile | New Topic | Forum Index | Help | Search 
Parked Jet With Engines Spinning  
User currently offlineFlybyguy From United States of America, joined Jun 2004, 1801 posts, RR: 1
Posted (10 years 3 months 2 weeks 2 hours ago) and read 6036 times:

All of the commercial aircraft I have seen parked at the gate prior to boarding had their engines spinning slowly (1 rotation every 6-7 seconds). These aircraft seem to have been parked for several hours so this is not the situation where an arriving aircraft becomes a departure flight as soon as its passengers deplane.

I was wondering if there is a technical reason why the engines remain spinning slowly while at the gate?


"Are you a pretender... or a thoroughbred?!" - Professor Matt Miller
23 replies: All unread, jump to last
 
User currently offlineJetMechMD80 From United States of America, joined Jul 2004, 380 posts, RR: 7
Reply 1, posted (10 years 3 months 2 weeks 2 hours ago) and read 6000 times:

Its just the wind, if its blowing up the tailpipe, it can make the engine turn backwards, you can see this when they start engines, the engine will stop spinning, and start turning the other direction. Some times if the wind is blowing hard enough, those blades can be turning pretty fast!!! That's why you see the TR's (thrust Reversers) deployed at the gate some times. It blocks the wind.

[Edited 2004-07-11 07:19:34]


"I get along great with nobody"~ Billy Idol
User currently offlineFly2HMO From , joined Dec 1969, posts, RR:
Reply 2, posted (10 years 3 months 2 weeks 1 hour ago) and read 5975 times:

It always surprises me to see that the engines are so easy to turn, even a slight brease will get them spinning in a jiffy.

Has anybody in here actually spinned a jet fan with his/her own hand? How loose are they? Do low bypass turbofans (eg. JT8D) spin as easily?

 Smile/happy/getting dizzy

[Edited 2004-07-11 07:50:51]

User currently offlineAaron747 From Japan, joined Aug 2003, 8182 posts, RR: 26
Reply 3, posted (10 years 3 months 2 weeks ago) and read 5941 times:

It's called windmilling. And yes, you can get the fan going rather easily by hand.


If you need someone to blame / throw a rock in the air / you'll hit someone guilty
User currently offlineAloges From Germany, joined Jan 2006, 8725 posts, RR: 43
Reply 4, posted (10 years 3 months 1 week 6 days 20 hours ago) and read 5788 times:

"Has anybody in here actually spinned a jet fan with his/her own hand? How loose are they?"

I've spun a Rolls-Royce Olympus with my own hand. It took a little bit of an effort, but that was probably due to mass inertia, not friction. By the way, I didn't see any "don't touch this" signs near that displayed pair of engines.



Walk together, talk together all ye peoples of the earth. Then, and only then, shall ye have peace.
User currently offlineQantasA332 From Australia, joined Dec 2003, 1500 posts, RR: 25
Reply 5, posted (10 years 3 months 1 week 6 days 19 hours ago) and read 5742 times:

As Aaron747 mentioned already, the tendency of jet engine fans to spin when not running is attributed to "windmilling." Windmilling is just that - the tendency of a propeller or jet fan or any other set of airfoils in that sort of arrangement to spin even when not being powered. It all has to do with the various components of airflow...

In normal operation, a propeller/fan is spun under power to achieve thrust. The spinning creates an apparent airflow directed parallel and opposite in direction to the rotation. Assuming the airplane is in forward flight, there's another flow component created by the freestream, which is approximately perpendicular to the rotation-induced flow. The resultant is obviously a flow directed partway between the two component flows:



Note that there is a "positive" AoA, which produces a forward lift (thrust).

Now, what if the engine stops? Well, due to drag (both parasitic and induced) and friction within the engine, the prop's/fan's rotational speed begins to decrease. Accordingly, the crosswise flow component decreases as well, moving the resultant flow so that AoA decreases. Eventually, the resultant flow is such that the AoA becomes "negative," producing backwards lift (drag). The (backwards) lift vector is oriented with a prop/fan rotation-wise component, so that it induces continued rotation:



Of course, a plane doesn't need to be flying to experience windmilling of its engines - wind will cause windmilling when on the ground, which is the spinning you (Flybyguy) were wondering about.

It's getting late here and I'm about to go to bed, so please forgive me if I left anything out...

Cheers,
QantasA332


User currently offlineMD11Engineer From Azerbaijan, joined Oct 2003, 14062 posts, RR: 62
Reply 6, posted (10 years 3 months 1 week 6 days 17 hours ago) and read 5677 times:

Carefull when spinning a fan with your hands. The fan blade edges are rather sharp and I´ve heard of people who got cut badly (didn´t happen to myself though).

Jan


User currently offlineFly2HMO From , joined Dec 1969, posts, RR:
Reply 7, posted (10 years 3 months 1 week 6 days 15 hours ago) and read 5595 times:

Isn't windmilling something you'd want to avoid since the oil pumps aren't running?

User currently offlineAloges From Germany, joined Jan 2006, 8725 posts, RR: 43
Reply 8, posted (10 years 3 months 1 week 6 days 14 hours ago) and read 5560 times:

"Isn't windmilling something you'd want to avoid since the oil pumps aren't running?"

I don't think a slowly windmilling engine needs that much lubrication, but I'm certainly no expert on this. Windmilling on the ramp is often avoided by engine inlet plugs or deployed reversers:

View Large View Medium
Click here for bigger photo!

Photo © Harri Koskinen
View Large View Medium
Click here for bigger photo!

Photo © Roman Doleys

Here's an extra goodie: an Il-62 taking off with an inlet plug in place.

View Large View Medium
Click here for bigger photo!

Photo © Jukka Huppunen




Walk together, talk together all ye peoples of the earth. Then, and only then, shall ye have peace.
User currently offlineMD11Engineer From Azerbaijan, joined Oct 2003, 14062 posts, RR: 62
Reply 9, posted (10 years 3 months 1 week 6 days 13 hours ago) and read 5479 times:

Windmilling on the ramp is much slower than in flight. The bearings are designed to withstand it.

Jan


User currently offline320tech From Turks and Caicos Islands, joined May 2004, 491 posts, RR: 5
Reply 10, posted (10 years 3 months 1 week 6 days 4 hours ago) and read 5269 times:

Spinning the blades by hand can be done for technical reasons, too. Examples - to check for free movement after maintenance; to turn the internal components during a boroscope inspection; to inspect blades for damage after an ingestion of rocks, birds, ice, whatever (have to be able to see second and third row of blades and stators); for gambling purposes (like roulette - 1 in 36 chance of winning)  Smile .

I have never seen anything to warn against spinning blades, except during spool-ups. There are limits to how many times you can spool an engine, but that has more to do with starter overheat limits than blade lubrication.



The primary function of the design engineer is to make things difficult for the manufacturer and impossible for the AME.
User currently offlineSinlock From United States of America, joined Dec 2000, 1650 posts, RR: 2
Reply 11, posted (10 years 3 months 1 week 6 days 3 hours ago) and read 5243 times:

Isn't windmilling something you'd want to avoid since the oil pumps aren't running?

Actualy there is the an issue with the JT8D when it's mounted on the 737-200. It was mandated that an aircraft if it spends more than 2 hours on the ground Inlet covers should be installed to prevent bearing damage (I think #2)



My Country can beat up your Country....
User currently offlineOly720man From United Kingdom, joined May 2004, 6811 posts, RR: 11
Reply 12, posted (10 years 3 months 1 week 5 days 21 hours ago) and read 5156 times:

I heard from someone that one engine type has to keep turning because, due to thermal effect as the engine cools after landing, it can set in a position that leaves the main shaft slightly bent so it can't be run up again until it's completely cool. Not good if you've got short turnrounds.

Incidentally I was once at KUL after a Mauritius 747SP arrived and the engine did lots of clanging and banging as it freewheeled in the breeze.

Andy



wheat and dairy can screw up your brain
User currently offlineModesto2 From United States of America, joined Jul 2000, 2812 posts, RR: 5
Reply 13, posted (10 years 3 months 1 week 5 days 18 hours ago) and read 5114 times:

I've spun one of JetBlue's A320 IAE V2527-A5 engines...easy, anyone can spin a fan. There's surprisingly very little resistance.

User currently offlineAuae From United States of America, joined Apr 2004, 296 posts, RR: 3
Reply 14, posted (10 years 3 months 1 week 5 days 18 hours ago) and read 5102 times:

It shouldn't be surprising that they are easy to spin, think about it. There is no obstruction to the blade turning other than bearing friction. Most airseals don't become effective until the blades or disks grow (creep) a little bit from spinning. And as far as bearing friction, you would expect there would be very little. We are talking about very high precision bearings here. You might think that the mass would be harder to spin than it is, but the long blades on the high bypass engines make a good lever arm. Hell, I have seen some engines windmilling fast enough I sure as hell wouldn't try to stop it by hand!

Shawn



Air transport is just a glorified bus operation. -Michael O'Leary, Ryanair's chief executive
User currently offlineWrenchBender From Canada, joined Feb 2004, 1779 posts, RR: 9
Reply 15, posted (10 years 3 months 1 week 5 days 16 hours ago) and read 5076 times:

Oly720man,
That is the Garrett powered King Air, the engine/prop had to be hand spun for about 3 minutes after shutdown. It allowed for main shaft cooling without taking on a permanent "set".

WrenchBender



Silly Pilot, Tricks are for kids.......
User currently offlineA/c train From United Kingdom, joined Jun 2001, 501 posts, RR: 4
Reply 16, posted (10 years 3 months 1 week 5 days 13 hours ago) and read 4999 times:

oly720man, what your hearing is the midspan shrouds clanking together as they go just over the 12 o clock pstn, you can actually see them doing it e.g, CFM-56.

User currently offlineAogdesk From United States of America, joined Jun 2004, 935 posts, RR: 3
Reply 17, posted (10 years 3 months 1 week 5 days 12 hours ago) and read 4984 times:

The noise of the blades clanking around is the top ones 'falling over' center. The blade retention design allows the blades to move outward with centrifugal force when the engine is running, which maintains a tight seal between the outer edge of the blade and the rubstrip. Very efficient. I've had many rampers come up to me near departure time and inform me that some of the blades are ready to fall out. A little mock surprise and a very concerned look on my face when I 'evaluate' the situation, and then a casual "I think it'll be alright, and if it ain't, thats why its got more than one engine" always makes things fun.

User currently offlineMD11Engineer From Azerbaijan, joined Oct 2003, 14062 posts, RR: 62
Reply 18, posted (10 years 3 months 1 week 5 days 12 hours ago) and read 4976 times:

The main reason the fan blades are attached with a loose fit is to let them move in case of turbulence and vortices hitting them. This prevents bending and cracking of the blades themselves. It is like an elastic mount, only that the centrifugal force keeps them aligned.

Jan


User currently offlineFlybyguy From United States of America, joined Jun 2004, 1801 posts, RR: 1
Reply 19, posted (10 years 3 months 1 week 5 days 7 hours ago) and read 4881 times:

Thanks everyone for your detailed and informative answers  Smile/happy/getting dizzy


"Are you a pretender... or a thoroughbred?!" - Professor Matt Miller
User currently offlineA/c train From United Kingdom, joined Jun 2001, 501 posts, RR: 4
Reply 20, posted (10 years 3 months 1 week 4 days 11 hours ago) and read 4719 times:

It should be noticed that modern engines with wide chord fan blades do not make this clanking sound as they do not have midspan shrouds and they are NOT a loose fit into the hub, as you would know if you had ever taken a set of fan blades out of a V2500/RB211-535-E4, they can be very tight and need some encouragement from a hammer handle to take out at times, good excercise,
If your interested in the mispan shrouds, they act as aerodynamic snubbers, notice on engines such as CF6-80C2 / CFM-56.
regards a/c


User currently offlineSmcmac32msn From United States of America, joined May 2004, 2211 posts, RR: 4
Reply 21, posted (10 years 3 months 1 week 4 days 9 hours ago) and read 4687 times:

That is the Garrett powered King Air, the engine/prop had to be hand spun for about 3 minutes after shutdown. It allowed for main shaft cooling without taking on a permanent "set".

King Airs don't need to be spun down, Garrett or not. The only aircrafts that I know need to be "spun down" are Cessna 441's (Conquest) and the Cheyenne 4's.



Hey Obama, keep the change! I want my dollar back.
User currently offlineG4doc2004 From United States of America, joined Feb 2004, 123 posts, RR: 0
Reply 22, posted (10 years 3 months 1 week 4 days 7 hours ago) and read 4656 times:

Smcmac32msn, I must correct you. I currently have among other jets in the corporate fleet I maintain, a KingAir B-100, msn BE-34, with the Garrett 331-252B engines. AND, in the Garrett/Allied Signal maintenance manual, it does call for "the propellers to be pulled thru several blades by hand after shutdown. This aids in the internal cooling of the high pressure turbine wheel and shaft." If you leave the battery switches on after shut down, you can see a noticeable temperature decrease on both TGT gauges after this exercise.


"Failure to prepare is preparing to fail"--Benjamin Franklin
User currently offlineAloha717200 From United States of America, joined Aug 2003, 4515 posts, RR: 15
Reply 23, posted (10 years 3 months 1 week 3 days 10 hours ago) and read 4546 times:

While I had an internship out at the airport we had a bizjet...I forget the type now...parked on the ramp during some heavy winter winds. I also heard some of that clanking and wonderd what it was. The engine wasn't on, but I looked down the intake and the fans were spinning a bit from the wind. I didn't realize that blades can actually move, I thought they were fixed.


We eventually towed that plane into the hangar because of snow. But it was interesting.


Top Of Page
Forum Index

Reply To This Topic Parked Jet With Engines Spinning
Username:
No username? Sign up now!
Password: 


Forgot Password? Be reminded.
Remember me on this computer (uses cookies)
  • Tech/Ops related posts only!
  • Not Tech/Ops related? Use the other forums
  • No adverts of any kind. This includes web pages.
  • No hostile language or criticizing of others.
  • Do not post copyright protected material.
  • Use relevant and describing topics.
  • Check if your post already been discussed.
  • Check your spelling!
  • DETAILED RULES
Add Images Add SmiliesPosting Help

Please check your spelling (press "Check Spelling" above)


Similar topics:More similar topics...
Stored/Parked Aircraft With Flaps Extended? posted Mon Nov 13 2006 04:59:56 by Warreng24
Questions About Operations With Engines Turned Off posted Fri Jun 24 2005 02:45:12 by BABUMUSHAI
747-300 With 3 Engines? posted Tue Mar 9 2004 10:47:43 by Bookin
Flight Over Oceans With 4 Engines? posted Sun Aug 24 2003 00:27:28 by Ajaaron
A Northwest DC9 With 9 Engines posted Sat Mar 1 2003 13:59:28 by BR715-A1-30
DC-10 With 2 Engines? posted Wed Jan 16 2002 00:15:49 by TechRep
Jetstream With Jet Engines? posted Tue Oct 23 2001 09:28:10 by AA777-200
RPM Of Jet Engines During Take-off posted Fri Jun 9 2006 18:00:26 by MerlinIIIB
Imagine The A318 With A321 Engines And Wings... posted Sun May 21 2006 20:54:37 by A342
Pre 1970's European Jet Engines posted Mon May 8 2006 18:21:41 by Baw2198

Sponsor Message:
Printer friendly format