flybyguy
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Parked Jet With Engines Spinning

Sun Jul 11, 2004 1:58 pm

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?
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JetMechMD80
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RE: Parked Jet With Engines Spinning

Sun Jul 11, 2004 2:16 pm

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]
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Fly2HMO
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RE: Parked Jet With Engines Spinning

Sun Jul 11, 2004 2:49 pm

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]
 
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Aaron747
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RE: Parked Jet With Engines Spinning

Sun Jul 11, 2004 3:51 pm

It's called windmilling. And yes, you can get the fan going rather easily by hand.
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aloges
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RE: Parked Jet With Engines Spinning

Sun Jul 11, 2004 8:02 pm

"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.
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QantasA332
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RE: Parked Jet With Engines Spinning

Sun Jul 11, 2004 9:08 pm

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,
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MD11Engineer
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RE: Parked Jet With Engines Spinning

Sun Jul 11, 2004 10:43 pm

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
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Fly2HMO
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RE: Parked Jet With Engines Spinning

Mon Jul 12, 2004 12:38 am

Isn't windmilling something you'd want to avoid since the oil pumps aren't running?
 
aloges
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RE: Parked Jet With Engines Spinning

Mon Jul 12, 2004 1:31 am

"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:

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MD11Engineer
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RE: Parked Jet With Engines Spinning

Mon Jul 12, 2004 2:58 am

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

Jan
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320tech
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RE: Parked Jet With Engines Spinning

Mon Jul 12, 2004 11:24 am

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.
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Sinlock
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RE: Parked Jet With Engines Spinning

Mon Jul 12, 2004 12:32 pm

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)
 
oly720man
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RE: Parked Jet With Engines Spinning

Mon Jul 12, 2004 6:55 pm

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
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modesto2
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RE: Parked Jet With Engines Spinning

Mon Jul 12, 2004 9:25 pm

I've spun one of JetBlue's A320 IAE V2527-A5 engines...easy, anyone can spin a fan. There's surprisingly very little resistance.
 
AUAE
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RE: Parked Jet With Engines Spinning

Mon Jul 12, 2004 9:39 pm

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
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WrenchBender
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RE: Parked Jet With Engines Spinning

Mon Jul 12, 2004 11:25 pm

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
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A/c train
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RE: Parked Jet With Engines Spinning

Tue Jul 13, 2004 2:54 am

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.
 
aogdesk
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RE: Parked Jet With Engines Spinning

Tue Jul 13, 2004 3:22 am

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.
 
MD11Engineer
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RE: Parked Jet With Engines Spinning

Tue Jul 13, 2004 3:31 am

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
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flybyguy
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RE: Parked Jet With Engines Spinning

Tue Jul 13, 2004 9:09 am

Thanks everyone for your detailed and informative answers  Smile/happy/getting dizzy
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A/c train
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RE: Parked Jet With Engines Spinning

Wed Jul 14, 2004 4:31 am

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.
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smcmac32msn
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RE: Parked Jet With Engines Spinning

Wed Jul 14, 2004 7:04 am

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.
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G4Doc2004
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RE: Parked Jet With Engines Spinning

Wed Jul 14, 2004 8:56 am

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.
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Aloha717200
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RE: Parked Jet With Engines Spinning

Thu Jul 15, 2004 5:53 am

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.

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