Flybyguy From United States of America, joined Jun 2004, 1809 posts, RR: 1 Posted (11 years 1 month 2 weeks 5 days 8 hours ago) and read 9606 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
JetMechMD80 From United States of America, joined Jul 2004, 380 posts, RR: 6
Reply 1, posted (11 years 1 month 2 weeks 5 days 7 hours ago) and read 9570 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.
Aloges From Germany, joined Jan 2006, 9082 posts, RR: 41
Reply 4, posted (11 years 1 month 2 weeks 5 days 2 hours ago) and read 9358 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.
QantasA332 From Australia, joined Dec 2003, 1500 posts, RR: 23
Reply 5, posted (11 years 1 month 2 weeks 5 days ago) and read 9312 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...
320tech From Turks and Caicos Islands, joined May 2004, 491 posts, RR: 5
Reply 10, posted (11 years 1 month 2 weeks 4 days 10 hours ago) and read 8839 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) .
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.
Sinlock From United States of America, joined Dec 2000, 1689 posts, RR: 2
Reply 11, posted (11 years 1 month 2 weeks 4 days 9 hours ago) and read 8813 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)
Oly720man From United Kingdom, joined May 2004, 6998 posts, RR: 11
Reply 12, posted (11 years 1 month 2 weeks 4 days 3 hours ago) and read 8726 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.
Auae From United States of America, joined Apr 2004, 296 posts, RR: 3
Reply 14, posted (11 years 1 month 2 weeks 4 days ago) and read 8672 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!
Air transport is just a glorified bus operation. -Michael O'Leary, Ryanair's chief executive
Aogdesk From United States of America, joined Jun 2004, 935 posts, RR: 3
Reply 17, posted (11 years 1 month 2 weeks 3 days 18 hours ago) and read 8554 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.
MD11Engineer From Germany, joined Oct 2003, 14616 posts, RR: 62
Reply 18, posted (11 years 1 month 2 weeks 3 days 18 hours ago) and read 8546 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.
A/c train From United Kingdom, joined Jun 2001, 501 posts, RR: 3
Reply 20, posted (11 years 1 month 2 weeks 2 days 17 hours ago) and read 8289 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.
G4doc2004 From United States of America, joined Feb 2004, 123 posts, RR: 0
Reply 22, posted (11 years 1 month 2 weeks 2 days 13 hours ago) and read 8226 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
Aloha717200 From United States of America, joined Aug 2003, 4540 posts, RR: 13
Reply 23, posted (11 years 1 month 2 weeks 1 day 16 hours ago) and read 8116 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.