Guessbb From Canada, joined Nov 2007, 1 posts, RR: 0 Posted (5 years 6 months 1 week 1 day 13 hours ago) and read 8087 times:
I am curious, as a friend and I are having a discussion as to why we have seen several turbofan equipped commercial aircraft on the tarmac either getting ready for boarding or just settled from landing, with the fan on the engine spinning counter clockwise, the opposite direction as to what I am traditionally used seeing to propel the plane forward.
Anyone care to explain? is it for reverse thrust, or is it because the engine is stalled so it freely moves in reverse?
KELPkid From United States of America, joined Nov 2005, 5930 posts, RR: 4 Reply 1, posted (5 years 6 months 1 week 1 day 12 hours ago) and read 8083 times:
Quoting Guessbb (Thread starter): Anyone care to explain? is it for reverse thrust, or is it because the engine is stalled so it freely moves in reverse?
Thanks for your time!
Well, Rolls-Royce engines turn the opposite way that P&W and GE engines do, I can tell you that much IIRC, Rollers turn clockwise as viewed from the front and the other two manufacturers turn counterclockwise...
Most likely, if it's during boarding, the fan blades are just windmilling (being blown by the wind) as the engines are shut down...if the wind is from the front of the plane, the fan blades will turn the "proper" way, however if it's a tail wind, they may just start turning backwards.
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
TepidHalibut From Iceland, joined Dec 2004, 206 posts, RR: 6 Reply 2, posted (5 years 6 months 1 week 1 day 12 hours ago) and read 8074 times:
Welcome to A.Net !
Not all engines spin the same way, so could just be a different engine from the ones you're used to.
If the engine isn't actually running, then the fan could just be windmilling. Bearings are often so good that even a minor breeze can result in reasonable rpms in either direction.
I don't believe any Turbofan engine actually reverses direction to reverse thrust. They (generally) just deflect the airflow to achieve thrust in the reverse direction : the fan and core still operate as normal.
I doubt it's a stalled engine :there would still be a significant airflow thru' the sore, driving the LP Turbine in the normal direction.
N231YE From , joined Dec 1969, posts, RR: Reply 3, posted (5 years 6 months 1 week 1 day 12 hours ago) and read 8067 times:
As other have mentioned, when the engine is not running, a slight breeze can turn the blades (windmill). Thus, the engine may appear to be operating (and in reverse) when its not.
HAWK21M From India, joined Jan 2001, 31201 posts, RR: 58 Reply 4, posted (5 years 6 months 1 week 1 day 12 hours ago) and read 8056 times:
Quoting Guessbb (Thread starter): with the fan on the engine spinning counter clockwise, the opposite direction as to what I am traditionally used seeing to propel the plane forward
Firstly the Engine Fan blades are Windmilling.
Secondly some operators have the Sequence of rotation Clockwise & others Counterclockwise.
A Tail wind on the Tarmac can cause this reverse to normal direction rotational effect too.
Brenintw From Taiwan, joined Jul 2006, 1446 posts, RR: 1 Reply 5, posted (5 years 6 months 1 week 1 day 7 hours ago) and read 7950 times:
Last week while I was sitting at HKG watching the apron movements, there was a 777-200 belonging to CX parked right outside the window. As I looked at it, the fan blades were moving counter-intuitively (i.e., the way they were turning would have blown air from the back to the front of the engine) ... I couldn't really figure why they were turning that way.
As I watched the pushback, I noticed that the blades slowed to a stop, and then started turning in the direction I expected them to (i.e., forcing air into the engine) ... and continued to speed up until they were just a blurred disk.
After some thought, I figured exactly what has been explained in this thread, that while the A/C was loading, the engines were simply windmilling, "in reverse," while they changed to the "correct" direction once the engine was started.
I'm tired of the A vs. B sniping. Neither make planes that shed wings randomly!
Avioniker From United States of America, joined Dec 2001, 1109 posts, RR: 11 Reply 6, posted (5 years 6 months 1 week 1 day 6 hours ago) and read 7931 times:
Pilotboi From United States of America, joined Sep 2007, 2366 posts, RR: 10 Reply 7, posted (5 years 6 months 1 week 1 day 5 hours ago) and read 7919 times:
What's even cooler is if they are spilling in reverse and they try to start the engines. First it slows down, then starts spinning the opposite (correct) way. Happened a bunch of times on the RJs here last month as we had a lot of days with crazy winds. A few times it had to be spinning around 20-25 RPM (yes I tried to time a rotation, just a rough estimate).
Tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 81 Reply 8, posted (5 years 6 months 1 week 1 day 5 hours ago) and read 7917 times:
Quoting TepidHalibut (Reply 2): I don't believe any Turbofan engine actually reverses direction to reverse thrust. They (generally) just deflect the airflow to achieve thrust in the reverse direction : the fan and core still operate as normal.
You actually can't operate a turbofan under reverse rotation...the airfoils aren't symmetric (leading edge to trailing edge) so the airflow would be a total disaster.
Plus, the fan is directly coupled to the low pressure compressor/turbine, so reversing the fan would turn the compressor into an expander, reversing flow through the whole combustion path, which would be very messy.
Pilotboi From United States of America, joined Sep 2007, 2366 posts, RR: 10 Reply 10, posted (5 years 6 months 1 week 14 hours ago) and read 7824 times:
Quoting BAe146QT (Reply 9): You're right of course, but it would demonstrate that what Guessbb saw wasn't reverse thrust...
737tdi From United States of America, joined Sep 2007, 465 posts, RR: 0 Reply 12, posted (5 years 6 months 1 week 1 hour ago) and read 7716 times:
As for the windmilling, if the wind is blowing too hard from the back most operators have a provision for starting the engine with the thrust reversers deployed. This helps prevent shearing the starter due to excessive loads, to stop the fan from windmilling backwards. The start sequence goes like this. Prior to engaging the start switch the reverser is deployed, then engage start switch, once N2 rotation is observed the reverser is stowed. I have had to do this a few times. F.Y.I.
FlyASAGuy2005 From United States of America, joined Sep 2007, 6515 posts, RR: 11 Reply 14, posted (5 years 6 months 6 days 16 hours ago) and read 7660 times:
So how does reversers work on a turbo-prop? You hear this real LOUD wosh sound (many ATR flights).
CAM2:"Lightning coming out of that one." CAM1: "What?"
F14D4ever From United States of America, joined May 2005, 319 posts, RR: 4 Reply 15, posted (5 years 6 months 6 days 16 hours ago) and read 7649 times:
Quoting 737tdi (Reply 12): ... most operators have a provision for starting the engine with the thrust reversers deployed ...
Very interesting (to a desk jockey like me).
Thanks for posting that.
TristarSteve From Sweden, joined Nov 2005, 3690 posts, RR: 34 Reply 16, posted (5 years 6 months 6 days 14 hours ago) and read 7625 times:
Quoting FlyASAGuy2005 (Reply 14): So how does reversers work on a turbo-prop? You hear this real LOUD wosh sound (many ATR flights).
The prop changes pitch so the airflow is reversed. Very effective.
Quoting 737tdi (Reply 12): most operators have a provision for starting the engine with the thrust reversers deployed.
What aircraft are you on? I have never seen it happen. I start up A320 and B777/Trent regularly in very strong tail winds, they start Ok, except the smoke out the back on light up flies over ne on the headset!!
ZBBYLW From Canada, joined Nov 2006, 1922 posts, RR: 7 Reply 17, posted (5 years 6 months 6 days 14 hours ago) and read 7616 times:
Quoting FlyASAGuy2005 (Reply 14): So how does reversers work on a turbo-prop? You hear this real LOUD wosh sound (many ATR flights).
Basically what happens here is this only happens on a constant speed prop. The props go from fine even finner to a point in which they have a negative pitch. Nothing changes in the actually turbine part of the engine. The props just go to a negative (reverse) pitch where the trust now becomes a forward vector.
After looking at your diagram I notice that it is just the bi-pass air that is being "revered". If you look at old turbo-jets (JT8D) it is a clam-shell style reverser because, well there is no bi-pass air. Has anyone thought of putting a clam shell on a turbo-fan engine. Putting it on the actual jet part of the engine in addition to the bi-pass section. If so has this been tried?
Ex52tech From United States of America, joined Dec 2006, 559 posts, RR: 1 Reply 18, posted (5 years 6 months 6 days 14 hours ago) and read 7612 times:
Quoting 737tdi (Reply 12): This helps prevent shearing the starter due to excessive loads, to stop the fan from windmilling backwards.
More on the lines of helping to prevent a hot start, I have started them up all the way to idle with the reverser deployed. Starters are pretty tough.
Some engines can have the starter crash engaged, around 20% N2, in the event of a high power compressor stall and corresponding over-temp that usually follows. The starter can be engaged to get some airflow moving to assist in cooling the engine down. The engine would be winding down because the operator had placed the fuel lever in the cutoff position at the time of the stall, trust me, if you have a high power stall, 9 out of 10 people will shut the engine down, and it's spelled out in the check list.
Quoting TristarSteve (Reply 16): What aircraft are you on? I have never seen it happen.
There are provisions for a reverser deployed start on most aircraft, it is not a normal practice, but it can be done. JT-9's had a penchant for hot starts with high tail winds, epically if the EVC is tired and doesn't allow the vanes to brake away from the full closed position until the N2 rpm is above 40%.
Quoting KELPkid (Reply 1): Well, Rolls-Royce engines turn the opposite way that P&W and GE engines do,
And....that would be backwards, but RR won't admit it.
"Saddest thing I ever witnessed....an airplane being scrapped"
Pilotboi From United States of America, joined Sep 2007, 2366 posts, RR: 10 Reply 19, posted (5 years 6 months 6 days 13 hours ago) and read 7605 times:
Quoting ZBBYLW (Reply 17): If you look at old turbo-jets (JT8D) it is a clam-shell style reverser because, well there is no bi-pass air.
Actually, the JT8D is a bypass engine, just a low one at 0.96:1. But your question still stands and would apply to hi-bypass engines. So are there any hi-bypass engines that use clamshells? Well I did a quick search and could not find one. I think they are generally impractical. I mean think if a 777 had a clamshell, it'd be HUGE! Anyway, if anyone finds one, let us know.
TristarSteve From Sweden, joined Nov 2005, 3690 posts, RR: 34 Reply 20, posted (5 years 6 months 6 days 13 hours ago) and read 7601 times:
Quoting ZBBYLW (Reply 17): Has anyone thought of putting a clam shell on a turbo-fan engine. Putting it on the actual jet part of the engine in addition to the bi-pass section. If so has this been tried?
The RB211-22B was designed with a clamshell type hot stream reverser. If you look at L1011 pictures from 1973 you can see the fittings. The air motor that drives the reverser still has a drive outlet for it. Because of this the air motor is too strong for the cold stream reverser, and if you get a hiccup in the system, the reverser motor will twist the teleflex drive cables apart. A nightmare for maint in the early days.
When the hot stream reverser was removed it left space for a redesigned afterbody on the engine and all engines were modified around 1978 as it provided significant fuel savings.
I never saw a hot stream reverser used, don't know if they ever entered service.
Ex52tech From United States of America, joined Dec 2006, 559 posts, RR: 1 Reply 21, posted (5 years 6 months 6 days 13 hours ago) and read 7590 times:
Quoting TristarSteve (Reply 20): The RB211-22B was designed with a clamshell type hot stream reverser.
We took them off of the DC-10-30's, -40's, and the early 741/742 JT-9's.......what a nightmare. Even the tail pipes that were deactivated and bolted shut were trouble.
"Saddest thing I ever witnessed....an airplane being scrapped"
Tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 81 Reply 22, posted (5 years 6 months 6 days 12 hours ago) and read 7582 times:
Quoting ZBBYLW (Reply 17): After looking at your diagram I notice that it is just the bi-pass air that is being "revered". If you look at old turbo-jets (JT8D) it is a clam-shell style reverser because, well there is no bi-pass air. Has anyone thought of putting a clam shell on a turbo-fan engine. Putting it on the actual jet part of the engine in addition to the bi-pass section. If so has this been tried?
People have thought of it (L1011, DC-10, etc. as described above) but it's not a very good idea. Clamshells for the cold stream flow on a high-bypass engine would be huge and having a mixed clamshell for the hotstream and cascade for the cold is a maintenance disaster.
The C-17 has full reversing of both hot and cold streams, but is uses cascade reversers for both.
Boeing767mech From United States of America, joined Dec 2000, 992 posts, RR: 3 Reply 23, posted (5 years 6 months 6 days 9 hours ago) and read 7550 times:
Quoting Tdscanuck (Reply 22): The C-17 has full reversing of both hot and cold streams, but is uses cascade reversers for both.
Does the C-5 A/B still use hot exhaust reversors, I know the C-5M re-engined with CF-6-80's have bybass duct resevsors only.
Tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 81 Reply 24, posted (5 years 6 months 6 days 8 hours ago) and read 7541 times:
Quoting Boeing767mech (Reply 23): Quoting Tdscanuck (Reply 22):
The C-17 has full reversing of both hot and cold streams, but is uses cascade reversers for both.
Does the C-5 A/B still use hot exhaust reversors
As far as I can tell, no. This picture shows what the bypass reversers look like deployed:
It doesn't look like there is any hot-section reversing in this photo, but it's so dark that it's tough to tell. As you can see, the sleeve line for the cold reverser is just aft of the circumfrential painted line on the fan case that you can see really well here:
As you can see, the hot section ends way aft of the split line in the hot section cowl. The only thing in the nacelle aft of the back of the engine is the very short bare metal nozzle, which I'm pretty sure is too small and thin to contain cascades.
Tom.
25 JetMech: Interesting, I have never seen this being done before. It would certainly stop the engine windmilling in reverse however! On a modern turbofan engine
26 Boeing767mech: From this overwhelming response of pictures I stand corrected, 30 hours without sleep will do it to you, I must have been thinking about the 747 and
27 Tdscanuck: You could still be right...I'm going off supposition based off some somewhat unclear pictures. It took ~400 photos in the a.net database before I fou
28 Avioniker: See if you can find a pic of N905NA landing. Last I saw it still had the core reverser and made pretty colors when stopping. Here's an old pic. http:/
29 KELPkid: The props go into beta, where the prop blades actually have a negative angle of attack. On the P&W PT6, it sounds like a loud growl...I can still hea
30 Ex52tech: I would be willing to bet that the turbine reversers are still operational on 905. They want everything they can get to stop that airplane if the nee
31 Avioniker: Actually it's much more a matter of the government not wanting to spend the money to upgrade the plane from the AA original build. (I can't remember
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IIRC, Rollers turn clockwise as viewed from the front and the other two manufacturers turn counterclockwise...
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