Sponsor Message:
Aviation Technical / Operations Forum
My Starred Topics | Profile | New Topic | Forum Index | Help | Search 
TurboFan Engine: Fan Operating In Reverse?  
User currently offlineGuessbb From Canada, joined Nov 2007, 1 posts, RR: 0
Posted (7 years 2 weeks 4 hours ago) and read 9818 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?

Thanks for your time!

31 replies: All unread, showing first 25:
 
User currently offlineKELPkid From United States of America, joined Nov 2005, 6428 posts, RR: 3
Reply 1, posted (7 years 2 weeks 4 hours ago) and read 9818 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  Smile 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 :-)
User currently offlineTepidHalibut From Iceland, joined Dec 2004, 210 posts, RR: 6
Reply 2, posted (7 years 2 weeks 4 hours ago) and read 9807 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.

Hope this helps.


User currently offlineN231YE From , joined Dec 1969, posts, RR:
Reply 3, posted (7 years 2 weeks 3 hours ago) and read 9799 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.

User currently offlineHAWK21M From India, joined Jan 2001, 31702 posts, RR: 56
Reply 4, posted (7 years 2 weeks 3 hours ago) and read 9788 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.

regds
MEL



Think of the brighter side!
User currently onlineBrenintw From Taiwan, joined Jul 2006, 1710 posts, RR: 1
Reply 5, posted (7 years 1 week 6 days 22 hours ago) and read 9681 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!
User currently offlineAvioniker From United States of America, joined Dec 2001, 1109 posts, RR: 11
Reply 6, posted (7 years 1 week 6 days 21 hours ago) and read 9662 times:

This might help
 
http://www.google.com/search?hl=en&q=how+a+thrust+reverser+works

Simple T/R Illustration


[Edited 2007-11-15 18:28:14]


One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533
User currently offlinePilotboi From United States of America, joined Sep 2007, 2366 posts, RR: 9
Reply 7, posted (7 years 1 week 6 days 21 hours ago) and read 9650 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).

Quoting Avioniker (Reply 6):
This might help

That doesn't really explain why fans spin backwards. When thrust reversers are deployed, fans still spin in the same direction as normal operation.


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 8, posted (7 years 1 week 6 days 20 hours ago) and read 9648 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.

Tom.


User currently offlineBAe146QT From United Kingdom, joined Sep 2006, 996 posts, RR: 0
Reply 9, posted (7 years 1 week 6 days 6 hours ago) and read 9559 times:



Quoting Pilotboi (Reply 7):
That doesn't really explain why fans spin backwards.

You're right of course, but it would demonstrate that what Guessbb saw wasn't reverse thrust...  Wink



Todos mis dominós son totalmente pegajosos
User currently offlinePilotboi From United States of America, joined Sep 2007, 2366 posts, RR: 9
Reply 10, posted (7 years 1 week 6 days 5 hours ago) and read 9555 times:



Quoting BAe146QT (Reply 9):
You're right of course, but it would demonstrate that what Guessbb saw wasn't reverse thrust...

Ah, so that's what he was trying to show. Okay.


User currently offlineBAe146QT From United Kingdom, joined Sep 2006, 996 posts, RR: 0
Reply 11, posted (7 years 1 week 6 days 5 hours ago) and read 9553 times:



Quoting Pilotboi (Reply 10):
Ah, so that's what he was trying to show. Okay.

Well I think so. That's the way I read it anyway.



Todos mis dominós son totalmente pegajosos
User currently offline737tdi From United States of America, joined Sep 2007, 923 posts, RR: 2
Reply 12, posted (7 years 1 week 5 days 16 hours ago) and read 9447 times:
Support Airliners.net - become a First Class Member!

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.

737tdi


User currently offlineHAWK21M From India, joined Jan 2001, 31702 posts, RR: 56
Reply 13, posted (7 years 1 week 5 days 10 hours ago) and read 9412 times:



Quoting 737tdi (Reply 12):
I have had to do this a few times. F.Y.I.

Which type.
regds
MEL



Think of the brighter side!
User currently offlineFlyASAGuy2005 From United States of America, joined Sep 2007, 7004 posts, RR: 7
Reply 14, posted (7 years 1 week 5 days 8 hours ago) and read 9391 times:

So how does reversers work on a turbo-prop? You hear this real LOUD wosh sound (many ATR flights).


What gets measured gets done.
User currently offlineF14D4ever From United States of America, joined May 2005, 319 posts, RR: 4
Reply 15, posted (7 years 1 week 5 days 7 hours ago) and read 9380 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.



"He is risen, as He said."
User currently offlineTristarSteve From Sweden, joined Nov 2005, 4052 posts, RR: 33
Reply 16, posted (7 years 1 week 5 days 6 hours ago) and read 9356 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!!


User currently offlineZBBYLW From Canada, joined Nov 2006, 1991 posts, RR: 6
Reply 17, posted (7 years 1 week 5 days 5 hours ago) and read 9347 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.

Quoting Avioniker (Reply 6):
This might help

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?

Cheers,

Chris



Keep the shinny side up!
User currently offlineEx52tech From United States of America, joined Dec 2006, 559 posts, RR: 1
Reply 18, posted (7 years 1 week 5 days 5 hours ago) and read 9343 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.
 Big grin



"Saddest thing I ever witnessed....an airplane being scrapped"
User currently offlinePilotboi From United States of America, joined Sep 2007, 2366 posts, RR: 9
Reply 19, posted (7 years 1 week 5 days 5 hours ago) and read 9336 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.


User currently offlineTristarSteve From Sweden, joined Nov 2005, 4052 posts, RR: 33
Reply 20, posted (7 years 1 week 5 days 4 hours ago) and read 9332 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.


User currently offlineEx52tech From United States of America, joined Dec 2006, 559 posts, RR: 1
Reply 21, posted (7 years 1 week 5 days 4 hours ago) and read 9321 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"
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 22, posted (7 years 1 week 5 days 3 hours ago) and read 9313 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.

Tom.


User currently offlineBoeing767mech From United States of America, joined Dec 2000, 1030 posts, RR: 3
Reply 23, posted (7 years 1 week 5 days ago) and read 9281 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.

David



Never under-estimate the predictably of stupidty
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 24, posted (7 years 1 week 4 days 23 hours ago) and read 9272 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:

View Large View Medium
Click here for bigger photo!

Photo © Den Pascoe


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:

View Large View Medium
Click here for bigger photo!

Photo © Kenneth C. Iwelumo



There is a very similar looking split line aft of the circumfrential line on the hot section cowl that's visible here:

View Large View Medium
Click here for bigger photo!

Photo © Frank C. Duarte Jr.



However, the clincher (for me) is this shot:

View Large View Medium
Click here for bigger photo!

Photo © Pete Hawk


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 Post contains links and images 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 Post contains images 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
Top Of Page
Forum Index

Reply To This Topic TurboFan Engine: Fan Operating In Reverse?
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...
Fan Blades In #2 Engine Of 3 Engine A/c? posted Sun Aug 8 2004 00:15:10 by WakeTurbulence
GE CF6 80E1 Engine Fan Specs posted Fri May 28 2004 21:31:00 by BWIA330
"One-Piece" Jet Engine Fan? posted Fri Dec 14 2001 21:08:06 by Mr Spaceman
F-28's Operating In US, Not Stage 3 posted Mon Jul 16 2001 09:01:47 by B737-112
Sparks In Engine On Reverse posted Thu Sep 14 2006 11:11:58 by RyDawg82
In Theory, Could A Jet Engine Do This? posted Tue Jun 26 2007 04:13:42 by TazzrassinIDA
Engine Drag In Engine Failure posted Wed Jun 20 2007 11:26:08 by A320ajm
Operating A Turboprop Engine posted Sun May 13 2007 06:39:47 by T56A15
Next Gen NB Engines : Contra Rotating Turbofan Fan posted Fri Apr 27 2007 13:25:59 by Keesje
In-flight Engine Fires-Up Or Down? posted Thu Jan 18 2007 22:47:37 by Foxbravo03
GE CF6 80E1 Engine Fan Specs posted Fri May 28 2004 21:31:00 by BWIA330
"One-Piece" Jet Engine Fan? posted Fri Dec 14 2001 21:08:06 by Mr Spaceman
F-28's Operating In US, Not Stage 3 posted Mon Jul 16 2001 09:01:47 by B737-112
Sparks In Engine On Reverse posted Thu Sep 14 2006 11:11:58 by RyDawg82
Engine MRO And Its Development In Todays Market posted Fri May 16 2008 02:40:35 by NicoEDDF
Engine In The Tail End Of A 747? posted Mon May 5 2008 00:37:58 by 1821
Why No Reverse Thrust On Landing In ATL? posted Fri Feb 8 2008 17:12:26 by BR715-A1-30

Sponsor Message:
Printer friendly format