Guessbb
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TurboFan Engine: Fan Operating In Reverse?

Thu Nov 15, 2007 7:24 pm

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!
 
KELPkid
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RE: TurboFan Engine: Fan Operating In Reverse?

Thu Nov 15, 2007 7:39 pm



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.
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tepidhalibut
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RE: TurboFan Engine: Fan Operating In Reverse?

Thu Nov 15, 2007 7:45 pm

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.
 
N231YE
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RE: TurboFan Engine: Fan Operating In Reverse?

Thu Nov 15, 2007 7:55 pm

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.
 
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HAWK21M
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RE: TurboFan Engine: Fan Operating In Reverse?

Thu Nov 15, 2007 8:29 pm



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
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BreninTW
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RE: TurboFan Engine: Fan Operating In Reverse?

Fri Nov 16, 2007 1:28 am

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.
 
avioniker
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RE: TurboFan Engine: Fan Operating In Reverse?

Fri Nov 16, 2007 2:24 am

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
 
pilotboi
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RE: TurboFan Engine: Fan Operating In Reverse?

Fri Nov 16, 2007 2:45 am

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.
 
tdscanuck
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RE: TurboFan Engine: Fan Operating In Reverse?

Fri Nov 16, 2007 2:49 am



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.
 
BAE146QT
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RE: TurboFan Engine: Fan Operating In Reverse?

Fri Nov 16, 2007 5:37 pm



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
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pilotboi
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RE: TurboFan Engine: Fan Operating In Reverse?

Fri Nov 16, 2007 5:58 pm



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.
 
BAE146QT
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RE: TurboFan Engine: Fan Operating In Reverse?

Fri Nov 16, 2007 6:03 pm



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.
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737tdi
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RE: TurboFan Engine: Fan Operating In Reverse?

Sat Nov 17, 2007 7:14 am

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
 
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HAWK21M
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RE: TurboFan Engine: Fan Operating In Reverse?

Sat Nov 17, 2007 1:46 pm



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

Which type.
regds
MEL
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FlyASAGuy2005
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RE: TurboFan Engine: Fan Operating In Reverse?

Sat Nov 17, 2007 3:36 pm

So how does reversers work on a turbo-prop? You hear this real LOUD wosh sound (many ATR flights).
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F14D4ever
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RE: TurboFan Engine: Fan Operating In Reverse?

Sat Nov 17, 2007 4:14 pm



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.
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Tristarsteve
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RE: TurboFan Engine: Fan Operating In Reverse?

Sat Nov 17, 2007 5:41 pm



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
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RE: TurboFan Engine: Fan Operating In Reverse?

Sat Nov 17, 2007 6:21 pm



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
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ex52tech
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RE: TurboFan Engine: Fan Operating In Reverse?

Sat Nov 17, 2007 6:33 pm



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.
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pilotboi
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RE: TurboFan Engine: Fan Operating In Reverse?

Sat Nov 17, 2007 6:43 pm



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
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RE: TurboFan Engine: Fan Operating In Reverse?

Sat Nov 17, 2007 6:54 pm



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
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RE: TurboFan Engine: Fan Operating In Reverse?

Sat Nov 17, 2007 7:21 pm



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.
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tdscanuck
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RE: TurboFan Engine: Fan Operating In Reverse?

Sat Nov 17, 2007 7:54 pm



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.
 
boeing767mech
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RE: TurboFan Engine: Fan Operating In Reverse?

Sat Nov 17, 2007 11:03 pm



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
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tdscanuck
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RE: TurboFan Engine: Fan Operating In Reverse?

Sat Nov 17, 2007 11:49 pm



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.
 
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jetmech
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RE: TurboFan Engine: Fan Operating In Reverse?

Sun Nov 18, 2007 12:23 am



Quoting 737tdi (Reply 12):

Interesting, I have never seen this being done before. It would certainly stop the engine windmilling in reverse however!

Quoting ZBBYLW (Reply 17):
Putting it on the actual jet part of the engine in addition to the bi-pass section. If so has this been tried?

On a modern turbofan engine, 75 to 80 % of the thrust is generated by the cold stream by-pass air, so the addition of clam-shells to reverse the core engine would only add a small amount of additional reverse thrust. Such a small gain would not be worth the additional weight and complexity.

Regards, JetMech
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boeing767mech
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RE: TurboFan Engine: Fan Operating In Reverse?

Sun Nov 18, 2007 1:15 am



Quoting Tdscanuck (Reply 24):
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

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

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

Photo © Frank C. Duarte Jr.



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


View Large View Medium

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.

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 saying c-5, And yes I know they don't use core reversors on the 747 anymore.

David
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tdscanuck
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RE: TurboFan Engine: Fan Operating In Reverse?

Sun Nov 18, 2007 3:55 am



Quoting Boeing767mech (Reply 26):

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 saying c-5

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 found a single one that had the reversers deployed at all...it's entirely possible there's a better one I missed that shows core reversers.

Tom.
 
avioniker
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RE: TurboFan Engine: Fan Operating In Reverse?

Sun Nov 18, 2007 4:42 am

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.
 Smile
Here's an old pic.
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KELPkid
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RE: TurboFan Engine: Fan Operating In Reverse?

Sun Nov 18, 2007 5:23 am



Quoting FlyASAGuy2005 (Reply 14):
So how does reversers work on a turbo-prop? You hear this real LOUD wosh sound (many ATR flights).

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 hear YV's Beech 1900's cycling the props 3 times into full beta before takeoff (as I did every day when I worked at LRU in college)...
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ex52tech
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RE: TurboFan Engine: Fan Operating In Reverse?

Sun Nov 18, 2007 4:22 pm



Quoting Avioniker (Reply 28):
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.

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 need arises.
"Saddest thing I ever witnessed....an airplane being scrapped"
 
avioniker
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RE: TurboFan Engine: Fan Operating In Reverse?

Sun Nov 18, 2007 6:03 pm



Quoting Ex52tech (Reply 30):
They want everything they can get to stop that airplane if the need arises.

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 if 911NA had the reversers on it when NASA bought it or not but it is also pretty much the same as it was when they got it from JAL, aside from the structural mods to support the piggyback mounts.)
You might be surprised to know that the shuttle doesn't weigh all that much compared to a full load of freight on that plane. Stopping isn't the problem as much as accelerating that big drag generator on top.

 Smile
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