reversers and a/c speed
Joined: Sep 2010
Aviation Qualifications: ATPL
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From: by the seaside
Low speed ingestion.
Had to use emergency reverse at low speed at the old Gotenborg on a DC 9 -51 to stop us sliding off the cliff at the end.
Engines surged repeatedly - characterised by a very loud bang as the airflow re established itself and ignited the fuel in the back end.
Reduced the thrust until the banging stopped (emergency reverse - white knuckle reverse).
Fortunately we hit some ice with some braking action otherwise I wouldn't be writing this.
Borescope check reviewed no damage.
Reverse is much more effective at high speed and ineffective at low speed - so if in doubt I always pulled a little too much when initially setting it.
Once watched a skipper reverse off a stand in lhr - not a good idea because of FOD and the risk of standing the aircraft on it's tail.
Couldn't be done on a VC 10 because of the castoring nosewheel.
Had to use emergency reverse at low speed at the old Gotenborg on a DC 9 -51 to stop us sliding off the cliff at the end.
Engines surged repeatedly - characterised by a very loud bang as the airflow re established itself and ignited the fuel in the back end.
Reduced the thrust until the banging stopped (emergency reverse - white knuckle reverse).
Fortunately we hit some ice with some braking action otherwise I wouldn't be writing this.
Borescope check reviewed no damage.
Reverse is much more effective at high speed and ineffective at low speed - so if in doubt I always pulled a little too much when initially setting it.
Once watched a skipper reverse off a stand in lhr - not a good idea because of FOD and the risk of standing the aircraft on it's tail.
Couldn't be done on a VC 10 because of the castoring nosewheel.
Joined: Feb 2008
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From: Wingham NSW Australia
Using Reverse Thrust.
Whether it is the "hot stream" or the "cold stream" mass air flow being diverted to provide reverse thrust the effective speed retardation is achieved by redirecting the gas or mass air flow foward, ideally fully opposite to the forward flow direction, but generally at around 45 degrees forward. It has been mentioned that it is inlet drag which is the primary contributor to speed reduction. This is not supported by either Rolls Royce (The Jet Engine) or General Electric (The Aircraft Gas Turbine Engine and its operation), each of whom stress the reversal of gas or cold stream flow as being the means by which speed is reduced with no mention of the effects of inlet drag. Reverse thrust is very effective in "washing off" speed to a value where less brake energy is required and is usually fully cancelled by 60 Kts to preclude re-ingestion of gas flow or ingestion of FOD.
Last edited by Old Fella; 30th December 2010 at 09:27. Reason: Spelling correction
Joined: Mar 2002
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From: Florida
This is not supported by either Rolls Royce (The Jet Engine) or General Electric (The Aircraft Gas Turbine Engine and its operation), each of whom stress the reversal of gas or cold stream flow as being the means by which speed is reduced with no mention of the effects of inlet drag.
Joined: Dec 2003
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From: South of Watford
Blind Pew
Couldn't be done on a VC 10 because of the castoring nosewheel.
There was a page in the VC10 Flying Manual detailing all the reasons why it "should not" be done. I think the risk of using brakes while reversing was the major concern (Tail on ground, nose in air...means no tea, unless the cabin crew have climbing gear.)
I have been present when a 3 or 4 point turn was required near the end of the runway before departure, (many many moons ago), it worked.
Joined: Feb 2008
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From: Wingham NSW Australia
Inlet drag
Lomapaseo. I did not say that either RR or GE dispute the effects of inlet drag. What I did say was that neither mention it when explaining how thrust reversal assists in slowing the aircraft. I mention inlet drag only because it has been cited by another contributor to the thread as being the primary contributor to slowing the aircraft and that the contribution of reversal of the gas flow, or fan discharge, is relatively small. This I find difficult to accept.
Regards
Old Fella
Regards
Old Fella
Last edited by Old Fella; 31st December 2010 at 00:11. Reason: Clarification
Joined: Nov 2007
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From: United Kingdom
Whilst that Inlet Drag does contribute to overall decelleration it's only a small effect. Old Fella is right to find it difficult to accept that Inlet Drag is the prime contributor... because it isn't!
Mustafagander and Old Fella are both on the right track and were right to challenge SNS3Guppy.
TCF
Mustafagander and Old Fella are both on the right track and were right to challenge SNS3Guppy.
TCF
Last edited by TheChitterneFlyer; 31st December 2010 at 08:52.
Joined: Jan 2004
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From: Stratosphere
four engine jock:
Question: 'Can you use Reverse Thrust in flight on a B727?'
Answer: No.
Though you can reverse the aircraft on the ground, blow back from the gate, do three point turns, etc. Have done all these, though one must be careful if using the #2 engine, as it is very likely to compressor stall. Three point turning can place a lot of strain on the nose gear.
Question: 'Can you use Reverse Thrust in flight on a B727?'
Answer: No.
Though you can reverse the aircraft on the ground, blow back from the gate, do three point turns, etc. Have done all these, though one must be careful if using the #2 engine, as it is very likely to compressor stall. Three point turning can place a lot of strain on the nose gear.
Joined: Oct 2005
Posts: 3,218
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From: USA
Lomapaseo didn't dispute the effects of inlet drag.
The formula for net thrust is gross thrust minus inlet drag. Pure and simple; take away the thrust, and you're left with drag.
While reversing the airflow accounts for some retarding force, it's not much at all.
Many reversers don't divert the airflow forward at all, and it's not the diverted airflow that's accounting for the retarding force; it's the inlet drag.
Take away the thrust, all you have left is the drag. Very simple.
The formula for net thrust is gross thrust minus inlet drag. Pure and simple; take away the thrust, and you're left with drag.
While reversing the airflow accounts for some retarding force, it's not much at all.
Many reversers don't divert the airflow forward at all, and it's not the diverted airflow that's accounting for the retarding force; it's the inlet drag.
Take away the thrust, all you have left is the drag. Very simple.
Joined: Nov 2007
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From: United Kingdom
SNS3Guppy; I stand corrected and have edited my previous post.
However, on the continuing saga of what provides the decelleration force from a thrust reverser...
Please explain how you have managed to remove "thrust" from your formula. Indeed, if you were to turn-off the fuel supply, you'd produce drag from a windmilling engine; however, this is not the case. If you were to shut-down an engine in-flight there isn't a huge ammount of drag to overcome. Now do the same with a reverser deployed; yes, there would be an increase in drag, but not as much as there would be with the engine running.
If, during flight, you experience an inadvertant thrust reverser deployment (accomanied by loss of airspeed, buffeting/vibration), most (if not all) Emergency Checklists instruct you to shut-down the engine. Why do you think this is? In this example we've removed the "thrust" and regained control of the aeroplane. Your theory implies that the removal of the thrust would further increase the drag!
With regard to thrust reverser design... indeed, not all thrust reverser systems divert the thrust in a forward direction (usually perpendicular to the free stream air); however, the net effect is the same. As Mustafagander has said, this produces a "barn door" type of effect to the free-stream airflow and, as trimotor has correctly stated, is proportional to TAS.
Happy New Year
TCF
However, on the continuing saga of what provides the decelleration force from a thrust reverser...
Please explain how you have managed to remove "thrust" from your formula. Indeed, if you were to turn-off the fuel supply, you'd produce drag from a windmilling engine; however, this is not the case. If you were to shut-down an engine in-flight there isn't a huge ammount of drag to overcome. Now do the same with a reverser deployed; yes, there would be an increase in drag, but not as much as there would be with the engine running.
If, during flight, you experience an inadvertant thrust reverser deployment (accomanied by loss of airspeed, buffeting/vibration), most (if not all) Emergency Checklists instruct you to shut-down the engine. Why do you think this is? In this example we've removed the "thrust" and regained control of the aeroplane. Your theory implies that the removal of the thrust would further increase the drag!
With regard to thrust reverser design... indeed, not all thrust reverser systems divert the thrust in a forward direction (usually perpendicular to the free stream air); however, the net effect is the same. As Mustafagander has said, this produces a "barn door" type of effect to the free-stream airflow and, as trimotor has correctly stated, is proportional to TAS.
Happy New Year
TCF
Joined: Feb 2008
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From: Wingham NSW Australia
Thrust reversal
SNS3Guppy. If you believe that the contribution made by reversing the gas flow and/or fan discharge is such a minor one in the reduction of forward speed, why is it that so much work goes into the design of the reversal system, whether it be clam shell, bucket or blocker doors? Further, if simply removing the thrust would lead to the desired speed reduction, why are the engines accelerated to the relatively high thrust setting attained at Maximum Reverse? Also, it is common for the thrust to be redirected forward by as much as 45 degrees. Just look at the cascade vanes on a RB211 or JT3D, or the buckets on a aircraft so equipped.
Per Ardua ad Astraeus
Joined: Mar 2000
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From: UK
Surely this is simple? To change the direction of motion of a mass requires a force. Turn a jet's airflow back on itself and you require a rearward (decelerating) force. QED? Exactly the same as 'adverse intake momentum drag' for the now deceased Harrier and other S/VTOLS
How the reverser force changes with a/c forward speed someone else will have to explain
How the reverser force changes with a/c forward speed someone else will have to explain
Joined: Aug 1998
Aviation Qualifications: ATPL
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From: Ex-pat Aussie in the UK
As Mustafagander has said, this produces a "barn door" type of effect to the free-stream airflow and, as trimotor has correctly stated, is proportional to TAS.
What happens to those molecules after they have left the engine is irrelevant, as those molecules (being in a free gas stream) have no way of transmitting any force back to the aircraft. Your "barn door" is built of gas - not wood!
