Maximum possible thrust-setting during reverse?
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"That must be Question of the year?"
This question came up during discussion of the RB199 engine performance with students at the university.
To give my friend, the responsible professor some hints, I have raised this question for him.
This question came up during discussion of the RB199 engine performance with students at the university.
To give my friend, the responsible professor some hints, I have raised this question for him.
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This question came up during discussion of the RB199 engine performance with students at the university
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Yes, of course, it was a professor of Aeronautics (and a former pilot).
The students are not taught upon some assumptions, but it happens that during the lessons some questions are coming up, which cannot be correctly answered without having the sufficient knowledge and background.
It is also impossible to have the relevant informations about military fighter aircraft handy at the university.
Information about the RB199 has not been taught there, but some discussions have been made about engine performance in comparison with other type of engines..
You can imagine that students are also interested in aeronautical details which are not all open to the public, but even those should be answered (if) correctly.
As my background is based on fixed- and rotary wing but not on high performance fighter aircraft, I thought to ask the specialists at this forum to get the answer.
Thank you for your respose.
The students are not taught upon some assumptions, but it happens that during the lessons some questions are coming up, which cannot be correctly answered without having the sufficient knowledge and background.
It is also impossible to have the relevant informations about military fighter aircraft handy at the university.
Information about the RB199 has not been taught there, but some discussions have been made about engine performance in comparison with other type of engines..
You can imagine that students are also interested in aeronautical details which are not all open to the public, but even those should be answered (if) correctly.
As my background is based on fixed- and rotary wing but not on high performance fighter aircraft, I thought to ask the specialists at this forum to get the answer.
Thank you for your respose.
Last edited by flighttest-engineer; 11th Aug 2009 at 13:51.
Bo Nalls, is there any maximum time period for which max dry thrust may be used with the reversers deployed?
Any problem with hot exhaust gas reingestion at low speed? Is there any issue regarding yaw instability if asymmetric reverse is used or if only 1 reverser deploys? Any corrective input to the nosewheel steering system, for example?
In large aircraft, full reverse thrust should be cancelled at around 50 KIAS (except in emergency) to avoid reingestion and possible FOD damage.
The original question was clearly reasonable; a pity that it received so many stupid comments in reply.
Any problem with hot exhaust gas reingestion at low speed? Is there any issue regarding yaw instability if asymmetric reverse is used or if only 1 reverser deploys? Any corrective input to the nosewheel steering system, for example?
In large aircraft, full reverse thrust should be cancelled at around 50 KIAS (except in emergency) to avoid reingestion and possible FOD damage.
The original question was clearly reasonable; a pity that it received so many stupid comments in reply.
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The duration of Thrust reverse deployment is indeed dependant on forward speed and power settings not any time limits. Idle TR is often used on long descending slope taxiways to save cooking the brake units. I know early Tornado had a re-ingest warning but not sure if it was continued in later life.
There should be no possibility of assymetric deployment as if the either engines buckets don't deploy within 0.25 seconds and be fully deployed within 1.25, they both cancel and illuminate a CWP warning. The system can be overidden to restore braking but obviously the Driver Airframe would then be ready and waiting for any yaw. This was especially important if the pilot had pre-selected Lift Dump before landing. This system deployed the TR and the spoilers, and also trimmed the taileron fully nose down as soon as the WOW switches made.
There used to be a problem at high speed of the venturi effect sucking the buckets off the stowed microswich which would illuminate a thrust reverse deployed warning in the cockpit; which according to one test pilot I spoke to concentrated his mind somewhat during a low and fast Terrain Following sortie. It was cured by increasing the 'throw' on the microswitch and pre loading the drive system from the TR motor to the buckets.
All the above is based on knowledge dragged from the depths of my memory and may have mellowed with time!
There should be no possibility of assymetric deployment as if the either engines buckets don't deploy within 0.25 seconds and be fully deployed within 1.25, they both cancel and illuminate a CWP warning. The system can be overidden to restore braking but obviously the Driver Airframe would then be ready and waiting for any yaw. This was especially important if the pilot had pre-selected Lift Dump before landing. This system deployed the TR and the spoilers, and also trimmed the taileron fully nose down as soon as the WOW switches made.
There used to be a problem at high speed of the venturi effect sucking the buckets off the stowed microswich which would illuminate a thrust reverse deployed warning in the cockpit; which according to one test pilot I spoke to concentrated his mind somewhat during a low and fast Terrain Following sortie. It was cured by increasing the 'throw' on the microswitch and pre loading the drive system from the TR motor to the buckets.
All the above is based on knowledge dragged from the depths of my memory and may have mellowed with time!
Last edited by country calls; 11th Aug 2009 at 12:34. Reason: Spelling not to scale
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Beagle
No max time period is quoted in the RTS. Normal TR can be used from 165kts down to re-ingest warning (approx 60kts), but in an emergency (high speed landing due to swept/flapless) TR can be engaged at idle power at 200kts, steadily increasing thrust until max dry is reached at 165kts.
Yes. TR re-ingest audio sounds at approx 60kts as a warning - it is upto the driver to cancel TR at this point! TR could however be used to bring the ac to a halt, if on slippery surfaces for example, but with an increased risk of FOD'ing the engines. As mentioned earlier, idle TR is sometimes used during taxy to avoid too much wheelbraking.
As mentioned above, the automatic system should not allow only one TR to deploy but there is an override function. When override is engaged the safety circuits are by-passed to permit the use of a single TR. Asymetric TR does come with limitations, most notably a max 10kt x-wind. However, other speed/throttle handling limits can be applied dependent upon whether the ac has asymmetric underwing stores, fuel in the uunderwing tanks or if the runway is flooded, wet or dry. Despite the engines being close to the centreline all of the above limits are there for reasons of lateral instability. The ac has a semi-sensible nosewheel steering/NSAS system though and this is normally used to counter any yaw during the slowdown, if all the above limits are adhered to. If not, then the ac can quickly bite, as has been evidenced historically.
NSAS= nosewheel steering augmentation system. NSAS takes inputs from the rudder pedal position and the yaw channel of the CSAS (fly by wire computer) and modifies the steering commands to counter any yaw during take-off/landing.
Country Calls
Close with your figures. 0.5 secs & 2 secs respectivly. And no auto trimming after touchdown.
is there any maximum time period for which max dry thrust may be used with the reversers deployed?
Any problem with hot exhaust gas reingestion at low speed?
Is there any issue regarding yaw instability if asymmetric reverse is used or if only 1 reverser deploys? Any corrective input to the nosewheel steering system, for example?
NSAS= nosewheel steering augmentation system. NSAS takes inputs from the rudder pedal position and the yaw channel of the CSAS (fly by wire computer) and modifies the steering commands to counter any yaw during take-off/landing.
Country Calls
There should be no possibility of assymetric deployment as if the either engines buckets don't deploy within 0.25 seconds and be fully deployed within 1.25, they both cancel and illuminate a CWP warning.
Last edited by Bo Nalls; 11th Aug 2009 at 14:12.
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BEagle, Contry Calls, Bo Nalls
Thanks for the intersting answers to my question.
I am familiar with the B757/B757/B777 thrust reversers, but there is always something intersting to learn about the different systems.
To me, real "Insiders" which make comments to "non stupid questions" do not give stupid answers.
Thank you all.
Thanks for the intersting answers to my question.
I am familiar with the B757/B757/B777 thrust reversers, but there is always something intersting to learn about the different systems.
To me, real "Insiders" which make comments to "non stupid questions" do not give stupid answers.
Thank you all.
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History of the Tornado Thrust reverser.
During early development, the TR system could be pre-armed in the air and power slammed to max at (or, if you were clever, just before) touch down. The resulting decel was impressive, (even for an ex-naval pilot!) Strakes had been designed into the upper buckets to avoid attachment to the fin of the forward gas flow, hopefully to avoid asymmetric attachment in crosswinds, which had already been seen in an early Viggen prang.
But random violent yawing at max RV was experienced and eventually, Tim Ferguson in prototype PO3, landing on a wet runway at Warton, lost it directionally, ran off the runway, and wrote the gear off.
MBB was responsible for the reverser and went back to the wind tunnels with MTU, and looked at it again. They decided to take the strakes OUT of the top buckets and allow attachment to the fin. HOWEVER, they failed, during the subsequent retesting, to note there was now a nasty nose-down pitching moment brought into play with selection of TR. Pitching moment had originally been controlled by a small lip at the exit ends of the top buckets and this had not been modified when the flow was changed with said strake removal. We found we were getting v high nosewheel loads with prearmed landings. By this stage in the development prog nobody was prepared to go back to play with the bucket lip geometry further and the powers that be said we will get round the problem by only allowing prearming if the touch down attitudes are low, so that there wasn't a large nose up attitude through which high pitch rates could be generated. I think the Service interpreted this as "No pre-arming at approach alpha's greater than x", where x was lower than the normal approach recommendation. So pre-arming TR was effectively emmasculated and one of the most impressive features of the a/c was buggered. At Warton Flight Ops we tried v hard to get a reprieve but the project was still politically fragile and our leaders didn't want to risk the effects of yet more development cost. It remains my saddest memory of the development programme.
So--normal op was max dry thrust at touch down, or after the said changes, after derotation, left on until the re-ingestion audio sounded at 50 knots. (Tests with chalk chips on the runway had showed reingestion at 40 knots)
After Tim's prang with PO3, and in addition to the removal of the upper bucket strakes, a feed was incorporated from the yaw stab sensor directly into the nosewheel steering system to kill any directional instability and it was then virtually poosible to keep you feet off the pedals during the roll-out!
But random violent yawing at max RV was experienced and eventually, Tim Ferguson in prototype PO3, landing on a wet runway at Warton, lost it directionally, ran off the runway, and wrote the gear off.
MBB was responsible for the reverser and went back to the wind tunnels with MTU, and looked at it again. They decided to take the strakes OUT of the top buckets and allow attachment to the fin. HOWEVER, they failed, during the subsequent retesting, to note there was now a nasty nose-down pitching moment brought into play with selection of TR. Pitching moment had originally been controlled by a small lip at the exit ends of the top buckets and this had not been modified when the flow was changed with said strake removal. We found we were getting v high nosewheel loads with prearmed landings. By this stage in the development prog nobody was prepared to go back to play with the bucket lip geometry further and the powers that be said we will get round the problem by only allowing prearming if the touch down attitudes are low, so that there wasn't a large nose up attitude through which high pitch rates could be generated. I think the Service interpreted this as "No pre-arming at approach alpha's greater than x", where x was lower than the normal approach recommendation. So pre-arming TR was effectively emmasculated and one of the most impressive features of the a/c was buggered. At Warton Flight Ops we tried v hard to get a reprieve but the project was still politically fragile and our leaders didn't want to risk the effects of yet more development cost. It remains my saddest memory of the development programme.
So--normal op was max dry thrust at touch down, or after the said changes, after derotation, left on until the re-ingestion audio sounded at 50 knots. (Tests with chalk chips on the runway had showed reingestion at 40 knots)
After Tim's prang with PO3, and in addition to the removal of the upper bucket strakes, a feed was incorporated from the yaw stab sensor directly into the nosewheel steering system to kill any directional instability and it was then virtually poosible to keep you feet off the pedals during the roll-out!
I recall seeing a most impressive Viggen demo at Paris in 1969. It landed and seemed to stop in roughly its own length, then roared off again in full a/b.
The Jaguar displays which followed seemed to lack something....
I always wondered why pre-armed TR landings were binned in the Tornado as it had been an original design requirement, if I'm not mistaken, Dave?
Changing the SOP is often a good idea. In the VC10, the old idea was 'slow introduction of full reverse' after an asymmetric landing. All this did was to ensure that the full reverse effect bit you in that gap between the rudder losing effectiveness and the nosewheel steering being usable - so often a routine asymmetric landing could become a bit of a waltz at around 100 KIAS.
I insisted on full reverse being applied rapidly as soon as the nosewheel was on the ground - spoilers and idle reverse as soon as the aircraft had touched down, then a slam to full reverse once the nosewheel was on and the flying pilot called for it. This meant that any initial asymmetric yawing moment could be countered far more easily, particularly as the fin still provided some 'directional' (OK, purists, I know it's actually part of the lateral stability quartic) stability. In addition, the greater decelerative force took the aircraft through the undesirable rudder / nosewheel steering speed regime much more quickly, so any divergence was more easily countered once the nosewheel steering could be used. This was particularly so during double asymmetric landings.
The Jaguar displays which followed seemed to lack something....
I always wondered why pre-armed TR landings were binned in the Tornado as it had been an original design requirement, if I'm not mistaken, Dave?
Changing the SOP is often a good idea. In the VC10, the old idea was 'slow introduction of full reverse' after an asymmetric landing. All this did was to ensure that the full reverse effect bit you in that gap between the rudder losing effectiveness and the nosewheel steering being usable - so often a routine asymmetric landing could become a bit of a waltz at around 100 KIAS.
I insisted on full reverse being applied rapidly as soon as the nosewheel was on the ground - spoilers and idle reverse as soon as the aircraft had touched down, then a slam to full reverse once the nosewheel was on and the flying pilot called for it. This meant that any initial asymmetric yawing moment could be countered far more easily, particularly as the fin still provided some 'directional' (OK, purists, I know it's actually part of the lateral stability quartic) stability. In addition, the greater decelerative force took the aircraft through the undesirable rudder / nosewheel steering speed regime much more quickly, so any divergence was more easily countered once the nosewheel steering could be used. This was particularly so during double asymmetric landings.
Last edited by BEagle; 11th Aug 2009 at 17:09.
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More reverse thrust
Yes, it was indeed a design requirement. in 1968/9 landing distance was defined as being no greater than clean take off distance, wet or dry.
I had a great mate in the Navy, the late Chris Comins (Fred's Five Sea Vixens, 1962) who later left and joined, i think Court Line with BAC 111's. I heard an interesting story later which i havn't been able to confirm, that landing at Luton(?) on snow or ice, he selected reverse, lost it directionally, did a complete 180 as he proceeded down the runway and cancelled RT to put on "forward" power to help decelerate! Any truth?
I had a great mate in the Navy, the late Chris Comins (Fred's Five Sea Vixens, 1962) who later left and joined, i think Court Line with BAC 111's. I heard an interesting story later which i havn't been able to confirm, that landing at Luton(?) on snow or ice, he selected reverse, lost it directionally, did a complete 180 as he proceeded down the runway and cancelled RT to put on "forward" power to help decelerate! Any truth?
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As previously stated PATRLs can be done - you just have to specifically brief it and then make sure that it is essentially a normal approach/landing with nothing untoward going on. I believe these extra restrictions were introduced after the Cyprus crash.
Of interest I was once in the back of a Tornado (before I was qualifeid on it) being flown by a Jag mate. TR failed on landing, override selected then both throttles were pushed to max dry with 1 bucket deployed. The ride down the runway was interesting - skipped around a bit but stayed essentially straight. We didn't slow down much, though!! [Usually when TR fails and override is selected both buckets come out normally].
Of interest I was once in the back of a Tornado (before I was qualifeid on it) being flown by a Jag mate. TR failed on landing, override selected then both throttles were pushed to max dry with 1 bucket deployed. The ride down the runway was interesting - skipped around a bit but stayed essentially straight. We didn't slow down much, though!! [Usually when TR fails and override is selected both buckets come out normally].
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Memory fading, hair turned grey, but I seem to recall that TR during taxi (pre take off) was not desirable - I think it was to maintain the integrity of the TR locking pins (which were checked by the see-off team) as there was a remote possibility of the buckets flapping open and awaving in the breeze.