Lift Engines Why??
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Lift Engines Why??
Why has the engineering and technological brilliance of the pegasus engine been shelved in favour of lift engines for the JSF. I thought that the pegasus had a super sonic version so why cart around the dead weight of a lift engine albeit a lift fan?
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FEBA,
I assume you mean the Plenum Chamber Burning variant that was to be fitted into the P1154. I don't beleive it ever got beyond captive running. PCB was essentially adding an afterburner to the forward nozzles - not sure that would fit with the stealth requirement. Any ex- Bristol-Siddeley types care to elaborate?
I assume you mean the Plenum Chamber Burning variant that was to be fitted into the P1154. I don't beleive it ever got beyond captive running. PCB was essentially adding an afterburner to the forward nozzles - not sure that would fit with the stealth requirement. Any ex- Bristol-Siddeley types care to elaborate?
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FEBA
Sometimes very simple questions are the devil to answer without writing a book, but here goes. It is all about the basics of thrust. Thrust comes from good old M x V. M is the mass of air taken in by the system and V is the speed that is imparted to the air. A Sea King’s rotor in the hover takes in a huge M and gives it a modest v while a Sea Harrier’s Pegasus takes in a modest m and gives it a relatively big V. They both weigh the same in a heavy hover, so both are using the same thrust (MxV) but the nature of that thrust (downwash) is quite different.
To fly supersonically you need an engine system that delivers air with a very big V and we only know how to do that by making said air pretty hot. Any fighter using reheat on takeoff makes that point.
Standing such a fighter vertically on its tail a few feet off the ground and letting it do a VTO is not on. It is not that the amount of thrust is wrong it is that the type of thrust (hot and fast) is not usable with rubber tyres underneath you and a solid ground surface of normal materials.
So the Pegasus could not have used PCB during VTO or VL. And the cold non PCB thrust was not enough to hover (and so VL) an aeroplane that was big enough to carry all the fuel needed to use the PCB for a sensible period of time during the sortie. Catch 22.
The Lockheed and Boeing JSF teams had to use the same basic F-119 engine. They could modify it as they saw fit for STOVL. Boeing added nozzles like a Harriers under the CG and shut down the rear exhaust for the hover. If you saw the Channel 4 progs you may have seen the dull red glow inside the Boeing nozzles. This hot air must not find its way into the intake or the engine will likely surge or worse. Boeing used a ‘jet- screen’ of coldish air tapped off before the combustion chambers and exhausted down in front of the hovering nozzles in an attempt to build a barrier between the really hot main exhaust and the nose intake. It proved somewhat marginal.
Lockheed decide to turn the engine into a sort of turbo prop on the downwind leg by using a shaft to drive a lift fan behind the cockpit. Complicated mechanically, but very efficient when it came to generating lift. Taking 26,000 shaft HP out from the engine reduced the thrust at the back by 6000lb. But that same SHP produced 16,000 lb of thrust when driving the fan. A gain of 10,000lb of hover capability over a pure jet. Rather more than an edge in a competition?
Finally, the Lockheed way enabled the huge column of cool fan air to robustly blow back the hot air from the rear when it tried to migrate forward. The IR film showing the way the cold column turns back the hot column as it tries to move forward under the aircraft is the stuff old Harrier guys can only dream about.
Regards
Sometimes very simple questions are the devil to answer without writing a book, but here goes. It is all about the basics of thrust. Thrust comes from good old M x V. M is the mass of air taken in by the system and V is the speed that is imparted to the air. A Sea King’s rotor in the hover takes in a huge M and gives it a modest v while a Sea Harrier’s Pegasus takes in a modest m and gives it a relatively big V. They both weigh the same in a heavy hover, so both are using the same thrust (MxV) but the nature of that thrust (downwash) is quite different.
To fly supersonically you need an engine system that delivers air with a very big V and we only know how to do that by making said air pretty hot. Any fighter using reheat on takeoff makes that point.
Standing such a fighter vertically on its tail a few feet off the ground and letting it do a VTO is not on. It is not that the amount of thrust is wrong it is that the type of thrust (hot and fast) is not usable with rubber tyres underneath you and a solid ground surface of normal materials.
So the Pegasus could not have used PCB during VTO or VL. And the cold non PCB thrust was not enough to hover (and so VL) an aeroplane that was big enough to carry all the fuel needed to use the PCB for a sensible period of time during the sortie. Catch 22.
The Lockheed and Boeing JSF teams had to use the same basic F-119 engine. They could modify it as they saw fit for STOVL. Boeing added nozzles like a Harriers under the CG and shut down the rear exhaust for the hover. If you saw the Channel 4 progs you may have seen the dull red glow inside the Boeing nozzles. This hot air must not find its way into the intake or the engine will likely surge or worse. Boeing used a ‘jet- screen’ of coldish air tapped off before the combustion chambers and exhausted down in front of the hovering nozzles in an attempt to build a barrier between the really hot main exhaust and the nose intake. It proved somewhat marginal.
Lockheed decide to turn the engine into a sort of turbo prop on the downwind leg by using a shaft to drive a lift fan behind the cockpit. Complicated mechanically, but very efficient when it came to generating lift. Taking 26,000 shaft HP out from the engine reduced the thrust at the back by 6000lb. But that same SHP produced 16,000 lb of thrust when driving the fan. A gain of 10,000lb of hover capability over a pure jet. Rather more than an edge in a competition?
Finally, the Lockheed way enabled the huge column of cool fan air to robustly blow back the hot air from the rear when it tried to migrate forward. The IR film showing the way the cold column turns back the hot column as it tries to move forward under the aircraft is the stuff old Harrier guys can only dream about.
Regards
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From my vantage point (sharing the test site with the PCB Harrier rig whilst working on more modest Gem engines) I can offer up my (irrelevant?) opinions of PCB in Harrier.
The plenum chamber burning concept actually ran during the mid eighties although I’m sure it was ex BS staff that pushed the original sixties BS concept with the MoD even though it appeared to have limited applications by the time it was tested.
In terms of stealth the Harrier has always had relatively poor IR characteristics, although not burdened with an afterburner, the position of the aft nozzles relative to the airframe results in a large area of (exhaust plume) washed structure that is elevated to reasonably high temperatures increasing the IR signature in terms of level and viewing aspect (certainly more of an issue today when considering 8-14 micron IR systems). This is just common sense and can be deduced purely from the engine/aircraft geometry and a minor grasp of Planck’s Law etc (it is no criticism of a sixties design working in the 21st century). Also, as anyone who has been involved in helicopter exhaust design can tell you, even modest temperature rises can have significant impact on structural life.
The front nozzles of the PCB Harrier were fitted with the reheat system so if ever the design had evolved to a flight demonstrator the material spec for structure aft of the nozzles would have been interesting. And IR stealth characteristics would have gone out the window.
The rig did demonstrate that hot gas recirculation was an issue (although in all honesty that surely should have been flagged as a very high risk at the design phase by having the hot nozzles so close to the intake) and naturally like the std Harrier the JSF aircraft have been designed to have the hot gas flow as far away from the intakes as possible (as featured in the C4 programme). No doubt some RR individual will explain that the rig allowed a useful understanding of hot gas management in the near field but the concept did appear awfully aircraft specific.
The other issue of using reheat as lift comes with ground erosion and the higher the gas velocities the more damage the ground experiences. This can even be a problem with proper runways so imagine the effect on a semi prepared strip and all the potential FOD hazards. (I believe that even the V22 has a potential ground erosion issue when the exhausts are in the vertical position). In short there will be a point at which lift thrust can not be increased by raising the jet velocity but mass flow will have to be increased instead (LM/BAE lift fan for example). I also seem to remember that the far field was also affect in terms of v.high ambient temperatures which I guess would dictate the distance that ground crew could operate in the vicinity of the aircraft.
On an aside – did anyone think that the C4 JSF programmes were “engineering-lite”? Certainly from the flight test aspect there was very little said about the massive telemetry / instrumentation support and how these were probably more critical for monitoring in-flight safety than the chase aircraft. Plus it used my least favourite phrase – “designed by computers” – that’s a bit like saying that the Harrier was designed by an HB pencil - rather than by engineers.
lhb
The plenum chamber burning concept actually ran during the mid eighties although I’m sure it was ex BS staff that pushed the original sixties BS concept with the MoD even though it appeared to have limited applications by the time it was tested.
In terms of stealth the Harrier has always had relatively poor IR characteristics, although not burdened with an afterburner, the position of the aft nozzles relative to the airframe results in a large area of (exhaust plume) washed structure that is elevated to reasonably high temperatures increasing the IR signature in terms of level and viewing aspect (certainly more of an issue today when considering 8-14 micron IR systems). This is just common sense and can be deduced purely from the engine/aircraft geometry and a minor grasp of Planck’s Law etc (it is no criticism of a sixties design working in the 21st century). Also, as anyone who has been involved in helicopter exhaust design can tell you, even modest temperature rises can have significant impact on structural life.
The front nozzles of the PCB Harrier were fitted with the reheat system so if ever the design had evolved to a flight demonstrator the material spec for structure aft of the nozzles would have been interesting. And IR stealth characteristics would have gone out the window.
The rig did demonstrate that hot gas recirculation was an issue (although in all honesty that surely should have been flagged as a very high risk at the design phase by having the hot nozzles so close to the intake) and naturally like the std Harrier the JSF aircraft have been designed to have the hot gas flow as far away from the intakes as possible (as featured in the C4 programme). No doubt some RR individual will explain that the rig allowed a useful understanding of hot gas management in the near field but the concept did appear awfully aircraft specific.
The other issue of using reheat as lift comes with ground erosion and the higher the gas velocities the more damage the ground experiences. This can even be a problem with proper runways so imagine the effect on a semi prepared strip and all the potential FOD hazards. (I believe that even the V22 has a potential ground erosion issue when the exhausts are in the vertical position). In short there will be a point at which lift thrust can not be increased by raising the jet velocity but mass flow will have to be increased instead (LM/BAE lift fan for example). I also seem to remember that the far field was also affect in terms of v.high ambient temperatures which I guess would dictate the distance that ground crew could operate in the vicinity of the aircraft.
On an aside – did anyone think that the C4 JSF programmes were “engineering-lite”? Certainly from the flight test aspect there was very little said about the massive telemetry / instrumentation support and how these were probably more critical for monitoring in-flight safety than the chase aircraft. Plus it used my least favourite phrase – “designed by computers” – that’s a bit like saying that the Harrier was designed by an HB pencil - rather than by engineers.
lhb
Pretty much most documentaries made in the last 10 years or more seem to target the hard-of-thinking audience and substitute gimmicky camera work, intrusive tappity tappity.....thud thud noise as background to every speaker & voiceover, strange..................
..............pauses in the voiceover to artificially build.........................
.......................... dramatic tension etc etc. Oh, and there's always the standard "...But suddenly something went wrong" melodrama.
..............pauses in the voiceover to artificially build.........................
.......................... dramatic tension etc etc. Oh, and there's always the standard "...But suddenly something went wrong" melodrama.
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Mr Farley sir,
It's an honour to have you reply to my question. I'm going to show it to my youngest son Harry, he has a burning ambition to fly a Harrier (strange since he's only nine). Whether he's able to fulfil his ambition matters little at this stage, what is important is that the Harrier is providing the right sort of vectored thrust in the schooling department. Harry has gone from the bottom to the top and is now a team leader as well as a talented rugby player.
I'm not an expert on the Harrier, all of my studies were done in Horsham library (I remember a picture of you trying to connect a Harrier to the extended arm of a JCB, what was all that about?) I do recall that Shorts lost out with lift engines and Hawker won with the Pegasus. Your last post adaquately explains why.
Thanks to you all
FEBA
It's an honour to have you reply to my question. I'm going to show it to my youngest son Harry, he has a burning ambition to fly a Harrier (strange since he's only nine). Whether he's able to fulfil his ambition matters little at this stage, what is important is that the Harrier is providing the right sort of vectored thrust in the schooling department. Harry has gone from the bottom to the top and is now a team leader as well as a talented rugby player.
I'm not an expert on the Harrier, all of my studies were done in Horsham library (I remember a picture of you trying to connect a Harrier to the extended arm of a JCB, what was all that about?) I do recall that Shorts lost out with lift engines and Hawker won with the Pegasus. Your last post adaquately explains why.
Thanks to you all
FEBA
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FEBA and Harry
Glad to hear Horsham has a good library. The hovering that was carried out alongside what I recall was originally an extended turntable ladder (courtesy of the local Fire Brigade) and later a large crane hired for a few days, was just to establish how accurately the Harrier could be hovered. We used basic visual cues that were provided by a simple wooden sight designed by Heinz Frick (another Dunsfold tp) as part of his Skyhook project. It turned out that one could keep the pilots head steady inside a 1 foot cube if you really tried and a 1 metre cube if you didn’t.
The issue of why the P1127/Pegasus concept was preferred over the SC1/RB108 lift jets involved a very different era when mechanical complexity, as well as the use of electrics in a flight critical role, was rather asking for trouble. So Hawkers decided you would not need a single volt to control the P1127 or Harrier family including the Sea Harrier. The necessary interconnection between the nozzles and the flaps with the Harrier IIs as well as the later use of a digital engine control system (correctly) put an end to that.
The SC1 was more pleasant to hover than the P1127 because there were no air intake effects trying to make the thing point tail to wind. But it was a nightmare to operate in the circuit, stopping and starting all those donks without a flight engineer.
Today a complex beast is not necessarily something to fear from a reliability point of view (think of your car compared to your Dad’s) and computers can act as your flight engineer. But as the JSF programme has shown there are many other factors affecting the optimum configuration, not least of which are stealth and hot gas recirculation.
But stick at it Harry, because when it is your turn the jet of the day will give you a lot of fun if you can only beat the other guys (and girls) into the cockpit
Regards
Glad to hear Horsham has a good library. The hovering that was carried out alongside what I recall was originally an extended turntable ladder (courtesy of the local Fire Brigade) and later a large crane hired for a few days, was just to establish how accurately the Harrier could be hovered. We used basic visual cues that were provided by a simple wooden sight designed by Heinz Frick (another Dunsfold tp) as part of his Skyhook project. It turned out that one could keep the pilots head steady inside a 1 foot cube if you really tried and a 1 metre cube if you didn’t.
The issue of why the P1127/Pegasus concept was preferred over the SC1/RB108 lift jets involved a very different era when mechanical complexity, as well as the use of electrics in a flight critical role, was rather asking for trouble. So Hawkers decided you would not need a single volt to control the P1127 or Harrier family including the Sea Harrier. The necessary interconnection between the nozzles and the flaps with the Harrier IIs as well as the later use of a digital engine control system (correctly) put an end to that.
The SC1 was more pleasant to hover than the P1127 because there were no air intake effects trying to make the thing point tail to wind. But it was a nightmare to operate in the circuit, stopping and starting all those donks without a flight engineer.
Today a complex beast is not necessarily something to fear from a reliability point of view (think of your car compared to your Dad’s) and computers can act as your flight engineer. But as the JSF programme has shown there are many other factors affecting the optimum configuration, not least of which are stealth and hot gas recirculation.
But stick at it Harry, because when it is your turn the jet of the day will give you a lot of fun if you can only beat the other guys (and girls) into the cockpit
Regards
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Mr Farley Sir
I'm intrigued. When the lift fan is redundant why is it absorbing so much power? What negative impact on useful payload does the lift engine impose. Also after reading the naval history of the Falklands War by Lt Cdr Brown RN a subsonic Harrier was a capable match for the supersonic Mirage (Kfir). The extra ordinary manoeuverability of the Harrier has been reported (and still reported after the FAA air wars against the French AF recently. Navy News APR) as a decisive factor in CAP's over the South Atlantic. So why do we need a supersonic fighter or even a supersonic STOVL fighter?
Taking 26,000 shaft HP out from the engine reduced the thrust at the back by 6000lb.
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FEBA
Sorry, but you have to read that sentence you pulled out as a quote in the context of the whole para:
Try reading the whole para again....
Lockheed decided to turn the engine into a sort of turbo prop on the downwind leg by using a shaft to drive a lift fan behind the cockpit. Complicated mechanically, but very efficient when it came to generating lift. Taking 26,000 shaft HP out from the engine reduced the thrust at the back by 6000lb. But that same SHP produced 16,000 lb of thrust when driving the fan. A gain of 10,000lb of hover capability over a pure jet. Rather more than an edge in a competition?
When the clutch is engaged and the fan starts to produce lift it is producing 16,000lb for the loss of only 6000lb at the back. That is VERY good news indeed. The fan is only driven in the circuit.
Sorry, but you have to read that sentence you pulled out as a quote in the context of the whole para:
Try reading the whole para again....
Lockheed decided to turn the engine into a sort of turbo prop on the downwind leg by using a shaft to drive a lift fan behind the cockpit. Complicated mechanically, but very efficient when it came to generating lift. Taking 26,000 shaft HP out from the engine reduced the thrust at the back by 6000lb. But that same SHP produced 16,000 lb of thrust when driving the fan. A gain of 10,000lb of hover capability over a pure jet. Rather more than an edge in a competition?
When the clutch is engaged and the fan starts to produce lift it is producing 16,000lb for the loss of only 6000lb at the back. That is VERY good news indeed. The fan is only driven in the circuit.
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The shaft driven fan is not intended to be available for combat. Most aircraft design decisions are compromises. The shaft driven fan is very efficient at low speeds and in the hover but has the drawback that it cannot be used in ACM.
The Harrier system (and the Boeing X-32B system) are much less efficient in the hover but can/could be used at higher forward speeds.
The Harrier system (and the Boeing X-32B system) are much less efficient in the hover but can/could be used at higher forward speeds.
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Thanks for that John, but wasnt the fact the Harrier could VIFF one of the reasons it was so good at low level combat. So the JSF has lost this ability, and would lose a dogfight with the Harrier.
Or have I got the wrong end of the stick, and although the JSF has vertical capabilities it is not a Harrier replacement, but a Jack of all Trades, and hopefully a master of some ot them.
Or have I got the wrong end of the stick, and although the JSF has vertical capabilities it is not a Harrier replacement, but a Jack of all Trades, and hopefully a master of some ot them.
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The issues behind your comments are not simple, even if they may appear so. Defence procurement agencies around the globe struggle with them – and often fail, so we are not going to get far in a few paragraphs here.
If your only wish is to win in every guns dogfight that you find yourself in, then you do need extremely good agility, aerodynamic departure resistance, an ability to disconnect the flight path vector from the attitude of the aircraft (which makes it harder for the enemy to anticipate what you are up to and so to plan their tactics) and either a lot of fuel or a low fuel consumption at full throttle so that you will not be the first to break out of the scrap.
In those circumstances a suitably skilled pilot (most important) with a Harrier or a thrust vectoring variant of the Su27/Su30 family should not get shot down and may even shoot the other guy down.
But you have to ask yourself what job you want to do when you are not in a dogfight. That leaves everything else aeroplanes are used for which is a lot of stuff. You also have to ask yourself what if the enemy can prevent you getting in close in the first place. What do you do then – go home and complain they would not play your war?
Well actually, the last thing you might do is go home, because there is a good chance you will die while you are wandering around looking for the dogfight you know you can win.
The Harrier is still a remarkable device, but today is no longer viable as other than a sophisticated low level night bomb truck or BVR interceptor that can operate from small sites. Still not things to sniff at perhaps, but you need a bit more than that for the next generation of kit that may still be around in 50 years time.
I am more than content that the JSF programme (at present) includes a variant than can still use small sites (land vertically) and at the same time be competitive in most of the up and away jobs and require no special training to fly in the circuit.
After all do we by cars that can see off anybody at the local traffic island but are not on for everything else we need them to do?
Regards
John
The issues behind your comments are not simple, even if they may appear so. Defence procurement agencies around the globe struggle with them – and often fail, so we are not going to get far in a few paragraphs here.
If your only wish is to win in every guns dogfight that you find yourself in, then you do need extremely good agility, aerodynamic departure resistance, an ability to disconnect the flight path vector from the attitude of the aircraft (which makes it harder for the enemy to anticipate what you are up to and so to plan their tactics) and either a lot of fuel or a low fuel consumption at full throttle so that you will not be the first to break out of the scrap.
In those circumstances a suitably skilled pilot (most important) with a Harrier or a thrust vectoring variant of the Su27/Su30 family should not get shot down and may even shoot the other guy down.
But you have to ask yourself what job you want to do when you are not in a dogfight. That leaves everything else aeroplanes are used for which is a lot of stuff. You also have to ask yourself what if the enemy can prevent you getting in close in the first place. What do you do then – go home and complain they would not play your war?
Well actually, the last thing you might do is go home, because there is a good chance you will die while you are wandering around looking for the dogfight you know you can win.
The Harrier is still a remarkable device, but today is no longer viable as other than a sophisticated low level night bomb truck or BVR interceptor that can operate from small sites. Still not things to sniff at perhaps, but you need a bit more than that for the next generation of kit that may still be around in 50 years time.
I am more than content that the JSF programme (at present) includes a variant than can still use small sites (land vertically) and at the same time be competitive in most of the up and away jobs and require no special training to fly in the circuit.
After all do we by cars that can see off anybody at the local traffic island but are not on for everything else we need them to do?
Regards
John
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John
Thankyou for you're explanation. It is my understanding the JSF while the next generation of plane, is to be used by all the Forces not just the Airforce. This means it is a one size fits all, and obviously does all the jobs well.
Using you're car analogy, does this mean we will have a plane that comes as is, in any colour as long as its black? After all car companies are moving towards less and less choices of model, back to the model T.
Thankyou for you're explanation. It is my understanding the JSF while the next generation of plane, is to be used by all the Forces not just the Airforce. This means it is a one size fits all, and obviously does all the jobs well.
Using you're car analogy, does this mean we will have a plane that comes as is, in any colour as long as its black? After all car companies are moving towards less and less choices of model, back to the model T.
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inaspin
Not quite one size fits all. In fact not al all really. The F-35 will come in three versions. The A is purely conventional to replace the F-16 fleets and has more fuel where the fan is in the B model. The B has the operating site flexibility of STOVL to replace all Harrier variants, while the C has a wing extension, stonger gear and a hook to replace the F-18A/B in navy ops. Commonality of over 70% has led to a real control of costs compared to needing three totally different aircraft.
Not quite one size fits all. In fact not al all really. The F-35 will come in three versions. The A is purely conventional to replace the F-16 fleets and has more fuel where the fan is in the B model. The B has the operating site flexibility of STOVL to replace all Harrier variants, while the C has a wing extension, stonger gear and a hook to replace the F-18A/B in navy ops. Commonality of over 70% has led to a real control of costs compared to needing three totally different aircraft.
