View Full Version : Differences between Airline & Fighter jet turbine engines?
:confused:Please excuse my naievety, but i was wondering what the initial differences are between the jet engines used by airliners and fighter jets are?
I understand that the Turbofan engine used on airliners produces propulsion from the gas turbine exhast and also through the bypass air caused by the fan, so surely a fighter jet engine's work the same way? But why does only the fighter jet's engine produce the super hot exhaust flame which can be easily seen (not during afterburn)?
This question has left me pondering for a while now, is it simply the difference between a economical and highly tuned engine?
Any words of wisdom would be great.
Fray
NutLoose
16th Jul 2007, 22:18
On an airliner the fan (high ratio bypass engine) is similar to a glorified Multi bladed propellor in a casing and this produces 70 % of the thrust of the engine, the core engine, I.E the pure Jet producing the remaining 30%.
On a fighter although it is a turbo jet the fan is nowhere near as big, or indeed producing anything like the amont of thrust of a fan engines fan as mentioned above............ the main core engine produces the thrust..
One of the main problems on a jet engine is the noise, the crackle you hear is the hot air expending rapidly rearwards rubbing against the still cold air of the atmosphere, ( Air Shear) remember the old 707 and VC10 as examples, thought the VC10 is an early generation turbojet... the resulting effect is noise, a fan engine pushes a blanket of warm air at speed ( remember it too has been compressed) rearwards at the same time and this cushions the 2 airflows mentioned and reduces the noise, also the fan produces a more effective thrust ratio so requires less fuel and therefore is more efficent.........
The flame would tend to be afterburner, you do not need to be in reheat to produce it...... The not so mighty Jaguar, ex RAF front line of late had a thing called Part Throttle Reheat......your engine normally has 100 % dry power (none reheat) then the wet power (reheat, which basically is squirting fuel into a shaped exhaust and lighting it...... lots of power lots of fuel burn) the Jaguar when in reheat could be pulled back to 80% dry power and the reheat remained alight, this did 2 things, it reduced the smoke signature of the aircraft in combat and allowed rapid introduction of reheat where in a combat situation seconds count.....................
See I knew all that Jaguar stuff would come in handy one day :hmm:
k12479
17th Jul 2007, 22:38
"On a fighter although it is a turbo jet" - I think generally all fighter engines nowadays are turbofans, but with a lower bypass ratio than airliner engines.
Originally civil & military engines were basically the same thing, RR Avon/Conway/Spey, etc but then they went their own ways with high bypass for civil use where speeds are lower, economy very important and engine diameter not so critical and low bypass for military use where high speed is important, economy less so and smaller diameters required.
cornish-stormrider
18th Jul 2007, 16:39
and space and things like ease of change, i.e can you do it in the snow with a few lads, a toolbox, two winches and a new donk.
Also things like ability to provide oil to the engine under differing and rapidly changing G loadings, flying inverted.........
Lots of things
Don't forget the M08 on the rb199 runs the blades rather hot.......
NutLoose
18th Jul 2007, 23:56
Did I put TurboJet.......... I must of been tired, I meant Turbofan........ see those 15 years in the RAF as a Sumpy did rot my brain away ;)
RB199 ppsssssssssssssttttttttttttttttttt........... never enamoured me to that hunk of junk... and the even bigger pile of Junk they were designed to Power.....
k12479
19th Jul 2007, 15:28
RB199 powers the Tornado? whats bad about it? Heard less than favourable comments on the Jaguar, but not Tornado.
BluntM8
20th Jul 2007, 08:02
I was under the impression that PTR on the Jag was primarily so that maintaining contact during AAR was possible as the jet got heavier?
Blunty
NutLoose
20th Jul 2007, 11:01
Hmm, I was taught on my course it was to aid rapid response of reheat in combat, not that it was any good at that anyway....... Chucking flames out the back end when refuelling, with the chances of spillage seems a odd idea, but then nothing would suprise me...... we could never figure out why the Jet Pipe air flow pressures ran lower than the Engine....
RB199 powers the Tornado? whats bad about it? Heard less than favourable comments on the Jaguar, but not Tornado.
Well as an ex Jag man I ain't gonna agree...... The Jag was a cheap to operate, go anywhere with the minimum of equipment and support... Also of late had capabilities that surpassed all the other aircraft in the ground attack scenario and was cheap to upgrade militarily speaking.....
I still cannot see why it went...... hours was a factor, but as Cosford was full of low houred versions, they could have been replaced by higher houred ones from service and brought up to the latest specs utilising equipment removed from those replacing them......
For everything you ever wanted to know about jet engines and more you should try and get hold of a book called 'The Jet Engine' by Rolls-Royce. Its very detailed without going into all the mathsy stuff too much. A library would be your best bet because the book costs a few hundred quid.
Rigga
20th Jul 2007, 20:08
My book cost me less than £20 in 1988! - though it did seem like hundreds at the time.
Even so, it is worth its weight in gold if you want to know about jet engines.
Finding a stockist is the worst thing - try googling for the title.
NutLoose
20th Jul 2007, 21:24
There is a lot of difference between the older versions of the Rolls Royce "The Jet Engine" and the newer versions........... If you can, get an older one... the newer version has in my eyes become a RR public relations, stroke, self advertising hype exercise, and a lot of the really useful has dissapeared, it's a shadow of it's former self... get one of the earlier additions to get some really useful information.......
The word Ebay springs to mind :)
winglit
21st Jul 2007, 03:37
Well as an ex Jag man I ain't gonna agree...... The Jag was a cheap to operate, go anywhere with the minimum of equipment and support... Also of late had capabilities that surpassed all the other aircraft in the ground attack scenario and was cheap to upgrade militarily speaking.....
Did you ever do any time in Incirlik? Cos if you did you might remember that when the day got hot, the only thing that would get a Jag off the ground was the curvature of the Earth. Good job they had a long runway!
As for the Swing-Wing-Arrow-Of-Death, why would they design an engine that would self destruct in the event of a DC bus power failure? (Saw it happen at Saints once....big boom!...lots of blades coming out the top of the detuners!)
As for the Queen of the skies, I do miss that crackle of four Conways at take-off power on a really cold day!
Another thing worth mentioning about the differences of jet vs fan is FOD. The smaller the bypass, the more susceptible they are to FOD damage.
I know I said a few hundred quid earlier, but I should have checked first. About £40 would be more accurate. D'oh
There's samples here, also might answer your question if you can make out the text.
http://www.rolls-royce.com/history/publications/jet_engine/section01.htm
The book that I have is from 1996 and is pretty good in describing most jet engine related stuff, it would be interesting to see what the changes between it and the latest book actually are.
NutLoose
21st Jul 2007, 23:52
winglitQuote:
Well as an ex Jag man I ain't gonna agree...... The Jag was a cheap to operate, go anywhere with the minimum of equipment and support... Also of late had capabilities that surpassed all the other aircraft in the ground attack scenario and was cheap to upgrade militarily speaking.....
Did you ever do any time in Incirlik? Cos if you did you might remember that when the day got hot, the only thing that would get a Jag off the ground was the curvature of the Earth. Good job they had a long runway!
As for the Swing-Wing-Arrow-Of-Death, why would they design an engine that would self destruct in the event of a DC bus power failure? (Saw it happen at Saints once....big boom!...lots of blades coming out the top of the detuners!)
As for the Queen of the skies, I do miss that crackle of four Conways at take-off power on a really cold day!
Another thing worth mentioning about the differences of jet vs fan is FOD. The smaller the bypass, the more susceptible they are to FOD damage.
My Jag time was spent in Germany with a few detachments thrown in.....
It was a time of turmoil with the German Jag squadrons departing and that jack of all trades master of none the flying flick knife arriving to take their places.....
The engine was designed to do that so they could climb and eject in an emergency.
I do remember the probs with them at the start, undemanded control inputs from flying close to radio masts, and then the nav who command ejected both parties If memory serves correctly when he thought they were suffering the above when in fact it wasnt..
I also did 6 or 7 years on the VC 10 too and was onboard whilst we were doing runs to ascertain why one engine was slow to accel..... we found out why when that spat its turbine stages out and up through the detuner... the vibration was staggering, being felt in the line office some distance away... the fireman on duty at the nose failed to see the fire and was dismayed as he retorted " 18 years in the RAF and never had a real aircraft fire, I finally get one, I am parked 80 ft away from it,facing it and I MISS IT"
k12479
22nd Jul 2007, 23:27
what's a 'detuner'?
NutLoose
23rd Jul 2007, 12:23
A Detuner
It is a device to reduce the noise footprint the aircraft makes, the VC10 detuner was 4 large tubes that you backed the aircraft engines up...
This structure was built up at the height of the engines and internally had a large curved plate with holes drilled in to let the airflow through, a bit like a collander... the tube was then angled up 90 % to the vertical so all the noise and exhaust was sent upwards to reduce the amount heard on the ground....... Indeed some airports you will see a sloped horizontal fencing looking similar to a set of open football stands, these to are angled to deflect the sound and exhaust upwards.
rockytan
24th Jul 2007, 18:53
the N in N1 ,Why the desiners decided to use N ?thanks
k12479
24th Jul 2007, 23:36
'n' is commonly the letter used for RPM in equations, maybe its because of that?
NutLoose
24th Jul 2007, 23:42
From my RAF training so long ago we were still using wing warping......... is the N not an abreviation of Nominal ? seems to ring a bell..
http://en.wikipedia.org/wiki/Nominal
as in
Nominal - The exact or ideal intended value for a specified parameter. Tolerances are specified as positive or negative values from this specified value.
Kitbag
16th Aug 2007, 15:46
Jaguar PTR was indeed introduced as for AAR. It was found that the need to throttle the RH engine back at altitude to prevent surging left the driver without sufficient fine control of his airspeed on the LH engine- effectively he would have been at max dry or thereabouts, and any further increase in airspeed would necessitate going to Min reheat. This made the aircraft too fast. :eek: Solution was to provide reheat from 80%Nh or Part Throttle Reheat, thus giving the driver a much greater degree of control at a critical point in AAR. :) To ease commonality it was applied to all Jaguar engines and only required a bit of extra electrical string as far as the airframe was concerned. Think its the only type to have that facility but then when you look at the origins of the FCU its hardly surprising that little extras had to be added on to make it a militarily user friendly engine :uhoh:.
Use of 'n' is indeed as a result of maths/physics equations indicating rotational speed.
I was told PTR was there to give the Jag driver the ability to get out of his parking space.;)
radicalrabit
31st Aug 2007, 14:20
ahhhhhh 54 SQN Coltishalll.. he hehe happy days ...............
camlobe
1st Sep 2007, 13:39
Thread resume (for a moment or two anyway).
One of the other differences is not immediatey apparant in the military word.
Longivity.
Military engines, whether piston or jet, have traditionally been highly stressed. This has been to ensure, as far as practible, most power from least size/weight. The drawback from operating highly stressed engines is that they don't last as long.
As an example, some of you may remember a number of years back, CFM and Rolls-Royce were alternating adverts in Flight singing their own praises as one of their respective engines had broken new records. For RR it was a 211. For CFM it was a 56. Each manufacturer proudly stated that their engine had accomplished 30,000 hours and seven years 'on-wing' if memory serves me well. I am not aware of any military, front-line engines being this long-legged. Cue someone telling me otherwise.
Both adverts stopped around the same time...
Off thread now. With regard to the Real B0llicks 199, what a piece of tripe. A three-spool engine. WOW. Unfortunately, the IP (intermediate pressure) spool was so inefficient, it failed to contribute to engine thrust (i.e. it only produced enough to power itself). Read excess weight and wear for no performance gain. Good design, that. This is why the EJ200 is a two-spool engine (as it was designed using the 199 as a starting block).
camlobe
rockytan
8th Jun 2022, 22:32
Thanks guys
Cat Techie
9th Jun 2022, 00:27
Jaguar PTR was indeed introduced as for AAR. It was found that the need to throttle the RH engine back at altitude to prevent surging left the driver without sufficient fine control of his airspeed on the LH engine- effectively he would have been at max dry or thereabouts, and any further increase in airspeed would necessitate going to Min reheat. This made the aircraft too fast. :eek: Solution was to provide reheat from 80%Nh or Part Throttle Reheat, thus giving the driver a much greater degree of control at a critical point in AAR. :) To ease commonality it was applied to all Jaguar engines and only required a bit of extra electrical string as far as the airframe was concerned. Think its the only type to have that facility but then when you look at the origins of the FCU its hardly surprising that little extras had to be added on to make it a militarily user friendly engine :uhoh:.
Use of 'n' is indeed as a result of maths/physics equations indicating rotational speed.
Utter balls. It was because the aeroplane above anything much above minimum landing mass had a hole on one engine thrust at max dry to maintain level flight and too much thrust at min reheat. Proven by the two incidents that happened to the French A single seater prototypes in May 1969. Brit test pilot saved the second by a blast of full reheat but scared the living daylights out of him. Jimmy Dell.
jolihokistix
9th Jun 2022, 02:04
*15-yr-old thread, resurrected by two words three posts above.
Cat Techie
9th Jun 2022, 09:11
*15-yr-old thread, resurrected by two words three posts above.
But some actual myths corrected.
NutLoose
9th Jun 2022, 13:56
Maybe he is a slow reader.
No one could explain why the Jag Reheat ran at a lower pressure than the main engine if i am remembering correctly. Neither Rolls Royce or the course staff.
Cat Techie
9th Jun 2022, 21:51
Maybe he is a slow reader.
No one could explain why the Jag Reheat ran at a lower pressure than the main engine if i am remembering correctly. Neither Rolls Royce or the course staff.
PV/T = C. Combined Gas Laws. Had a J41 type course exam question that was never in the type course requiring the theoretical knowledge of that (Mod 15 stuff) to answer it. 9 blokes on the course, I was the only bloke to get it right ( Was a temp at P3 over P2.5 or something like that, then we all moaned why it was in the exam!)
NutLoose
10th Jun 2022, 11:33
Similar we did an exam at Halton and it mentioned Cotter pins that you naturally think push bike pedals, as we know the Americans call the split pin a cotter pin and it was mentioned in the Gnome maintenance manuals that of course we had access too, but were not in the engine build section that we did installing the turbines too etc.