PAK-FA and FGFA Thread

shiv · Post by **shiv** » 18 Aug 2011 19:20

She's graceful and seems to have plenty of reserve power.
http://www.youtube.com/watch?v=zDtqDWiqSLc

girish.r · Post by **girish.r** » 18 Aug 2011 19:49

shiv wrote:She's graceful and seems to have plenty of reserve power.
http://www.youtube.com/watch?v=zDtqDWiqSLc

Certainly. Appears that the pilot is ensuring minimal stress to the air frame.

A very nice video though.

shiv · Post by **shiv** » 18 Aug 2011 19:56

girish.r wrote: Certainly. Appears that the pilot is ensuring minimal stress to the air frame.

Absolutely. My guess - 4G

Kanson · Post by **Kanson** » 18 Aug 2011 20:22

vina wrote:It is quite obvious to everyone, that the rear part of the plane with the current engines is just an interim/initial set of engines with no stealth or anything to get the plane into flight test. The 5th gen engine and the stealth treatment in the rear has to come sometime for all aspect stealth. There could still be some way to go for this. This 2015 date seems ultra aggressive and at best this could be the entry into service with low rate of production, of the half baked version that is currently flying and when the 5th gen engines and in 2018/2020 when the engines and stuff are mature enough, a sort of MLU/ MKII version of that with all aspect stealth and all bells and whistles will enter service.

Yes it is quite obvious that the current engine seen in flying displays are not the final 5th gen engine meant for PAK-FA. While we can think about various entry dates, do we have "Progress Report Card" for the prospective engine enumerating the milestones achieved so far? Is there any confirmative report on the engine which will fly by 2015 or 2018 or 2020? Pls do share if there is one. Thank You.

Kanson · Post by **Kanson** » 18 Aug 2011 20:25

shiv wrote:She's graceful and seems to have plenty of reserve power.
http://www.youtube.com/watch?v=zDtqDWiqSLc

Plenty of reserve power with the use of afterburner? Check around 3:30 and other points, one can see blue flame.

SaiK · Post by **SaiK** » 18 Aug 2011 20:36

I am thinking the stealth skins for the engine section would be part of engine LRU, so it would be just plug and play stealth!? blending would be the only design consideration left.

shiv · Post by **shiv** » 18 Aug 2011 20:45

Kanson wrote:
Plenty of reserve power with the use of afterburner? Check around 3:30 and other points, one can see blue flame.

In fact the engine shows a blue flame every time it is seen from the back, Check at 0:48, 0:58, 1:22, 2:03, 2:48, 3:24 and 4:16. Either the afterburner was on throughout the 7 minute display or the engine simply shows a blue Primus stove flame from the back

Austin · Post by **Austin** » 18 Aug 2011 22:34

Kanson wrote:Is there any confirmative report on the engine which will fly by 2015 or 2018 or 2020? Pls do share if there is one. Thank You.

There is this so called "Item 127", which is supposedly going to be rated at 180 kN (~ 18T ) that was mentioned by Saturn designer which is under testing , the current engine 117 is rated at 149 kN (15T )

Having said that just raw thrust does not speak much for an engine unless we know the T/W ratio and key parameter like specific fuel consumption , TBO and engine life and it would take the right amount of all these parameter that would make a great engine supplemented by the right cost.

We also need to know about empty weight , fuel ,payload ,ntow , mtow and similar parameter of the aircraft and how the aerodynamics would all come together to compliment the engine ,a great engine can get bogged down by bad aerodynamics and materials and vice verse.

Right now there is no real hard data coming from Sukhoi on the aircraft parameters or its engine , some scanty data on payload , some on radars , some scanty information on PMF/FGFA and other minor facts are known.

Hence at this stage all data are just educated speculation unless some real data starts flowing from Sukhoi and i am afraid that would be long time to come.

neerajb · Post by **neerajb** » 19 Aug 2011 00:08

shiv wrote:
Kanson wrote:
Plenty of reserve power with the use of afterburner? Check around 3:30 and other points, one can see blue flame.
In fact the engine shows a blue flame every time it is seen from the back, Check at 0:48, 0:58, 1:22, 2:03, 2:48, 3:24 and 4:16. Either the afterburner was on throughout the 7 minute display or the engine simply shows a blue Primus stove flame from the back

It's got to be the afterburner. This is how the engine looks from nozzle side.

The red thing at the center is tail cone, the three rings are afterburner flame holder and at the background, the turbine look alikes are vanes to straighten the flow behind the turbines. With all this stuff in between and a long jet pipe accommodating the afterburner flame, it is not possible to see the fuel burning in combustion chambers.

Cheers....

shiv · Post by **shiv** » 19 Aug 2011 06:57

Kanson wrote: Plenty of reserve power with the use of afterburner? Check around 3:30 and other points, one can see blue flame.

Technically hot gases in the jet pipe behind the turbine blades could be visible to the naked eye or to a camera that picked up those wavelengths. I say this because I have distinctly seen flame in videos in situations where one would not expect an after burner. Or else afterburners are commonplace in low speed, low level airshow displays and the judgement of "reserve power" or no reserve power cannot be reached from merely seeing the afterburner on.

Check these videos: Tornado, F-15 and F-22 Raptor

This Tornado has just pulled out from a steep dive at 1:48 and is turning to the right after that. Yet the backside of both engines are glowing and it's not as if he needs burners to climb again because he does not go into a climb
http://www.youtube.com/watch?v=rkwy-GUvaLc

The F-15 video below again shows what appears to be burners on for a huge proportion of the display - even in level flight manoeuvres without a subsequent climb requiring more power.
http://www.youtube.com/watch?v=LQ-ZEwJUKAI

And here is the F-22 that has a 10 foot flame coming out of each engine almost throughout the demo including, spectacularly at 2:40 when the flame points up while the nose is down.
http://www.youtube.com/watch?v=84n6EvUAbvw

Austin · Post by **Austin** » 19 Aug 2011 08:05

shiv wrote:
girish.r wrote: Certainly. Appears that the pilot is ensuring minimal stress to the air frame.
Absolutely. My guess - 4G

Shiv impressive you are very close , it was doing 5G link

negi · Post by **negi** » 19 Aug 2011 08:49

If one will look into the tail pipe directly then flame can be seen even if the ABs are not engaged (depending on the throttle setting). Also isn't blue flame an indicator of complete combustion i.e. ABs are not engaged.

shiv · Post by **shiv** » 19 Aug 2011 09:09

negi wrote:If one will look into the tail pipe directly then flame can be seen even if the ABs are not engaged (depending on the throttle setting).

This is what I thought. The gases are glowing hot inside the engine in front of the blades. For them to stop glowing they have to transfer all the heat to the blades in a span of a few tens of centimeters as they pass though the gaps between spinning blades. So a glow could be visible at a high power setting minus afterburner. I remember seeing (can't recall if it was live or a video) a non afterburning civilian airliner engine that revealed a glow directly behind - but deep in the engine exhaust and not spilling out like an afterburner glow.

Singha · Post by **Singha** » 19 Aug 2011 09:15

I am expecting a bigger engine and 2D tvc nozzle - all aspect stealth is something one needs in a air superiority mission when enemy awacs are always lurking. they have been playing with flat nozzles for quite some time now...prolly before the 3d nozzle....this is their chance to take some risks and change the ballgame instead of just coasting on su27 work.

http://t3.gstatic.com/images?q=tbn:ANd9 ... gGwVVp9htw

bring it on!

negi · Post by **negi** » 19 Aug 2011 09:19

^ Irc they were having issues like loss of thrust when experimenting with a 2D rectangular nozzle.

Singha · Post by **Singha** » 19 Aug 2011 09:22

its been said 15% thrust is lost and thats where the big 35,000-45,000lb class 5th gen engines provide enough power to still have a excellent t:w ratio. the F22 will suffer from the same physics and seems to be doing fine for 20 yrs now.

pakfa is a heavier bird than f22. if the f22 has 35000lb afterburning motors, to achieve the same t:w the pakfa might need something around 40000-45000lb as a wild guess. if Rus is unable to attain this level and still maintain good sfc , they might call it a day and fall back to the proven 3D but improve it for stealth and durability maybe.

vina · Post by **vina** » 19 Aug 2011 09:51

shiv wrote:And here is the F-22 that has a 10 foot flame coming out of each engine almost throughout the demo including, spectacularly at 2:40 when the flame points up while the nose is down.
http://www.youtube.com/watch?v=84n6EvUAbvw

My Gosh. What a stunning routine and done so effortlessly. Just watch it at around 10;00, just as it is is lifting of and flying parellel to the runway, the stabilizers deflect up to push the tail down and get the nose up (a pretty large deflection, not too much airspeed, just a few knots above take off) and the plane promptly goes vertical, accelerating straight up like a rocket, and then dumps speed at the top and the tail-planes deflect downwards, lifting the tail and dropping the nose. It does all the routines we see the MKI do , but far more tightly and much less labored.

Those PW F119 engines are really something. The F22 has such massive excess thrust and looks like a thrust to weight ratio far north of 1:1 to do such a thing. In an air to air fight, that plane should be simply brilliant.

The PAKFA is a bigger plane (around 4 m longer) . While the extra range is good and will give us expeditionary capability to take the fight deep into Panda land if need be and is the perfect choice for us, I am not sure that plane will be as nimble as the F22 in a knife fight . Matching the F22 T:W ratio will take some doing. The Chinese Golden Dragon /F20 ding-dong sure as hell is noway going to get close. The chinese simply dont have the engines for that sort of thing.

Austin · Post by **Austin** » 19 Aug 2011 10:28

The F-22 seems to be using afterburner for most of its display which gives it sheer raw power under its command , on the contrary I hardly recollect MKI ever with its lower thrust compared to F119 using A/B during its display at AI . I could be wrong but I dont recollect MKI ever using A/B

shiv · Post by **shiv** » 19 Aug 2011 13:39

vina wrote:
My Gosh. What a stunning routine and done so effortlessly.

Vinaji - just a little surprised that you hadn't seen that rather old F-22 video. But if you want to compare it with something which is perhaps not as spectacular, but which beats anything else that I have even seen with my own eyes - it is this MiG 29 OVT display from Aero India 2007. Sorry about the video quality - but the performance was spectacular - including flying at AoA of >90 degrees and recovery to level flight, double somersaults etc.

http://www.youtube.com/watch?v=k2teSidNYVA

But there is a side-by side two frame comparison of the F-22 and Su-30 which was on YouTube but taken off due to copyright reasons. However I saved it on my HDD. You really need to see that. Maybe I can put it up for d/l somewhere. Let's see. All my Sundar-Bandar comparison videos were inspired by that

shiv · Post by **shiv** » 19 Aug 2011 14:27

Su-30 and F-22 side by side airshow comparison
https://rapidshare.com/files/307404329/su30-f22.flv

Kanson · Post by **Kanson** » 19 Aug 2011 16:40

Austin wrote:
Kanson wrote:Is there any confirmative report on the engine which will fly by 2015 or 2018 or 2020? Pls do share if there is one. Thank You.
There is this so called "Item 127", which is supposedly going to be rated at 180 kN (~ 18T ) that was mentioned by Saturn designer which is under testing , the current engine 117 is rated at 149 kN (15T )
Having said that just raw thrust does not speak much for an engine unless we know the T/W ratio and key parameter like specific fuel consumption , TBO and engine life and it would take the right amount of all these parameter that would make a great engine supplemented by the right cost.
......
Right now there is no real hard data coming from Sukhoi on the aircraft parameters or its engine , some scanty data on payload , some on radars , some scanty information on PMF/FGFA and other minor facts are known.
Hence at this stage all data are just educated speculation unless some real data starts flowing from Sukhoi and i am afraid that would be long time to come.

Thank You Austin. Pls do share these details when and where they are available.

Kanson · Post by **Kanson** » 19 Aug 2011 16:55

shiv wrote:
Kanson wrote: Plenty of reserve power with the use of afterburner? Check around 3:30 and other points, one can see blue flame.
Technically hot gases in the jet pipe behind the turbine blades could be visible to the naked eye or to a camera that picked up those wavelengths. I say this because I have distinctly seen flame in videos in situations where one would not expect an after burner. Or else afterburners are commonplace in low speed, low level airshow displays and the judgement of "reserve power" or no reserve power cannot be reached from merely seeing the afterburner on.

Check these videos: Tornado, F-15 and F-22 Raptor

This Tornado has just pulled out from a steep dive at 1:48 and is turning to the right after that. Yet the backside of both engines are glowing and it's not as if he needs burners to climb again because he does not go into a climb
http://www.youtube.com/watch?v=rkwy-GUvaLc

The F-15 video below again shows what appears to be burners on for a huge proportion of the display - even in level flight manoeuvres without a subsequent climb requiring more power.
http://www.youtube.com/watch?v=LQ-ZEwJUKAI

And here is the F-22 that has a 10 foot flame coming out of each engine almost throughout the demo including, spectacularly at 2:40 when the flame points up while the nose is down.
http://www.youtube.com/watch?v=84n6EvUAbvw

If I could read you correctly, according to your opinion, these hot flames seen behind the tail of the engine is not from afterburner. I respect your opinion, Sire, however, it is common knowledge that the flames generated in the combustion chamber of the engine is controlled in such fashion that it won't reach the turbine blades nor touches the wall of the chamber let alone these flames pass through these turbine blades and appear behind the tail of the engine. In the MAKS 2011 PAK-FA show uploaded in youtube, you can see flames behind the tail. That is only possible with afterburner.

With the use afterburner, you are approaching maximum installed thrust of the aircraft, so where is the question of plenty of reserve power remaining.

Singha · Post by **Singha** » 19 Aug 2011 16:59

2 x 18T engine, internal weapons carriage, wing body lifting fuselage blending, spherical maws, next gen IRST, L-band and X-band fuselage arrays, big internal fuel tanks...gotta love the concept.

perhaps afterburner is preferred in airshows because at those low levels and speeds neither is it generating high lift from wings or any margin of safety if wing lift breaks down.

Austin · Post by **Austin** » 19 Aug 2011 17:01

The after burner if used should produce yellow and long tail of flame , you can see that very distinctly in F-22 video.

PAK-FA or even the MKI vs F-22 comparision video does not show similar use of after burner , blue flame can be seen inside the engine of pak-fa.

Austin · Post by **Austin** » 19 Aug 2011 17:03

shiv wrote:Su-30 and F-22 side by side airshow comparison
https://rapidshare.com/files/307404329/su30-f22.flv

At some place F-22 manouvers are more crisp and tight , some manouvers appears to have been done better by MKI , the F-22 engages afterburner at some places while for MKI i dont see it using afterburner.

shiv · Post by **shiv** » 19 Aug 2011 18:20

Kanson wrote:Sire, however, it is common knowledge that the flames generated in the combustion chamber of the engine is controlled in such fashion that it won't reach the turbine blades nor touches the wall of the chamber let alone these flames pass through these turbine blades and appear behind the tail of the engine.

We have a small difference of opinion simply because what is "common knowledge" for you does not extend to ignorant fossils like me.

Maybe I am out of date but I was taught that "flames" means hot gases. That means that "hot gases" and "flames" are exactly the same. No difference except that all hot gases give off light whose wavelength depends on the temperature. When the wavelength of light that hot gases emit falls within the visible spectrum that can be detected by the human eye those hot gases are called "flames" (in fizziks anyway). When hot gases reach certain temperatures, and depending on what is in those hot gases - typically carbon dioxide, carbon monoxide, oxygen, water vapor, oxides of nitrogen and maybe some more complex hydrocarbons from incomplete combustion of aviation fuel and lubricants the hot gases begin to produce radiation wavelengths shorter than infra red that become visible to the human eye.

What is "visible" at the back of the engine is only a function of the temperature of the gases that come out and the equipment that detects that. One often finds video cameras picking up some infra red and may show "flames" where the naked eye cannot see them but the naked eye may well detect yellow and blue flame when gases of sufficient temperature manage to get into the jet pipe behind the blades.

I do not want to make this into a controversy but I believe you are completely wrong when you say that "flames" (hot gases actually) of the combustion chamber do not touch the walls. That is not possible except when the engine is turned off, in which case there are no flames/hot gases.

Hitesh · Post by **Hitesh** » 19 Aug 2011 18:44

How many missiles can PAK-FA carry internally? Raptor can carry 6 medium range missiles in its central bay and 2 short range missiles in its two side bays. Wiki says that PAK-FA may only carry four internally and has 6 external hardpoints. If that is the case, Raptor still has an edge over PAK-FA by virtue of being able to carry more missiles internally.

How much does PAK-FA weigh? Wiki says 26k kgs. Raptor is 29.3k kgs. Raptor's engines have more thrust in dry and afterburner mode. Wiki says that Russians designed it to be more manuverable but less stealthy than Raptor.

From the initial data, it seems that Raptor would have an edge over PAK-FA.

merlin · Post by **merlin** » 19 Aug 2011 19:26

Hitesh wrote:From the initial data, it seems that Raptor would have an edge over PAK-FA.

If they meet each other in battle. If they don't its irrelevant IMHO.

shiv · Post by **shiv** » 19 Aug 2011 19:35

One post to settle the issue. The video below is of the car that broke the world land speed record using Rolls Royce Spey jet engines. There is no afterburner pipe or nozzles. A glow is seen from the engine all the way - including a faint glow as the car slows down after breaking the sound barrier.

http://www.youtube.com/watch?v=LKQ-xj5C2m8

Jet engines can be seen to glow from behind even without afterburner.

neerajb · Post by **neerajb** » 19 Aug 2011 19:41

Shiv Saar, IMO the tornado video you have posted shows afterburner. The flame outside the nozzle is not the only evidence of afterburner usage (during day time the flame outside the nozzle is not that visible), nozzle size too indicates whether mil power is being used or the afterburner. In the tornado video, the nozzles are wide open to the max which happens only at startup or when the afterburners are engaged. Intrestingly the tornado shuts off the burner at the end of the turn, the glow fades away and the nozzle shrinks. Couldn't figure out nozzle sizes in the F-15 video. You can check the glow in MKI takeoffs during red flag as well. No glow is visible prior to brake release and burner engagement, only after that the burner kicks in with the nozzle opening up and flames exiting from the back of it.

Here is another F-16 video which shows the transition from mil power to afterburner. The nozzle converges and expands and no glow is seen at mil power.

[youtube]GcFWP_TvNjM&feature=related[/youtube]

Between the combustion chamber and the nozzle, We have 2-3 turbine stages, 1-2 stator vane stage/s, tail cone blasting cold (relatively) air from the compressor back to the turbines, multi-stage afterburner assembly and a long jet pipe blocking the fire.

Cheers....

Austin · Post by **Austin** » 19 Aug 2011 20:02

Close up of nozzle while in-flight

http://cdn-www.airliners.net/aviation-p ... 970458.jpg
http://www.sukhoi.ru/forum/attachment.p ... 1313679731

neerajb · Post by **neerajb** » 19 Aug 2011 20:03

Thrust SSC used afterburning jet engines. Looking more closely shows con/di nozzles at the back of engine cowl. One can actually see them changing aperture.

Cheers....

shiv · Post by **shiv** » 19 Aug 2011 20:11

neerajb wrote: You can check the glow in MKI takeoffs during red flag as well. No glow is visible prior to brake release and burner engagement, only after that the burner kicks in with the nozzle opening up and flames exiting from the back of it.

Saar a glow is visible in non afterburning airliner engines at high power when viewed directly from the back. The Spey video above shows a lot of flame but airliner engines just show a faint hint of flame near the rim. It's just that such views are very rare because no one can get up close and personal to a jet engine's backside at takeoff. The mere presence of glow is not proof of afterburning. I am still looking for a suitable video showing just that in a civil airliner engine - I have seen one in some advertisement.

The other point is the false argument that the PAKFA does not have thrust to spare because it is allegedly using afterburners. Clearly that argument is a bogey because if the PAKFA was seen using afterburners so are the Tornado, F-15 and F-22, so all of them are short of thrust.

neerajb · Post by **neerajb** » 19 Aug 2011 20:12

Austin wrote:Close up of nozzle while in-flight

http://cdn-www.airliners.net/aviation-p ... 970458.jpg
http://www.sukhoi.ru/forum/attachment.p ... 1313679731

Su-30 MKI of IAF afterburning.

http://www.flickr.com/photos/bwjones/2831576285/

Cheers....

shiv · Post by **shiv** » 20 Aug 2011 07:01

Folks - need to correct misconceptions and remove myths.

It is possible to see glowing gases deep inside a jet pipe behind the rotating blades in a non afterburning engine running at high power. The presence of visible flame inside the jet pipe is not proof of afterburning - although a bright flame shooting back out of the exhaust is indicative of afterburning (as long as no special IR sensitive cameras are being used)

The first half of this video shows the Rolls Royce Derwent engine. Check the glow, and later the rotating blades as they come to a halt.
http://www.youtube.com/watch?v=moX6Lps9w70

The video below shows an Al-31 ground test. At 1 min 08 sec you can see flame deep in the jet pipe without afterburning. At 1:32 you can see what the diameter of the exhaust nozzle is and the intensity of the flame with afterburning.
http://www.youtube.com/watch?v=wsJyC5g4IBo&NR=1

Singha · Post by **Singha** » 20 Aug 2011 08:29

> Wiki says that PAK-FA may only carry four internally and has 6 external hardpoints

what wiki? pakfa has two tandem internal bays for atleast 8 bvr AAMs internally and 2 side bays near wing root for perhaps 4 wvr aams. total = 12

and it combat radius/loiter time will be more than raptor for sure given its large area of wing body blending creating room for tankage.

there is no proof / pic if the pakfa will have external hardpoints - the two prototypes have no external pylons. it should be able to carry 4 big LGBs internally/ 8 x AASM-250 with a stealthy designator pod put semi conformally under the chin in our FPGA version.

external stealth weapon carriers are coming for all (incl SH)...a couple of these could potentially add another 6-8 AASM-250 style weapons (imagine the Rafale 2nd pylons wrapped into a stealth container - it carries 3xAASM). this would push the load of small bombs near the 20 mark of SU30 but with adv of stealthy carriage.

Hitesh · Post by **Hitesh** » 20 Aug 2011 08:59

Singha, thanks for responding to my post. If you don't mind, can you paste the link so I can look up the data from your source?

vina · Post by **vina** » 20 Aug 2011 10:00

shiv wrote:What is "visible" at the back of the engine is only a function of the temperature of the gases that come out and the equipment that detects that. One often finds video cameras picking up some infra red and may show "flames" where the naked eye cannot see them but the naked eye may well detect yellow and blue flame when gases of sufficient temperature manage to get into the jet pipe behind the blades.

True. I think the key is to see the after burner spray bars and see if they are lit bright orange or not. But typically blue flame seen (naked eye.. camera sensors are different) in the nozzle typically means that the burner is probably lit, sort of like the flame you can see in your LPG stove at hit hits the vessel bottom, especially from the side like in airshows.. If you look for a couple of inches above the stove, from the side, the flame wont be blue, coz it has lost heat. I would think the same would be true for the combustors, though you if you look from right astern of the engine (which like you said would be simply suicidal, even from quite a distance, if you dont fancy getting roasted, even cars and still be be thrown up flying), you could probably see right into the flame holders

Thanks for the Sundar vs Bandar flv file. Yes, the MKI and the F22 are amazing acrobats. It is just that the F22 with it's higher specific excess installed thrust can do those things from situations (like take off and accelerate vertically like a rocket just after wheels up for instance), where an MKI might need to build up a bit of speed before truing it.

I do not want to make this into a controversy but I believe you are completely wrong when you say that "flames" (hot gases actually) of the combustion chamber do not touch the walls. That is not possible except when the engine is turned off, in which case there are no flames/hot gases.

The combustion chambers have dilution holes to make sure that that the the temperature at the walls are lower than that at the max of the flame front. The whole damn thing will melt otherwise.

What happens when the combustion products which are at very high temp and pressure (P proportional to T..flip sides of the same coin) goes through the turbine is this. The turbine is a Reaction turbine ( google /wiki for reaction vs impulse turbine to know the diff), so there will be pressure (and hence temp) drop across the turbine. A significant drop in fact in just a few inches and that is not due to the "heat transfer" to the turbine blades, but rather that the bulk of the energy is converted into work and it is the remaining energy that is present after the turbines (still hot and at higher pressure) , that is accelerated through the nozzles (all the while dropping in pressure and temp), while it comes out through the Musharraf at the ambient atmospheric , pressure as a very high velocity and still somewhat hot jet.

shiv · Post by **shiv** » 20 Aug 2011 10:13

vina wrote:It is just that the F22 with it's higher specific excess installed thrust can do those things from situations (like take off and accelerate vertically like a rocket just after wheels up for instance), where an MKI might need to build up a bit of speed before truing it.

Some modern aeromodels which enjoy that sort of excess thrust can do amazing stuff - but I find them boring. But watching an F-22 dance that way is enchanting. The plane is able to stand comfortably on its tail and still retain control

The combustion chambers have dilution holes to make sure that that the the temperature at the walls are lower than that at the max of the flame front. The whole damn thing will melt otherwise.

Technical query. The hot part of the engine is also the highest pressure part. How would higher pressures be achieved outside of those dilution holes ensuring that cool air flows in rather than hot gas flowing out of those holes - or do the dilution holes do just that - let gas expand out and be relatively cooler at the periphery at the expense of some loss of efficiency.

In fact i thought that cool air flowing outside the combustion chamber wall would cool the walls enough to prevent meltdown.

vina · Post by **vina** » 20 Aug 2011 10:59

shiv wrote:Some modern aeromodels which enjoy that sort of excess thrust can do amazing stuff - but I find them boring. But watching an F-22 dance that way is enchanting. The plane is able to stand comfortably on its tail and still retain control

Not just aero models, but some real planes ! In fact, the most acrobatic Sukhoi is which of these : Su27, SU30, Pak-FA ? The answer is none of the above,but Sukhoi 26. Just check out some of the YouTube videos of that puppy and it's younger litter. Simply mind blowing.

Technical query. The hot part of the engine is also the highest pressure part. How would higher pressures be achieved outside of those dilution holes ensuring that cool air flows in rather than hot gas flowing out of those holes - or do the dilution holes do just that - let gas expand out and be relatively cooler at the periphery at the expense of some loss of efficiency.

The highest pressure part is just after the last stage of the HP compressor. The pressure inside the combustors is typically 3 to 5% lower than that, especially where the flame is present and combustion happens , and the colder dense air flows fed into the combustor and also comes in through the dilution holes to keep that thing cool.

In fact i thought that cool air flowing outside the combustion chamber wall would cool the walls enough to prevent meltdown.

No, if the combustion happens on the metal surface of the combustor, it will melt the walls. Air simply does not have that kind of heat transfer capacity (ok, in air cooled engines you have huge fins, like in your motorcycle, that increase the area of the surface multifold, enough for even air to be able to keep it from melting, but impractical in a gas turbine), and even in a car, the cylinder walls and cooled by the water /fluid in the cooling system to keep it from melting.