Jetengine question

[fve]

What is the purpose of the 'things' al the way at the back of jet-fighter planes exausts?
I mean the camera-aperture-alikes which are now adays also moving for thrust vectoring.
What is the reason for opening and closing of them?
Why have I only seen them on fighters?

Frans

[Arclight]

Summarized from my ever extensive notes on my aircraft maintenance course:

This is in regards to operation on Concorde.

In the transonic airspeed range, when re-heat is initiated, the jet pipe pressure would rise beyond limits and cause an engine surge if the propelling nozzle area remained unaltered. There is a need to increase the exhaust nozzle area to control the gas pressure. The propelling nozzle has a series of 36 moveable flaps connected by links which are actuated by pneumatic rams. As the reheat fuel flow increases, the rams progressively open the flaps to control the jet pipe pressure to compressor delivery ratio within safe limits. This ensures the nozzle expands the gas to the correct velocity without creating an excessive back pressure on the engine and altering the selected engine RPM

However in supersonic range, re-heat is cancelled but engine power will be set to high. The propelling nozzle will be choked with exhaust gases since it is a convergent duct. The secondary nozzle consists of 18 moveable flaps (this is what you can see) which free float in response to the primary exhaust flow pattern to form a divergent section aft of the convergent primary nozzle. The exhaust gases therefore expand in this section and accelerate and produces a jet-stream velocity increase behind the choked nozzle. It therefore makes maximum use of existing pressure thrust.

In much much simpler terms, imagine pushing a rubber ball into a funnel. The radial expansion of the ball would push on the inclined surface of the funnel and produce a reaction force, which then ejects the ball.

Hope this helps rather than confuses you

[fve]

Arclight thank you for responding.
If I can digest this within one week I shall be happy.
Is it maybe for noise reasons only fighters are aloud to use this system?

I googled on 'jetengine simulation' and found next link:
http://www.grc.nasa.gov/WWW/k-12/airplane/ngnsim.html

This is probably known by you but just in case.
Looking at what is offered there I realise again how little I realy understand of the underlying principles.

Cheers

Frans

[Arclight]

Ahh yes, NASA have some useful material for studying the principles of a gas turbine engine! That website will help you with many things

From my understanding, it hasn't been used in the civilian world since Concorde for a lot of reasons such as noise regulations and fuel economy purposes. I'm not 100% sure of the exact reasons though but I bet those 2 reasons I put fall into it somewhere. Perhaps someone else can shed some light on the reasons why the system isn't used on airliners anymore since it's not really my area of expertise.

You'll be able to digest it after reading it several times over so don't be too worried about what I wrote. It's taken me 3 years so far of intensive studying to understand many aspects of aircraft (other than the obvious basics) and I still have a year left of my course. It's difficult but extremely interesting

[bigjuicyspider]

Arclight, kind of off subject, but you wouldn't know anything about a JT8D would you? I've been looking for a curve of N1 versus EPR specifically for the -7 variant, and just thought I'd ask if a student of such things might have one (on the off chance).

[Arclight]

Just had a look and I have nothing Spider.

Closest I could come to was a few cutaways of a JT8D-200 and a a small bit of history regarding P&W.

Not a lot around on the web it seems either.

[CAPFlyer]

Let me look through my 707, 737 and 727 stuff this evening. I might have something. If not I'll see if any of my mechanic contacts have something.

Sent from my SAMSUNG-SGH-I747 using Tapatalk 2

[CAPFlyer]

Back on the original question in the meantime (still looking through a bunch of books), here's a video from "Agent JayZ" of S&S Turbines up in Canada put up a great video showing the operation of the J79 "Turkey Feathers" (which is what you're talking about) in action from idle to full Afterburner and talks about how they work.

http://youtu.be/x5ccK94IvsA?hd=1

And a video of the nozzle in action -

http://youtu.be/qZKKeg7lOr8

[fve]

Thanks very much CAPflyer!
Right now it is 3AM overhere so I look forward to enjoy it when I wake up.

Cheers,

Frans

[fve]

Very nice videos, especialy the 1st one.
The temperature isolation is something I also never thought of.

Thanks both of you, at least now I have an overall picture of what is going on.

[Arclight]

Agree on the first video, great to see close up operation!

The second video, while also great, has taught me that there is now only one way I ever want to clear snow from my street again!

[seaniam81]

Yes Jay's videos are great.

[CAPFlyer]

Arclight, kind of off subject, but you wouldn't know anything about a JT8D would you? I've been looking for a curve of N1 versus EPR specifically for the -7 variant, and just thought I'd ask if a student of such things might have one (on the off chance).

Sorry to be so long coming back to you on this, but I can't find any charts like what you're looking for. Unfortunately, I'm not finding any references to it in the maintenance manuals for the JT8D either, so I don't know who might have one. The closest I come is the power and cruise control charts and I guess when doing the test cell runs, they just check against those and assume that it's operating right if it's making the right power at the given settings.

[bigjuicyspider]

No, no problem at all. Thanks for looking though

[Kiwi Spitfire]

I'm not 100% sure of the exact reasons though but I bet those 2 reasons I put fall into it somewhere. Perhaps someone else can shed some light on the reasons why the system isn't used on airliners anymore since it's not really my area of expertise.

Pretty old - but might as well fire it out there. Airliners generate the majority of their thrust using a ducted fan arrangement these days - much more efficient that a pure thrust jet engine, all the energy in the exhaust is extracted by the final turbine stages and used to drive the big-ass fan up the front. Hence there is no real reason to get too excited about perfectly tuning the jetpipe opening, thats just needed for pure jet engines.

Altering the jet pipe size is as you described nicely further up, we used to measure the very tip of the tailpipe (only place it actually matters) on the MB339CB's we had in the AF to ensure it was both an equal circle in shape, and was the right size down to the .001 of an inch based on our ground run figures for TIT (smaller size = more thrust but hotter engine/higher pressure). We would alter the size of the opening by installing small wedges right at the end of the tailpipe, but the automatic nozzles in AB engines are pretty sweet!