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Enthusiast Joined: Aug 02, 2006 Posts: 65 Status: Offline

Hello!

It is time to continue my engine question topics Embarassed

I have examined during past few weeks jet engines and everything which has something to do with them. I ran across "As the throttles advance toward MIL, the nozzles are moving to their fully closed position." this sentence which I didn't completely understand. http://www.f-15estrikeeagle.com/navigat ... nology.htm
So does it means that for example at 50% of thrust the nozzles are not fully closed etc? I have also heard that the nozzle is constantly adjusted. I decided to ask this question because I didn't found any specific explanation.

Thanks for your time!
Best regards
_Viper_

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Elite 1K Joined: May 26, 2005 Posts: 1152

IANAPE (I am not a propulsion engineer) but:

It's complicated, but it comes down to you want to have the highest exit velocity and mass airflow as possible for the given throttle position. Ever put your thumb over a garden hose to spray water farther? Same sort of thing. The more you choke down the nozzle, the higher the exit velocity (to a point).

A couple of very good "intro" pages to read, courtesy of NASA:

http://www.grc.nasa.gov/WWW/K-12/airplane/thrsteq.html
http://www.grc.nasa.gov/WWW/K-12/airplane/mflchk.html

Enthusiast Joined: Jul 03, 2007 Posts: 97 Status: Offline

It is not a direct 1 to 1 correlation, but as the operator moves the throttle handle from idle (15-20 degrees PLA) to mil power (83-90 degrees PLA), the engine control moves the nozzle from 100% open at idle to fully closed at mil power. During transition from mil to augmentation the engine control begins to open the nozzle again from fully closed at 91 degrees PLA (stage 1 aug.) to fully open again at stage 11 augmentation (130 degrees PLA).

Newbie Joined: Oct 10, 2007 Posts: 1 Status: Offline

Hello _Viper_
Like Guysmiley said but with a little more technical explanation. The exhaust nozzle is a variable position duct. This means that at different Throttle Lever Angles (TLA) input by the pilot (or whoever is moving the throttle) the nozzle will open and close as needed to produce thrust based on specific engine pressure ratios, based on things like altitude, barometric pressure, and temperature. The exhaust duct will close (convergent duct) and open (divergent duct) to maximize thrust output . To answer your question, the nozzle shouldnt be closed fully at 50% thrust, but it all depends on atmospheric conditions. The aircraft and engine compute data and adjust the nozzle position as necessary. I hope this helps.

Dan

Idle RPM is greater than 50%, isn't it? I never ran one, but seems to me it was in the 65% range. The nozzle closed pretty rapidly above idle, and I think it was fully closed at about 80%. I could well be wrong. I was merely a pointyhead who also happened to be an expediter and ProSuper on drill weekends and watched a lot of launches.
The nozzle had to open for afterburner (oops, sorry...augmenter) operation because the back pressure with it closed would cause engine RPM to deteriorate.

Enthusiast Joined: Jul 03, 2007 Posts: 97 Status: Offline

Actually, at least for PW engines, the proper acronym is PLA, for Power Lever Angle, and the augmentor is called a convergent/divergent balance-beam nozzle. The term convergent/divergent does not refer to the action or movement of the nozzle, but to the inner configuration, or shape. The inner panels, or flaps, upstream, are convergent flaps and the downstream flaps are divergent flaps. No matter what PLA setting the engine is at there is some convergence, or reduced diameter, in the nozzle a little forward of the axial mid-point, and then a divergence producing something of an hourglass profile. The term balance beam is due to the fulcrum point for the convergent flaps being roughly in the center of the flap, balancing the internal pressure on the flap thereby greatly reducing the actuator force required to open and close the nozzle. An unbalanced nozzle on an engine in the F100 class would require roughly 120k/lbs of force to close, while a balanced nozzle only requires about 25k/lbs

Enthusiast Joined: Aug 02, 2006 Posts: 65 Status: Offline

I found this short video clip last night. At least it reveals to me how quickly the nozzle actually closes after the pilot pushes the throttle to the max AB. According to my sources it should take about 7 sec to reach max AB from idle power which can also be seen. To be honest I have never seen a photo from a running engine where the nozzle is somewhere between fully open or closed.
http://www.youtube.com/watch?v=iOY209NAjIg

Enthusiast Joined: Jul 03, 2007 Posts: 97 Status: Offline

Yes, maximum time allowed from ground idle to 90% max thrust is 7.5 seconds, 13 seconds to 98%. A good running engine will usually do better.

Salute!

Good stuff, on the whole, you wrenchbenders (heh heh).

As with some early, over-powered beasts like the VooDoo, the nozzles were kept open on the ground when PLA was way back. Kept taxi speed under control.

Some of the jets I flew didn't use that feature when WOW, but others did. That was so's we could slow down easier in the landing pattern.

Engaging the 'burner (only Brits call the damned thing an augmentor) is a different story, as you all have pointed out.

The early motors like I flew in the Deuce and VooDoo didn't have the cosmic convergent-divergent nozzles controlled by anything, and they simply opened wide when lighting the burner, then sort of "floated" once you had all the gas and flames coming out the rear end.

You could tell when the nozzles opened, as thrust decreased immensely until the burner lit with a huge "bang".

For the afficianados, stand at approach end and listen for the nozzles opening and closing if the pilot is making significant PLA changes. The older motors had rotary accuators around the nozzles driven by teleflex drive cable. Hence, the "ziiiing, zaaaaang whirring sound).

later,

gums sends ....

Gums wrote:

For the afficianados, stand at approach end and listen for the nozzles opening and closing if the pilot is making significant PLA changes. The older motors had rotary accuators around the nozzles driven by teleflex drive cable. Hence, the "ziiiing, zaaaaang whirring sound).

Dating yourself there Gums.... J57 with Iris Nozzles? Cheers

As for the "zip zip" noises; you'll still hear them from any PW powered Viper.

The F100's CENC "Convergent Exhaust Nozzle Control" uses high pressure air from the N2 compresor. It is an air-motor that drives shafts that turn the actuators. The actuators are "screw-jacks" that extend/retract as they are turned.

The "zip" noise is the air being exhausted overboard from the CENC as it moves the nozzle.

The nozzle has roughly half the exit area when closed than when open (100%)

The CENC can move the nozzle from full open to closed in less than a second. That is 341 revolutions end to end. WARNING -- Don't get your fingers in the turkey feathers when one moves Exclamation

Further, as Gums mentioned above, the F100s have a "reduced ground idle" feature that opens the nozzle a bit more at Idle on the ground than in the air. This is based off the WOW switch as he mentioned.

So for anyone who can see a Viper in the distance, or can hear it but not clearly see it's features. Listen...

If you hear the Zip Zip when the power is changed it is PW.
If you don't hear anything during power changes, it is a GE.
(GE uses oil pressure as hydraulic fluid in the nozzle actuators)

For those who want to know what's all involved with nozzle ratios and positions, check out this graphic from NASA. Just to give you an idea what's involved to find the optimum Aj (Area Jet) for any given power setting. This is some of the magic worked by the engine's control computer a few times per second... Shocked

As for spooling time and engine acceleration from IDLE to MAX, the new engines take less than 5 seconds. The PW-229 will actually start lighting up the burner before the engine reaches MIL so that they both reach full power at almost the same time. Shaves a second or so off the total time. Cool

Keep 'em flyin' Thumb

Enthusiast Joined: Aug 02, 2006 Posts: 65 Status: Offline

_Viper_ wrote:

To be honest I have never seen a photo from a running engine where the nozzle is somewhere between fully open or closed.

I quote myself because now I knew what I was looking for Razz

http://www.airliners.net/open.file/1177035/L/
http://www.airliners.net/open.file/0955522/L/
http://www.airliners.net/open.file/0802319/L/
These pics are from the GE engine powered Vipers but there are plenty of stuff about PW powered birds too. What I learned from all of you is that the nozzle position depends on lots of variable things. When it comes to GE-engines I can notice that the nozzle isn't fully open even at full AB. Thanks again to all of you.

Next engine question....

Enthusiast Joined: Aug 02, 2006 Posts: 65 Status: Offline

That_Engine_Guy wrote:

Next engine question....

Hmm, probably not in the near future I guess Mr. Green

By the way I just found this awesome nighttime engine test photo http://www.f-16.net/index.php?name=PNph ... e&start=15
It looks like the engine is at full AB but my eyes might lie.

Enthusiast Joined: Aug 02, 2006 Posts: 65 Status: Offline

I should also fly Falcon 4.0 AF to see how well I can notice these things in a simulation.

That_Engine_Guy wrote:

Next engine question....

The block 15's would dump fuel at shutdown, we had to put a bucket under the jet to catch it, now I noticed the block 25-52 no longer does that, why?

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Question about engine nozzles