Low Observability of the F-135 engine

[spazsinbad]

I'm working on a laptop which sometimes can be exceedingly slow along with this forum and yet 'soon' the 'catapult PDF' will be uploaded which may answer some future questions. Yes the doodad at the front that is connected to the aircraft that subsequently pulls the aircraft down the catapult track is called the shuttle. There is a more explicit video some where (would you believe in a catapult thread?) why we are talking about this in an LO Engyn thread I have yet to divine but - whatever. Meanwhile a Shornet Video Catapulting in SloMo: watch the 'bouncing baby boyo'

And I have not seen any other info about a 'problem' with F-35C catapult bounce - sure they bounce ANDbut so do other aircraft. Ashore the F-35C has been tested on all the catapults including EMALS why have we not heard about this before?

Here we go - first a slomo first catapult afloat... OK maybe one of the first.... AND lastly but not LEASTLY - HOWL!!!!!!!

[spazsinbad]

You didn't ask for it & no one wants it so here it is.... All you can read about F-35C & some Hornet Catapult Info in 105pp.

[sferrin]

'sferrin' speak in riddles if you must but I have no clue to what you are referring by using 'it'. I could guess but won't.

"It" being the notion of using an energized nose-gear strut to assist with rotation to shorten takeoff. That's what they tested late 80s/early90s on an F-5 or T-38. The tests were successful but then I never heard another word about it.

[sferrin]

And I have not seen any other info about a 'problem' with F-35C catapult bounce - sure they bounce ANDbut so do other aircraft. Ashore the F-35C has been tested on all the catapults including EMALS why have we not heard about this before?

Because we typically don't hear about something until we hear about something. (Which may not be the moment it's noticed. Also, maybe they thought it was something they could live with -all aircraft "bounce" to one degree or another- but subsequently decided they couldn't.) I'll see if I can track it down later. I'm certain it's from the recent report that's been making the rounds though.

[vanshilar]

Because we typically don't hear about something until we hear about something. (Which may not be the moment it's noticed. Also, maybe they thought it was something they could live with -all aircraft "bounce" to one degree or another- but subsequently decided they couldn't.) I'll see if I can track it down later. I'm certain it's from the recent report that's been making the rounds though.

Or because Gilmore is trying to find new things to complain about to try to show the F-35 program is falling behind as it continues to resolve previously-mentioned problems.

Two years from now he'll be complaining about the shade of gray being used.

[spazsinbad]

'sferrin' speak in riddles if you must but I have no clue to what you are referring by using 'it'. I could guess but won't.

"It" being the notion of using an energized nose-gear strut to assist with rotation to shorten takeoff. That's what they tested late 80s/early90s on an F-5 or T-38. The tests were successful but then I never heard another word about it.

This is what 'johnwill' wrote on another thread recently & pointed to - albeit indirectly - with this reference on page 2:

viewtopic.php?f=60&t=52449&p=356203&hilit=complication#p356203

"Exactly the same functionality for both airplanes [F-35C & Shornet]. As far as I know, all Navy catapult launched airplanes that use nose gear launching use the same process. Bridle launched do not, because they do not compress the nose gear. Even the F-111B I helped test 48 years ago had the same compressed strut design.

Notice the launch bar is connected to the shuttle at a 45 deg angle, so when the the bar pulls on the gear it is pulling forward and down. Look at the F-35C videos and you will notice the airplane nose drops abruptly and oscillates a couple of times as the airplane starts to move. The strut has compressed the air/oil in the cylinder until it hits bottom, then the tire is compressed and causes the oscillation. When the shuttle releases the launch bar, the tire expands and the shock strut is extended, both of which force the nose of the airplane upward to get the desired AoA."

viewtopic.php?f=60&t=52449&p=356206&hilit=compressed#p356206

[johnwill]

I was wrong about the tire being the source of the oscillation. Clearly the strut is also oscillating.
It's easy to modify the oscillation. Just change the diameter of the orifices used to provide damping to the strut. Nitrogen pressure that provides spring force can also be adjusted. What makes it difficult is conflicting requirements for the damping at the start of the cat sequence, rotation, and landing. Like everything else in airplane design, comprises are necessary.

[sferrin]

Going back to the engine, what exactly is happening here at 4:58? The engine goes from what appears to be maximum afterburner to I don't know what. Almost as if the engine explodes and the whole back end of the fuselage is blocked out by a massive fireball. I'm not saying the engine malfunctioned or anything but something definitely changes there at 4:58, and it stays this way for the rest of the launch and climb out. You can even hear a noticeable *pop*.

[spazsinbad]

In several places now there is mention of the F-35C variable afterburner especially codified for catapulting to alleviate 'pop stalls', if steam is ingested during the catapult stroke, especially at beginning as the aircraft starts down the track, one would guess. This variable afterburner was built in from the start from the experience with the Super Hornet which had variable afterburner added AFTER it was first built to again alleviate pop stalls experienced without this function. In the case of the Shornet the A/B is at 125% power before launch going to full A/B at 150% by the end of the stroke. AFAIK we can only guess that a similar situation applies to the F-35C during catapulting. There are now several videos which reference this change from part A/B to full A/B on this forum, usually accompanied by the air guitars I guess to add or listen to the pop stall - if it is there - but of course the noise is what they measure.

In the video I hear a few things particularly the shuttle hitting the end of the catapult track as it is stopped by the water brake. One may wonder how well the sound is accurately synchronised to the video action also. Remember the noise probably overwhelms the video sound recording at that particular time with some sound clipping due to loudness levels etc. The 'clacking' constant rhythmic sound - to me - sounds like igniters sparking off as in engine start but of course that engine has started - this 'clacking' could be audio clipping noise but only my guess. Video CLIP will be on Utube.... now.

Another guess (I have never been near an afterburning engine) perhaps the metal is expanding noisily? These guys know:

See no evil, Hear no evil, Speak no evil but FEEL THE EVIL

http://www.dtic.mil/ndia/2007test/Fischer_SessionH4.pdf (6.4Mb)

[eloise]

In the case of the Shornet the A/B is at 125% power before launch going to full A/B at 150% by the end of the stroke

125% afterburner ? like higher than the maximum ?

[spazsinbad]

Agree - deliberately confusing - because I'm tired of repeating this post so here goes.... 2 page article PDF attached.

JBD Testing A Key Step For Joint Strike Fighter
18 Jul 2011 Amy Butler; Aviation Week & Space Technology

"...Even without the more extensive data provided by today’s sensor array, Super Hornet engineers gained valuable experience during JBD trials that led to a change in how the aircraft is launched. During testing, hot air was inadvertently
recirculated into the air intake of the Super Hornet, prompting a “pop stall,” or hiccup in the airflow for the propulsion system. The result was a dangerous fireball coughing from the back of the Super Hornet, says Briggs.

The design fix was the creation of a limited afterburner setting for launch. Engineers crafted software such that the engine is at 122% of military power when a pilot sets it to afterburner. By the time the jet reaches the edge of the deck, the system automatically opens the throttle to full afterburner at 150% of power without intervention by the pilot, says Briggs.

Having completed the first phase of JBD trials with a single F-35C, engineers are eager to test a more realistic scenario with one aircraft in front of the deflector and one behind.

Because of this lesson, the limited afterburner setting was designed into the F-35 in its infancy...."

Source: http://www.navair.navy.mil/lakehurst/nl ... esting.pdf

[sferrin]

Yes, but that flame at 4:57 is anything but "minimum after burner". Looks more like maximum here:





What's going on at 4:58 looks like something else.

[spazsinbad]

Bewdy is in the aye of the beholden - in my video clip at second 18 or thereabouts I see the burner 'popping' to full some short distance down the track. It is similar to the day time video where the same phenomena seen. However if you see the same video differently then so be it.

[sferrin]

It's not a matter of what I see, the picture speaks for itself. 4:58 is very different than 4:57. 4:57 looks more like what you'd see as "maximum" on the test videos I posted. 4:58 looks like. . .more.

[optimist]

to me it looks like the auto light meter on the camera is being overexposed. As the plane moves, it gets a better look up the pipe, goes overexposed, it then re-adjusts