Low Observability of the F-135 engine

[spazsinbad]

SCREECH - I thought it was an AFB somewhere?

viewtopic.php?f=56&t=15287&p=193006&hilit=Screech#p193006

Screech, the F135 and the JSF Engine War
17 Mar 2011 Graham Warwick

"For those of us who thought screech was the noise made by GE/Rolls and Pratt & Whitney in their war of words over the JSF second engine, here's the background to comments made this week about screech problems with the F-35's F135 engine.

Testifying before the House Armed Services Committee on Tuesday, JSF program executive officer Adm David Venlet said afterburner screech on the F135, which prevents the engine from sustaining full thrust, "caused us to avoid certain portions of the flight envelope." Instead, F-35s have flown to other points in the envelope to keep flight-test going. Kits are being installed to overcome the problem, he said.

So what is screech and what's the fix?

Pratt says screech is a phenomenon caused by pressure pulsations in the afterburner at low altitude and high speed. The problem was discovered during development testing around March 2009, having previously been encountered - and solved - in the F-22's F119 engine, from which the F135 is derived.

Pratt points out that the F119 and F135 are the only production engines with stealthy augmentors. Their design eliminates conventional spray bars and flame holders and integrates multi-zone reheat fuel injection into curved vanes that block the line-of-sight to the turbine.

Building on its experience with the F119, the fix for the F135 includes "minor hardware changes to the fuel system, reduced aerodynamic leakages and upgraded software," says Pratt, adding that the modified engine "now provides full max augmented thrust throughout the flight envelope."

A kit has been developed for flight-test engines, and two have been modified. The production configuration will be validated this year in both the CTOL/CV and STOVL variants of the F135, Pratt says.

I have asked GE/Rolls whether their F136 has a screech-free stealthy augmentor. Watch this space for their answer."

Source: http://www.aviationweek.com/aw/blogs/de ... 0491972939

[sferrin]

I did have to laugh. All those guys with the sound gear look like they're hold guitars and with the guitar music playing.

BTW that bouncing you see there from 0:03 - 0:18 or so apparently is noticeable enough that it's a "must fix" issue. Don't recall where I read that specifically, but it was in some article today or yesterday. Probably in the one where they were implying a "cover up" on the F-35's progress.

[spazsinbad]

The chaps with 'air guitars' (sound recorders) are noted in the F-35B DT-III thread here: viewtopic.php?f=22&t=52450&p=356960&hilit=CERTIFIED#p356960 & viewtopic.php?f=22&t=52450&p=356578&hilit=grab#p356578 & viewtopic.php?f=22&t=52450&p=356506&hilit=guitars#p356506

Yes the bouncing catapult pilots have been commented upon ever since carrier catapulting videos were made available - first I have heard about it being a problem on the DOT&E 'maus92' memo thread. A thread about the wheels/nosegear says the NG is storing energy during the cat stroke to help lift the nose up OFF the catapult. I'll go find it. Look at this post & entries above also....

viewtopic.php?f=60&t=52449&p=356203&hilit=complication#p356203
&
viewtopic.php?f=60&t=52449&p=356205&hilit=Robust#p356205

Look at 'milestone F-35C' thread for slomo catapult pilot jerking action - one pilot even forgets to secure the visor which flies up under stress.

[neptune]

I did have to laugh. All those guys with the sound gear look like they're hold guitars and with the guitar music playing.

BTW that bouncing you see there from 0:03 - 0:18 or so apparently is noticeable enough that it's a "must fix" issue. Don't recall where I read that specifically, but it was in some article today or yesterday. Probably in the one where they were implying a "cover up" on the F-35's progress.

...the bouncing appears to initiate at the release of the cat and stabilizes as the F-135 goes to max throttle...per Spaz's visor video..if the pressure on the nose strut is modulated then this indeed could be tuned out by software....

[spazsinbad]

One would have to read the last to posts at the URLs in my last post above to gain detail about F-35C (& Shornet) nose gear. Without GILMORE saying more about what the problem is with the nose gear (I have not seen any other official negative comment about it) then we can only guess about cause/effect/cure or whatever is needed. Meanwhile I petition GILMORE to fix the bouncing boys in the Shornets as seen in this video:

[neptune]

One would have to read the last to posts at the URLs in my last post above to gain detail about F-35C (& Shornet) nose gear. Without GILMORE saying more about what the problem is with the nose gear (I have not seen any other official negative comment about it) then we can only guess about cause/effect/cure or whatever is needed. Meanwhile I petition GILMORE to fix the bouncing boys in the Shornets as seen in this video:

...alas, they have no software to fix, me thinks...

[sferrin]

A thread about the wheels/nosegear says the NG is storing energy during the cat stroke to help lift the nose up OFF the catapult. I'll go find it. Look at this post & entries above also....

Not sure I follow. There was a program years ago (I believe involving a T-38/F-5) where they used a modified nose gear that was energized and caused the aircraft to essentially, "pop a wheelie" to assist with rotation, enabling shorter takeoffs. Is this the kind of thing you're talking about?

[barrelnut]

From 0:43 onwards you can see what's inside the tailpipe.

[spazsinbad]

A thread about the wheels/nosegear says the NG is storing energy during the cat stroke to help lift the nose up OFF the catapult. I'll go find it. Look at this post & entries above also....

Not sure I follow. There was a program years ago (I believe involving a T-38/F-5) where they used a modified nose gear that was energized and caused the aircraft to essentially, "pop a wheelie" to assist with rotation, enabling shorter takeoffs. Is this the kind of thing you're talking about?

Not sure what you have said IF you have read the references quoted. To be clear in the case of the F-35C on the catapult.

At this forum URL there is this: viewtopic.php?f=60&t=52449&p=356205&hilit=Robust#p356205

Complex & Robust
Flight International F-35 Special ? 2014? Magazine

"Mark Ayton explains the highly complex landing gear systems used on the F-35...

...The nose gear of the CV variant is a dual stage gas over oil cantilever strut with a staged air curve that provides a source of high energy, which helps the aircraft to achieve adequate angle of attack when released from the catapult during take-off from the aircraft carrier....

...There are two reasons for having a staged shock strut for the nose gear on the F-35C CV variant.... The second is to store energy gained from the compression of the strut under the high pressure effect of the catapult. When the catapult lets go of the launch bar, the energy is released, providing a rotation that helps achieve the angle of attack necessary to get off the deck.

Similarly when the aircraft hits the deck on landing the strut is compressed and energy is stored to help rotate the aeroplane and get it back off the deck if the arrestor cables are missed and a ‘go-around’ or ‘bolter’ is required. Bolter is the term used when the aircraft’s tail hook misses the arrestor cables on the carrier deck forcing the pilot to go around for another landing...."

Source: Not now at original URL - download article here: download/file.php?id=23692 (PDF 1.44Mb)

[spazsinbad]

'barrelnut' Thanks for headsup about A/B & nosegear compression on catapult - here is a slow motion clip from that video:

[sferrin]

From 0:43 onwards you can see what's inside the tailpipe.

Looks like sparks in the exhaust there at 4:52. And like a friggin' meteor at 4:58. Like the whole a$$ end of the plane lit up. It's actually lighting up the wingtip vortices at 5:09

[sferrin]

Not sure what you have said IF you have read the references quoted. To be clear in the case of the F-35C on the catapult.

Sounds like we might be talking about the same kind of thing. I didn't know it ever went anywhere.

[steve2267]

At 6:36 in the video, there is a good view of the nosegear extending during a cat shot. Is this extension the "push" mentioned above to help the aircraft achieve the proper AOA for takeoff?

F-35 Lightning II-CVN-69 Aircraft carrier @ 6:36 on Youtube

[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.

"...I didn't know it ever went anywhere."

To answer 'steve2267': You will have to imagine the forces unleashed at the end of the catapult stroke when the shuttle is stopped by the water brake while the aircraft - having been dragged without remorse nose down - is suddenly released. Then while the nose gear is still on the deck the energy stored in that gear pushes the nose up as the aircraft flies off the deck. Yes the nose gear drops down but no energy released in that - the NG is off the deck then and just gravity drops.

I'm making a catapult PDF to fit the file size limit here and found this pic which MAY show the nose wheel just as it is lifting off the deck - I cannot say with certainty however.

http://3.bp.blogspot.com/--8f_CuL0iX8/V ... 8d53_b.jpg

[steve2267]

Stoopid questions:

Just to be clear, after the catapult shuttlecock (or whatever you call the doodad dragging the aircraft by its nosegear) releases the nosegear, the nosegear is compressed. As the aircraft rotates, the energy stored in the compressed nosegear strut is magically released and the nosegear exerts an upward thrusting force on the aircraft resulting in an additional nose-up pitching moment, thus helping the aircraft reach the correct AOA / takeoff pitch attitude -- correct?

After watching another video of the nosegear engaging the cat shuttlecock, I see that the bar from the nosegear to the shuttlecock is at a downward angle, so that as the shuttle moves forward, dragging the aircraft behind it, it is also pulling down on the nosegear, compressing it.

I saw the bouncing (i.e. vertical oscillations) of the aircraft at the beginning of the cat shot, but just thought that the F-35C bounces a little more than other aircraft. I recalled other aircraft oscillating too, but maybe not as much. Is it truly "a problem"?