Mini S-duct?

Military aircraft - Post cold war aircraft, including for example B-2, Gripen, F-18E/F Super Hornet, Rafale, and Typhoon.
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Orangeburst

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Unread post03 Sep 2019, 21:10

sprstdlyscottsmn wrote:Actually I think the Caveat is the intake of the SR-71, which produced half the thrust of the whole engine above Mach 3.


Not sure I follow...did it actually become like a ramjet or you mean it lost 50% thrust?
What % thrust would typically be acceptable loses from test stand to install? Seems like with a clean stealth design you could just oversize the inlet, although I guess this would incur unwanted mass and volume.
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sprstdlyscottsmn

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Unread post03 Sep 2019, 23:20

Orangeburst wrote:Not sure I follow...did it actually become like a ramjet or you mean it lost 50% thrust?
What % thrust would typically be acceptable loses from test stand to install? Seems like with a clean stealth design you could just oversize the inlet, although I guess this would incur unwanted mass and volume.

The pressure (at Mach 3) on the back (interior) of the inlet spike was so much higher than the pressure against the front (exterior) of the inlet spike that the spike PULLED the plane forward, producing half of the forward thrust, before the airflow even got to the compressor stage.

Typically installation of an inlet can produce ~10±% loss. In the case of the F-35 it was 7%.
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Unread post04 Sep 2019, 02:28

sprstdlyscottsmn wrote:
Orangeburst wrote:Not sure I follow...did it actually become like a ramjet or you mean it lost 50% thrust?
What % thrust would typically be acceptable loses from test stand to install? Seems like with a clean stealth design you could just oversize the inlet, although I guess this would incur unwanted mass and volume.

The pressure (at Mach 3) on the back (interior) of the inlet spike was so much higher than the pressure against the front (exterior) of the inlet spike that the spike PULLED the plane forward, producing half of the forward thrust, before the airflow even got to the compressor stage.

Typically installation of an inlet can produce ~10±% loss. In the case of the F-35 it was 7%.


I will have to look further into this very interesting Delta-P.
Thank you sir.
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sprstdlyscottsmn

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Unread post04 Sep 2019, 04:16

https://www.youtube.com/watch?v=F3ao5SCedIk

plus...

" The propulsive thrust distribution between these components changed with flight speed: at Mach 2.2 inlet 13% – engine 73% – ejector 14%; at Mach 3.0+ inlet 54% – engine 17.6% – ejector 28.4%" - F-12 Series Aircraft Propulsion System Performance and Development". David H Campbell J. Aircraft Vol II NO 11 November 1974

Ejector in this case is the afterburner.
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Unread post05 Sep 2019, 04:18

sprstdlyscottsmn wrote:https://www.youtube.com/watch?v=F3ao5SCedIk

plus...

" The propulsive thrust distribution between these components changed with flight speed: at Mach 2.2 inlet 13% – engine 73% – ejector 14%; at Mach 3.0+ inlet 54% – engine 17.6% – ejector 28.4%" - F-12 Series Aircraft Propulsion System Performance and Development". David H Campbell J. Aircraft Vol II NO 11 November 1974

Ejector in this case is the afterburner.


Thanks but my mind is turning to mush. The video does not really go into detail about the forward push from the spike as far as I can tell. I did see some comments in this video and others from people involved in the program who spoke about this spike push but did not give details. I get the dynamic diffuser part and ramjet effect, which provides extra compression ratio pre turbojet. I have a pretty good understanding of dynamic centrifugal compressors, IGV's and pressure ratios as I have been in the industry for decades, but am having difficulty in seeing how a higher pressure would only push forward and not be expanding towards the backend in an equilibrium way. I am probably thinking of just static pressure in the spike that deadheads and need to think mass air flow velocity at pressure towards the spike, ie just like blasting an air nozzle at the back end of spike to push it forward. Maybe this is where the bypass doors come into play. But it still seems like it is flow toward the back that is actually performing the forward thrust. Pressure is just potential energy but flow from pressure is kinetic. Seems like it breaks the law of conservation of energy to me..lol
Anywho, you are a much brighter guy than me. I always read your stuff, but mostly Over My Head. Thanks dude.
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Unread post05 Sep 2019, 05:01

Okay, let's talk conservation. Total air pressure stays "constant". When dynamic pressure (caused by moving air) increases the static pressure (pressure perpendicular to a surface) decreases. Every time air passes through a compression shock wave (such as those seen in the video) the total pressure rises. By the time the air gets to the back of the spike, and to the bypass tubes, it has been compressed by shockwaves and slowed to subsonic speeds. This means the static pressure on the back of the spike, increased by both compression waves and reduced speed, is far greater than the static pressure on the front of the spike (which has only gone through one shockwave and is still very supersonic [high dynamic pressure and low static pressure]). Hope that was clear as mud.
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Unread post05 Sep 2019, 11:18

I'm always amazed what kinds of aircraft and spacecraft were designed and made in 1960s mostly using pen, paper and basic wind tunnels. SR-71/A-12 is definitely one of the most awesome aircraft ever.
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Unread post06 Sep 2019, 00:10

hornetfinn wrote:I'm always amazed what kinds of aircraft and spacecraft were designed and made in 1960s mostly using pen, paper and basic wind tunnels. SR-71/A-12 is definitely one of the most awesome aircraft ever.


Brilliant engineers back in the day. Just doing some slight reading on the J58 got me reading some fascinating comments. This link has very interesting info on the J58 as well as this story on the Concorde:

https://www.pprune.org/archive/index.php/t-506122.html

"And from that same thread, a demonstration of how one of Clive's slide rule-toting colleagues wowed the USAF brass and Rockwell's B1 engineers:

If I may, I would now like to mention the 'some oil lamps and diesel oil' story. This is a true story told to me by Dr Ted Talbot, the father of the Concorde Intake, brilliant aerodynamicist and all round amazing gentleman. Ted had been invited in 1975 to speak to the US test pilots at Edwards Air Force Base in California, and after he landed he was invited to take a tour through one of the top secret hangars there, and in this hangar were a few glistening Mach 2.5 design B1A development aircraft. Now Ted had heard that Rockwell were having major difficulties with the engine intakes, and obviously had more than a passing interest in such things, and was allowed to take a close look. Just above and slightly forward of each intake he observed several beautiful made precision total pressure probes mounted under the wings, and although he had a good idea what they were for, said nothing at the time.
That evening, Ted gives his presentation speech to the assembled Test pilots, explaining in fair detail how the Concorde engine intake operated, and that the fact that unlike most other supersonic designs, the engine power was more or less freely variable at Mach 2 and above, even to the extent that if necessary the throttle could be closed all the way to the idle stop. There allegedly many gasps of amazement and disbelief in the room at this, and one B1A pilot was heard to ask his boss 'why the hell can't WE do that John'?. (It should be borne in mind here that the 'traditional' way of slowing down Mach 2+ aircraft is not to touch the throttles initially, and just cut the afterburners. If you don't do it this way many designs will drive into unstart and even flame-out).
After the audience had asked Ted several questions about Concorde, Ted was then invited to ask the assembled USAF and Grumman personnel about the B1A programme, which would be honestly answered within the confines of security considerations. Ted said that he only had one real point to raise; 'I see that you are having major difficulties with wing boundary level interference at the engine inlets'. There was now a gasp of horror from various members of the USAF entourage, 'That's top secret, how the hell do you know that?'. Ted chortled 'it's easy, I saw that you have a multitude of precision pressure sensors under the wing forward of the intakes, that I assume are to measure the wing boundary flows'. Ted then unhelpfully comes up with 'Oh, and you've got the design completely wrong, your intakes are mounted sideways, and that allows the intake shocks to rip into the wing boundary layer, which will completely screw up your inlets at high supersonic speeds. That in my opinion is where most of your problems lie, with wing boundary level interference, but I think that your probes for measuring boundary layer are beautiful, we never had such things'. According to Ted there was not so much uproar at the meeting as much as horror and amazement that this (even then) quite senior in years British aerodynamicist had in a few seconds observed the fundamental design flaw in an otherwise superb but top secret aircraft, and could even see what they were trying to do about it. Ted was asked, 'so you had no boundary layer issues with Concorde then?' Oh we had a few, mainly with the diverter section mounted above the intake' replies Ted, 'but we sorted out the problems relatively easily. 'You said that you did not use precision pressure probes under the wing to measure boundary layer flow fields, so what DID you use then?', asks a Rockwell designer. 'Some oil lamps and diesel oil' replies Ted. The room is now filled with laughter from all those assembled, but Ted shouts 'I am serious, it's an old wind tunnel trick. You mix up diesel oil with lamp black, which you then paint over the wing surface forward of the intakes, where it forms a really thick 'goo', which sticks like glue to the wing'. The pilots in particular seem quite fascinated now, and Ted goes on; 'You fly in as cold air that you can find (we flew out of Tangiers and Casablanca) and flew as fast as you could. As the skin temperature increases with Mach number, the diesel and lamp black 'paint goo' becomes quite fluid, and start to follow the boundary layer flow field. You then decelerated as rapidly as possible, and the flow field 'picture; is frozen into the now again solid 'goo'. After we landed we just took lots of pictures, repeated the process for several flights until we know everything that we needed to know about our difficulties. After doing some redesign work we then repeated the exercise again several times, eventually proving that we'd got things right'. The audience asked Ted if this technique might help them with the B1A, but he replied that although it might help them with accurately illustrating the problem, in his opinion it was irelevant, 'because the intakes are the wrong way round'.
The B1A intake problems were never resolved, and in 1977 the project was cancelled, due to performance and cost issues. However the project was reborn as the B1B, not entering service until 1986. Although an amazing aircraft, with astonishing low altitude performance and capability, it is a fixed intake design, limited to Mach 1.6 at altitude. Ted was right it seems."
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Unread post06 Sep 2019, 00:18

sprstdlyscottsmn wrote:Okay, let's talk conservation. Total air pressure stays "constant". When dynamic pressure (caused by moving air) increases the static pressure (pressure perpendicular to a surface) decreases. Every time air passes through a compression shock wave (such as those seen in the video) the total pressure rises. By the time the air gets to the back of the spike, and to the bypass tubes, it has been compressed by shockwaves and slowed to subsonic speeds. This means the static pressure on the back of the spike, increased by both compression waves and reduced speed, is far greater than the static pressure on the front of the spike (which has only gone through one shockwave and is still very supersonic [high dynamic pressure and low static pressure]). Hope that was clear as mud.



I get how the pressure is created, just cannot see how pressure differential, ie potential energy, achieves thrust. I am probably a lost cause..
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sprstdlyscottsmn

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Unread post06 Sep 2019, 00:25

What a fantastic story! Thanks for sharing that. That is the kind of engineer I want to be when I'm older.
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Unread post06 Sep 2019, 04:13

Wouldn't with ADVENT a supercruise-capable B-1B would be possible due to the complexity of the intake being moved inside the engine itself?
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Unread post06 Sep 2019, 04:59

No need to wait for Advent. F119 could do it.
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Unread post06 Sep 2019, 13:49

Orangeburst wrote:The B1A intake problems were never resolved, and in 1977 the project was cancelled, due to performance and cost issues. However the project was reborn as the B1B, not entering service until 1986. Although an amazing aircraft, with astonishing low altitude performance and capability, it is a fixed intake design, limited to Mach 1.6 at altitude. Ted was right it seems."


:bang: The B-1A reached Mach 2.22. The B-1Bs top speed has Z-E-R-O to do with the fact that the inlets are oriented vertically. I wonder if anybody pointed out that the Mach 3 XB-70 also had it's inlets the "wrong way round".
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Unread post06 Sep 2019, 18:36

sferrin wrote:
Orangeburst wrote:The B1A intake problems were never resolved, and in 1977 the project was cancelled, due to performance and cost issues. However the project was reborn as the B1B, not entering service until 1986. Although an amazing aircraft, with astonishing low altitude performance and capability, it is a fixed intake design, limited to Mach 1.6 at altitude. Ted was right it seems."


:bang: The B-1A reached Mach 2.22. The B-1Bs top speed has Z-E-R-O to do with the fact that the inlets are oriented vertically. I wonder if anybody pointed out that the Mach 3 XB-70 also had it's inlets the "wrong way round".


Where does it say that the B-1A did not reach M2.2? Does not discount that they were not having inlet problems at speed. I don't know and just passing along an anecdote, probably from his book.
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krorvik

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Unread post06 Sep 2019, 20:40

Orangeburst wrote:I get how the pressure is created, just cannot see how pressure differential, ie potential energy, achieves thrust. I am probably a lost cause..


Others would need to weigh in here, but isn't it in fact mostly kinetic energy? The pressure is not creating the thrust itself, it is rather the difference in kinetic energy exerted by the gas on each side. Like "regular" lift on a wing.
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