Blackbird
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Supersonic Inlets / Number Of Shockwaves

Wed Aug 20, 2008 2:13 am

What's the typical and maximum mach number you can get with an inlet...

-That uses 1 shockwave to slow the flow to subsonic-speed?
-That uses 2 shockwaves to slow the flow to subsonic-speed?
-That uses 4 shockwaves to slow the airflow to subsonic-speed?
-That uses 8 shockwaves to slow the airflow to subsonic-speed?


Blackbird
 
tdscanuck
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RE: Supersonic Inlets / Number Of Shockwaves

Wed Aug 20, 2008 7:12 am



Quoting Blackbird (Thread starter):
What's the typical and maximum mach number you can get with an inlet...

-That uses 1 shockwave to slow the flow to subsonic-speed?
-That uses 2 shockwaves to slow the flow to subsonic-speed?
-That uses 4 shockwaves to slow the airflow to subsonic-speed?
-That uses 8 shockwaves to slow the airflow to subsonic-speed?

There's no maximum...they'll all work. You'll just have really horrible pressure losses (which can be overcome with excess thrust). For really high Mach numbers you'll have a heating problem with a really inefficient inlet, but I suppose you could build a carbon-carbon inlet if you really wanted to.

Typical is a whole 'nother matter. I would expect nobody uses 1 shock wave on current designs anymore...old MIG's probably did, with the round flat inlets, so that's probably good up into the Mach 1.5 region. Things like the F-14 and Foxbat could go Mach 2-2.5 with multi-shock inlets (I think they were 4). Offhand I can't think of any that use 8, although the SR-71 might have.

At some point the pressure losses get too bad, even with a nice inlet, which is why you switch over to scramjets and skip the whole issue (and pick up a bunch of others).

Tom.
 
Blackbird
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RE: Supersonic Inlets / Number Of Shockwaves

Thu Aug 21, 2008 11:26 pm

Tom,

Quote:
There's no maximum...they'll all work.

I thought the inlet would become so inefficient that it would be useless after awhile (in terms of thrust being so low and drag becoming so high)

Quote:
You'll just have really horrible pressure losses (which can be overcome with excess thrust). For really high Mach numbers you'll have a heating problem with a really inefficient inlet, but I suppose you could build a carbon-carbon inlet if you really wanted to.

The pressure losses are a result of the shock-wave formation I would assume? Wouldn't the airflow become more turbulent the more powerful the shockwaves as well?

[quote]Typical is a whole 'nother matter. I would expect nobody uses 1 shock wave on current designs anymore...old MIG's probably did, with the round flat inlets, so that's probably good up into the Mach 1.5 region.

I would have figured you could have made it to 2... didn't the F-16 use a pitot-type inlet?

Quote:
Things like the F-14 and Foxbat could go Mach 2-2.5 with multi-shock inlets (I think they were 4).

I don't know what the number of shockwaves in the MiG-25's inlets are, but 4-shocks sounds about correct for the F-14.

Regarding high-performance inlets, many high-performance inlet-designs often use porous ramps and surfaces and such to remove turbulent airflow. That I assume would improve performance over the typical figures for a multi-shock inlet (ie. the theoretical maximum efficiency for a 4-shock duct with porous ramps would be more efficient than the theoretical maximum for a 4-shock duct without porous ramps)

Quote:
Offhand I can't think of any that use 8, although the SR-71 might have.

Okay, could you give me a guess as to what mach number you could get to with a 90-percent efficiency assuming the inlet uses substantial amount of porous-surface?


BTW: I don't know how many shocks the A-12/YF-12/SR-71/M-21 use either -- the aircraft I'm asking questions about is not the Blackbird.
 
tdscanuck
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RE: Supersonic Inlets / Number Of Shockwaves

Fri Aug 22, 2008 2:16 am



Quoting Blackbird (Reply 2):

Quote:
There's no maximum...they'll all work.

I thought the inlet would become so inefficient that it would be useless after awhile (in terms of thrust being so low and drag becoming so high)

There's certainly a climbing inefficiency, but you're never going to shock down to below ambient pressure (with any normally shaped inlet) so you can still make positive thrust. It's just a question of how large an engine are you willing to pack around.

Quoting Blackbird (Reply 2):
I would have figured you could have made it to 2... didn't the F-16 use a pitot-type inlet?

I'm not sure on the F-16...that's a weird one that could go either way. I found an AIAA paper that discusses an improved F-16 inlet for the J79 engine and that one is definitely a multi-shock, but it's not clear if the original one was or not. The drag improvement with the multi-shock is dramatic, so I would hazard a guess that the original wasn't, but was on the bad edge of the performance curve when it got up to Mach 2.

Quoting Blackbird (Reply 2):
Okay, could you give me a guess as to what mach number you could get to with a 90-percent efficiency assuming the inlet uses substantial amount of porous-surface?

I really haven't the foggiest, since it really depends what the engine is. Can you tolerate augmetors (afterburner) during cruise or do you need to supercruise? Etc.

Tom.
 
Blackbird
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RE: Supersonic Inlets / Number Of Shockwaves

Sat Aug 23, 2008 3:59 am



Quote:
I really haven't the foggiest, since it really depends what the engine is. Can you tolerate augmetors (afterburner) during cruise or do you need to supercruise? Etc.

Okay, the engine is a single-spool turbojet with a relatively low pressure-ratio (~9:1), subsonic intake-velocity, with a modulating afterburner, with low AB settings used for cruise.


Blackbird
 
tdscanuck
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RE: Supersonic Inlets / Number Of Shockwaves

Sat Aug 23, 2008 4:51 am



Quoting Blackbird (Reply 4):
Okay, the engine is a single-spool turbojet with a relatively low pressure-ratio (~9:1), subsonic intake-velocity, with a modulating afterburner, with low AB settings used for cruise.

Unfortunately, I can't help you. I was trying to suggest the types of questions that would need to be answered to provide the Mach number you're looking for. However, even with the answers to those questions, I don't know how to do the detailed calculations. I have a pretty good conceptual grasp of supersonic aerodynamics but I don't have the detailed skills to do much beyond very simple numeric estimates. If we're lucky, somebody on the forum does.

What you're describing sounds like some known applications that span the whole range from about Mach 1.2 to about 3.5, so I imagine the theoretical limit is even higher than that.

Tom.
 
Blackbird
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RE: Supersonic Inlets / Number Of Shockwaves

Sat Aug 23, 2008 5:15 pm

Oh well, I gave it a shot...

By the way, do reflected shocks count as individual shockwaves?

(I would assume so right? Either way, I made a mistake -- the diagram I saw was overly simplistic, and didn't show the reflected waves. Either way I checked a book I have, and I count a total of 13 to 14 shockwaves minimum if you count the reflections. There could be 15 or 16, but the drawing also shows the airflow pattern in terms of the flow directions into the duct, and it's hard to tell which are which, however those 2-3 extra lines that I counted seem to match the flow-directions into the duct, not the shockwave -- so, a count of either 13 or 14 seems more accurate)


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vikkyvik
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RE: Supersonic Inlets / Number Of Shockwaves

Sat Aug 23, 2008 7:08 pm



Quoting Blackbird (Reply 6):

By the way, do reflected shocks count as individual shockwaves?

There's no particular reason I can see why they wouldn't. They act as their own oblique shock waves.
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
tdscanuck
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RE: Supersonic Inlets / Number Of Shockwaves

Sat Aug 23, 2008 7:33 pm



Quoting Blackbird (Reply 6):
By the way, do reflected shocks count as individual shockwaves?

As far as I know, yes. The shock relationships hold whether it's a reflected or non-reflected shock, so from the point of view of air passing through them they're the same thing (albeit at different angles).

Tom.
 
Blackbird
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RE: Supersonic Inlets / Number Of Shockwaves

Sat Aug 23, 2008 9:22 pm

That's a lot of shocks then


Blackbird

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