Blackbird
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Shockwave Position And Subsonic/Supersonic LE Flow

Thu Apr 24, 2008 2:04 am

I know that if the shockwave sweep angle is less than the sweep angle of the wing, the airflow over the leading edge behaves as if it's subsonic. I also know if the shock sweeps beyond the angle of the wing and crosses over the leading edge the wing behaves totally supersonic.

What I don't know is how does the airflow behave if the airflow over the leading-edge if the shockwave is equal to the sweep of the wing, but in front of it (like on the XB-70, it has a 72-degree sweep apex, which then quickly tapers to the 65-degree wing-LE -- the shock sweep equals the wings)


Andrea Kent
 
tdscanuck
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RE: Shockwave Position And Subsonic/Supersonic LE Flow

Thu Apr 24, 2008 7:06 am



Quoting Blackbird (Thread starter):
What I don't know is how does the airflow behave if the airflow over the leading-edge if the shockwave is equal to the sweep of the wing, but in front of it (like on the XB-70, it has a 72-degree sweep apex, which then quickly tapers to the 65-degree wing-LE -- the shock sweep equals the wings)

With something like the XB-70, you're talking about oblique shock waves and the flow is supersonic on both sides of the shockwave. The wing should be seeing entirely supersonic flow.

Tom.
 
Blackbird
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RE: Shockwave Position And Subsonic/Supersonic LE

Thu Apr 24, 2008 6:34 pm

I remember reading something like if the shockwave was in front of the wing you'd get a subsonic flow component or something over the leading edge. If I recall that's why supersonic planes used conical-camber and some highly swept wings used blunt leading-edges. From what I remember it didn't matter if the wave was a normal or oblique shock.

I think it had something to do with the angle of the airflow to the shock or something.


Andrea Kent

[Edited 2008-04-24 11:34:54]
 
tdscanuck
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RE: Shockwave Position And Subsonic/Supersonic LE Flow

Fri Apr 25, 2008 12:54 am



Quoting Blackbird (Reply 2):
I remember reading something like if the shockwave was in front of the wing you'd get a subsonic flow component or something over the leading edge. If I recall that's why supersonic planes used conical-camber and some highly swept wings used blunt leading-edges. From what I remember it didn't matter if the wave was a normal or oblique shock.

I think it had something to do with the angle of the airflow to the shock or something.

The angle of the airflow to the shock is the difference between a normal and oblique shock, so if one matters then so does the other.

Tom.
 
wingscrubber
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RE: Shockwave Position And Subsonic/Supersonic LE Flow

Fri Apr 25, 2008 2:03 am

andrea, you're describing transonic flow. The only way you would be pushing a shockwave with subsonic regions of flow behind it is close to, but not exceeding Mach 1. A sub-sonic airliner however may see supersonic flow over the top of an airfoil at its critical Mach no. The easiest way to visualize a shockwave is to imagine the bow wave of a boat.
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Blackbird
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RE: Shockwave Position And Subsonic/Supersonic LE Flow

Fri Apr 25, 2008 2:13 am

Then why can you use blunt leading edges and conical cambers on highly swept wings/delta-wings while going supersonic and not have outrageously high drag??


Andrea Kent
 
tdscanuck
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RE: Shockwave Position And Subsonic/Supersonic LE Flow

Fri Apr 25, 2008 3:01 am



Quoting Blackbird (Reply 5):
Then why can you use blunt leading edges and conical cambers on highly swept wings/delta-wings while going supersonic and not have outrageously high drag??

Conical camber really shouldn't be much of a factor...from the flow's point of view, it just looks like a changing angle of attack across the span.

Blunt leading edges is more interesting...I suspect that it's because, on a highly swept wing, a blunt leading edge looks sharper. For very fast aircraft there's another issue that kicks in...aerodynamic heating. A sharp leading edge typically has an attached shock wave, which causes huge heat transfer. The sharp profile has low high area/volume ratio and hence lousy heat capacity...you could melt the edges if you're not careful. A blunt profile will throw a detached shockwave which, although it's draggier, causes far less heat transfer into the structure and is easier to design to withstand the heating. This is why, for example, ICBM's tend to have blunt noses even though they're going something like Mach 30 when they come back down.

Tom.
 
Blackbird
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RE: Shockwave Position And Subsonic/Supersonic LE Flow

Fri Apr 25, 2008 3:21 am

Tdscanuck,

How much knowledge do you have on supersonic aircraft design?


Andrea Kent
 
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RE: Shockwave Position And Subsonic/Supersonic LE Flow

Fri Apr 25, 2008 6:42 am



Quoting Blackbird (Reply 7):
How much knowledge do you have on supersonic aircraft design?

Err...relative to what? I've got a BSE in aeronautical engineering (also mechanical engineering, but that doesn't study supersonic flows much) and I'm about half way through my MS in aeronautical engineering. Naturally, that encompasses a fair amount of aerodynamics and aircraft design study. My day job doesn't involve supersonic design (lots of transonic duct flow though), so I don't know nearly as much as the guys who do it for a living. But I've had good professors, read a lot, and remember what I read.

Tom.
 
tdscanuck
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RE: Shockwave Position And Subsonic/Supersonic LE Flow

Fri Apr 25, 2008 6:52 am



Quoting Tdscanuck (Reply 6):
The sharp profile has low high area/volume ratio and hence lousy heat capacity...you could melt the edges if you're not careful.

That'll teach me to change tacks in mid-sentence. That should have said "high area/volume" ratio. Lots of area, little volume...high conductive and radiative heat transfer and not much structure to absorb it.

Tom.
 
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RE: Shockwave Position And Subsonic/Supersonic LE

Fri Apr 25, 2008 11:34 pm

Andrea, supersonic aircraft do incur outrageously high drag, that's why they fly at outrageously high altitude with outrageously powerful engines. Remember supersonic aerodynamics do not obey the same rules as subsonic air, and if still in doubt about induced supersonic drag, take a look at the Thrust SSC (supersonic car). It's as aerodynically slippery as you can get with NO wings, using the engines from the Mach 2 capable F4 Phantom and it barely broke the sound barrier.
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Blackbird
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RE: Shockwave Position And Subsonic/Supersonic LE Flow

Sat Apr 26, 2008 5:44 am

Wingscrubber,

I'm aware drag is much higher at supersonic speeds than subsonic.


Andrea Kent
 
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RE: Shockwave Position And Subsonic/Supersonic LE Flow

Sun Apr 27, 2008 9:30 pm

well Im sorry to sound patronizing but if you ask an outragiously stupid question you get an outragiously stupid answer  Smile
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Blackbird
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RE: Shockwave Position And Subsonic/Supersonic LE Flow

Mon Apr 28, 2008 1:29 am

I remember reading in a book about the Concorde that the airflow along the leading edge was subsonic with cambered areas to recoup "suction losses" -- the wing is highly swept and does not look like it would form a normal shock, so it would only form an oblique shock. Then how is the leading edge subsonic?


Andrea Kent
 
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RE: Shockwave Position And Subsonic/Supersonic LE Flow

Mon Apr 28, 2008 3:46 am



Quoting Blackbird (Reply 13):
I remember reading in a book about the Concorde that the airflow along the leading edge was subsonic with cambered areas to recoup "suction losses" -- the wing is highly swept and does not look like it would form a normal shock, so it would only form an oblique shock. Then how is the leading edge subsonic?

If you're flying supersonic, you shouldn't have subsonic flow anywhere except at the stagnation points (and in the engine inlets). The flow leaving the airfoils needs to match freestream fairly closely. The book may have meant that the velocity component normal to the leading edge is subsonic.

Tom.
 
Blackbird
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RE: Shockwave Position And Subsonic/Supersonic LE Flow

Mon Apr 28, 2008 4:03 am

Tom,

What does that mean "the velocity component normal to the leading edge is subsonic?"


Andrea Kent
 
tdscanuck
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RE: Shockwave Position And Subsonic/Supersonic LE Flow

Tue Apr 29, 2008 2:34 am



Quoting Blackbird (Reply 15):

What does that mean "the velocity component normal to the leading edge is subsonic?"

Velocity is a vector. You can decompose a vector into components along any coordinate system you choose. One coordinate system that can be convenient is one where one axis is normal (square) to the leading edge and the other is parallel to the leading edge. The flow component normal to the leading edge is the one that "sees" the airfoil. The airfoil is almost invisible to the component parallel to the leading edge because the change in wing geometry in the spanwise direction is much more gradual than the change in the chordwise direction.

For highly swept wings and moderate Mach numbers, the component of velocity that's square to the leading edge may be subsonic.

Tom.
 
Blackbird
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RE: Shockwave Position And Subsonic/Supersonic LE Flow

Tue Apr 29, 2008 2:59 am

How do you define highly-swept (~60 degrees, ~75-degrees, etc), and moderate mach numbers? (Mach 1, or 2, or like Mach 1-3)


Andrea Kent
BTW: I think this is what I was reading about... if the wingroot shockwave sweep-angle is moderately less than the sweep of the wing's leading edge the velocity component normal to the leading edge could be subsonic. Although I'm not sure exactly.
 
sovietjet
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RE: Shockwave Position And Subsonic/Supersonic LE Flow

Tue Apr 29, 2008 3:43 am

I don't think there is a definition per se for highly swept....I think anything above 60 degrees should be considered that way but of course different people will have different limits.

Same with moderate mach number...as long as you're not reaching hypersonic flow which starts at around Mach 5 the mach number can be considered moderate.

You're questions are interesting...I suggest you pick up a copy of Anderson's "Fundamentals of Aerodynamics"...I bet it will answer a lot of your questions.
 
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RE: Shockwave Position And Subsonic/Supersonic LE Flow

Tue Apr 29, 2008 4:48 am

Soviet Jet

I have heard great things about that book actually.

You do realize I don't have an infinite amount of money at my disposal, and I have other things to do for the time being than buy a 185 dollar book. I'll get around to it though.


BTW: Anybody have any rule of thumbs for this phenomenon -- subsonic velocity-component normal to the leading edge at supersonic speeds, vs wing-sweep and shock-angle?


Andrea Kent
 
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RE: Shockwave Position And Subsonic/Supersonic LE Flow

Tue Apr 29, 2008 5:34 am

Andrea, no need to get snappy, it was merely a suggestion. I have that book and I got it for much less than 185  Smile. If you ever really get serious about buying it, try out half.com or amazon I found it for $85 not $185.
 
vikkyvik
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RE: Shockwave Position And Subsonic/Supersonic LE Flow

Tue Apr 29, 2008 5:48 am



Quoting Blackbird (Reply 19):
You do realize I don't have an infinite amount of money at my disposal, and I have other things to do for the time being than buy a 185 dollar book. I'll get around to it though.



Quoting Sovietjet (Reply 18):
You're questions are interesting...I suggest you pick up a copy of Anderson's "Fundamentals of Aerodynamics"...I bet it will answer a lot of your questions.

That's fine if you have other things to do. But people will be less and less likely to answer your questions if you don't do some research on your own.

"Fundamentals of Aerodynamics" is sitting on my bed right next to me as I'm typing this.

Blackbird, I got it for something like $120. The school bookstore was selling it for $190 or so, but you can get it cheaper online.

Not to mention, you could probably find it at a university engineering library.

That said, I'm not exactly sure what your question is. There are standard tables for oblique shock angles, relative to the incoming freestream Mach number and the flow diversion angle (i.e. around a wedge-shaped object). I wouldn't be surprised if you could find them somewhere online.
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