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 Shockwave Position And Subsonic/Supersonic LE Flow
 Blackbird From , joined Dec 1969, posts, RR: Posted Wed Apr 23 2008 19:04:29 UTC (8 years 1 month 3 days 12 hours ago) and read 4299 times:

 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 From Canada, joined Jan 2006, 12710 posts, RR: 78 Reply 1, posted Thu Apr 24 2008 00:06:24 UTC (8 years 1 month 3 days 7 hours ago) and read 4283 times:

 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 From , joined Dec 1969, posts, RR: Reply 2, posted Thu Apr 24 2008 11:34:26 UTC (8 years 1 month 2 days 20 hours ago) and read 4259 times:

 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 From Canada, joined Jan 2006, 12710 posts, RR: 78 Reply 3, posted Thu Apr 24 2008 17:54:59 UTC (8 years 1 month 2 days 13 hours ago) and read 4240 times:

 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 From UK - England, joined Sep 2001, 862 posts, RR: 0 Reply 4, posted Thu Apr 24 2008 19:03:34 UTC (8 years 1 month 2 days 12 hours ago) and read 4236 times:

 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 From , joined Dec 1969, posts, RR: Reply 5, posted Thu Apr 24 2008 19:13:25 UTC (8 years 1 month 2 days 12 hours ago) and read 4235 times:

 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 From Canada, joined Jan 2006, 12710 posts, RR: 78 Reply 6, posted Thu Apr 24 2008 20:01:42 UTC (8 years 1 month 2 days 11 hours ago) and read 4230 times:

 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 From , joined Dec 1969, posts, RR: Reply 7, posted Thu Apr 24 2008 20:21:17 UTC (8 years 1 month 2 days 11 hours ago) and read 4229 times:

 Tdscanuck, How much knowledge do you have on supersonic aircraft design? Andrea Kent
 Tdscanuck From Canada, joined Jan 2006, 12710 posts, RR: 78 Reply 8, posted Thu Apr 24 2008 23:42:40 UTC (8 years 1 month 2 days 8 hours ago) and read 4213 times:

 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 From Canada, joined Jan 2006, 12710 posts, RR: 78 Reply 9, posted Thu Apr 24 2008 23:52:38 UTC (8 years 1 month 2 days 7 hours ago) and read 4211 times:

 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.

 Wingscrubber From UK - England, joined Sep 2001, 862 posts, RR: 0 Reply 10, posted Fri Apr 25 2008 16:34:03 UTC (8 years 1 month 1 day 15 hours ago) and read 4178 times:

 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 From , joined Dec 1969, posts, RR: Reply 11, posted Fri Apr 25 2008 22:44:05 UTC (8 years 1 month 1 day 9 hours ago) and read 4164 times:

 Wingscrubber, I'm aware drag is much higher at supersonic speeds than subsonic. Andrea Kent
 Wingscrubber From UK - England, joined Sep 2001, 862 posts, RR: 0 Reply 12, posted Sun Apr 27 2008 14:30:52 UTC (8 years 4 weeks 1 day 17 hours ago) and read 4135 times:

 well Im sorry to sound patronizing but if you ask an outragiously stupid question you get an outragiously stupid answer
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 Blackbird From , joined Dec 1969, posts, RR: Reply 13, posted Sun Apr 27 2008 18:29:58 UTC (8 years 4 weeks 1 day 13 hours ago) and read 4127 times:

 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
 Tdscanuck From Canada, joined Jan 2006, 12710 posts, RR: 78 Reply 14, posted Sun Apr 27 2008 20:46:42 UTC (8 years 4 weeks 1 day 11 hours ago) and read 4120 times:

 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 From , joined Dec 1969, posts, RR: Reply 15, posted Sun Apr 27 2008 21:03:42 UTC (8 years 4 weeks 1 day 10 hours ago) and read 4118 times:

 Tom, What does that mean "the velocity component normal to the leading edge is subsonic?" Andrea Kent
 Tdscanuck From Canada, joined Jan 2006, 12710 posts, RR: 78 Reply 16, posted Mon Apr 28 2008 19:34:04 UTC (8 years 4 weeks 12 hours ago) and read 4082 times:

 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 From , joined Dec 1969, posts, RR: Reply 17, posted Mon Apr 28 2008 19:59:44 UTC (8 years 4 weeks 11 hours ago) and read 4081 times:

 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 From Bulgaria, joined Mar 2003, 2734 posts, RR: 15 Reply 18, posted Mon Apr 28 2008 20:43:13 UTC (8 years 4 weeks 11 hours ago) and read 4077 times:

 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.
 Blackbird From , joined Dec 1969, posts, RR: Reply 19, posted Mon Apr 28 2008 21:48:47 UTC (8 years 4 weeks 10 hours ago) and read 4071 times:

 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
 Sovietjet From Bulgaria, joined Mar 2003, 2734 posts, RR: 15 Reply 20, posted Mon Apr 28 2008 22:34:29 UTC (8 years 4 weeks 9 hours ago) and read 4069 times:

 Andrea, no need to get snappy, it was merely a suggestion. I have that book and I got it for much less than 185  . If you ever really get serious about buying it, try out half.com or amazon I found it for \$85 not \$185.
 Vikkyvik From United States of America, joined Jul 2003, 12133 posts, RR: 24 Reply 21, posted Mon Apr 28 2008 22:48:20 UTC (8 years 4 weeks 9 hours ago) and read 4067 times:

 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.

 I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
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