jhooper
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Supersonic Flight

Sat Jun 07, 2003 7:18 am

Why is it so difficult for an aircraft to exceed Mach 1?
Last year 1,944 New Yorkers saw something and said something.
 
Ikarus
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RE: Supersonic Flight

Sat Jun 07, 2003 10:14 am

When an aircraft flies through air, the air molecules have to be moved out of the way. Air is not space - it is matter, full of gas molecules. Mach 1 is more or less the (maximum) speed that they can move at. If an object passes faster than that, it does not merely shift them out of the way by moving them up and down a bit around itself, it does not glide through the air. Instead, it's like it's hitting a wall of air and dragging a giant curtain along with it. A shockwave does just that: It accelerates, within a thickness of just a few molecules, a wall of air in the direction of movement of the object, not just laterally out of the way.

Let's look at it from the point of view of the supersonic plane:
Ahead of it, the air is approaching at a speed greater than Mach 1. When the air hits the nose, it cannot shift up or down quick enough, so the air is jerked forward, forming a shockwave, and dragging adjacent air molecules forward with it.

This is probably an oversimplification, and quite frankly I am tired and not sure whether that explanation is quite accurate (after all, the thing about air molecules meeting up behind a wing and having a longer distance above the wing than below the wing is pure BS, too, so I tend to distrust these explanations I got as kid... perhaps someone will embarass me in a few moments and give a better or truer explanation as to the causes of shockwaves)

But essentially, at Mach 1, shockwaves form. These accelerate large portions of air very quickly and require a lot of energy to sustain. There's a good reason for the boom of Concorde and other supersonic planes, and that very boom costs energy.

This has several results: Firstly, the drag rises significantly due to the required energy - so much that most airplane engines cannot keep up the speed of the plane. Secondly, the shockwaves and supersonic flow can cause structural problems (the increased drag can shear off engines and stress the wings beyond their limits), and they can also cause loss of control. Therefore flying above Mach 1 in a craft designed for subsonic operations is highly undesirable...

Regards

Ikarus
 
AWspicious
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RE: Supersonic Flight

Sat Jun 07, 2003 5:11 pm

Interesting this topic was posted. Just today I was wondering if it's at all possible to achieve supersonic flight without creating a sonic boom. Will there possible be a day when modern (future) technology find a way to avoid that one major controversial side-effect?

A.W.
Nevermind political correctness - Envision using your turn signals!
 
Ikarus
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RE: Supersonic Flight

Sat Jun 07, 2003 7:28 pm

awspicious:

In theory, there may be, though I doubt it.

Why could there be such a thing? A lot of what I described is based on one- or twodimensional theory. In 2D, nothing can go supersonic without a shock. But what has been found is that in the threedimensional realm (i.e. reality) it is sometimes possible to avoid shockwaves, although only very close to Mach 1 and I do not fully understand why or how.

The other thing is that shock waves (and therefore sonic booms) aren't all of the same strengths. Concorde was designed in a very traditional way. It's entire shape is designed with shockwaves in mind: A strong shockwave starting at the nose, and perhaps one when the flow joins together again behind the plane. That is traditional and relatively efficient, drag-wise.

But some people think that a future supersonic transport design would be approached differently. By attempting to make full use of threedimensional effects, it would be possible to design a plane with weaker shockwaves, less of a boom (though still there, the boom might be less noisy). Or an asymmetric shock distribution, with more stronger shockwaves above the plane, but weaker ones below it. I doubt that any future supersonic transport designs would fly at Mach 2 over land, but Mach 1.4 or 1.5 may well be possible.

Indeed, the Sonic Cruiser design is likely to have been strongly influenced by shockwave engineering. Supersonic Business Jets may be a reality before the next public transport plane, and if they are, expect them to look different from today's planes.

All in all, there's still a lot of research going on. But who knows, in 20 years we might see supersonic transport be a reality once again, this time better and more widespread....

Regards

Ikarus
 
Guest

RE: Supersonic Flight

Sat Jun 07, 2003 10:14 pm

As one who has spent thousands of hour near Mach 1.0 but never exceeded it, I was fascinated by a couple of experiences that I had while riding on B727s several years ago. On two separate occasions, while sitting at window seats, I was able to see the small shadow of the shockwave on the wings upper surface. And, by golly, it looked exactly like the ones in the wind tunnels that we've all seen.

With the upcoming demise of Concord, it looks like I will not be able to achieve one of my life's goals - to cross the Atlantic on Concord and return on a sailing ship. Hmmm... My dream may yet live. Anyone know of a privately owned T38 with a refueling probe?

Jetguy
 
Ikarus
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RE: Supersonic Flight

Sun Jun 08, 2003 12:23 am

Jetguy:
You can actually see the shadow of shockwaves on most wings of most jetliners, if the conditions are right (generally that means the sun is almost vertically above the aircraft - noon!) I've also seen the shockwaves (not shadows, shockwaves themselves!) around the engine inlet of a Germanwings A319 when descending towards CGN - I've got about a dozen photos of that because I got too excited... Big grin

These shocks are recompression waves - the air accelerates to supersonic flow without causing a shockwave (that is actually possible! I almost forgot!), but when decelerating from supersonic to subsonic flow, shockwaves always occur. So basically that means if you could design a plane to fly supersonically that accelerates the flow into supersonic regimes without a shock, and then decelerates the flow to something quite close to Mach 1 prior to a recompression shock forming, the shockwaves would be minimal, and the plane would be not much more noisy than today's airliners. However, I doubt that such a design is physically possible....

Vortices and shockwaves. Those are aerodynamic phenomena I love to look out for!

Regards

Ikarus
 
gigneil
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RE: Supersonic Flight

Sun Jun 08, 2003 4:35 am

The bizjet folks are really looking into the boomless supersonic transport thing... its not that it will actually BE boomless, but the boom will not reach the ground at an audible level.
 
Sonic
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RE: Supersonic Flight

Mon Jun 09, 2003 4:32 am

I am not sure of this, but would supersonic boomless bizjet/SST really help? Since the ban is of supersonic flight over land, not of booms (acceleration and deceleration). If the ban would be only for acceleration and decleration over land, then I believe Concorde would have been much more popular - such flights as LHR-DXB (booms over English Chanell and Persian Gulf) would be possible, thus usage of Concorde could be expanded to Europe-Middle East and other same distance destinations - many major cities are near oceans or seas thus SST could decelerate-accelerate over them and then turn to destination.
If you are saying that countries will drop ban when boomless SST is made, I very doubt this - if Russia will create such a plane, then probably few countries will remove bans, if Airbus will do so, probably USA won't remove ban and thus would limit the use of the aircraft severely. If Boeing would build it, some EU states and (probably) Russia wouldn't remove ban or lag the removing of it.
 
Bellerophon
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RE: Supersonic Flight

Mon Jun 09, 2003 5:31 am

Sonic

If I have understood you correctly, you seem to think sonic booms only occur during acceleration or deceleration.

If this is what you think, then you are not correct.

An aircraft in supersonic flight will trail a sonic boom behind it throughout its flight.

Regards

Bellerophon
 
Ikarus
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RE: Supersonic Flight

Mon Jun 09, 2003 6:34 am

Sorry if my explanation gave the wrong impression...

Booms (shockwaves) occur at all times when an object flies faster than the speed of sound.

When I talk of accelerating & decelerating the air, I don't mean that the plane itself needs to be accelerating or decelerating. Put it like this: You walk at constant speed. While you walk, you shift air out of the way, it passes beside you and behind you but not through you. So you take still air, accelerate it so that it moves around you. If you were to walk at the speed of sound, the air would not just smoothly be shifted out of the way, it would actually encounter almost infinite acceleration, the shock, and the velocity would jump up instantaneously. That would happen at all times while you walk supersonically - because you keep accelerating different bits of air all along your path.

Regards

Ikarus
 
Guest

RE: Supersonic Flight

Mon Jun 09, 2003 7:59 am

Icarus,
I felt exactly the same way - it was exciting to see it. It's just that 99.9% of my flight time in high speed aircraft is sitting in one of the two front seats. To be able to sit back in the cabin, at a window seat, and actually see the shockwave was a real treat.

Jetguy
 
B2707SST
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RE: Supersonic Flight

Mon Jun 09, 2003 10:16 am

For a thorough but understandable guide to supersonic aerodynamics, I recommend Larry Reithmaier's Mach One and Beyond. One note that hasn't come up is that most supersonic designs require the wing leading edge to be clear of the shock wave at cruising speed. Since the angle of the shock increases with Mach number, faster aircraft require higher angles of sweep, often in the form of a delta wing, which is relatively inefficient at low speeds. Variable-sweep wings, another way to get around the problem, are heavy and mechanically complex.

Air friction at supersonic speeds can also demand exotic construction materials: at Mach 2, Concorde gets away with a mostly aluminum airframe, which can handle 200 degree (F) skin temperatures, but designers may have to go to titanium for anything much faster. By Mach 2.7, kinetic heating rises to 500 degrees (F) and Mach 3 generates temperatures of 750 degrees.

Some research has been done on low-boom designs, especially in trying to break the strong bow and tail shocks that cause the typical double-boom into a series of weaker shockwaves that might be audible as a roar or rumble instead of a sharp clap. I recall that low-boom designs involved unappealing aerodynamic tradeoffs, which compromised operating costs and profitability.

For a while in the early 1960s, engineers hoped that cruising altitudes of 60,000 - 70,000 feet would dissipate an SST's boom to acceptable levels. Tests with the XB-70 and SR-71 showed this not to be the case and the FAA reluctantly conceded that SSTs would not be able to fly over land. The Europeans and Sud Aviation in particular were much more optimistic, with the French holding out for a medium-range Concorde as late as 1964, and they tended to minimize the sonic boom problem until Concorde tests over western England and Scotland caused a public uproar.

Usually sonic booms don't reach the ground until Mach 1.15 is exceeded, because temperature layers in the atmosphere tend to refract the booms away from the ground, but this speed is too low to get out of the high-drag transonic region. Boeing mentioned a Sonic Cruiser "sweet spot" at Mach 1.2, which would probably cause an audible boom under most conditions. A light, small bizjet might get away with a low-boom design, but I still doubt it would be able to fly US transcon. A large 300-seat SST certainly will be limited to over-water supersonic flight for the foreseeable future.

--B2707SST

[Edited 2003-06-09 03:25:32]
Keynes is dead and we are living in his long run.
 
Bellerophon
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RE: Supersonic Flight

Mon Jun 09, 2003 11:21 am

B2707SST

I haven’t seen the book you mention, could you let me have the ISBN number please?

For Concorde, the skin temperature limit at M2.04 is 127°C or 261°F.

Mach 3 would have been ideal from an engine point of view, but as you say, the material to be used in Concorde’s construction, an aluminium alloy, was not suitable for the temperatures which would have been encountered of around 333°C or 629°F. The rise in temperature with increasing Mach (Ram Rise) is most definitely non-linear.

At the design stage, the cruise Mach number was initially set at M2.20, but this was later reduced to M2.04, because of problems keeping the fuel, oil and passenger cabin cool, and BA cruise climb Concorde at M2.00, to allow a small margin beneath the limit.

A titanium built Concorde would have handled the temperatures at Mach 3, from a purely structural point of view, with ease. The problems these temperatures would have caused in a host of other areas could not have been overcome in a civilian aircraft at that time, and would still be extremely difficult today.

Regards

Bellerophon
 
B2707SST
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RE: Supersonic Flight

Mon Jun 09, 2003 12:17 pm

Bellerophon:

The ISBN is 0070520216 and can be found at Amazon.com. The content is rather basic for an aviation professional -- the author discusses subjects like compressible flows, transonic pitch-down, normal/oblique/expansion shocks, airfoil types, wing sweep, and kinetic heating -- but it still makes a good reference. Appendices on the SR-71 and Lockheed's L-2000 SST entry are nice additions.

--B2707SST

Keynes is dead and we are living in his long run.
 
Bellerophon
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RE: Supersonic Flight

Mon Jun 09, 2003 6:23 pm

B2707SST

Many thanks

Bellerophon
 
Klaus
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Bellerophon

Wed Jun 11, 2003 10:31 am

Bellerophon: Mach 3 would have been ideal from an engine point of view

Why?

Or did you mean that it would have been the maximum attainable speed with the engine technology available?
 
Guest

RE: Supersonic Flight

Thu Jun 12, 2003 12:16 am

As I understand it, the big challenge with supersonic flight from the engine perspective is difficulties encountered with a gas turbine using supersonic air. Currently, the air must be slowed to transonic speeds in order for it to be usable.

Perhaps I should check my copy of Aerodynamics for Naval Aviators. Partly I just wanted to post something to say "hey" to everyone, as I have not posted anything for three weeks.

-Normal
 
Klaus
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NormalSpeed

Thu Jun 12, 2003 12:30 am

Hi again!  Big grin

I´m aware of the basic problems in supersonic engine/inlet design... What I would like to know is why Mach 3 would have been better than Mach 2 from an engine point of view...
 
Bellerophon
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RE: Supersonic Flight

Fri Jun 13, 2003 11:43 am

Klaus

You ask...Why?, a good question!

Occasionally I write something, which I either know or believe to be correct, but which, as I write it, I rather hope I am not asked to justify.

When I wrote…Mach 3 would have been ideal from an engine point of view… that was one of those occasions, and I also have to admit, in retrospect, I did not make it clear that I was talking theoretically.

I will do my best to answer, but let me say at the outset that I am not an expert on the RR Snecma Olympus 593 Mk 610 engine, and so I have consulted BAe and RR technical notes for information.

It would have been better, and probably more accurate, had I said …Mach 3 would have been ideal thermodynamically from a theoretical engine point of view….

The ideal engine for cruise at moderate supersonics speeds is a relatively low pressure ratio pure turbo jet engine. Pure jet because we need the engine to be capable of being installed in a slender nacelle, and relatively low pressure ratio because the intake itself will make a major contribution to total compression, about 7:1 at Mach 2.

At Mach 2, due to the design of the intake and exhaust systems, the core engine need only develop about 50% of the total thrust required, as the intake system and the exhaust system both contribute 25% each. This allows Concorde to fly with the re-heats off at Mach 2, substantially reducing fuel flows and increasing range.

However, this simple picture is complicated by two main points. Firstly, such an engine, designed and optimised for supersonic cruise, will probably not be powerful enough for take-off or transonic acceleration, and secondly, fuel consumption, whilst flying away from the design cruise regime cannot be ignored.

The first point can be solved by using re-heats as a method of augmenting basic engine thrust levels efficiently, with the advantage that it does not increase nacelle frontal area and has very low internal losses.

The second point however leads to a compromise being reached, and a higher pressure ratio engine being used than would be suggested by Mach 2 cruise considerations alone.

If we now take this engine to Mach 3, this total compression ratio rises to about 34:1, in theory a substantial achievement, which should result in even less thrust being required from the core engine, leading to my original comment.

However there would be problems in other areas, principally that while this very high pressure ratio is, in theory, desirable thermodynamically, the temperature rise taking place during compression, combined with the physical properties of the materials used in the compressor blades, would limit the amount of fuel which could be burned, which would in turn limit the specific thrust.

Put simply, the compressor blades wouldn’t have handled the temperatures, let alone the turbine blades.

So, as temperature was causing major problems in other areas of the aircraft design, such as cabin air cooling, fuel, oil and hydraulic fluid temperatures, and the design Mach number kept coming down, eventually to Mach 2.04, the engine designers saw no need to solve the problems of Mach 3, as the airframe was never going to get there.

…Or did you mean that it would have been the maximum attainable speed with the engine technology available?...

Even at Mach 2, the latter stages of the compressor are made of a nickel-based alloy, normally reserved for the turbine area, and though the engines could have handled the temperatures at speeds a little above Mach 2, it seems unlikely that Mach 3 could have been achieved at that time, with the materials then available, however desirable it may have been theoretically.

I’m sorry if I confused you initially, and I hope that this explanation has not made matters even worse.

Regards

Bellerophon


The following sources gratefully acknowledged:

Chris Orlebar “The Concorde Story”
British Aerospace PLC Training Centre “Concorde Conversion Lecture Notes”



 
Klaus
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Joined: Wed Jul 11, 2001 7:41 am

Bellerophon

Sat Jun 14, 2003 9:11 am

Bellerophon: You ask...Why?, a good question!

But always so easy to ask!  Wink/being sarcastic


Bellerophon: It would have been better, and probably more accurate, had I said ..Mach 3 would have been ideal thermodynamically from a theoretical engine point of view...

Meaning highest resulting thrust relative to fuel flow?

I had been thinking of the drag issues and the total fuel consumption (per distance) and that´s why I was a bit surprised...

As far as I understand, with increasing speed the rotating compressor/turbine component would gradually be replaced by a ramjet-style compressor when looking for the most efficient solution... the Concorde inlet design acting as a kind of "low-speed" ram-precompressor at supersonic speeds... Or is that way off the mark?


Bellerophon: I’m sorry if I confused you initially, and I hope that this explanation has not made matters even worse.

No, not all that much.  Smile
I´ve been following this particular topic with interest for some time, so I believe I´ve got a general picture of what´s going on. And you´ve certainly added more substance to it. Thank you for the explanations.


I don´t know if that information is available at all (much of it might still be classified), but it might be interesting to know how much of the theoretical efficiency gains at higher mach speeds can actually be realized in "real life". I would think of the experience with the XB-70 and the SR-71... I´d guess the limiting factors you described would have affected them even more seriously than Concorde.
 
Bellerophon
Posts: 516
Joined: Thu May 09, 2002 10:12 am

RE: Supersonic Flight

Sat Jun 14, 2003 11:42 am

Klaus

...Meaning highest resulting thrust relative to fuel flow?

Yes, because of the aerodynamic design of the intake system, more and more compression is done naturally as Mach number increases, reducing the amount of work the turbine has to do. The turbine does not need to extract as much energy from the jet flow in order to produce the required amount of compression, leading to a higher thrust output for a given fuel flow.

...with increasing speed the rotating compressor/turbine component would gradually be replaced by a ramjet-style compressor...

Yes, in theory you could have an engine with no rotating parts, if the airflow into a correctly designed intake were fast enough. The aerodynamics of the intake would produce all the compression you needed.

The problem is, how do you get the aircraft to go that fast in the first place, and, crucially, how do you deal with the extremely high temperatures produced in the rotating assemblies of a conventional jet engine whilst you are doing it.

Regards

Bellerophon
 
Klaus
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Joined: Wed Jul 11, 2001 7:41 am

Bellerophon

Sun Jun 15, 2003 9:28 pm

As far as I´ve heard, there is ongoing research into ramjet technology; It just seems unlikely that any of that will benefit civil aviation in the foreseeable future.  Sad

I love watching Ferraris zip by on the Autobahn, even if I can very well live without one for myself. Big grin
It´s just good to know someone´s always pushing the boundaries... And losing that aspect in civil aviation is a bit sad.

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