UAL747
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What Exactly Happens At Mach 1.00?

Wed May 24, 2006 9:06 am

I have a stupid question:


Why is this number 768 mph, better known as Mach 1 such a big deal for modern day jetliners? Why does it cost and use so much more energy to fly at Mach 1 than at Mach .84? Why can't conventional super-critical aft loaded wings such as on the 777 stand Mach 1 + operations. Lord knows they have the power plants (from a thrust standpoint) to push it well over Mach 1. (BTW, I've flown over Mach 1.00 in a 777, but considering we had a helluva tailwind going from IAH-CDG over the Atlantic, it doesn't count).

UAL
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foilcat
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RE: What Exactly Happens At Mach 1.00?

Wed May 24, 2006 10:47 am

When the speed closes to sound barrier, Mach 1, the theories of the aerodynamic will be totally different. Therefore, the design of the aircraft has to be coped with these requirements.
 
corey07850
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RE: What Exactly Happens At Mach 1.00?

Wed May 24, 2006 10:57 am

Quoting UAL747 (Thread starter):
Lord knows they have the power plants (from a thrust standpoint) to push it well over Mach 1

Well they might have the power, but this is actually one of the main things to consider when building a supersonic aircraft (other than the aerodynamics of course)... Supersonic flow through the engine can and will tear an engine apart. Therefore it's imperative that the flow be reduced to subsonic speeds before reaching the fan/1st stage compressor
 
AA737-823
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RE: What Exactly Happens At Mach 1.00?

Wed May 24, 2006 10:59 am

Basically, Bernoulli's equation becomes, well... inverted kinda.
Normally, pressure decreases as velocity increases. Not so in supersonic fluid flow.

Further, it's not 768mph. Supersonic flow is temperature dependent. It changes with changes in temperature.

It has nothing to do with supercritical wing design. To do supersonic, a typical wing would have to, uh, break.

Additionally, as far as powerplants go, you can't use a turbofan to do supersonic flight. You have to have a convergent-divergent inlet duct, and that rules out turbofan-family engines. Gotta be pure turbojet, or close to it. That's one reason the Concorde was such a pig- four engines screaming, plus the necessary afterburners. FUEL HOG!

Further, you did NOT fly over mach 1 on a 777. You may have exceeded the speed of sound in terms of GROUND SPEED, but you were not travelling at mach 1. Mach has only to do with airspeed. If I found a 800mph jetstream, I couldn't very well claim to have flown at mach 1 in my Cessna Squawk Chicken, could I?
 Smile
 
MarkC
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RE: What Exactly Happens At Mach 1.00?

Wed May 24, 2006 11:07 am

You never flew at mach 1. Ground and airspeed are very different things.

Over mach 1, things are backwards aerodynamically. The best example is a convergent / divergent nozzle in a rocket engine. In subsonic flow, a normal convergent (area getting smaller) nozzle accelerates the mass flow, then after the flow becomes supersonic, the nozzle design changes to be divergent (area getting larger), but this further accelerates the flow.

Its also why engines are designed to be in total subsonic flow. Parts of airplanes can be made to live in both, but its a compromise, and not efficient in both. The last bit is an answer to your question.
 
UAL747
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RE: What Exactly Happens At Mach 1.00?

Wed May 24, 2006 1:25 pm

Quoting MarkC (Reply 4):
You never flew at mach 1. Ground and airspeed are very different things

I'm quite aware of that fact, notice the (we had a big tailwind). I do know that on the monitors it's displayed as groundspeed, not airspeed.

UAL
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sllevin
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RE: What Exactly Happens At Mach 1.00?

Wed May 24, 2006 3:16 pm

Sustained flight at precisely Mach 1 is virtually impossible, because of shockwave stagnation.

Broadly speaking, you can fly at M.97 or M1.03, but you don't operate for sustained periods in the transonic realm -- it's very inefficient at best.

Steve
 
iwok
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RE: What Exactly Happens At Mach 1.00?

Wed May 24, 2006 3:42 pm

Quoting UAL747 (Thread starter):
Why does it cost and use so much more energy to fly at Mach 1 than at Mach .84?

This is a very good question. There are several reasons for this; friction being a key issue. I heard once that at super-cruise, the amount of heat generated on the Concorde skin necessitated that almost 10% of its engine output was dedicated to the cooling system! The heat was of course generated by the friction of still air hitting the supercruising fuselage and wings.

Another issue has to do with optimization. In other words, the wings that you need to sustain supercriuse are very different from those which enable good landing and takeoff performance. You don't need much wing in super sonic flight to stay up; but you need more to land. Maybe a swing wing design would offer better aerodynamic performance at sub and super sonic speeds; but I guess this is an expensive and MX heavy option.

iwok
 
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kc135topboom
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RE: What Exactly Happens At Mach 1.00?

Wed May 24, 2006 4:05 pm

There have been incidents where large or commerical aircraft exceeded Mach. 1. But those were usually in a dive of some type and significant structual damage occured, as well as changing all the engines.

The TW B-727 that the pilots were playing with the leading edge slats (over New York state?) was one incident. They were cruising at FL370 and finally recovered the aircraft around 8,000'. At times, they had as much as 85 degrees of nose down pitch, and the aircraft reached Mach. 1.04, IIRC. IIRC, the China B-747 that "fell" over the Pacific might have exceeded, or got very close to Mach. 1.0, too. There were 4 incidents of it happening in KC-135s, as well. One of those had the wing tips curled up some 15 degrees.

Didn't it happen to a B-707 or DC-8 in the 1960s, too?
 
sebolino
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RE: What Exactly Happens At Mach 1.00?

Wed May 24, 2006 4:13 pm

Well, I don't know much about fluid dynamics, but the problem seems to be that the plane flies faster than the pression wave it induces. So there must be a kind of "accumulation" of pression on the nose of the plane which acts like a barrier.
 
Geo772
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RE: What Exactly Happens At Mach 1.00?

Wed May 24, 2006 4:33 pm

Quoting Sebolino (Reply 9):
Well, I don't know much about fluid dynamics, but the problem seems to be that the plane flies faster than the pression wave it induces. So there must be a kind of "accumulation" of pression on the nose of the plane which acts like a barrier.

Spot on there, once you hit mach one the wave is perpendicular to the front of the aircraft and you get a nice sonic boom. The second boom incidentally is caused by the tail. As mach number increases the mach cone narrows. It is important that this mach cone does not meet the leading edge of the wing, or any other structure for that matter as it would appear to be a brick wall. This is why the high speed test vehicles that Nasa and others use have such narrow wingspans.
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RE: What Exactly Happens At Mach 1.00?

Wed May 24, 2006 4:51 pm

Quoting AA737-823 (Reply 3):
Additionally, as far as powerplants go, you can't use a turbofan to do supersonic flight. You have to have a convergent-divergent inlet duct, and that rules out turbofan-family engines.

To put it straight...in fact most contemporary supersonic military airplanes use turbofan engines, albeit with very modest bypass ratios.

What it takes is an inlet that decelerates intake airflow to subsonic speed before it reaches the engine inlet face. Such an intake may either be of fixed or variable geometry. The latter type is significantly more efficient at speeds in excess of, say, Mach 1.4 but adds complexity in terms of kinematics and control.
 
oly720man
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RE: What Exactly Happens At Mach 1.00?

Thu May 25, 2006 12:36 am

http://www.landspeed.com/images/archive/thrustshock.jpg

This is a pic of Thrust SSC at M1 and a bit. The bow shock is more or less perpendicular to the direction of the car, and extends quite a long way (150 ft or so).

Quoting Geo772 (Reply 10):
It is important that this mach cone does not meet the leading edge of the wing, or any other structure for that matter as it would appear to be a brick wall.

Not quite... The very high pressure variation across the shock can have an impact on the structure and control of the aircraft and, going fast enough, there is a significant temperature rise across the shock that will affect the structure as well. The Bell X-15 almost suffered structural failure at the fin root because of the very high temperatures generated behind the shock wave there. Because it was a vertical fin the shock wave was extremely strong.
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Oryx
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RE: What Exactly Happens At Mach 1.00?

Thu May 25, 2006 1:26 am

Quoting Iwok (Reply 7):
The heat was of course generated by the friction of still air hitting the supercruising fuselage and wings.

Nope, it is not the friction that heats up the air but the compression. If you look at thermal images the points where it is the hottest are the places where the air is the slowest. These points are called stagnation points (or lines).

On idea which helps to understand this phenomenon is that the sound waves propagating from the plane contain energy. The energy must have come from somewhere. The only possible source are the engines. So the engines have to increase the power output in order to deliver the energy which is dissipated in the sound waves.
 
3201
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RE: What Exactly Happens At Mach 1.00?

Thu May 25, 2006 3:13 am

Lots of talk on this thread is on second or third order effects or the resulting vehicle design aspects, not the fundamental difference in the physics between subsonic and supersonic flow. If the question is "what exactly happens at Mach 1," I think the OP is looking for a description of the physical change.

The explanation I like about the fundamental physical difference is that when you're flying subsonic, the air molecules can "hear" you coming (through the same mechanism as sound travels, thus propogating at the "speed of sound"), and are gracefully shepherded out of your way, and when you're flying supersonic, they can't, and are violently shoved out of your way, at resulting greater energy cost.
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777236ER
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RE: What Exactly Happens At Mach 1.00?

Thu May 25, 2006 3:57 am

Quoting UAL747 (Thread starter):
Why can't conventional super-critical aft loaded wings such as on the 777 stand Mach 1 + operations

The super-critical nature of modern airfoil tends to delay and reduce transonic drag rise, by decreasing the strength of the normal shock on the airfoil, and moving it aft. The normal shock tends to separate the boundary layer, so reducing the strength of the shock and moving it aft reduces shock-induced separation and thus wave drag. The aim of making a supercritical section is to get it to look as much like a von Karman ogive as possible.

That being said, when the freestream is sonic, the flow over the entirety of the airfoil remains sonic, negating all the benefits of a supercritical section.
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deltadc9
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RE: What Exactly Happens At Mach 1.00?

Thu May 25, 2006 5:07 am

Quoting Iwok (Reply 7):
I heard once that at super-cruise, the amount of heat generated on the Concorde skin necessitated that almost 10% of its engine output was dedicated to the cooling system!

I thought supercruise was supersonic without afterburner.

This poster says that Concorde used afterburners, if so, how could it carry that much fuel?

Quoting AA737-823 (Reply 3):
That's one reason the Concorde was such a pig- four engines screaming, plus the necessary afterburners. FUEL HOG!

So what is the real deal here?
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vikkyvik
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RE: What Exactly Happens At Mach 1.00?

Thu May 25, 2006 5:53 am

Quoting DeltaDC9 (Reply 16):
I thought supercruise was supersonic without afterburner.

This poster says that Concorde used afterburners, if so, how could it carry that much fuel?

Supercruise is indeed cruising supersonic without using afterburners. However, you can use afterburners to accelerate up to your cruise speed (and for takeoff), which Concorde did. At some point past Mach 1 (not sure when), Concorde would shut off the afterburners.

~Vik

EDIT:

You're generally going to hit your max drag in the transonic region (~M0.85-M1.15). At higher supersonic speeds drag will likely decrease (though not to subsonic levels, if I remember correctly. Hence the need for afterburners while going transonic.

[Edited 2006-05-24 22:55:11]
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wagz
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RE: What Exactly Happens At Mach 1.00?

Thu May 25, 2006 5:58 am

Quoting DeltaDC9 (Reply 16):
I thought supercruise was supersonic without afterburner.

This poster says that Concorde used afterburners, if so, how could it carry that much fuel?

Concorde did indeed use afterburners, but only at takeoff and during acceleration from "normal" cruise through Mach 1 up I beleive Mach 1.7. At this point afterburners were shut off and the aircraft would supercruise. All in all the afterburners were needed only for these relatively breif portions of flight.
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Starlionblue
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RE: What Exactly Happens At Mach 1.00?

Thu May 25, 2006 6:03 am

BA didn't use afterburners. They used reheat.  Wink
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Filton
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RE: What Exactly Happens At Mach 1.00?

Thu May 25, 2006 8:16 am

My first post...

Quoting UAL747 (Thread starter):
Why does it cost and use so much more energy to fly at Mach 1 than at Mach .84?

Big version: Width: 358 Height: 271 File size: 12kb
Courtesy of Anderson



As people have said, drag in the transonic region is high, then reduces.

This is also why M 0.84ish is the top speed of most airliners. At the thickest section of the chord, the airspeed accelerates (to greater than M 1 if above IAS of ~M 0.84), so you get local shockwaves and the associated drag. Thinner wings or increased sweep angle are the main ways to reduce this. Both cause problems.
 
David L
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RE: What Exactly Happens At Mach 1.00?

Thu May 25, 2006 8:23 am

Quoting Filton (Reply 20):
My first post...

Welcome. How appropriate that with a username like that your first post should be Concorde-related.  Smile
 
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Starlionblue
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RE: What Exactly Happens At Mach 1.00?

Thu May 25, 2006 9:01 am

From one of my very favorite movies:

There was a demon that lived in the air. They said whoever challenged him would die. Their controls would freeze up, their planes would buffet wildly, and they would disintegrate. The demon lived at Mach 1 on the meter, seven hundred and fifty miles an hour, where the air could no longer move out of the way. He lived behind a barrier through which they said no man could ever pass. They called it the sound barrier.
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zeke
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RE: What Exactly Happens At Mach 1.00?

Thu May 25, 2006 10:21 am

Quoting UAL747 (Thread starter):
Why is this number 768 mph, better known as Mach 1 such a big deal for modern day jetliners? Why does it cost and use so much more energy to fly at Mach 1 than at Mach .84? Why can't conventional super-critical aft loaded wings such as on the 777 stand Mach 1 + operations. Lord knows they have the power plants (from a thrust standpoint) to push it well over Mach 1. (BTW, I've flown over Mach 1.00 in a 777, but considering we had a helluva tailwind going from IAH-CDG over the Atlantic, it doesn't count).

I dont have time to go into a detailed reply today, however most jet transport aircraft operating today operate in the trans-sonic region, some airflow over the aircraft is supersonic.

For some time aircraft have had mach trimmers installed to counter some of the local supersonic flow effects.

Generally this is the order things will happen over the wing :

Below mach 1, around .75-.78 the airflow top of the wing will accelerate to reach mach 1, a small normal shock forms, as speed increases the magnitude of this shock increases and moves towards the trailing edge.

At the same time under the wing as the speed is increased (closer to M 0.9) a small shock will also form, sometime after the shock on the upper surface as the acceleration over the bottom is not as pronounced.

Approaching mach 1 (say M0.95), the shocks on the upper and lower surfaces meet at the trailing edge, as speed is increased further a new shock in-front of the wing will form.

At M 1.05 a new detached shock is in-front of the wing.

Brief as I can be at the moment, the speeds at which shocks form above is a function of aerofoil shape, wing sweep, fuselage and engine interference, newer aerofoils are flatter (super critical) delaying such effects a little.
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deltadc9
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RE: What Exactly Happens At Mach 1.00?

Sat May 27, 2006 12:48 am

Quoting Wagz (Reply 18):
Concorde did indeed use afterburners, but only at takeoff and during acceleration from "normal" cruise through Mach 1 up I beleive Mach 1.7. At this point afterburners were shut off and the aircraft would supercruise. All in all the afterburners were needed only for these relatively breif portions of flight.



Quoting Vikkyvik (Reply 17):
Supercruise is indeed cruising supersonic without using afterburners. However, you can use afterburners to accelerate up to your cruise speed (and for takeoff), which Concorde did. At some point past Mach 1 (not sure when), Concorde would shut off the afterburners.

Thanks guys, never came across that info before.
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ftrguy
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RE: What Exactly Happens At Mach 1.00?

Sat May 27, 2006 12:40 pm

To answer the question "What Exactly Happens at Mach 1.00" in an aircraft that is designed to do it, absoultely nothing. Having gone M1.0+ many times in a fighter jet, the only thing you notice is the mach meter says 1.XX. You can definetly feel the drag though when you take the jet out of afterburner. You pretty much lurch forward in your seat like you were putting the breaks on in your car.
 
HaveBlue
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RE: What Exactly Happens At Mach 1.00?

Sat May 27, 2006 1:23 pm

Ahhhh the supercruise misconception again... the F-22 is NOT the first plane to be able to go supersonic without afterburner. I know for a fact that the F-4 could would do it regularly and certainly the 14/15/16 can as well. Supercruise is the ability to sustain 'high' mach numbers without the burner on, which is apparently a capability of the Raptor. And as far as that goes, the SR-71 was supercruising all day long  Smile
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Starlionblue
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RE: What Exactly Happens At Mach 1.00?

Sat May 27, 2006 9:18 pm

Quoting HaveBlue (Reply 26):
And as far as that goes, the SR-71 was supercruising all day long

AFAIK the SR-71 used only the burners, effectively becoming a ramjet at high speeds.
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vikkyvik
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RE: What Exactly Happens At Mach 1.00?

Sat May 27, 2006 11:43 pm

Quoting HaveBlue (Reply 26):
the F-22 is NOT the first plane to be able to go supersonic without afterburner.

I didn't see anyone mention the F-22 anywhere.....?  Smile
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wingscrubber
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RE: What Exactly Happens At Mach 1.00?

Sun May 28, 2006 4:20 am

I thought it might help to add an illustration to the topic...

View Large View Medium
Click here for bigger photo!

Photo © Guilherme Bystronski - UK Airshow Review

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Pihero
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RE: What Exactly Happens At Mach 1.00?

Sun May 28, 2006 6:00 am

Actually, there is a much more dramatic pic here.

Quoting Ftrguy (Reply 25):
To answer the question "What Exactly Happens at Mach 1.00" in an aircraft that is designed to do it, absolutely nothing.

Spot on, and Zeke describes the physics very well, though I'm not quite sure that on modern airliners any part of the aircraft sees a supersonic flow at normal cruising Mach numbers as it would become rapidly too costly in terms of drag-->and costs.
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ftrguy
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RE: What Exactly Happens At Mach 1.00?

Sun May 28, 2006 11:41 am

Quoting Ftrguy (Reply 25):
To answer the question "What Exactly Happens at Mach 1.00" in an aircraft that is designed to do it, absoultely nothing.

Just to clarify. I know many things are happening outside the aircraft. I was referring to what the pilot will notice when passing the sound barrier.
 
viv
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RE: What Exactly Happens At Mach 1.00?

Wed May 31, 2006 9:55 pm

Quoting UAL747 (Thread starter):
I've flown over Mach 1.00 in a 777, but considering we had a helluva tailwind going from IAH-CDG over the Atlantic, it doesn't count

What nonsense. You are confusing ground speed with air speed. Mach numbers have nothing to do with ground speed.
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twal1011727
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RE: What Exactly Happens At Mach 1.00?

Wed May 31, 2006 11:14 pm

Quoting KC135TopBoom (Reply 8):
The TW B-727 that the pilots were playing with the leading edge slats (over New York state?) was one incident.

That was an investigator that threw this out during the investigation.
but never proven.

My dad gets red faced with anger and foams at the mouth when he hears about this garbage.
(retired TWA Capt 1956-1987)
So watch out or other TWA pilots will burn you up.

The theory with this was to pull the leading edge control circuit breaker then put out trailing edge flaps to less than 2 degrees and this ///could/// make you cruise faster at the same engine thrust. My thought is the extra lift would be negated by the increase in drag - which is a by-product of lift.
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TurkishWings
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RE: What Exactly Happens At Mach 1.00?

Wed May 31, 2006 11:52 pm

Can someone please tell me how many km/h is Mach 1?
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3201
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RE: What Exactly Happens At Mach 1.00?

Thu Jun 01, 2006 12:03 am

Quoting TurkishWings (Reply 34):
Can someone please tell me how many km/h is Mach 1?

It changes with temperature, so there is no single number.

In m/s, it is the square root of 402 * temperature in Kelvin (C + 273). To convert to km/h, just multiply by 3.6.

So at 25C, it is about 1250 km/h. But at 36000', the temperature is more likely to be -55C, where it is about 1065 km/h.
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TurkishWings
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RE: What Exactly Happens At Mach 1.00?

Thu Jun 01, 2006 12:30 am

Quoting 3201 (Reply 35):
So at 25C, it is about 1250 km/h. But at 36000', the temperature is more likely to be -55C, where it is about 1065 km/h.

Wow. That's it? I flew 1150 km/hr between IST and BKK on an A-340 of TK. Also with TK, I flew AMM-IST on a 737-500 over 1100 km/h.
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Starlionblue
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RE: What Exactly Happens At Mach 1.00?

Thu Jun 01, 2006 1:31 am

Quoting TurkishWings (Reply 36):
Quoting 3201 (Reply 35):
So at 25C, it is about 1250 km/h. But at 36000', the temperature is more likely to be -55C, where it is about 1065 km/h.

Wow. That's it? I flew 1150 km/hr between IST and BKK on an A-340 of TK. Also with TK, I flew AMM-IST on a 737-500 over 1100 km/h.

As we like to repeat here in tech_ops, that's ground speed, meaning speed of the plane relative to the ground. Mach number is measured as the speed of the object through the medium in which it is immersed, in this case the air. You were never supersonic since the air was also traveling relative to the ground in the same direction as you were. Also known as a tailwind.
"There are no stupid questions, but there are a lot of inquisitive idiots."
 
777236ER
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RE: What Exactly Happens At Mach 1.00?

Thu Jun 01, 2006 8:31 pm

Quoting Starlionblue (Reply 37):
Mach number is measured as the speed of the object through the medium in which it is immersed, in this case the air.

Mach number isn't the speed, it's the speed nondimensionalised with respect to the speed of sound through the medium.
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David L
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RE: What Exactly Happens At Mach 1.00?

Fri Jun 02, 2006 2:58 am

Quoting 777236ER (Reply 38):
it's the speed nondimensionalised with respect to the speed of sound

Or you could say it's the ratio of your airspeed to that of sound in the air in which you're flying.

I'm sure "nondimensionalising" is illegal in some countries.
 
Tristarsteve
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RE: What Exactly Happens At Mach 1.00?

Fri Jun 02, 2006 4:29 am

Reading this thread I am again surprised that Concorde worked. All those analogue computors controlling things designed in the early 60s, and it worked. I only flew on Concorde once, but the most remarkable thing was the complete lack of sensation going supersonic.
 
David L
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RE: What Exactly Happens At Mach 1.00?

Fri Jun 02, 2006 4:38 am

Quoting TristarSteve (Reply 40):
I only flew on Concorde once, but the most remarkable thing was the complete lack of sensation going supersonic.

True. I wanted my money back... well, maybe not! Fortunately I'd read quite a bit about it before I tried it so I wasn't in the least bit disappointed. The two very gentle nudges in the back from the reheats just prior to Mach 1 and the extreme braking on landing were about the only "tactile" clues you got.
 
texfly101
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RE: What Exactly Happens At Mach 1.00?

Fri Jun 02, 2006 6:57 am

Quoting AA737-823 (Reply 3):
Further, it's not 768mph. Supersonic flow is temperature dependent. It changes with changes in temperature.

True, the Mach number is a dimensionless ratio of the speed of sound to the true relative airspeed, not indicated or groundspeed, of the object. It is also dependent on the medium thru which the object is passing. It varies from planet to planet due to the differences in atmospheres.

Quoting Oly720man (Reply 12):
The Bell X-15 almost suffered structural failure at the fin root because of the very high temperatures generated behind the shock wave there

I believe you're referring to the North American X-15A-2 that set the modern day speed record (although unofficial) of Mach 6.07. And yes, the airplane was retired due to structural damage.
But to get back to the question, beginning in the transonic region, local area shock waves start to form at selected areas that are experiencing localized increase in flow rates due to their shape. This is the rise of drag that is shown on the curve. This requires additional power to overcome this increased drag. This drag is specifically due to the effect of compressiblity. At speeds lower than the transonic range, air remains basically uncompressed and flows over the surface. When the flow regime nears the speed of sound (Mach 1), the flow is no longer uncompressed and the air goes thru a local compression that is evidenced by a shockwave. At Mach 1, the leading shock waves converge to the singular point that is seen in the photographs. All protuberences will have a shock wave. If you look at a special type of photograph of a vehicle in a wind tunnel, you will see a shock wave at all protuberences. The power requirements to pass thru both the transonic region and Mach 1 is proportional to the power required to compress the volume of air affected by the shock wave. And that is a great amount of power in relation to subsonic flow. So all supersonic aircraft are designed to both delay the onset of this drag and also to be shaped to minimize the amount of drag, hence area ruled fuselages, symmetrical airfoils and swept wings. Area ruling was one of the aerodynamic design elements that allowed the relatively low powered turbojets of the late 40's and 50's to push their airframes past Mach 1. See the development of the Convair F-102 for an excellent history of just such an advance.
As far as why certain engines can't go past Mach 1, this is a subject that is too long for this column. Just be aware that all jet engines depend on their inlet structures to both contain the shock wave and then to slow the air down below Mach 1 prior to it entering the compressor stage. Also, propellers are not able normally able to travel faster than Mach 1 due to the shockwave being greater than their available power (this is a gross simplification), even tho tip speeds sometimes exceeds Mach 1. This is why the Russian Tu-95 Bear is so noisy that its debilitating to its crew. Its contra rotating propellers are all producing sonic booms. 14 hours of sonic booms...ouch!
 
prebennorholm
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RE: What Exactly Happens At Mach 1.00?

Fri Jun 02, 2006 8:01 am

The physics related to sonic speed fills many feet of book shelves. And what has been learned about it has been a very difficult process.

Many wise words have been written here in this thread, but it only touches very small parts of the whole problem.

Let me tell a few details from the early period of learning about supersonic speed. In spring 1947 colonel (later general) Albert Boyd, head of flight test division of the USAF, was searching for test pilots to pilot the Bell X-1 research plane which was hoped to be capable of supersonic speed.

The reason was that Bell's own test pilot had got scared of the plane after a few flights at high subsonic speed. So scared that he demanded a special bonus of $25,000 to continue the program. Which Mr. Bell refused to pay.

One day colonel Boyd had one of his test pilots, captain Charles E. (Chuck) Yeager, standing at attention in his office for an interview about his ability to take part in the program.

One question from colonel Boyd was like this: Some scientists believe that at speed of sound air pressure goes to infinite. Captain, do you know what that means?"

Captain Yeager answered "Yes Sir".

They never discussed the meaning of that. They both knew that it would mean that the plane would explode.

Colonel Boyd continued: "We are not going there step by step, but inch by inch. But I believe that it is possible".

That tells us how little the cleverest men in the world knew about supersonic flight before it had been accomplished. And consequently how complicated the real physics about supersonic speed are.

In the following I am going to explain supersonic flight in an extremely shortened version, in just one sentence of a few words. I am prepared that for that I will get slammed right here in this thread by the more knowledged readers. Anyway, I can't resist. And I will insist that it will give some readers a feeling about what supersonic flight is all about. The sentence comes here:

Imagine that at speed of sound and beyond the air masses behaves as if it is frozen solid.

No easy bicycle ride there. More like a coal miner making his way while extending his underground tunnel.

Vastly simplified, and far away from the whole truth of course, but much less so than you might imagine.

It is completely impossible that the compressors and turbines in a turbine engine can work at supersonic air flow inside the engine. Luckily there are a few things which make life easier. First of all the intake can be designed in a way so it slows down the air. When it gets compressed, then the air gets heated, and the sonic speed goes up in absolute values. Having passed all compressor stages, and having been compressed to maybe 20 bar, then the internal airspeed will be only a minor fraction of sonic speed, maybe only in the region of 100 mph in the combustion chamber. That was nice!

Only while passing, and after passing the turbine stages do the heavily compressed gasses expand enormously and accellerate to great speed.

Accellerating a given number of pounds of air to a given speed is an action which creates a similar reaction in the opposite direction. The reaction is the engine thrust.

That's also the reason why in Swedish language a jet engine is called a "reaktionsmotor".

During the Bell X-1 test program they encountered a lot of troubles which Chuck Yeager sometimes happened to share with his wife. One day Mrs. Glennis Yeager asked Chuck: "Why is it so important to break down that sound barrier?" Chuck answered: "Because then planes can go faster".

That was also an extreme simplification. Supersonic flight is one hell of a lot more than just going faster.

Before I get too far off topic I will wish you all goodnight, and please don't slam me completely out of shape for the "solid air".

[Edited 2006-06-02 01:05:14]
Always keep your number of landings equal to your number of take-offs
 
prebennorholm
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RE: What Exactly Happens At Mach 1.00?

Fri Jun 02, 2006 8:32 am

Quoting Texfly101 (Reply 42):
It [Mach 1 speed] is also dependent on the medium thru which the object is passing. It varies from planet to planet due to the differences in atmospheres.

True! That reminds me about the Viking landers, the first vehicles to make a soft landing on the planet Mars. They deployed their brake chutes at roughly local Mach 3 speed. That was the first time ever that something like a parachute was operated at supersonic speed.

That was in a very cold atmosphere of almost 100% CO2 and at great altitude at an ambient pressure of a tiny fraction of one percent of atmospheric pressure at sea level on planet Earth.

Some NASA scientists were very worried how those brake chutes would behave. But since both Viking 1 and Viking 2 landed safely, then they must have got it right.
Always keep your number of landings equal to your number of take-offs
 
DeltaGuy
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RE: What Exactly Happens At Mach 1.00?

Fri Jun 02, 2006 8:39 am

Quoting AA737-823 (Reply 3):
Additionally, as far as powerplants go, you can't use a turbofan to do supersonic flight. You have to have a convergent-divergent inlet duct, and that rules out turbofan-family engines. Gotta be pure turbojet, or close to it.

Back in the old days maybe, but a turbofan will work fine in today's fighter applications. Granted, there isn't a whole hell of alot of secondary air, and the Inlet Fan isn't overly large either. (Our turbofans look alot like turbojets- to the untrained eye, they'd look the same from the outside) That's why you don't see a massive fan sitting on the front of a tac air jet engine, massive bypass doesn't produces speed...raw combusted air does.

Quoting Rheinbote (Reply 11):
To put it straight...in fact most contemporary supersonic military airplanes use turbofan engines, albeit with very modest bypass ratios. What it takes is an inlet that decelerates intake airflow to subsonic speed before it reaches the engine inlet face. Such an intake may either be of fixed or variable geometry. The latter type is significantly more efficient at speeds in excess of, say, Mach 1.4 but adds complexity in terms of kinematics and control.[/

All supersonic jet engines are of very low bypass- meaning, there is a minimal amount of secondary air routed through the case, and most of the air is used for combustion. The Turbofan system still works well in supersonic applications as it is efficient and that secondary air is good for cooling...it all meets anyways in the augmentor (afterburner) section, and gets burned just the same.

In the F-15's P&W F100 engines that I work on, three of the N1 stator blade sections and two of the N2 stator have variable pitch blades, to vary the amount of air entering the engine for combustion. Their position is determined by the DEEC (basically the computer of the motor), and by inputs from the throttle....if you're going into full augmentation, they'll open wide for that extra air, etc etc. Same principle applies pretty much for any modern-day turbofan engine inside most tactical aircraft.

Anyways, enough engine lessons for one day. Good thread!

DeltaGuy
"The cockpit, what is it?" "It's the little room in the front of the plane where the pilot sits, but that's not importan
 
B2707SST
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RE: What Exactly Happens At Mach 1.00?

Fri Jun 02, 2006 1:16 pm

If you're interested in the physics and engineering challenges of transonic and supersonic flight, Larry Reithmaier's Mach 1 and Beyond is a fantastic introduction. It manages to be thorough and informative while remaining very readable for the lay person.

http://www.amazon.com/gp/product/007...2-2801067-4558428?%5Fencoding=UTF8

--B2707SST
Keynes is dead and we are living in his long run.
 
HaveBlue
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RE: What Exactly Happens At Mach 1.00?

Fri Jun 02, 2006 3:40 pm

Quoting Texfly101 (Reply 42):
At Mach 1, the leading shock waves converge to the singular point that is seen in the photographs.

Not to nitpick, but I have plenty of pictures of that shockwave from subsonic aircraft. They are fast, and the local airflow is supersonic, but I have pix of 70' conical shockwaves enveloping an entire F-14 from the cockpit back and he was definitely not at Mach 1.  Smile Among many other less impressive but equally full shockwave pictures.
Here Here for Severe Clear!
 
texfly101
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RE: What Exactly Happens At Mach 1.00?

Sat Jun 03, 2006 12:16 am

Quoting HaveBlue (Reply 47):
Not to nitpick, but I have plenty of pictures of that shockwave from subsonic aircraft. They are fast, and the local airflow is supersonic, but I have pix of 70' conical shockwaves enveloping an entire F-14 from the cockpit back and he was definitely not at Mach 1. Smile Among many other less impressive but equally full shockwave pictures.

Yes, you are correct. I agree and they are definitely evidence of the local shockwave at the points of localized atmospheric compression. This compression starts in the transonic range, say at greater than Mach .8 depending on local conditions. That induces the entrained water to become visible as water vapor, which is what we typically see as the shock wave. One of the "myths" that still makes Mach 1 so mysterious was that prior to Chuck's X-1 flight, the "Sound Barrier" was something that people really thought existed. There is no barrier, only a phase condition of the fluid where we transistion from a non-compressed state to a compressed state. This is rather described as an "increased pressure area". Looking at a set of steam tables, add in some air measurements, and doing some bernoulli type of mathematics (don't ask unless you're a masochist or a physics major as it was a long two semesters) you can predict very close what will happen. This visiblity of the shockwave is typical of where the greatest compression is taking effect. That is typically at the lead protuberence until passing thru Mach 1 and then it typically translates to the point at which the greatest cross section of the object is, usually somewhere around mid wing, hence area ruling, swept wings, etc for aircraft that operate in the transonic range (a necessity for supersonic aircraft). There are still shockwaves everywhere on the structure as photos show of wind tunnel objects. Take a look at
http://www3.imperial.ac.uk/hypersonics/gallery/flowvisualisation
A very interesting subject. Look at the papers and feel free to enter the wide wonderful world of aerodynamics.
Just as a side note, the one of the reasons for the 747 having a high cruise speed is that the bulge, occuring before the wing, helps the area rule of the fuselage by giving a smoother increase in cross section which helps delay the onset of transonic drag. That enables the 747 to cruise at a higher mach number
 
GDB
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RE: What Exactly Happens At Mach 1.00?

Sun Jun 04, 2006 12:21 am

Excellent thread.

Some Concorde related points.
Indeed reheat was used, for about 70-90 seconds on take off, overland crusing speed was Mach 0.95, where it was very inefficient.

Overwater, reheats on, two at a time (so only two barely perceptible nudges-bad luck for those who wanted anything dramatic!), Mach 1 without a sensation-not on my 7 flights, on one I did not know we were supersonic and glanced at the cabin display and saw Mach 1.02.

Reheat on for around 10 mins, taking us from Mach 0.95 @ 28.000 ft, to Mach 1.7 @ around 47.000 feet.
The aircraft was very efficient at Mach 2 and above 50.000 feet.

TristarSteve, you were right to be amazed that analouge computers were running your seemless supersonic flight-because in one vital area, that was computer critical, they were not.
The intake control system, to slow the air from up to Mach 2.02, to Mach 0.45, in about 11 feet, using two intake ramps, was originally an analouge system.
This was the early 60's, digital would have been preferred, but then that would mean a system taking up a third of the cabin.

The two Prototypes (or as I prefer it-'Technology Demonstrators'), that flew in 1969, had the analogue intake system.
Testing soon showed it would not be sufficient, too insensitive, for reliable commercial service, too many failures.
Luckily, by 1970, the microelectronic revolution had begun, it would now be possible to have small digitial systems, 8 of them, one for each of the two intake ramps per engine.

When I saw 'small', I mean each one was about the length of a carton of 200 cigarettes, housed in rear avionic bays, alongside CVR and FDR.
Just ready for testing on the first UK Pre-Production G-AXDN (or as I prefer it-'Prototype'), in 1973, the system was just ready for service in Jan 1976.

But throughout the service life, the Air Intake Control System (AICU), was something that required in depth knowledge and expertise, in BA we were lucky there.
Beside me is a 'simple' guide to the AICU, produced by one of our Engineers in 1993, this basic look at AICU operation, fault finding, maintenance, is A4 sized and about two inches thick.

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