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What Exactly Happens At Mach 1.00?  
User currently offlineUAL747 From , joined Dec 1969, posts, RR:
Posted (8 years 3 months 10 hours ago) and read 7040 times:

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

54 replies: All unread, showing first 25:
 
User currently offlineFoilcat From Hong Kong, joined Mar 2001, 131 posts, RR: 0
Reply 1, posted (8 years 3 months 8 hours ago) and read 6977 times:

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.

User currently offlineCorey07850 From United States of America, joined Feb 2004, 2527 posts, RR: 5
Reply 2, posted (8 years 3 months 8 hours ago) and read 6951 times:

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


User currently offlineAA737-823 From United States of America, joined Mar 2000, 5765 posts, RR: 11
Reply 3, posted (8 years 3 months 8 hours ago) and read 6952 times:

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


User currently offlineMarkC From United States of America, joined Apr 2006, 259 posts, RR: 0
Reply 4, posted (8 years 3 months 8 hours ago) and read 6929 times:

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.


User currently offlineUAL747 From , joined Dec 1969, posts, RR:
Reply 5, posted (8 years 3 months 5 hours ago) and read 6800 times:

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


User currently offlineSllevin From United States of America, joined Jan 2002, 3376 posts, RR: 6
Reply 6, posted (8 years 3 months 3 hours ago) and read 6733 times:

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


User currently offlineIwok From Sweden, joined Jan 2005, 1108 posts, RR: 0
Reply 7, posted (8 years 3 months 3 hours ago) and read 6708 times:

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


User currently offlineKC135TopBoom From United States of America, joined Jan 2005, 12134 posts, RR: 51
Reply 8, posted (8 years 3 months 3 hours ago) and read 6688 times:

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?


User currently offlineSebolino From France, joined May 2001, 3681 posts, RR: 4
Reply 9, posted (8 years 3 months 3 hours ago) and read 6676 times:

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.

User currently offlineGeo772 From United Kingdom, joined Jul 2004, 519 posts, RR: 0
Reply 10, posted (8 years 3 months 2 hours ago) and read 6652 times:

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.



Flown on A300B4/600,A319/20/21,A332/3,A343,B727,B732/3/4/5/6/7/8,B741/2/4,B752/3,B762/3,B772/3,DC10,L1011-200,VC10,MD80,
User currently offlineRheinbote From Germany, joined May 2006, 1968 posts, RR: 52
Reply 11, posted (8 years 3 months 2 hours ago) and read 6620 times:

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.


User currently offlineOly720man From United Kingdom, joined May 2004, 6694 posts, RR: 11
Reply 12, posted (8 years 2 months 4 weeks 1 day 18 hours ago) and read 6422 times:

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.



wheat and dairy can screw up your brain
User currently offlineOryx From Germany, joined Nov 2005, 126 posts, RR: 0
Reply 13, posted (8 years 2 months 4 weeks 1 day 17 hours ago) and read 6399 times:

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.


User currently offline3201 From , joined Dec 1969, posts, RR:
Reply 14, posted (8 years 2 months 4 weeks 1 day 16 hours ago) and read 6354 times:

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.


User currently offline777236ER From , joined Dec 1969, posts, RR:
Reply 15, posted (8 years 2 months 4 weeks 1 day 15 hours ago) and read 6331 times:

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.


User currently offlineDeltaDC9 From United States of America, joined Apr 2006, 2844 posts, RR: 4
Reply 16, posted (8 years 2 months 4 weeks 1 day 14 hours ago) and read 6309 times:

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?



Dont take life too seriously because you will never get out of it alive - Bugs Bunny
User currently offlineVikkyvik From United States of America, joined Jul 2003, 9897 posts, RR: 26
Reply 17, posted (8 years 2 months 4 weeks 1 day 13 hours ago) and read 6288 times:
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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]


"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlineWagz From United States of America, joined Mar 2003, 516 posts, RR: 16
Reply 18, posted (8 years 2 months 4 weeks 1 day 13 hours ago) and read 6277 times:
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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.



I think Big Foot is blurry, Its not the photographers fault. Theres a large out of focus monster roaming the countryside
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17014 posts, RR: 67
Reply 19, posted (8 years 2 months 4 weeks 1 day 13 hours ago) and read 6275 times:

BA didn't use afterburners. They used reheat.  Wink


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineFilton From United Kingdom, joined May 2006, 51 posts, RR: 0
Reply 20, posted (8 years 2 months 4 weeks 1 day 10 hours ago) and read 6221 times:

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.


User currently offlineDavid L From United Kingdom, joined May 1999, 9524 posts, RR: 42
Reply 21, posted (8 years 2 months 4 weeks 1 day 10 hours ago) and read 6216 times:

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

Welcome. How appropriate that with a username like that your first post should be Concorde-related.  Smile


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17014 posts, RR: 67
Reply 22, posted (8 years 2 months 4 weeks 1 day 10 hours ago) and read 6204 times:

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.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineZeke From Hong Kong, joined Dec 2006, 8988 posts, RR: 75
Reply 23, posted (8 years 2 months 4 weeks 1 day 8 hours ago) and read 6185 times:

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.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlineDeltaDC9 From United States of America, joined Apr 2006, 2844 posts, RR: 4
Reply 24, posted (8 years 2 months 3 weeks 6 days 18 hours ago) and read 6007 times:

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.



Dont take life too seriously because you will never get out of it alive - Bugs Bunny
25 Ftrguy : 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
26 Post contains images HaveBlue : 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 t
27 Starlionblue : AFAIK the SR-71 used only the burners, effectively becoming a ramjet at high speeds.
28 Post contains images Vikkyvik : I didn't see anyone mention the F-22 anywhere.....?
29 Post contains links and images Wingscrubber : I thought it might help to add an illustration to the topic... View Large View MediumPhoto © Guilherme Bystronski - UK Airshow Review
30 Post contains links Pihero : Actually, there is a much more dramatic pic here. Spot on, and Zeke describes the physics very well, though I'm not quite sure that on modern airliner
31 Ftrguy : 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.
32 Viv : What nonsense. You are confusing ground speed with air speed. Mach numbers have nothing to do with ground speed.
33 TWAL1011727 : 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
34 TurkishWings : Can someone please tell me how many km/h is Mach 1?
35 3201 : 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
36 TurkishWings : 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.
37 Starlionblue : 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 o
38 777236ER : Mach number isn't the speed, it's the speed nondimensionalised with respect to the speed of sound through the medium.
39 David L : 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 som
40 TristarSteve : Reading this thread I am again surprised that Concorde worked. All those analogue computors controlling things designed in the early 60s, and it worke
41 David L : 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 disappointe
42 Texfly101 : 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 i
43 Prebennorholm : 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 wo
44 Prebennorholm : 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 rou
45 DeltaGuy : 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
46 Post contains links B2707SST : 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
47 Post contains images HaveBlue : 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 hav
48 Post contains links Texfly101 : Yes, you are correct. I agree and they are definitely evidence of the local shockwave at the points of localized atmospheric compression. This compre
49 GDB : 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, whe
50 Post contains images Baroque : Does this make you feel at home? I always understood that this was a key factor in choosing a M2 cruise for Concorde. That, and the drag curve shown
51 Oryx : That depends on how far you want to get. Two items preventing the use of conventional engines are the geometry of the inlet and the outflow velocity.
52 Baroque : Thank you for those explanations. Although bypass air does not help you at Mach 2.0, even a little bit could help on take-off and any sub-sonic secto
53 Oryx : At Ma = .98 .... 1.02 many problems of higher velocities are avoided. The straight inlet duct and a lot of the modules of subsonic engines can still b
54 Baroque : quote=Oryx,reply=53]did't say that. It does help but only in moderate numbers. [/quote] Sorry, and I meant to put a "much" as in "does not help much".
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