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TripleDelta
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Modern Airliners Breaking The Speed Of Sound?

Wed Jul 28, 2004 3:55 am

"A DC-8-40 became the first jet airliner to exceed the speed of sound when, in 1961, it reached Mach 1.012 (667 mph) in a shallow dive"
- The Flier's Handbook, Pan Books, 1978.

Are modern airliners of the Airbus, Boeing or McDonnell Douglas variety able to perform the above without structural damage (at least for short periods)? I understand that the slim DC-8 fuselage did not give as much drag as today's "fat jets".
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MITaero
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RE: Modern Airliners Breaking The Speed Of Sound?

Wed Jul 28, 2004 4:46 am

Yes, I believe they are tested for 'upset' conditions (i.e. a dive), but someone else might be able to give a better answer.
 
aloges
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RE: Modern Airliners Breaking The Speed Of Sound?

Wed Jul 28, 2004 5:29 am

There is a story of a KAL 747SP going supersonic somewhere south of Alaska, and it suffered structural damage, IIRC, part of one horizontal stabiliser was torn apart. I can't find it anywhere, though.

Edit: I found it. It was a China Airlines 747SP, not a Korean Air Lines one:
http://www2.ntsb.gov/ntsb/brief.asp?ev_id=20001214X35672&key=1

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[Edited 2004-07-27 22:42:26]
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cancidas
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RE: Modern Airliners Breaking The Speed Of Sound?

Wed Jul 28, 2004 6:29 am

the citation x test pilots have had to break it when flying that plane. i mean, it soo close...
"...cannot the kingdom of salvation take me home."
 
QantasA332
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RE: Modern Airliners Breaking The Speed Of Sound?

Wed Jul 28, 2004 6:32 am

I would say yes, 'normal' airliners can exceed Mach 1.0 for short periods of time (and are tested under those conditions). In almost all cases, that could only really happen in a dive. But in the event that a plane was diving, for whatever reason, you certainly wouldn't want it to reach M1 and start breaking up.

As MITaero said, someone should be able to provide a more conclusive answer.

Cheers,
QantasA332
 
avioniker
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RE: Modern Airliners Breaking The Speed Of Sound?

Wed Jul 28, 2004 7:08 am

They shouldn't but there's an ex-TWA 727 and an ex-Evergreen 747 that have both done rolls and supersonic excursions. the 727 was pretty beat up as the gear had to be lowered to get things back to reality but the 747 had 1.084 Mach on the FDR and not much but the seat cushions were hurt.
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airplay
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RE: Modern Airliners Breaking The Speed Of Sound?

Wed Jul 28, 2004 7:15 am

I would say yes, 'normal' airliners can exceed Mach 1.0 for short periods of time (and are tested under those conditions).

Tested? Not to my knowledge. Why would you test a subsonic airliner at transonic or supersonic speeds? Just for kicks?

This is akin to testing an automobile at the bottom of the ocean.
 
MITaero
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RE: Modern Airliners Breaking The Speed Of Sound?

Wed Jul 28, 2004 7:22 am

Not really. I'm pretty sure and have heard (here at Boeing) that they are tested at above Mach 1. What happens if an aircraft enters a dive for some (horrible) reason? Should it break up, or should the pilots have a chance to recover?

BTW, that is a false analogy.
 
QantasA332
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RE: Modern Airliners Breaking The Speed Of Sound?

Wed Jul 28, 2004 7:50 am

Tested? Not to my knowledge. Why would you test a subsonic airliner at transonic or supersonic speeds? Just for kicks?

Like MITaero said, what happens if the airliner in question ever enters a dive?! After all, there's a decent chance that that could happen somewhere along the line... You need to know how the aircraft will behave in that situation, so testing is a very reasonable thing to do.

This is akin to testing an automobile at the bottom of the ocean.

Not really. If you drive your car into the ocean, you drown - who cares how the car behaves? Enter a dive in an airliner and reach M1, and you're still alive and well (worrying whether the plane will break up or not, no doubt).

Cheers,
QantasA332
 
prebennorholm
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RE: Modern Airliners Breaking The Speed Of Sound?

Wed Jul 28, 2004 8:03 am

Agree with Airplay. Subsonic airliners are certainly not designed to go supersonic, neither is that a part of the test program.

But any jet airliner would aerodynamically be able to break the sound barrier in a dive. Whether they break up doing so, or to what degree they get damaged, or if they suffer damage at all, that will depend very much upon the altitude. Also at supersonic speed is thin air a lot less violent than thick air.

Most airliners will already in the transonic region suffer from ineffective controls, especially ailerons and elevator. Also they are not designed to shift center of gravity like the Concorde was.

Some airliners will be able to compensate for ineffective elevator control using the tailplane trim, much the same way as Chuck Yeager did on the Bell X-1 in 1947. But that will of course only help if the tailplane doesn't separate.

Supersonic flight is not just "going faster". It is flying in accordance with entirely different physical laws. Subsonic airliners are optimized for what they are designed for and do not take supersonic laws into account.
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MITaero
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RE: Modern Airliners Breaking The Speed Of Sound?

Wed Jul 28, 2004 8:57 am

No one is arguing that subsonic aircraft are not designed for subsonic conditions. Further, Qantas and I are familiar with the physics of supersonic flows. Although a plane will perform horribly for M>1, it will not be an uncontrollable brick. How did the DC-8 mentioned in the first post pull out of its dive?

I do know that Boeing performs many off-design flight tests, and I believe overspeed testing is part of these. (Overspeed as in Mach number, not just "going faster".)
 
airplay
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RE: Modern Airliners Breaking The Speed Of Sound?

Wed Jul 28, 2004 10:07 am

Believe me....there is no certification requirement to exceed Mach 1 to test aircraft designed for operation in the subsonic range.

Although a plane will perform horribly for M>1, it will not be an uncontrollable brick.

Tell that to some of the first supersonic test pilots.

Not really. If you drive your car into the ocean, you drown - who cares how the car behaves?

OK...if you exceed the design limits of your airplane including Vmo/Mmo then the manufacturer how the airplane behaves or if you die. So what's the point here?

 
POSITIVE RATE
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Supersonic Airliners

Wed Jul 28, 2004 12:10 pm

There have been a few cases of airliners going wupersonic in regular pax service- unfortunately the end result was the destruction of the a/c and the loss of all on board. A few that come to mind: Silk Air 737, Egypt Air 767,PSA BA146. In all these cases the plane was in a supersonic nosedive and brokeup around the 10,000 feet mark.
 
learpilot
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RE: Modern Airliners Breaking The Speed Of Sound?

Wed Jul 28, 2004 1:16 pm

the citation x test pilots have had to break it when flying that plane. i mean, it soo close...

The Citation X was indeed flown slightly past Mach 1 by the Cessna test pilots. I've been told that it flies fine right up to Mach 1, and the only reason Mmo is .92 is because the FAA requires a .08 cushion between Mmo and Mach 1.

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TripleDelta
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RE: Modern Airliners Breaking The Speed Of Sound?

Wed Jul 28, 2004 9:05 pm

MITaero: well, the book doesn't say how it pulled out, but it does not mention that it was destroyed either, so I presume it pulled out okay. The DC-8 was certified in 1959., and the -40 appeared a year or so later I think, and since this happened in 1961, it could have been a part of some test programme. Probably someone, somewhere was interested what would happen when a subsonic airliner broke the speed of speed of sound. Those was the early years of jets and not much data on that was available I suppose.
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HaveBlue
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RE: Modern Airliners Breaking The Speed Of Sound?

Wed Jul 28, 2004 9:24 pm

The Alaskan Airline MD-80 off the coast of California was supersonic and inverted wasn't it? It's the only one that came to mind that wasn't mentioned so far.
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SlamClick
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RE: Modern Airliners Breaking The Speed Of Sound?

Wed Jul 28, 2004 11:11 pm

I'm pretty sure that the test pilots at Boeing or Airbus commercial divisions do not just run the plane up through the number to see what it will do. After all, there are no ejection seats, even in prototype aircraft. Just how brave do you think these guys are? And if Bill Allen made it very clear to Tex Johnston that he'd just done the last-ever roll in a Boeing 707, what do you think the board of directors would say about taking them supersonic and the risk of bad publicity if it breaks apart?

No, we can be pretty sure that the engineers will define an envelope for the plane and the test pilots will explore that envelope in very methodical fashion.

Then the envelope gets shrunk down to match us ordinary pilots in the form of "Limitations" in the AFM.

The DC-8 going supersonic was very carefully worked out beforehand. The whole story of it is up on the web somewhere. You can probably find a link to it by searching hear regarding "DC-8" and "supersonic." It was not just some cowboy saying "watch this."
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citationjet
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RE: Modern Airliners Breaking The Speed Of Sound?

Thu Jul 29, 2004 2:19 am

The Citation X did indeed first exceed Mach 1 on September 26, 2004. The prototype aircraft (N750CX) was flying with the chase plane, a T-38 (N638TC) owned by Chuck Thornton.

Cessna has an Executive conference room named "Mach 1", which displays a picture of both aircraft. This conference room is near my office.
The Citation X Mmo is 0.92. The margin above Mmo is 0.07, therefore Md is Mach 0.99, which had to be demonstrated for certification, flutter clearance, etc.
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MITaero
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RE: Modern Airliners Breaking The Speed Of Sound?

Thu Jul 29, 2004 2:23 am

You know what - after thinking about it, I bet they do overspeed testing under M=1. My bad.
 
HaveBlue
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RE: Modern Airliners Breaking The Speed Of Sound?

Thu Jul 29, 2004 2:26 am

CitationJet, I'm assuming you meant Sept 26, 2003  Smile

Interesting nonetheless.
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citationjet
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RE: Modern Airliners Breaking The Speed Of Sound?

Thu Jul 29, 2004 10:06 pm

HaveBlue,

The correct date for the Citation X breaking Mach 1 was Sept 26, 1994, not 2004.
The Citation X received Type Certification May 31, 1996.


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[Edited 2004-07-29 15:09:23]
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MrFord
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RE: Modern Airliners Breaking The Speed Of Sound?

Fri Dec 24, 2004 7:31 am

Here's a link about the supersonic DC-8.

http://www.dc8.org/library/supersonic/index.php

It was indeed in a test flight.

"After climbing to an altitude of 52,090 feet, the DC-8-42 series aircraft attained a maximum speed of Mach 1.012 or 660 mph while in a controlled dive through 41,088 feet. The purpose of the flight was to collect data on a new leading-edge design for the wing."
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aeroweanie
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RE: Modern Airliners Breaking The Speed Of Sound?

Fri Dec 24, 2004 8:13 am

Airliners certified under FAA FAR 25 have a designated Maximum Operating Mach number (Mmo) that can be found on their Type Certificate Data Sheets (see: http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/MainFrame?OpenFrameSet). This varies from M=.73 (the BAE 146) to M=.92 (the Boeing 747). Beyond this, there is a Dive Mach number (Md). The airliner, in certification, has to be safely taken to this speed (FAR 25.251b). Before roughly 1990, Md was at a minimum Mmo+.05 and Md is now at a minimum Mmo+.07 (FAR 25.335b). This is the speed that an airliner is likely to achieve in an upset maneuver. Some airliners have Md speeds greater than that called for in the regulations. As far as I know, only the Boeing 767 has its dive speed listed on its Type Certificate. The rest are manufacturer's proprietary data.

Md is the highest speed that the airliner has been demonstrated to safely achieve. They can go faster, but this has not been demonstrated to be safe.
 
SlamClick
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RE: Modern Airliners Breaking The Speed Of Sound?

Fri Dec 24, 2004 10:00 am

Not to nitpick, but at that altitude, assuming standard temperature I'd make Mach 1.012 to be 641.24mph, not 660.

Happiness is not seeing another trite Ste. Maarten photo all week long.
 
raivavae
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RE: Modern Airliners Breaking The Speed Of Sound?

Fri Dec 24, 2004 10:19 am

Exceeding M1.0 has nothing to do with a dive, if you dive you reduce altitude, so you have temperature increase, and a greater speed of sound, as a(kt)=39vT(°K)
it is when you will cross the conjunction altitude, around FL 240/260 for many jets that, you'll meet max VMO and destroy your aircraft.
if you fly over M1.0 above this altitude, all that can happened is buffeting, then high speed stall and usually spin.
as shown by the DC8 test flights they had to fly over the certified altitudes and moreover the tropopause altitude where temperature is constant to be able to fly above M1.0
 
777236ER
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RE: Modern Airliners Breaking The Speed Of Sound?

Fri Dec 24, 2004 10:23 am

Not to nitpick, but at that altitude, assuming standard temperature I'd make Mach 1.012 to be 641.24mph, not 660.

Uh, above the tropopause at 10km, the temperature is constant to about 22km. The speed of sound is 295.07 m/s throughout that, so the speed of sound at 41,088 feet (or 52,090 feet) is about 660 mph.
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777236ER
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RE: Modern Airliners Breaking The Speed Of Sound?

Fri Dec 24, 2004 10:27 am

Exceeding M1.0 has nothing to do with a dive, if you dive you reduce altitude, so you have temperature increase, and a greater speed of sound, as a(kt)=39vT(°K)

Not nessecarily, neglecting viscous effects, the temperature remains pretty constant above 32,000 feet or so to well above 70,000 feet, so if you're diving from above an altitude of 32,000 feet as your velocity increases your Mach number will increase directly proportionally.
Your bone's got a little machine
 
broke
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RE: Modern Airliners Breaking The Speed Of Sound?

Fri Dec 24, 2004 11:50 am

The large frontal area of today's high by-pass turbofan engines makes it extremely difficult for them to exceed M 1.

Even the EgyptAir 767 that went down near Nantucket did not exceed M1.

In addition, most airliners have an aerodynamic characteristic known as Mach Tuck. As the airflow over the wing approaches or exceeds M1 the center of lift moves aft and the nose goes down.

These airplanes have a system that will increase the nose up trim of the horizontal stabilizers to counter this and it is usually an automatic function. It's called the Mach Trim System on Long beach built airplanes.
 
SlamClick
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RE: Modern Airliners Breaking The Speed Of Sound?

Fri Dec 24, 2004 12:16 pm

"Standard atmosphere" would have the trop height at about 36200' (not 10KM) and the temperature approximately -57.4oC or 214.6oK above that. I have Douglas performance charts that show exactly that figure to have been used in their performance engineering at about the time of this flight.




Happiness is not seeing another trite Ste. Maarten photo all week long.
 
SlamClick
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RE: Modern Airliners Breaking The Speed Of Sound?

Fri Dec 24, 2004 12:22 pm

The rationale behind a dive to accelerate through M1 would be just that - trading altitude for speed that the engine thrust might not be able to deliver.

It is my understanding that a common technique for getting "through the number" in an underpowered airplane is to initiate a climb, then push over the top to yield a bit less than one G, but still slightly positive. This will unweight the wings and cut induced drag drastically allowing a quick acceleration through the sonic region. You will then end up slightly supersonic and in not too nose-low an attitude. Recovery from there should be easy enough.

You would certainly not want to attempt it in a steep dive in a fragile airplane. The research aircraft like the X-1 and Douglas D-558-2 had enough thrust to go super in a steep climb. The X-15 had so much thrust that it probably pushed the earth and its atmosphere away at supersonic speed!
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
speedracer1407
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RE: Modern Airliners Breaking The Speed Of Sound?

Fri Dec 24, 2004 4:56 pm

Regarding the analogy of testing a car under water, I think the point has been missed...at least with this specific analogy. Car manufacturers routinely test their products beyond their design limits, and specifically, beyond top speed expectations. This ensures that, should some fool drive down hill with a tail wind in a car that's not intended for such speed, it won't suddenly fly off the road or blow up. This is, of course, oversimplified, but wouldn't airline manufacturers do the same? I understand that steep-dive experimental explorations into supersonic speed in a 747, for example, could result in the needless death of test pilots, but the information gained from testing such extremes in "controlled" situations seems valuable since various operational circumstance COULD produce such a situation, and we hope that an airline manufacturer accounts for this possibility so that an unexpected dive doesn't result in instant structural failure. Just a thought.
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raivavae
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RE: Modern Airliners Breaking The Speed Of Sound?

Fri Dec 24, 2004 7:24 pm

wrong,
you can not compare cars and aircrafts, as the first one are tested on their mechanical resistance, the others is physical, there is nothing to do against the laws of physics.
 
777236ER
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RE: Modern Airliners Breaking The Speed Of Sound?

Fri Dec 24, 2004 9:03 pm

"Standard atmosphere" would have the trop height at about 36200' (not 10KM) and the temperature approximately -57.4oC or 214.6oK above that. I have Douglas performance charts that show exactly that figure to have been used in their performance engineering at about the time of this flight.

Makes no difference in this case, the aircraft went supersonic at 41088ft, and Mach 1 at that altitude is 660mph.
Your bone's got a little machine
 
Bellerophon
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RE: Modern Airliners Breaking The Speed Of Sound?

Fri Dec 24, 2004 11:00 pm

Raivavae

…Exceeding M1.0 has nothing to do with a dive, if you dive you reduce altitude, so you have temperature increase…

Not correct.

Above the tropopause, where the air temperature stays constant, a dive will not result in an increase in air temperature, only an increase in TAS and therefore an increase in Mach number.

Below the tropopause, in a dive, even though air temperature increases (and therefore the local speed of sound increases) the speed of sound only increases by 88 kts between 36,000 ft and sea level.

In a dive, the rapid conversion of potential energy (height) into kinetic energy (speed) means that the TAS of the aircraft can be increased much faster than the gentle rise in the local speed of sound, and it is quite easy to keep the Mach number increasing in a dive.

Right up to the point you lose control or the wings come off!


777236ER

I agree with your figures.

Mach 1.0, at 41,088 ft, in ISA conditions with an OAT of -56.5°C, is:

574 kts
660 mph
968 fps
1062 kph

which would make the TAS of the DC-8-40, at M1.012, 668 mph, almost exactly as given in the Flier’s Handbook.


SlamClick

Good post, I endorse what you have written.

…engineers will define an envelope for the plane and the test pilots will explore that envelope in very methodical fashion…

Test pilots who deliberately go outside the test envelope tend not to stay employed as test pilots these days.

…Then the envelope gets shrunk down to match us ordinary pilots in the form of "Limitations" in the AFM…

Giving us everyday pilots a comfortable margin of safety in normal operations.

…I'm pretty sure that the test pilots at Boeing or Airbus commercial divisions do not just run the plane up through the number to see what it will do…

Mainly because they know what is likely to happen, in the transonic region, to an aircraft not specifically designed for that regime.

Leaving aside test flights conducted as part of a properly researched and planned flight test program, the idea of deliberately going supersonic, in a subsonic passenger aircraft, strikes me as being one of the most dangerous and stupid things you can do in an aircraft.

Merry Christmas to all

Bellerophon
 
raivavae
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RE: Modern Airliners Breaking The Speed Of Sound?

Sat Dec 25, 2004 12:17 am

I do not agree with you,
A dive will increase your Tas by 0.2kt/second for each 1% slope.
Diving more than 5°=8% would reduce your speed by increasing drag significantly.
Saying 8%, would be 1.6kt/sec, which is 96 kt/min, but at this speed, you'll lose 5.000ft/min, reaching conjunction altitude and VMO before exceeding M1.0.
Aircraft are cruising at tropopause altitudes, about FL330, there is no way for a regular commercial aircraft to everspeed M1.0
 
777236ER
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RE: Modern Airliners Breaking The Speed Of Sound?

Sat Dec 25, 2004 2:36 am

A dive will increase your Tas by 0.2kt/second for each 1% slope.

Assuming thrust equaling drag, and lift equaling weight, I get the acceleration due to a 1% (0.9 degree) dive to be gsin0.9 = 0.154 m/s = 0.3 kt/s. To accelerate from 550kts TAS (say) to 574kts (speed of sound in tropopause) would take 80 seconds. If the aircraft were cruising at 550kt
s TAS (=282.9 m/s), a 1% slope is only 266.6 m/min = 874 feet/min, and the aircraft will only lose 1165 feet in height to gain those extra 24kts needed to go supersonic. Starting from 40,000, the aircraft will end up at 38835 - throughout which the speed of sound is constant.

I might be wrong. I don't get where you get the acceleration to be 0.2 kt/s with a 1% slope.
Your bone's got a little machine
 
SlamClick
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RE: Modern Airliners Breaking The Speed Of Sound?

Sat Dec 25, 2004 2:52 am

I guess the real point here is this: There is no point in designing an airplane to go above mach one unless it goes well above mach one. So we had Concorde doing mach two and losing money but being operated for national pride or some such thing. If you are going to solve the myriad engineering problems associated with flight through this regime and make money you are an authentic genius. The so-called National Space Plane seems to have been stillborn. I guess the bottom line is that hypersonic speeds within the atmosphere are sort of like streamlining an oxcart. What is the point?

Anyway, most modern jetliners are designed for transonic flight (0.75 to 1.3 Mach) but with a maximum free-stream mach number less than one. That is likely to remain true for a long time to come as profitability is better served by other efforts besides increasing cruise speed.
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Bellerophon
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RE: Modern Airliners Breaking The Speed Of Sound?

Sat Dec 25, 2004 10:24 am

Raivavae

…I do not agree with you…

That is your privilege, but, with respect, you are still wrong.  Smile

...Diving more than 5°=8% would reduce your speed by increasing drag significantly...

I'm not sure what you mean by that.

If you mean that the increase in drag will not allow the aircraft to accelerate, then you are not correct.

Whilst the total drag may well increase in a dive, the steeper the angle of the dive, the greater the acceleration and the faster the aircraft speed, right up until the point the aircraft achieves terminal velocity in a vertical dive.

Only at that point will the aircraft not accelerate any more due to drag.


…reaching conjunction altitude and VMO before exceeding M1.0….

Again I’m not sure what you mean by that, but as we are talking about deliberately exceeding MMO, why worry about exceeding VMO, which is probably less dangerous, though still stupid?


…Saying 8%, would be 1.6kt/sec, which is 96 kt/min, but at this speed, you'll lose 5.000ft/min…

And so easily exceed M1.0 long before reaching the tropopause, thank you for doing the maths to prove my point!

You can also read the post by 777236ER to see his calculations.

I must caution however, against trying to use simple maths and subsonic aerodynamics in the transonic and supersonic regions.

Many pilots died, and many aircraft were lost, whilst scientists discovered the special laws applying there. It is a highly specialised flight regime and virtually everything changes there, which it was why it was called the Sound Barrier for so many years.


Je suis sûr que votre Anglais est bien mieux que mon Français, mais je voudrais vous souhaiter un Joyeux Noël.

Bellerophon
 
raivavae
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RE: Modern Airliners Breaking The Speed Of Sound?

Sat Dec 25, 2004 10:45 pm

Bellerophon,

calculations of 777236ER are wrong, as he says 1%slope=0.9°!!!! 1%=0.6°
and he is making calculations with only 1% slope dive, I am talking about a 8% dive.

Exceeding VMO is much more dangerous than exceeding M1.0

An airliner would cruise around 506kt at M0.88, so it's a 70kt acceleration needed to reach M1.0 at the same altitude, as it takes about a minute to do so in a 5° dive ,altitude would be 5000ft lower and temperature 10°C greater, so M1.0 speed will be 10kt higher, it's finally a 80kt at least acceleration needed, and you'll meet VMO before M1.0, then below conjunction altitude your Mach number will decrease.

Dive more than 5° will significantly increase drag, try in your bath to place your hand with an important angle, you'll need a lot of power to have your hand moving as fast as when it's flat.
Diving with a great angle, will give you a important(excessive) vertical speed but not a greater horizontal speed.

Your french is very good, congratulations.
Merry Christmas to you as well.
 
777236ER
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RE: Modern Airliners Breaking The Speed Of Sound?

Sat Dec 25, 2004 10:58 pm

calculations of 777236ER are wrong, as he says 1%slope=0.9°!!!! 1%=0.6°

How do you work that out? A 100% slope is 90 degrees, a 50% slope is 45 degrees, so a 1% slope is 0.9 degrees.

An airliner would cruise around 506kt at M0.88, so it's a 70kt acceleration needed to reach M1.0 at the same altitude, as it takes about a minute to do so in a 5° dive ,altitude would be 5000ft lower and temperature 10°C greater

The temperature is constant above 36000ft, so the speed of sound wouldn't change.

Dive more than 5° will significantly increase drag, try in your bath to place your hand with an important angle, you'll need a lot of power to have your hand moving as fast as when it's flat.
Diving with a great angle, will give you a important(excessive) vertical speed but not a greater horizontal speed


I don't get what you're saying here. When the aircraft is diving, the drag drag due the wing is the same, varying only with speed not pitch. As for transonic drag, at M0.85, you're already quite up there, and it won't make things too draggy not to reach M1.0 before temperature rises.

Anyway, the point is, I think it's safe to say you can make a fast airliner go supersonic from cruise without too much effort - all you need a shallowish dive.
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raivavae
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RE: Modern Airliners Breaking The Speed Of Sound?

Sat Dec 25, 2004 11:07 pm

as you an read in the DC8 article above M1.0
it took 11.000ft above the tropopause to reach this speed, it's not so easy to accelerate above M1.0
 
777236ER
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RE: Modern Airliners Breaking The Speed Of Sound?

Sat Dec 25, 2004 11:44 pm

It was in a 'controlled dive', which could mean they reduced thrust, or added drag using spoilers etc. The numbers seem to imply it's pretty easy.
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raivavae
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RE: Modern Airliners Breaking The Speed Of Sound?

Sun Dec 26, 2004 12:51 am

reducing thrust an exceeding M1.0??!!!!

I am sorry to ask what is your occupation?

Slopes have nothing to do with mathematics, but with geometry, especially trigonometry, it's hard to explain in english, but as an example a 3°ILS slope=5%
 
SlamClick
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RE: Modern Airliners Breaking The Speed Of Sound?

Sun Dec 26, 2004 12:52 am

A 50% slope is not 45o

The way to calculate slope in percent is RISE over RUN.

That is, for a 50% slope, every hundred feet of linear (not horizontal) travel would yield a fifty foot climb or descent. Not going to dig out a sine/cosine table but it is not forty five degrees
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777236ER
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RE: Modern Airliners Breaking The Speed Of Sound?

Sun Dec 26, 2004 2:23 am

reducing thrust an exceeding M1.0??!!!!

I am sorry to ask what is your occupation?


Listen, simply because I made one mistake, as opposed to the whole myriad that you have apparently made, doesn't require you to insult me by questioning my job, with the obvious inference that I am not worthy enough to calculate what I am calculating. I got a 1st with honours from ICL in aeronautical engineering - I am not going to tell you my job because it's simply none of your business.

And yes, reducing thrust and exceeding M1. When diving, it's certainly possible for the contribution of weight to exceed any reduction of thrust parallel to the direction of travel.

And yes, 1% slope is 0.6 degrees, I made a mistake. But that only changes the acceleration due to the dive from 0.3kt/s to 0.26kt/s, which would only change the time taken in my calculation to go from 550kt to 574kt and thus supersonic from 80 to 92 seconds.
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raivavae
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RE: Modern Airliners Breaking The Speed Of Sound?

Sun Dec 26, 2004 3:49 am

Shall I answer ?

Asking for your occupation was not an insult to me, sorry for that, I was just wondering who could say 1%=0.9° lets say you were tired.
 
777236ER
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RE: Modern Airliners Breaking The Speed Of Sound?

Sun Dec 26, 2004 3:57 am

I was just wondering who could say 1%=0.9° lets say you were tired.

No, I simply made a mistake. Considering you haven't explained where any of your numbers or ideas have come from, I don't think you can really preach about accuracy.
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raivavae
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RE: Modern Airliners Breaking The Speed Of Sound?

Sun Dec 26, 2004 4:43 am

my numbers comes from my personal knowledges, that come from MERMOZ school in Paris and the "Guide pratique du Pilote de Ligne", that every french commercial pilot is holding in its case.
 
777236ER
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RE: Modern Airliners Breaking The Speed Of Sound?

Sun Dec 26, 2004 4:56 am

You don't seem to appreciate that the temperature is constant above 36000ft, and here...

Dive more than 5° will significantly increase drag, try in your bath to place your hand with an important angle, you'll need a lot of power to have your hand moving as fast as when it's flat.

...you seem really really confused. In the cases we're talking about, the aircraft doesn't continue horizontally, but travels along the attitude its pitched at, relative to the ground.

Maybe you need to brush up on your personal knowledge.
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raivavae
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RE: Modern Airliners Breaking The Speed Of Sound?

Sun Dec 26, 2004 5:35 am

I fully agree with the fact that temperature is constant above 36000ft, but airliners are flying rarely above FL380, they usually fly from FL300 to FL360, so when diving temperature WILL increase.

I am afraid you are not making the difference between conjunction altitude and tropopause, can you please tell me for a check ?


if "the aircraft does not continue horizontally", does it mean it's going backward?????? (or is it falling like a rock?)


Everything I clear to me and in my mind, it's just that it's not so easy to say it in english sometimes.

the example about the bath, is to tell you that with a 10° pitch down attitude or more, you'll increase grad signicantly, for sure the aircraft is going down as well !

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