B2707SST
Posts: 1287
Joined: Wed Apr 23, 2003 5:25 am

Re: Concorde economics vs. 787/A330/777/A321LR

Mon Jun 10, 2019 1:57 am

Phillipzu wrote:
B2707SST, as a person with no real knowledge about aviation, can you explain to me what a cold air pocket does to an aircraft?


This seems like a simple question, but it's a good one since there were actually several things going on at once in this case:

  • Cold air is denser than warm air, so it provides more lift. In this case, when Concorde flew into a colder air pocket, the wings suddenly generated more lift than they had been previously.
  • Denser cold air also produces more engine thrust than less dense warm air, so the engines would have suddenly generated extra power.
  • Most importantly, the speed of sound varies directly with air temperature (and only weakly with air pressure). Sound travels slower in cold air and faster in warm air. This is why the speed of sound at standard conditions is about 760 mph at sea level but only about 660 mph at high altitudes, where air temperatures are much colder.

The first two effects are not negligible but probably much less significant than the third point. This is because Concorde had a "Mach Hold" autopilot mode that left the engines at full dry thrust and maintained cruising speed (Mach 2 to 2.04 in normal conditions) by varying the pitch of the aircraft, and thus its climb rate. When outside air temperature suddenly decreases, the speed of sound falls while the airspeed does not, so the aircraft's Mach number is suddenly higher. The autopilot responds to the situation by pitching up to bleed off the excess Mach number. Apparently the temperature drop in this case was so large that the pitch-up maneuver resulted in Trubshaw's "barely controllable climb."

As Trubshaw mentions, the autopilot logic was modified after this incident. I am not certain but suspect the result was the "Max Cruise" mode, which acted like Mach Hold except that it WOULD use the auto-throttle system to reduce engine thrust rather than simply pitch up in overspeed situations. In commercial service, Max Cruise was typically used for the majority of the flight.

See http://www.concordesst.com/autopilot.html for more technical details on Concorde's various autopilot modes.
Keynes is dead and we are living in his long run.
 
Phillipzu
Posts: 8
Joined: Tue Jun 04, 2019 12:29 pm

Re: Concorde economics vs. 787/A330/777/A321LR

Mon Jun 10, 2019 9:12 am

B2707SST wrote:
Phillipzu wrote:
B2707SST, as a person with no real knowledge about aviation, can you explain to me what a cold air pocket does to an aircraft?


This seems like a simple question, but it's a good one since there were actually several things going on at once in this case:

  • Cold air is denser than warm air, so it provides more lift. In this case, when Concorde flew into a colder air pocket, the wings suddenly generated more lift than they had been previously.
  • Denser cold air also produces more engine thrust than less dense warm air, so the engines would have suddenly generated extra power.
  • Most importantly, the speed of sound varies directly with air temperature (and only weakly with air pressure). Sound travels slower in cold air and faster in warm air. This is why the speed of sound at standard conditions is about 760 mph at sea level but only about 660 mph at high altitudes, where air temperatures are much colder.

The first two effects are not negligible but probably much less significant than the third point. This is because Concorde had a "Mach Hold" autopilot mode that left the engines at full dry thrust and maintained cruising speed (Mach 2 to 2.04 in normal conditions) by varying the pitch of the aircraft, and thus its climb rate. When outside air temperature suddenly decreases, the speed of sound falls while the airspeed does not, so the aircraft's Mach number is suddenly higher. The autopilot responds to the situation by pitching up to bleed off the excess Mach number. Apparently the temperature drop in this case was so large that the pitch-up maneuver resulted in Trubshaw's "barely controllable climb."

As Trubshaw mentions, the autopilot logic was modified after this incident. I am not certain but suspect the result was the "Max Cruise" mode, which acted like Mach Hold except that it WOULD use the auto-throttle system to reduce engine thrust rather than simply pitch up in overspeed situations. In commercial service, Max Cruise was typically used for the majority of the flight.

See http://www.concordesst.com/autopilot.html for more technical details on Concorde's various autopilot modes.


Oh, okay! That actually makes sense, since warmth goes up, especially in cold environments.

Thanks! :D
 
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cpd
Posts: 5899
Joined: Sat Jun 28, 2008 4:46 am

Re: Concorde economics vs. 787/A330/777/A321LR

Tue Jun 11, 2019 1:44 pm

Phillipzu wrote:
B2707SST wrote:
cpd wrote:

You can head shake all you want, but it is the truth. If weather conditions intervened, it would and could drift back down to a lower altitude. That is the way it worked.

And a kudo to the post directly above - interesting.


Glad you found it interesting! You're right regarding cruise-climb - it would be more accurate to say the aircraft climbed relative to a standard density altitude as it burned off fuel. Deviations in temperature from standard atmosphere could and did cause changes in absolute altitude, including descents if temperatures rose.

In fact, test pilot Brian Trubshaw recounts in his book that during a demonstration flight for the Shah, Concorde was cruising at Mach 2 when it suddenly encountered an unusually cold air pocket. "The autopilot could not compete with the change and commenced a rapid climb, followed by an even more rapid descent during which [indicated] air speed rose to 550 kt, 20 kt above the normal limit." This prompted a change in autopilot logic that increased the use of autothrottle and decreased the use of pitch to maintain Mach number following sudden changes in air temperature.


B2707SST, as a person with no real knowledge about aviation, can you explain to me what a cold air pocket does to an aircraft?


In those colder upper air temperatures (say ISA -05) the jet engines run much better, among other helpful effects. So off she goes climbing very well. This is why production Concorde aircraft have those engine rating settings, among other safeguards (like bringing in the auto throttle if necessary) to limit the possibility of those temperature induced excessive climbing/descents.

Ordinarily the auto throttle would be armed ahead of the super cruise but not “active”, ie, the “auto throttle Mach hold” (there is an AP one too) would not be doing anything, unless the plane should encounter either a very cold temperature area, or the other problem, very hot temperatures.

For cold temperatures, This is particularly important around the Middle East and the equatorial regions. That’s another bit for you to research about. Particularly read about tropopause and troposphere.

For sure, that plane had a pretty clever autopilot system.

Edit: already a good reply. Didn’t see it.
 
Zeke2517
Posts: 32
Joined: Wed Jan 09, 2013 4:29 pm

Re: Concorde economics vs. 787/A330/777/A321LR

Tue Jun 11, 2019 2:03 pm

Phillipzu wrote:
B2707SST wrote:
cpd wrote:

You can head shake all you want, but it is the truth. If weather conditions intervened, it would and could drift back down to a lower altitude. That is the way it worked.

And a kudo to the post directly above - interesting.


Glad you found it interesting! You're right regarding cruise-climb - it would be more accurate to say the aircraft climbed relative to a standard density altitude as it burned off fuel. Deviations in temperature from standard atmosphere could and did cause changes in absolute altitude, including descents if temperatures rose.

In fact, test pilot Brian Trubshaw recounts in his book that during a demonstration flight for the Shah, Concorde was cruising at Mach 2 when it suddenly encountered an unusually cold air pocket. "The autopilot could not compete with the change and commenced a rapid climb, followed by an even more rapid descent during which [indicated] air speed rose to 550 kt, 20 kt above the normal limit." This prompted a change in autopilot logic that increased the use of autothrottle and decreased the use of pitch to maintain Mach number following sudden changes in air temperature.


B2707SST, as a person with no real knowledge about aviation, can you explain to me what a cold air pocket does to an aircraft?


Simply put, it makes the air denser so indicated airspeed will increase and barometric altitude will decrease. The autopilot will then try to go to where it thinks it should be, which might involve pulling some crazy moves.
 
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Pellegrine
Posts: 2210
Joined: Thu Mar 29, 2007 10:19 am

Re: Concorde economics vs. 787/A330/777/A321LR

Sun Jun 16, 2019 11:35 pm

RJMAZ wrote:
Seat prices for a supersonic aircraft would easily be $2000+ for London to New York.


HUH? The last say 5-7 years of operation JFK-LHR-JFK was over $10,000.
oh boy, here we go!!!
 
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Pellegrine
Posts: 2210
Joined: Thu Mar 29, 2007 10:19 am

Re: Concorde economics vs. 787/A330/777/A321LR

Sun Jun 16, 2019 11:50 pm

cpd wrote:
Good grief- some of the hottest parts on the exterior of the most famous Lockheed plane of them all were composite materials. They stood up to the demands of M3.0+ flight, sometimes even faster for some incredibly long missions.

Missions that were far longer than any Concorde flight and had more heating/cooling cycles as well.


Nope, the leading edges, chines, exhaust fairings and nose were all titanium alloy. SR-71 was 85% titanium alloy and 15% polymer composite. Composites were used on cooler parts of the airframe. Interesting tidbit, SR-71 could fly Mach 3.5-3.6, although 3.2 is the official number. What limited it was the shock wave coming off the nose getting caught up in the engine air intakes, and well temperature at the tip.
oh boy, here we go!!!
 
stephanwintner
Posts: 69
Joined: Thu Jan 03, 2019 5:04 pm

Re: Concorde economics vs. 787/A330/777/A321LR

Wed Jun 19, 2019 11:43 pm

RJMAZ wrote:
With digital design and testing it would be quicker to develop.


Hahahahah Clearly you've not used Catia or Enovia.

Turning off the sarcasm, I'm not at all convinced modern design methods are quicker. I believe we can analyze in much greater depth, hence we engineers do. E-mail and Powerpoint are wonderful devices for spreading chaos and misinformation, and modern PLM systems are terrible time wasters. Speaking from experience on the 787, and elsewhere, there is a reason engineering can't seem to get it right, especially compared to the pace of development in the early jet age.

That said, we do go into more depth analytically, and do probably need less testing. Then again performance goals are more challenging, which in turn requires more effort. Software has become more complex as well.

Shrug. But no, it's not faster overall, at least in this engineers experience.

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