Talk:Coffin corner (aerodynamics)

Example

Would be good to add examples of altitudes for different models of planes. —Preceding unsigned comment added by 64.114.83.92 (talk) 07:06, 8 April 2008 (UTC)

Mach Tuck definition

Definition of Mach Tuck is not entirely accurate. Two things cause it: 1.) The loss of the downwash off of the back of the wing as the flow starts to seperate 2.) As the airplane gets very close to the sound barrier, the upwash goes away causing a shift in the center of pressure (from the 25% chord mark, and to the 50% mark on a straight wing, however the degree of shift varies with wing-sweep, generally the higher the better) —Preceding unsigned comment added by AVKent882 (talk • contribs) 05:00, 19 June 2008 (UTC)

This is great information. May I suggest you WP:Be bold, and make the correction yourself? Remember to include citations of reliable sources. You might want to improve the Mach tuck article too. --Jdlh | Talk 07:58, 20 June 2008 (UTC)

Diagram

I think this article would be greatly improved by a diagram, although I don't have the necessary knowledge to create one. 92.10.190.188 (talk) 16:19, 9 June 2009 (UTC)

The illustrative diagram :[1]Patelurology2 (talk) 03:34, 23 June 2009 (UTC)

Another Illustration......: [2]Patelurology2 (talk) 03:40, 23 June 2009 (UTC)Patelurology2 (talk) 14:38, 25 June 2009 (UTC)

Wiki-German link......... [3]Patelurology2 (talk) 17:51, 23 June 2009 (UTC) Needs translation

Translation Program..... [4]Patelurology2 (talk) 18:04, 23 June 2009 (UTC) Needs more work

Wiki-German Page Side by Side to English [[5]]Patelurology2 (talk) 19:26, 29 June 2009 (UTC)

The diagram that is now in the english article is completely in contradiction with the text and the german article. I believe that the diagram is wrong on all counts and should be erased/replaced, but I am no specialist of aeronautics. 128.178.17.157 (talk) 13:44, 20 January 2011 (UTC)

I am a semi retired airline pilot and instructor, I was captain on jet airliners to which this article is of relevance. In my opinion, the picture, CoffinCorner.png (in the german version) is much more suitable for this article. Nolween (talk) 16:39, 19 January 2012 (UTC)

I would also have to also agree. The diagram is not well explained by the article and is not a very obvious or traditional example. First, it is confusing for the article to describe the coffin corner as the convergence of stall speed and critical mach number, and then show a chart that goes well into the supersonic range (obviously from a fighter jet or similar). I think it would be more appropriate in general for the coffin corner to be described as the convergence of the stall and max speed, where the max speed is limited by either the critical mach in a subsonic aircraft or high speed buffeting/structural/engine limitations in supersonic aircraft. Second, this is an overall unusual flight envelope and does not depict the coffin corner condition very well. Something is limiting the max altitude (likely engine flameout) and cuts off the chart, superseding the stall speed as a limitation so the stall and mach limits never converge like in traditional charts. Also, since the mach limit in the example is well above the stall speed at the 1G flight altitude limit, the described peril of the coffin corner is not very apparent. Many aircraft are able to climb well into their coffin corner region where stall and max become very close, but this aircraft has hundreds of knots of separation between the two. Examples of more traditional envelopes on page 28 [6]

128.158.1.164 (talk) 22:51, 9 August 2013 (UTC)

More about temperature

This page says that warmer air means a higher speed of sound, so that Mach tuck occurs at a higher speed, the coffin corner is looser, and the aircraft is safer. In forums about Air France 447, pilots have said that suddenly encountering warmer can be a big problem. Can someone explain this? I'm only guessing, but I suppose that the problem with warmer air is that it is less dense and so provides less lift. So an aircraft could be flying just fine with perfect airspeed information and no turbulence, but if it suddenly flies into warmer air, it does not have enough lift so it stalls, unless the pilot compensates quickly enough by increasing speed. Is that the problem with warm air? —Preceding unsigned comment added by 188.33.16.101 (talk) 14:13, 15 June 2009 (UTC)

The speed of sound is proportional to the square root of the air temperature (lookup the article for the exact formula). The flight Mach Number is the ratio of the flight speed to the speed of sound. Consequently, if an aircraft suddenly encounters warmer air, at a constant flight speed, the flight Mach Number will fall. The Mach Tuck phenomenon, when the centre of lift moves downstream on the airfoil, causing a nose down pitching moment, occurs at a particular flight Mach Number (when there is significant supersonic flow over the suction surface of the airfoil). Therefore, if the Flight Mach Number has fallen because the air temperature has increased, the danger of Mach Tuck will diminish.

You're right to point out that if the air density falls, as it would if air temperature increased at constant pressure, the stall speed will increase. However, in general I think pilot's are wary of encountering warm air because of the other phenomena likely to be met. Changes of temperature are likely to be coupled with large scale turbulent air movements which could be hazardous. I'm not an airline pilot so I cannot say for sure, but I would guess that this is the reason why they are wary of encountering warmer air. —Preceding unsigned comment added by 86.147.29.254 (talk) 11:41, 11 July 2009 (UTC)

Contradiction

I added a {{Citation needed}} after the parenthetical comment about indicated airspeed. See the detailed reason in the tag. -84user (talk) 19:44, 4 April 2011 (UTC)

Thanks for drawing our attention to that contradiction. I have made some changes that I hope will go some way to resolving the problem. See diff. I have left your {{citation needed}} tag in place. Are there any remaining problems with the tagged paragraph, and if so what are they? Dolphin (t) 22:53, 4 April 2011 (UTC)

Thanks, it's clearer now, so I removed the tag. A non-specialist reader such as myself still needs to carefuly read the linked articles to determine exactly what the terms mean, but that is something for those articles to address. -84user (talk) 12:46, 5 April 2011 (UTC)

Air France 447

There is a lot of discussion about how Air France 447 may have run into coffin corner issues. Is it worth mentioning AF447 in this article? Noel (talk) 17:21, 19 June 2011 (UTC)

If reliable, secondary sources exist, and someone is willing to cite those sources in the article, then it is worth mentioning AF447 in this article. However, if it is only speculation, or the only sources are primary sources or internet chat rooms etc. then it isn't the quality of information that an encyclopedia looks for. Wikipedia would prefer to wait a year and publish some high-quality information supported by reliable sources, rather than get in early and publish statements that prove to be unfounded. Dolphin (t) 00:13, 20 June 2011 (UTC)

Air france 447 did not enter into the coffin corner envelope. They did stall the plane, but without approaching the critical mach number. Hope this clears that up. Nolween (talk) 16:03, 19 January 2012 (UTC)

Aerodynamic basis

The following comment was added to the article by 132.3.61.68 on 30 November 2011. I removed the comment and pasted it here for discussion. Dolphin (t) 04:39, 30 November 2011 (UTC)

The picture is not actually depicting a coffin corner or Q-corner. The paragraphs describe the phenomenon well, but the picture is misleading. If you look at where the Q-corner is annotated in this picture, you will see that slowing down is an option. A coffin corner is characteristically described as the singular point where slowing down causes stall and speeding up causes an exceedance of the mach limit. Descending (into warmer and denser air) at that singular (equivalent) airspeed/mach combination into a flight regime where the stall (equivalent) airspeed is less than the mach limit is the only option to get out of the coffin corner. The max altitude line on this aircraft's flight envelope prevents it from having a coffin corner. However, you can imagine where it would be if you extrapolate the stall line up along its trajectory and do the same for the "top speed" (should read "mach limit") line (ie extend the stall line up and right and extend the mach line straight up). The point at which those two extended lines would meet would be the coffin corner. —Preceding unsigned comment added by 132.3.61.68 (talk) 02:43, 30 November 2011‎

I agree; the diagram doesn't depict a true coffin corner. Dolphin (t) 04:43, 30 November 2011 (UTC)

Consequences

"The FAA is concerned that as jet aircraft become more common, less experienced pilots will be flying those aircraft closer to the altitude of their coffin corners, and that catastrophic accidents will occur as a result.[2]"

This phrase is a misinterprtation of the AC quoted in the reference. The sentence suggests that the FAA are expecting lesser experienced pilots to start a trend of flying high performance aircraft closer to their limiting altitudes, which will result in an increase in catastrophic accidents. There is no mention of this in the circular. The FAA published the AC after a series of crashes of high performance aircraft operating at high altitudes to which no definite cause could be attributed, as the aircraft involved suffered near total destruction. They suspected that unfamiliarity with high altitude flight (not simply flight into coffin corner) may have played a role. The purpose of the AC is to encourage comprehensive training curricula at flight training organisations in high altitude/high performance flight operations, particularly for, but not limited to, pilots who have never never flown those types of aircraft before. This sentence would be more appropriate : "Following a series of crashes of high performance aircraft operating at high altitudes to which no definite cause could be attributed, as the aircraft involved suffered near total destruction, the FAA published an Advisory Circular establishing guidelines for improved aircrew training in high altitude operations in high performance aircraft. The circular includes a comprehensive explanation of aerodynamic effects of, and operations near Coffin corner." Nolween (talk) 13:14, 19 January 2012 (UTC)

Classic Wiki

This article states: "operations near Coffin corner" Umgawa, me likes Random capitalization and no use of Articles. — Preceding unsigned comment added by 192.158.61.141 (talk) 20:21, 23 March 2012 (UTC)

Vs0 is the wrong reference for stall speed

Vs0 is the published stall speed in unaccelerated level flight in the landing configuration. It makes no sense at all to talk about the coffin corner -- a high altitude phenomenon -- when referencing an aircraft in landing configuration (typically defined as flaps and gear down, but depends on aircraft).

Either Vs1 or Vs is probably a slightly better reference. Vs1 is for a "specified configuration" and is normally for level cruise flight. — Preceding unsigned comment added by James.d.carlson (talk • contribs) 22:57, 28 January 2017 (UTC)