Nfield From United Kingdom, joined Dec 2002, 38 posts, RR: 0 Posted (6 years 11 months 1 week 4 days 3 hours ago) and read 6899 times:
Can anyone explain the meaning of the phrase Coffin Corner? I've heard it mentioned in connection with the B47 Stratojet at altitude. What is it and what do you do to get out of it?
Zauberfloete From Austria, joined Nov 2000, 296 posts, RR: 0 Reply 1, posted (6 years 11 months 1 week 4 days 1 hour ago) and read 6843 times:
The higher an aircraft flies, the thinner the air is.
This affects the performance of an aircraft.
As the air gets thinner the necessary speed to avoid a low speed stall increases. You can recognize that with a PC flight simulator. Lets say you are cruising with Mach 0.84 on FL340. If you climb to FL410, with a steady Mach number of 0.84 you will notice that your indicated air speed decreases. That gives you an idea of when you would reach a point where the Mach cruise will not provide enough lift for your given aircraft. I don't know how to describe that better in short terms. In short words: the higher you fly, the more speed you need to create sufficient lift to keep the aircraft flying.
If your altitude is that high, that your max. cruise speed is below the (low speed) stall speed, a low speed stall will occur.
But there is another stall possible, it is called high speed stall. If your aircraft exceeds its maximum cruise speed, this high speed stall will occur. Because the geometry of your your aircraft wing is designed only for a certain range of speeds. Normally the wing is not designed to perform transsonic or supersonic flight.
That means on any given aircraft in any situation there is always a highspeed and a lowspeeds stallspeed. What amount this speeds have is related to your aircrafts weight and altitude. Generally spoken the gap between low speed stall and high speed stall decreases with increasing altitude.
That means that there can be a situation on a very high altitude where a small decrease in speed would lead to a low speed stall, and a small increase in speed to a high speed stall. The usable range of speed would be very small. That situation is called coffins corner.
In that situation a clear air turbulence for instance could result in a stall easily. Because a small change of airspeed would result in a high or low speed stall.
If a (low speed) stall occurs, the pilots will lower the nose of the aircraft, to gain speed in a descent. Once the aircraft gained a sufficient amount of speed, they will slowly and carefully rise the nose of the aircraft to level out the dive.
2912n From United States, joined Oct 2001, 2013 posts, RR: 12 Reply 2, posted (6 years 11 months 1 week 3 days 22 hours ago) and read 6783 times:
What it comes down to is a point where you have painted yourself into a corner...all the variables are against you and pretty much anything you do will just compound the problem. ie...you are going to crash.
Broke From United States, joined Apr 2002, 1300 posts, RR: 5 Reply 3, posted (6 years 11 months 1 week 3 days 18 hours ago) and read 6787 times:
Coffin corner is the point in air density (altitude) and air temperature where the airplane's stall speed and its Mach buffet speed are very close or the same.
Subsonic airplanes are not intended to fly at or above their critical Mach number. The critical Mach number is the speed in terms of the speed of sound where the airflow over the wing goes in excess of Mach 1.
An example of an airplane getting into trouble in the coffin corner is the U-2 flying at high altitude. Here the stall speed is very high (due to low air density) and the critical Mach number is very low (due to low ambient temperature).
The airplane is placed in a left turn. The result can be that the airflow over the left wing will low down below the stall speed and the airflow over the right wing will increase to the critical Mach number. So the inside wing stalls and the outside wing is in Mach buffet.
At 70,000+ feet, the gap between stall speed and critical Mach number for the U-2 can be only about 5 kts.
It takes a very good pilot to fly under these conditions and do it for hours. A gentle hand with the ability to maintain your concentration for long periods of time.
JohnM From United States, joined Feb 2001, 266 posts, RR: 1 Reply 4, posted (6 years 11 months 1 week 3 days 11 hours ago) and read 6682 times:
A former co worker who was a U-2 autopilot troop told me the same thing about the 5kts between stall and max speed at altitude. He told me that autopilot was a must have system for a mission for that reason.
EssentialPowr From United States, joined Sep 2000, 1820 posts, RR: 3 Reply 5, posted (6 years 11 months 1 week 2 days 8 hours ago) and read 6672 times:
Broke provided a good response.
I would add that, in a plot of max altitude (vert axis) versus max speed (horizontal axis) for a given a/c, the "coffin corner" is a point located at the uppermost, right side of the plot, mathematically called a "corner point" in any 2 dimensional plot.
If the speed coordinate is exceeded, a high speed (mach buffet) stall will occur. If the alt coord is exceeded, a classic "low speed" stall will occur.
That is the "coffin corner".
I have no clue what 2912n was trying to describe...
Prebennorholm From Denmark, joined Mar 2000, 5117 posts, RR: 55 Reply 9, posted (6 years 11 months 5 days 14 hours ago) and read 6578 times:
The reason for the strange name is in fact quite obvious.
For all planes you can draw a graph of the flight envelope with Mach number on the X axis and altitude on the Y axis. There will be two lines, one for minumum and one for maximum values. The result will be that you draw a "box" which has a shape which for many planes actually looks like a coffin.
And coffin corner is simply when you operate your plane in the upper rightmost corner of that graph of your flight envelope - where any decrease of speed would make you fall out of the sky, and any increase of speed would produce a Mach buffet - and make you fall out of the sky.
When talking about a five knots gap for the U-2, then it is just an accepted operational "safety margin". It could just go a little higher, an then the gap would soon be zero.
The big problem is to constantly know what your gap is. It changes all the time as the plane gets lighter as fuel is burned.
Flying the U-2 all way up at coffin corner is further complicated by the fact that a very slight sideslip will cause an engine flameout since at 70,000 feet or so the engine will suffer from oxygen starvation if the flow is not directly into the intakes. And it has to descend to some dangerous 50,000 feet to relight the engine.
Regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs, Preben Norholm
Essentialpowr From United States, joined Sep 2000, 1820 posts, RR: 3 Reply 10, posted (6 years 10 months 3 weeks 5 days 9 hours ago) and read 6524 times:
Preben,
You get the "overly extra redundant" description award. Congrats on restating what had already been thoroughly explained....
Sponsor Message: 
From United Kingdom, joined Dec 2002, 38 posts, 
From Austria, joined Nov 2000, 296 posts, 
From United States, joined Oct 2001, 2013 posts, 
From Sweden, joined Mar 2008, 0 posts, 
From Denmark, joined Mar 2000, 5117 posts, 
Sponsor Message: 
Printer friendly format
