Coffin corner

[blee256]

Can some explain to me coffin's corner. I've read about it in a book, but i still dont fully understand. I know it has to do with small margin of error, and stall speed. thanks.

[averyrm]

Planes like the U2 can fly so high that their stall speed rises very close to their Mmo, or Vne. Basically, fly too fast, overspeed the plane, fly too slow stall and possibly enter an unrecoverable spin or something fun like that.

[Foxcow]

At altitude in a bank, one wing will stall buffet while the other mach buffets. Thats what I call coffin corner

[pscraig]

If you plot the stall speed vs altitude and mmo (vne) vs altitude on a graph, they meet at the top of a triangle at the top of the graph. That is coffin corner, also called 'Q corner" in some circles.

[B]

Fly faster = Mach tuck
Fly slower = Stall

A stall in a T tail becomes so deep that you can not recover
Mach tuck is not recoverable in most aircraft

Just what I heard

[seagull]

What you heard is incorrect on both counts.

Even the Lears are recoverable, but to make them easier (without special techniques) they added those ventral fins on the aft fuselage.

As for tuck, it is recoverable, the controls are very heavy, but not an issue in a hydraulic aircraft and in most modern designs, there is little to no tuck anyway. Even if you don't have the elevator authority at altitude it will become recoverable at lower altitude as the IAS relative the mach number becomes higher, higher Q nets greater authority in terms of g/stick force.

[Looking4Lower]

Here is a straightforward definition, directly out of the Flightsafety High-Speed Aerodynamics training manual:

"The term coffin corner is used to describe the situation in which an airplane has climbed to such a high altitude that the difference between the low-speed stall buffet and the high-speed Mach buffet is only a few knots. Most turbine-powered business airplanes do not have enough thrust to climb to the coffin corner altitude."

"An example of this is illustrated in Figure 1-8. Note that the difference between low-speed buffet at 43,000 feet and the high-speed Mach buffet at 43,000 feet is approximately 40 knots indicated airspeed."


So...in a nutshell, your IAS starts to get squeezed between "redline" barberpole Vmo/Mmo speed and the stall speed, which creeps upward. And indeed, the ventral 'delta fins' on some of the later Lears are there to prevent a deep stall condition, and they also help dampen out Dutch Roll tendency.

[ananoman]

I think many find this confusing because they do not understand why the margin between stall speed and Mmo decreases with altitude.

Most people think of stall speed as an indicated altitude, because they are used to looking at an airspeed indicator during normal flight operations. As altitude increases the indicated stall speed will remain the same, but the true airspeed at which the airplane will stall increases dramatically.

The true airspeed of Mmo on the other hand will decrease slightly with altitude. The speed of sound is directly proportional to temperature. So it will decrease from 662 knots at sea level (15 C) to 574 knots above 40,000' (-56.5 C).

If you have an airplane with an Mmo of .80 then you are looking at a max true airspeed of about 460 knots above 40,000. Stall speed varies with weight, but you can count on stall speeds of over 100 knots indicated at normal landing weights when in the clean configuration. At higher weights it can be much increased. At 41,000' a 117 knot indicated stall speed translates into around 240 knots of true airspeed. So in this example we have a 220 knot margin. (If you go up to 50,000' then your stall speed is up to around 300 kts true)

Jets will have information in the flight manual on low and high speed buffit margins for weight and temperature. They will usually include a 'buffer' on each side for maneuvering or slight turbulence. So you may have a graph that indicates a 1.5 G stall margin and a 1.3 G high speed buffet. They will also include information on the highest altitude you should climb to given weight and temperature, and this may have more to do with climb rate than stall margin.

Although 'coffin corner' is usually not much of an issue in most aircraft, there have been instances of people stalling a jet because they tried to climb too high when they were heavy.

[shinysideup]

What you heard is incorrect on both counts.

Even the Lears are recoverable, but to make them easier (without special techniques) they added those ventral fins on the aft fuselage.

As for tuck, it is recoverable, the controls are very heavy, but not an issue in a hydraulic aircraft and in most modern designs, there is little to no tuck anyway. Even if you don't have the elevator authority at altitude it will become recoverable at lower altitude as the IAS relative the mach number becomes higher, higher Q nets greater authority in terms of g/stick force.

Very well put, I tried to reply to this before, but could not find the correct response in my head. Actually it was there, but could not find the words. Excellent response.

[blee256]

thanks guys, you have been very helpful.

[Tram]

How would you.. "fly out of coffin corner?"

[Minuteman]

Originally Posted by Tram

How would you.. "fly out of coffin corner?"

Easy, just pull back on the stick as far as it'll go.

[tgrayson]

Originally Posted by ananoman

As altitude increases the indicated stall speed will remain the same,

Stall speed in IAS increases with Mach number, too, because CLmax decreases with Mach number (at least, past .3 Mach or so.)

[Tram]

So - for real, if one were asked "how do you fly out of coffin corner?"

What would be the correct answer, other than "very carefully?"

[SteveC]

Descend to a "safer" altitude while carefully controlling airspeed.

[Tram]

Gratzi..

[dakovich]

during sim training on the CRJ200 our instructor demoed a mach tuck situation and we couldn't recover the airplane. ended up with the red screen of death. maybe it was a simism but man it is not a situation i want to be in ever again, especially if it was in a real airplane. it was very interseting and unsetteling to see how small a margin of safety you have when operating at high altitudes when it comes to the whole coffin-corner discussion. the CRJ is certified to 41,000' but the saftey margins up there (if you can actually get it that high) are just unacceptable.

[musicmixer]

Originally Posted by blee256

Can some explain to me coffin's corner. I've read about it in a book, but i still dont fully understand. I know it has to do with small margin of error, and stall speed. thanks.

When you are at altitude and you see the top and bottom checkerboards on your PFD closing in on each other you are approaching the CC. If they meet you can guess what is going to happen. I've been taught if you see this happening desend 4000'.