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## Coffin corner

• Spinrecovery
• Topic Author

### Spinrecovery created the topic: Coffin corner

Hi all,
I am aware that coffin corner occurs at absolute ceiling and when the stalling speed of an airplane is equal to critical mach number(lowest mach number at which the airflow over the wing becomes supersonic but does not exceed it) but I just don't see the relationship between stalling speed and critical mach number and how they contribute to coffin corner.
Does stalling speed decrease with altitude? Some books state that stalling speed decreases with altitude because there is less air molecules impacting the surface and the indicated airspeed at which a wing stalls decreases while TAS increases, other books suggest indicated airspeed a wing would stall remains constant and TAS increases. Could someone please kindly explain in details for the questions mentioned above? One last question, what is a region of reverse command and can it happen at any speeds?
Cheers

• Posts: 2311

### bobtait replied the topic: Coffin corner

Indicated Air Airspeed is actually not a measure of speed at all, it is a measure of dynamic pressure. That is, it is a measure the kinetic energy the air possesses as it enters the pitot. Kinetic energy is a function of both velocity mass. That is the true speed and the air density [mass of air entering the pitot]. Stalling IAS does not change with height because, by definition, it represents the minimum kinetic energy of the airstream required to enable the wings to produce lift equal to weight. That doesn't change with height. Since air density drops with height, if the kinetic energy of the airstream is to remain constant, TAS must increase to compensate.

For all aircraft, there is an IAS that would require the least power to maintain level flight. It is possible to fly level below that speed, but more power will be required to maintain level flight. Once the level flight IAS drops below that, you are operating in the region of reverse command. [By the way, most GA aircraft would be so close to the stall in that region that it is more a matter for bar-room conversation than actual flight].

There was, I believe, a problem of control reversal encountered in early flight tests in very high speed dives which resulted in deformation of the entire wing, but that had nothing to do with the region of reverse command as described above.

Bob

• Spinrecovery
• Topic Author

### Spinrecovery replied the topic: Coffin corner

What about the coffin corner? I read the books and researched from the internet and I still do not fully understand why coffin corner occurs and why does it have anything to do with Mcrit beside from the fact the stalling speed is equal to Mcrit when it occurs.
Could you please explain in a layman's term the concept of coffin corner?
Cheers

• Posts: 2311

### bobtait replied the topic: Coffin corner

The stall speed is pretty well known to GA pilots as the minimum speed at which the wings can no longer produce the necessary lift and therefore below which control will be lost. The critical mach number is the maximum speed at which the airflow over the top of the wing remains sub-sonic. For wings that were not specifically designed for supersonic flight, if the critical mach number is exceeded, the airflow over the top of the wing can separate and shock waves are generated. In other words you have painted yourself into a corner. Any slower and you lose control because you stall, any faster and you lose control because of flow separation and shock waves usually accompanied by a dramatic shift in centre of pressure causing a pronounced nose-down pitch. You are in an 'option depleted' situation. The actual range of available flying speeds is decreasing with altitude. In the end, the stall IAS speed and the critical mach number [which always corresponds to a particular IAS] will become the same and controlled flight is no longer possible.

Bob

• Mister W

### Mister W replied the topic: Coffin corner

Beat me to it, Bob.
Here are a couple of photos of the Primary Flight Display I took of my sim today for this thread. The real 737 will do the same thing.
First photo is a gross weight of 65tons @ FL400 and M0.79 which. The line at 248kts is where Mach buffet begins. The yellow line at 221kts is the begin of stall buffet if the aircraft is manoeuvring. So your Buffet boundary is 19kts above pre-stall buffet and 8kts below the onset of Mach buffet.

The second photo, I have increased the weight to 68tons. You can see that the 3ton increase has dramatically reduced the Buffet boundary down to within a few knots either of our cruise speed. This is pretty close to Coffin corner and would be flerting with disaster if it was real. This is why long haul airliners will step climb or cruise climb. They wait until they burn off enough fuel (and wieght) to make the next level.

The investigation into the Air france 447 disaster revealled that the co-pilot pulled back on the joystick after the autopilot disconnected sending the aircraft into a climb. The aircraft then climbed to an altitude it was too heavy for and stalled. We'll save swept wing stall characteristics and why the nose didn't pitch down for another time.

This is part of the fun stuff you learn in ATPL aerodynamics.

Hope this helps.

Regards,
Mister W

Just out of interest. The TAS in these photos is just out of view and showing 450ktas.