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mixomax
21st Oct 2003, 12:39
Hello all,

Anyone of you familar with this "Coffin's corner"?

I've tried on many search engine,and yet,still no joy!!!

Thank you so very much in advance for help....

PA-28-180
21st Oct 2003, 12:49
Cant give you a real technical answer...but I beleive that it refers to a situation where cruise speed and stall speed become almost equal- at high altitude.

Jamesel
21st Oct 2003, 13:55
Mixomax..

The term "coffin corner" refers to the point in an aeroplanes' performance envelope where the stall speed has increased to the same value as the Mach buffet, due to an increased altitude. In practical terms, the aircraft cannot climb any higher, regardless of power, due to the aerodynamic effects, thus it is sometimes referred to as the aerodynamic ceiling as opposed to the more normal (power limited) service or absolute ceiling.

Stall speed increases at altitude (more noticeable above 20,000')due to the reduction in Reynolds Number (crudely put - the viscosity of the air).
Mach buffet occurs as the airfoil accelerates the flow over the wing to the speed of sound, even though the rest of the aeroplane is at an indicated airspeed of less than Mach 1. The ailerons will possibly go to full deflection, rapidly each way (snatching), and Mach tuck (where the nose starts coming down regardless of the elevator position or airspeed) may occur, worsening the situation. Of course stalling an aeroplane isn't the best way to guarantee control over the next couple of seconds, either. In the coffin corner, on the recovery the aircraft may accelerate into Mach tuck.

At an altitude close the the limit, a turn will increase the effects, the flow over the faster (outside of turn) wing going more supersonic with more turbulence from the shock wave, and the flow over the slower (inner) wing slowing down. The machine is literally going both too fast and too slow at the same time - and it is only going to come down.

Either way, the aeroplane is not in controlled flight anymore - hence the term - coffin corner.

Common turbulence at high altitude can cause the same problems. Any commercial airliner pilot will compute (or have computed for him) the minimum and max airspeeds for the weight and altitude the airliner will be flying at, with an appropriate buffer for safety & comfort.

It is really only a problem for near gross weight, high flying jets, (or record setting gliders) the U-2 pilots were probably the first to name & popularise this situation, most previous jets being power limited in altitude.


Inappropriate commas, brackets and spelling due to lack of ability:O, sorry

411A
21st Oct 2003, 16:17
'Handling the Big Jets' (Davies) will give you the answer, if you can still find a copy in print.

First found in the early 50's, an area to be avoided.
A few early heavy jet transports (707, DC-8's) found the problem with engines (Pratt&Whitney JT4A) that would get 'em to high altitude, only to find that the maneuvering margin was very low.

A bad combination.

aviate1138
21st Oct 2003, 16:30
411A said...
A few early heavy jet transports (707, DC-8's) found the problem with engines (Pratt&Whitney JT4A) that would get 'em to high altitude, only to find that the maneuvering margin was very low.

Aviate added...
The early Lear Jets [ in the hands of almost untrained, wealthy private owners ] sometimes wound up on the Far Side and no prisoners were taken! With the excess power available it would get to Vne and beyond in straight and level flight very quickly. Flying by the Numbers really meant what it said.

Aviate 1138

Crossunder
21st Oct 2003, 18:08
...and I always thought that this Coffin's Corner was where the undertakers meet for a drink?

PAXboy
21st Oct 2003, 19:40
At an altitude close the the limit, a turn will increase the effects, the flow over the faster (outside of turn) wing going more supersonic with more turbulence from the shock wave, and the flow over the slower (inner) wing slowing down. The machine is literally going both too fast and too slow at the same time - and it is only going to come down. [dredging memory] Is this the reason why Concorde has to fly dead straight in high speed cruise? Because, if she turns even slightly, then all of these forces will leap out at her?

noisy
21st Oct 2003, 20:26
No, this aircraft was carefully designed not to have Mach buffet problems.

:D :D :D

PAXboy
22nd Oct 2003, 20:44
Interesting. When on Conc (August, this year) I noticed that the jump to light speed [sorry, acceleration to supersonic ;) ] was utterly smooth. When we dropped back through the sound barrier, there was a distinct shimmy and the a/c rocked - ever so slightly - fore and aft. It was the only such motion that I noticed on the flight.

fly-half
23rd Oct 2003, 22:18
It's referred to as the corner because it's shown in the corner of a graph of Altitude against Mach#. Anybody know how to paste graphs on here? There's an idea!

With respect to the altitude parameters, it's between the Manouvre Ceiling (below) and the Aerodynamic Ceiling (above). With respect to the Mach# it is between the 1g Low Speed Buffet and the 1.3g LSB, between those two altitudes. If any form of buffet in encountered, decrease altitude(still maintaining speed by closing throttle with a lowering of the nose).

Again, look it uo in a book somewhere, it's the area in the corner of the graph that you don't want to enter!

mixomax
25th Oct 2003, 11:14
Thank you very much gentlemen for all the infos.
It is always reassuring to meet well educated fly-boys...
Fly safe,
Mixomax

Slasher
26th Oct 2003, 09:06
If you want to talk to someone with real intimate knowlege of coffin corner, talk to a U2 pilot!

Ignition Override
26th Oct 2003, 14:33
How about the fastest plane, the SR-71? Does it have this corner?

The fastest man in the world has been my FO here a few times. I'm glad to have the chance to be the non-flying pilot for him on alternating legs. He was an F-4 Instructor Pilot, then flew the SR-71. He finished his Air Force career on the corporate-style G-III at Andrews AFB.

And E. is humble, soft-spoken and religious, definitely not above speaking with other pilots, no matter what their background or present aircraft. This can not be said for some pilots who "streak' around in the DC-10 or 747 (many are too self-impressed with their paychecks). I can ask E. about the corner, but my impressions are that the SR was limited more by temperature, controlling the c.g. with fuel transfer, and inlet airflow control. Even 65,000' was nowhere near the max altitude.:)

Pardon the comments which are unrelated to the "corner", but the irony of such a pilot's humility is never lost on me.

Chimbu chuckles
26th Oct 2003, 21:27
Mixomax

I'm no expert but here's my understanding.

The lower limit of controlled flight is obviously the 1g stall speed.

The upper allowed speed limit is Mmo limited by buffers below the speed at which mach (high speed) buffet occurrs.

Stall speed increases with altitude and weight.

Vmo/Mmo is not effected by weight.

At altitudes above 30000' the 1g stall speed starts to increase significantly while the limiting mach number starts reducing the allowed IAS.

Where these two meet on a graph is 'coffin corner'.

At this point an aircraft can have only a few knots between low speed buffet (stalling due to increased Vs due altitude) and high speed buffet (where localised airflow around various parts of the aircraft, not just over the wings, first exceeds the speed of sound.

Clearly aerofoil shape will have some effect...I well remember dragging an F28 to 35000' after a MTOW departure and sitting there experiencing almost imperceptable low speed buffet until we burned off some fuel. When we banked into a turn the buffet increased because we were no longer in 1g flight but something in excess of that in a 25 degree angle of bank...our effective weight was increased therefore our stall speed increased.

Clearly a stall at high altitude with its attendant reduced aerodynamic damping (due thin air) is something that would really get your attention!

At the high speed end you can get mach tuck where the nose keeps going down and there's SFA you can do about it. This is caused by the center of lift moving aft when airflow around an aerofoil becomes transonic. The faster you go the worse it gets. This is also a function of aerofoil shape, as discovered by the Test Pilots in the High speed flight in the UK during the war. It's made worse by swept wings where the thicker inboard sections (forward of the tips) go transonic first.

Mach tuck is offset by Mach Trim which automatically changes the stab trim angle of incidence to trim more and more nose up as you approach Mmo. While conducting post C Check test flights on a Falcon Corporate jet I was required to exceed Mmo by a set amount and it took a fair bit of muscle to hold the control column forward against the mach trim as we slid through mmo and into high speed buffet in a full power dive from 37000'...twas very interesting excercise ;) :}

Concorde, SR71 etc would not have any worries about high speed buffet as they are, most assuredly, designed to go at multiples of the speed of sound. Low speed buffet would be a different matter entirely. I don't know, but strongly suspect, that this is the reason for Concordes Mach 2.0 emergency descent in case of engine failure.

Hope this helps.

Chuck.

used2flyboeing
22nd Nov 2003, 02:53
the 707 is still a very "hot bird" even by modern standards, its low altitude speed cannot be matched - hence the AirForces continued interest in it as a platform.. IE low level flying under radar etc.. As far as Coffin Corner - who cares - so you stall it at FL35 - you got 35K to sort it back out - BAAAAAAAAAbay ..