Coffin Corner

[airfoilmod]

No Kiwi

reducing power and increasing pitch won't "fly". The increase in AoA is what causes the Stall, obvious. CC, "perceptually" sounds like a perfect balance of all vectors. Contrarily, it is a very dynamic configuration. No "Trades" left. Reducing power, Stall; +AoA, Stall; Fart, Stall.

Aviators don't "land" on the boat.

If the definition of Coffin Corner is "anything you do results in departing aerodynamic flight", then the thing you propose will cause departure. Defining terms is the first step in a discussion? Unless, like ssg you insist on ignoring the terms and proposing situations that don't meet the standards of a civil discussion. What if? Let's say, consider this. A fool's argument. Starting a talk without understanding the proposal is like launching with the control lock fixed and the pitot covered.

Airfoil

[Lemurian]

In a modern Airliner you can't get near "coffin corner" without busting MMO so we should coin a new term for the limiting alt where MMO and AoA stall coincide.

No, normally it is reached at a much lower Mach than Mmo.

Actually, the term "coffin corner" refers to a very small window between mach buffet and stall, not loss of control.

That is the accepted concept nowadays, in the FMS era.

Airfoil you can reduce power and use attitude to maintain airspeed. Airspeed is the critical factor.

Very much so and anyone who's used a HUD can testify to that principle.

Coffin corner in fact is a sensationalist description of approaching the *maximum reachable ceiling* at a given aircraft weight.
The Turcat formula shows that there is a point at which Cl max will be reached and will prevent a further climb whatever the available thrust as one would hit at the same time high and low stall.
It is to be remarked that this ceiling would be attained at only one Mach number.
Now, an airliner is normally operated at altitudes where there is a g factor protection ( as g increases the "apparent" weight ) and one would find those graphs in any QRH. Those protected graphs also exist ,in an easily exploitable form, in the FMS computations of MAX ALT.
So, in real airliner pilot's life, one would be flying with a reduced spread of Mach numbers and that spread will increase at the same altitude as we burn fuel and the airplane gets lighter and lighter. For instrance on a 60T A-320, that altitude (Mach and stall buffet) is 39,000 ft / Mach .78.
At 37,000 ft,flight is possible between .73 and .82 Mach (Mmo)
At 35,000 ft, it is between .70 and .82 Mach.....etc... etc....
From this, the inference is that the best *escape* will be to slowly reduce power (it's almost infinitesimal in the beginning) and start losing altitude at the Mach number one was at. And, yes, maintain Mach with the pitch control (what we do ususally in FL CH or OPEN DES, btw), and very soon, the margins will increase on both "sides" of that Mach.

[airfoilmod]

Lemurian

Wonderful explanation. Coffin Corner is sensationalized. I think you describe how to avoid it perfectly. My point is that, by definition, once you meet its parameters, no escape is possible. If one reduces power, infinitessimaly as you will, you will infinitessimaly Stall. I don't disagree with anything you write, but it ignore's the definition of the concept, sensational or not. Neither argument is wrong. You are certainly a very qualified Captain, you have my complete respect. This is a forum, not a cockpit.

Respectfully, Airfoil

[gr8shandini]

Airfoil,

You're normally on top of the aerodynamics questions, but I'm gonna have to disagree with you on this one. Take the term "coffin corner" out of the picture and think about maintaining an airspeed while reducing power. You do this every time you begin a descent for landing. Pull the power back, nose over slightly and you can maintain any speed you want right out to Vne. Or 1.05 Vstall for that matter. Just control airspeed with pitch and vertical speed with power. Standard "backside" stuff that you learned way back in your PPL days.

I'm a bit intrigued, actually, at how often the coffin corner comes up in this forum. It's fun to think about when you're talking U2s and such, but it happens at altitudes so far above what's reasonable for flight efficiency, I can't imagine an airline driver getting anywhere near it in normal ops.

[BOAC]

I think a few here are getting confused between 'coffin corner' and airliner max altitude, and as 'airfoil' rightly says, the real CC is a 'dead' end. There was no controllable way out of a true CC. An airliner, however, with a few knots between high speed and low speed control problems is a different kettle of fish, and of course we can all fly our way out of that. If the U2 had a 10kt margin as BA said, that also was not 'CC', but just a place to be, how shall I put it, a leetle bit careful. It is not really clear what the OP meant when the thread opened.

I suggest that those 'younger' posters do a bit of historical research on the early jets and their problems, particularly the early straightwing mil jets. A lot of useful lessons were learned.

[FE Hoppy]

Lemurian

I find the most telling sentence in your explanation to be :

It is to be remarked that this ceiling would be attained at only one Mach number.

Would it be correct to say that at a higher Mach number one would for the same mass and altitude require a lower Cl to maintain 1 g flight and therfore the limiting factor is in fact thrust?

With more thrust at the same altitude a higher Mach number could be obtained and thus a lower Cl.

Could recovery from this ceiling not be safely be obtained by lowering the nose and making an accelerating descent. As in doing so Cl is reduced and therefore high speed margin increased?

[airfoilmod]

gr8shandini

Everything you write, along with Lemurian, is indisputable. However, you are also not fully taking in the definition of Coffin Corner. You both describe manouvers that pull you back from the precipice. Reducing power inside the box causes a Stall, which is the A/C's honest response to lost lift; it wants to gain speed and refly. But you are at Vne, so the Stall becomes disastrous. Likewise, if you alter Pitch up or down you tumble out of control. BOAC said it best: it is a "one way street".

best, Airfoil

[Lemurian]

I can't imagine an airline driver getting anywhere near it in normal ops.

It has happened a few times in the pre-FMC era : an underestimation of weight would be enough to be too high . A jet upset is another example when the airplane is submitted to a g-factor higher than planned...
An incident where the influence of flying too close to the max aerodynamic ceiling had a part was the China Air 747 : High altitude, high AoA and an engine flame out....made interesting aerobatics.

This is a forum, not a cockpit.

...and it should reflect what we experience in our lives in the cockpit.
That term that you readily coined as CC has absolutely no official existence in our manuals, but we use terms like :
"Buffet Ceiling",. "Buffet Onset", "Buffet Limited Altitudes", "En-Route Maneuver limits"...etc...CC is just club bar crap (but it came from real-life experience of those gallant test pilots of the fifties who went up there to find out ) and its official name is "LIFT CEILING"

Coffin Corner "sounds hard to reach". Only if you can't imagine flying well past it.

You can't fly past it unless you tried to zoom it from a lower altitude, but you'd be stalling anyway...
This discussion is a perfect demonstration on how imprecise, unofficial undefined concepts can spoil a technological approach to a problem : is a corner a zone or a dot ?
If it's a zone, refer to the buffet margins graph of your airplane
If it's the summit of your curve - i.e a dot -, you are talking of a situation that is best described as accidental severe upset....see your training manual or get the Boeing or Airbus texts on upset recovery.

But you are at Vne, so the Stall becomes disastrous. Likewise, if you alter Pitch up or down you tumble out of control.

And now you start your usual uninformed blurb : You are NOWHERE NEAR Vne, as the parameter that causes the phenomenon is not Mach number, M, but the Clmax . M˛ part of the lift equation....Therefore, your explanation is in error...

[fantom]

Lemurian...

Please pm me if you were tcx.

[gr8shandini]

I guess it all depends on how you define when you're "in a corner." When I was punished in grade school, we weren't required to actually touch the line where the walls come together, so being in the vicinity is close enough to use the term in my mind.

I suppose that if you were flying an aircraft that had both a wicked Mach tuck tendency and deplorable stall characteristics, the very tip of the coffin corner situation could be truly harrowing. But as was stated earlier, in order to get to that singular point where you're well and truly screwed, you'd have to make a very deliberate effort at getting there as the critical speeds converge (barring some crazy NF-104 type zoom maneuver that punts you into ballistic territory). Not something you're likely to wander into.

And if either of the two critical cases prove to be controllable, you could either ride the stall or put up with the Mach buffet until you're low enough to have some breathing room again.

[Lemurian]

Would it be correct to say that at a higher Mach number one would for the same mass and altitude require a lower Cl to maintain 1 g flight and therefore the limiting factor is in fact thrust?

Absolutely correct. That's the gist of high altitude flying.

With more thrust at the same altitude a higher Mach number could be obtained and thus a lower Cl.

Yes, but at our lift ceiling, we are at Clmax. No level flight is achievable, whatever the thrust.

Could recovery from this ceiling not be safely be obtained by lowering the nose and making an accelerating descent. As in doing so Cl is reduced and therefore high speed margin increased?

Yes, as immediately upon lowering the nose - and begining the descent-, one is in fact reducing the AoA, hence the Cl and therefore escape the situation. Acceleration comes a bit later, as we see the buffet margins widening with diminishing altitude.
Google a very simple paper made by Airbus with "Getting to grips with aircraft performance". A very nice paper for reminding oneself of basic theory.

[Brian Abraham]

BOAC - Sorry, been away, hence late reply. Pat Halloran was an F-84 pilot with some 100 combat missions over Korea when he joined the Air Force’s U-2 program in 1957. Before moving on to fly the SR-71 Blackbird, Halloran logged 1,600 hours in the U-2. Here’s what he has to say about piloting it.

The Coffin Corner

“[Maximum and minimum airspeeds] began to converge as you went higher and higher. It wasn’t too bad when you first leveled off, but as you continued through the flight and [went] higher, the indicated airspeeds would get lower and lower, and pretty soon you were down to, oh, maybe a 10-knot window in which you had to continue the rest of the flight. If you exceeded or bounced off the limits, either too fast or too slow, you got pretty much the same indication, so it was difficult to tell which you were encountering: approaching stall or approaching Mach limit.”

AmericanHeritage.com / U-2

It is believed that at least three aircraft were lost (in the USA) due to loss of control resulting from transgressing the buffet boundary. The aircraft had low structual limits and was particularly fragile.

The Chuck Yeager Approach
To recover from CC.Tumble in an uncontrolled manner out of the sky until you hit thicker air. Ultimately you will recognise the uncontrolled state has now entered a recognisable aerobatic state (spin or inverted spin). Apply standard recovey technique. Simple.

I guess you are talking of Chuck's efforts in the NF-104. Didn't turn out too well for him, his only possible recovery action was to bail out. Lucky to survive considering the injuries/circumstances of the bail out. Accident showed his good side, coolness under stress, and his bad side, impetuosity.
NF104 | Spin, Crash & Rescue

[gr8shandini]

Actually, I heard another, not nearly as favorable account of the NF-104 incident from an engineer on the program. It's hard to say where the truth lies as Yeager has a - let's say - disagreeable personality and makes lots of enemies.

According to this guy, Yeager actually lost control of the aircraft twice. It was known that the F-104 had an unrecoverable spin mode with the engine out (apparently, the motor was big enough to generate a stabilizing gyroscopic force) and since the a flameout was a high probability, the NF-104s were fitted with spin chutes. So after the initial departure from controlled flight, he deployed the chute and recovered the aircraft. Instead of descending with the chute deployed until he reached the re-light envelope as they briefed, he decided to cut the chute to get down faster. But without hydraulics, the flight controls were still configured in the position that caused the initial spin and he immediately entered the second one which he had to bail out from.

Between the unauthorized record attempt (he didn't quite steal the airplane like in "The Right Stuff," but it's close) and the failure to follow procedure for the spin recovery / relight, it would have been end of career for any other pilot. But I guess being a national hero buys you a few "get out of jail" cards.

[Pugilistic Animus]

Chuck at 84 flies sick kids [Make -a - Wish] in a Ford Tri-motor---he still puts up about 200 hrs a year---so, I guess he's repentant---still a noteworthy aviator today---can't comment on his personality, but he still does good work


PA

[BOAC]

Thanks for that Brian - I bow to the U2 guys' experiences of course but was just observing that 10kts is not THAT tight a margin, but I guess they may have flown just that little bit higher - I would and, of course, they were not just 'sitting there' looking at the view!

[airfoilmod]

General Yeager

Flies out of O-17, Grass Valley, I see him occasionally coming and going, usually on Saturday mornings when he flies his friend Bart Riebe's Aviat Husky with a friend. He has a well developed command presence, people generally just go quiet when he enters the room or walks on the Ramp. In my experience, he wouldn' talk to just anyone who isn't with media. He is gruff, but very articulate with a real West Virginia accent. Looks very well for his years. He settled here years ago after retirement, his wife, Glennis, was born here (She has passed away). I've stood next to him and listened, (closely), he is a true hero, quite reserved, and keeps to himself in the neighborhood.

Airfoil

[SNS3Guppy]

No, normally it is reached at a much lower Mach than Mmo.

An aircraft at it's performance ceiling isn't at it's upper buffet margin for any reason but coincidence. When an aircraft reaches it's maximum ceiling, given available power and temperature, it's limited in further climb by power. It is not limited by an upper buffet margin. While one mach number may be the magic number for getting to that altitude, that one mach number does NOT represent the maximum speed. A higher mach number will be achievable (assuming one is not in "coffin corner"). This higher mach number's maximum value will be determined by the buffet boundary and will be the upper limiting number.

Simply because the airplane can't climb any more doesn't mean it's reached it's upper limiting value. You're right; the airplane will generally run out of performance prior to reaching Vmo, or such limiting number at which mach effects such as buffeting shall be found. The fact is that most airliners and even many corporate jets simply don't have the power to push themselves to "coffin corner." Certainly at some point they run out of available power and can't climb any more, and certainly a point may be reached where the best angle for climb coincides with or arrives very close to the stall point; the aircrat has reached it's absolute ceiling. This will seldom approximate or equate to the "coffin corner," in which the mach buffet boundary is very close to the low speed buffet (or departure, as the case may be).

[Brian Abraham]

10kts is not THAT tight a margin

Perhaps some other aircraft characteristic made it a tight margin. In all the writings of U2 ops the buffet margin is something oft mentioned and with much respect paid. But then again, perhaps it was skygod talk meant to invoke awe.

[BOAC]

meant to invoke awe

- mere mention of U2 does that alone for me. I would imagine any sort of control loss at that low IAS and with high inertia is something to be very wary of!

[chornedsnorkack]

Let´s have a look at it this way:

A plane at a given altitude (air density) and low Mach number has a Cl which depends on angle of attack, and that Cl has a maximum. That is where the plane stalls: any increase of AoA causes drop of lift. And decrease of AoA also causes decrease of lift. A plane flies at stall speed when its total weight is equal to the local maximum of lift for that air density and true airspeed. Right?

Note that at stall, Cd is substantial. A plane can have lower Cd, and bigger L/D, by flying faster. It is perfectly possible for a plane to have no thrust at all, or to have thrust which, at stall, is less than the drag at stall.

Now, with changes of Mach number the Cl will change. Generally, in nearsonic region, it decreases, so that supersonic Clmax is lower than subsonic Clmax. But not zero.

In nearsonic region the Clmax decreases with Mach number so fast that the product Clmax times V squared has a local maximum - increase of airspeed decreases the maximum lift over the given airspeed.

This is a local maximum - in supersonic range the Clmax may decrease further, but the lift shall increase with V squared, without bound.

The local maximum of lift equalling the total weight defines the coffin corner. Right?

Now, the available thrust changes depending on air density and true airspeed. Since the coffin corner is at stall (a high drag condition anyway), it makes sense that the engines may not provide the thrust necessary to reach and sustain coffin corner altitudes...