coffin corner

[jumper52]

Been looking for the complete definition of the coffin corner!!! Can someone clear this one up for me.

 

[banned username 2]

ok, here is a short and brief description...

As altitude increases so does the indicated airspeed of the onset of stall buffet...

As altitude increases the indicated airspeed that Mach buffet appears decreases...

When you get to the aerodynamic limit (altitude) of an airfoil, these 2 speeds are one in the same.... As you get higher these speeds get closer and closer, when you look at them on a speed vs. altitude chart they create a corner... when you get into that corner and exceed the speeds you usually die, hence the name "Coffin Corner"

Very simplified, but kinda to the point...

Hope this helps!

 

[Timebuilder]

"Coffin corner" refers to the fact that stall speed and mach speed are closer together at high altitudes. The name comes from the converging lines where the speeds are plotted on a graph.

 

[Rush Limbaugh]

"Coffin corner" occurs when a formation of fighters are working a container pattern on a conventional range and one delays the turn off target and the next one turns on time. Both fighters have a conflict at the next turn....oh wait....wrong forum

 

[El Cid Av8or]

Take a look at this web page:

http://142.26.194.131/aerodynamics1/High-Speed/Page6.html

Basically, the airplane goes fast enough that the airflow over the wings is no long "attached" to the wing. All lift is lost, the nose points down and like Falcon Capt. said, you're history. The only way to get out of it is to slow the airplane down, if you can. To avoid it, don't go over the critical Mach number for the airplane you're flying.

When designing an airfoil, engineers want to increase the camber of the wing to get more lift so the aircraft can carry more weight. Too much camber and you will have a greater chance of mach tuck when you get to cruising altitude and airspeed. One of those Catch-22 scenarios.

Timebuilder's explanation is very good, and it's simple. (One more reason I have to get out of a technical field!

 

[Timebuilder]

Basically, the airplane goes fast enough that the airflow over the wings is no long "attached" to the wing. All lift is lost, the nose points down and like Falcon Capt. said, you're history. The only way to get out of it is to slow the airplane down, if you can. To avoid it, don't go over the critical Mach number for the airplane you're flying.

I think that what you have described is know as Mach Tuck. The center of lift is literally pushed toward the rear of the wing, causing the nose-down moment.

The coffin corner, as it is called, does not lead to a real problem unless you allow you speed to vary so much that you reach stall speed or mach tuck, depending whether there was an decrease or increase in speed. Linda Pendelton has some observations about this in her book Flying Jets.

One of the "once a day" tests in the Lear is the Mach Trim test. If we lose the mach trim system, we must not fly faster than .74 mach in order to be certain that good control of the aircraft can be maintained. In the transonic region, control is very important!

 

[avbug]

Timebuilder,

I believe you're flying the Lear 35. The mach trim requirement for .74 on that airplane was a certification issue. Stick forces become light enough at Mmo that they do not fall within that required to meet Part 25 certification requirements. This occurs above .78 Mach, and is the reason the autopilot is required.

Part of the function of mach trim is to take up the mach tuck tendencies in the airplane, which are also associated with the lessening of stick force.

Mach tuck tendency in that airplane is extremely mild. I have flown the 35 at 410 at Mmo without an operative puller or autopilot, and without overspeed warning, and it flies very nicely. I did this legally, I should add...I wasn't flying a broken airplane around.

The only purpose for the mach trim is to meet certification requirements.

Coffin Corner is reached at a certain altitude. In most light jets, the airplane can't go high enough to reach this altitude. This is the case in the Learjet.

Coffin corner is bounded by aerodynamic (and engine) stall on the one hand, and mach effects on the other. It's important to note that coffin corner does not represent the margin betwenn aerodynamic/engine stall, and Mmo. It represents the actual margin between aerodynamic/engine stall, and the onset of mach effects. This occurs at a high altitude, and is often higher than most aircraft can reach.

The U2 operates in this regime, with only a very little margin at altitude between aerodynamic boundaries. Most of us never even come close.

 

[Timebuilder]

Thanks for the additional input. I think we're on the same page. I agree, I've flown the 35 in the flight levels for the experience, and it does fly nicely, indeed.

As you indicated, Linda P. says that the coffin corner is not an issue for most operations. 450 is as high as I've gone, but I have a feeling that you have gone higher. Just a guess, though.

You don't have any U2 or SR-71 time do you? I wouldn't be surprised, since it seems like you've flown everything else!

 

[El Cid Av8or]

This is what can happen when you're reading aviation material when you should probably be sleeping.

Thanks, Timebuilder. I thought the two were more related than they are. It's a good thing I don't have to worry to much about this in the Arrow or the Baron. Care to take me for a ride in the Lear so I can get a better understanding of it?

 

[avbug]

No U2 time, and I can only recognize one two out of three times on a good day. EssArSeventyWhat?

 

[flexlrpilot357]

It's the altitude and airspeed where, for a given airfoil, you'll:

stall if you go any slower, and:

hit Mmo if you go any faster.

 

[flywithastick]

I remember hearing a story about some major airline 727 crew trying some trick extending the flaps or slats slightly at altitude (near coffin corner?) 20-30 years ago. As I remember the story, they lost some pieces but landed OK. Anyone remember this? What was the deal?

 

[tothelineplz]

I swear an aging Captain once told me he recognized the numbers on a competitors triassic and swore that was the one. Assuming you're talking about the same thing. The S/O pushed in the CB's after he got back to the deck without consulting with his boss who was demonstrating something to his FO. But that's just how I remember it.

 

[tothelineplz]

Been there.

To all you up and comers...CC is nothing to be afraind of. Respected and understood, yes. And it's not that difficult a concept. Actually had the privilege on a non-rev to go there with a Captain who knew the three holer probably better than the folks in Seattley. But what happens on the flight deck stays on the flight deck. Now I understand CC. Because some people learn best by doing.

 

[flywithastick]

The S/O pushed in the CB's after he got back to the deck without consulting with his boss who was demonstrating something to his FO.

That sounds familiar... was it they pulled the slat cb and extended flaps a little? when the cb was reset, the slats deployed and were torn off?

 

[tothelineplz]

Something like that. But I find it hard to believe the slats and LE Flaps would be "torn off" as you put it. Maybe in Airport '77. I can't even begin to speculate about what happened. Just saying what I remember hearing. Airflow problems may exist if the flaps are extended above 20,000 feet. That's what the manual says. I suppose it would be directly proportionate to the amount of "smash" the crew had.

 

[tothelineplz]

By the way...flywithastick all you want but would you mind not quoting me without including the entire text. You remind me of a journalist. Thanks

To all readers:

Please insert the following sentence in FRONT of the quote by stickwhateverhisnameis...

I swear an aging captain once told me.

See how that changes the context? Please respect that and don't change the context of my posts. Thanks agani!

 

[Timebuilder]

Journalist. Ouch.

 

[flywithastick]

By the way Thanks To all readers:

Please change the context of my posts. Thanks agani!

Sure... no problem!

 

[tothelineplz]

Check that...not journalist...cartoon character...Johnny Bravo...WHATEVER!