VNE and the "coffin corner"?

ADP wrote:

The coffin corner is that place in an aircraft's speed and altitude range
where low speed stall runs into high speed mach buffet.
It has little to do with VNE.

Yes, it has a lot to do with Vne. It is the place where the stall speed
runs into *something* limiting the high speed. The upper speed limit is
Vne by definition, whatever the cause of this limit may be. Note that
Vne may vary with altitude, temperature, whatever. This red line is just
a simplification.

The mach number matters because in real life, the coffin corner was
relevant to military aircraft only. Their Vne was typically set by mach
speed.

Stefan

Yes, it has a lot to do with Vne. It is the place where the stall speed
runs into *something* limiting the high speed. The upper speed limit is
Vne by definition, whatever the cause of this limit may be. Note that
Vne may vary with altitude, temperature, whatever. This red line is just
a simplification.

The mach number matters because in real life, the coffin corner was
relevant to military aircraft only. Their Vne was typically set by mach
speed.

Stefan


I think now you splitting hairs. But if you really want to, Vne does not
even apply to most aircraft for which the "coffin corner" is an issue. The
"red line" on the airspeed indicator is not fixed and moves, it is the
lesser of Vmo or Mmo, which is the maximum operating speed (akin to but not
technically the same as Vne) or mach number.

Ivan

IIRC, the U2 pilots used to flirt with that corner for hours.

Marc Till

ADP a écrit:
Well,
You all almost have it right.
The coffin corner is that place in an aircraft's speed and altitude range
where low speed stall runs into high speed mach buffet.
It has little to do with VNE.
Mach buffet occurs when some part of the aircraft structure causes airflow
separation in the transonic region.
It is almost never caused by the wing per se but usually by some projection
such as a flap track, aileron edge, tail surface, etc.
Mach buffet is also not a stall per se but simply separation turbulence.
Obviously, going faster would result in a full high speed stall.
It is true that you can get into trouble by getting so high that you can't
slow down because of stall and can't speed up because of mach buffet.
Most older transport aircraft have tables that tell you what the limits are
between stall and mach buffet for a given weight and altitude.
The 727, for example, could fly at 39,000 feet but it required a light
airplane and smooth air. It also left as few as 15 kts between stall and
buffet.

The U-2 was really the ultimate motor glider. If I remember correctly, the
wing was basically a beefed-up sailplane wing and the whole aircraft had an
original design life of about 150 hours. It seems to have passed that with
ease. With the J-75 engine, the U-2 had a coffin corner of about 100,000
ft.
I wonder what the glide ratio was?

Cheers,

Allan

"Jim" wrote in message
...

Still thinking about VNE and whether it is usually stated as a TAS
rather than an IAS (one must read the POH to be sure, of course).

I've gotten the notion, probably from comments I've not understood
very well, that the "coffin corner" is the intersection of stall speed
as an IAS indication on the airspeed indicator, and VNE understood as
a TAS and thus occurs at a decreasing IAS with altitude.

I guess the consequence of this notion is that as aircraft altitude
goes up the stall speed TAS goes up to ultimately bump into
the VNE TAS. If VNE is published as an IAS, like stall speed,
then stall speed and VNE would never converge.

Or maybe it was the Manuevering TAS that
could bump into the VNE TAS.

IS this what is sometimes referred to as the "coffin corner"?

"Paul Repacholi" wrote in message
...
Jim writes:

Still thinking about VNE and whether it is usually stated as a TAS
rather than an IAS (one must read the POH to be sure, of course).

I've gotten the notion, probably from comments I've not understood
very well, that the "coffin corner" is the intersection of stall
speed as an IAS indication on the airspeed indicator, and VNE
understood as a TAS and thus occurs at a decreasing IAS with
altitude.

I guess the consequence of this notion is that as aircraft altitude
goes up the stall speed TAS goes up to ultimately bump into the VNE
TAS. If VNE is published as an IAS, like stall speed, then stall
speed and VNE would never converge.

Or maybe it was the Manuevering TAS that could bump into the VNE
TAS.

IS this what is sometimes referred to as the "coffin corner"?

As you get higher, power drops off so you CAN'T go faster, or you get
Mach limited, and IAS drops so you get closer to stall. So, you can't
go faster, and you can't go slower or you stall. As soon as you have
the slightest load increase or slight turn, stall, and probably spin.

You do not have to go anywhere near Vne or Va, just get high enough.

There's a story in here
http://www.nationalgeographic.com/wave/index.html

Click the plus sign on the 747.

Frank Whiteley
Colorado