VNE and the "coffin corner"?

[Jim]

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"?

[Mark James Boyd]

In article >,
Jim > wrote:
>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"?

I think "coffin corner" is a totally different phrase relating to
high speed stall buffet. At some high mach number at high altitude,
a shockwave causes airflow separation from the wing and a
stall.

There is also for the wing a low speed "traditional" stall
which occurs with too much angle of attack.

There is some TAS where the curves of stall speed and
max mach number intersect. This happens at some altitude.
At that altitude and above, the aircraft WILL stall,
one way or the other, despite precise speed control.

I'm told the U-2 used to fly around at 80 kts IAS
at this coffin corner, and the FL was determined by
how much turbulence was expected (flying right at the
intersection altitude can only be done at perfect
airspeed in perfectly smooth air).

Another issue is designing an aircraft shape where
all the parts (underbelly, top, tail surfaces, wings) stall
at the same mach mach number (high and maybe low speed).
This prevents shaking and buffeting and allows a higher
mach speed, and better efficiency, and apparently
higher altitudes before reaching "coffin corner" or
equivalent "coffin corners" for turbulent conditions.

I watched a video at a 737 ground school about a
captain who flew a 737 into coffin corner, hit
some turbulence, low speed stalled, and spun the
737. After spinning through many thousands of feet,
he recovered. I heard the galley was out of
liquor a few minutes later...

I don't believe "coffin corner" is from high speed
separation over the wing over the wing.
But there may be some relation as the wavi air
goes over the wing and the rather narrow control
surface and waves over the aileron or elevator,
causing it to stall, engage, stall, engage, etc.
I.e. flutter.

So maybe flutter is related to narrowness of
glider control surfaces? And therefore each
control surface has its own little tiny
"coffin corner"? Who knows...

[W.J. \(Bill\) Dean \(U.K.\).]

I have always understood that "coffin corner" is when the TAS for the stall
increases to meet the onset of compressibility or whatever which occurs at
Mach 1. I understand that the TAS of Mach 1 does not change with the
reduced density at altitude, whereas of course the TAS of the stall
increases.

I am sure I remember reading years ago that "coffin corner" was the
limitation for the maximum altitude for operating the Boeing B47.

W.J. (Bill) Dean (U.K.).
Remove "ic" to reply.

>
> "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 Manoeuvring TAS that could bump into the VNE TAS.
>
> Is this what is sometimes referred to as the "coffin corner"?
>

[Mark James Boyd]

In article <3fdf566a$1@darkstar>, Mark James Boyd > wrote:
>
>I don't believe "coffin corner" is from high speed
>separation over the wing over the wing.
I should have said "I don't believe it's from separation over
the wing of a glider."
The glider's don't go that fast, don't have a
shockwave, and don't reach anything near the
effects of mach.

[Mark Brown]

Jim,

You hit the nail on the head in paragraph three then
confused yourself! As you climb your Stall TAS increases
however VNE TAS doesnt. So they will run into each
other at very high altitudes or at least give you a
very small airspeed envelope to operate in. It is
this fact that you have to remember. Always think
in TAS and you cant go far wrong. IAS is only a value
that we see and unless you convert it accordingly to
TAS at altitude you may as well forget it.

At 18:00 16 December 2003, Jim wrote:
>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'?
>

[nafod40]

Jim wrote:
> 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.

VNE is due to two effects, compressibility (TAS affected), and force of
the wind (IAS). Stall speed is an IAS thing.

The corner is when you can go no slower (stall/IAS) and no faster
(compressibility/TAS). Bad place to be.

[Jim]

On 16 Dec 2003 18:40:56 GMT, Mark Brown
> wrote:

>Jim,
>
>You hit the nail on the head in paragraph three then
>confused yourself! As you climb your Stall TAS increases
>however VNE TAS doesnt. So they will run into each
>other at very high altitudes or at least give you a
>very small airspeed envelope to operate in. It is
>this fact that you have to remember. Always think
>in TAS and you cant go far wrong. IAS is only a value
>that we see and unless you convert it accordingly to
>TAS at altitude you may as well forget it.
>

Thank you Mark. I think I understand it now.

[Paul Repacholi]

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.

--
Paul Repacholi 1 Crescent Rd.,
+61 (08) 9257-1001 Kalamunda.
West Australia 6076
comp.os.vms,- The Older, Grumpier Slashdot
Raw, Cooked or Well-done, it's all half baked.
EPIC, The Architecture of the future, always has been, always will be.

[Stefan]

Jim wrote:

>> It is this fact that you have to remember.

You can climb pretty high in an average glider before you "have to
remember" this. If you actually find a wave to climb that high, I guess
you rather have to remember the the boiling point of water...

Stefan

[ADP]

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"?

[Stefan]

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

[Ivan Kahn]

>
> 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

[Marc Till]

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"?
>
>
>

[F.L. Whiteley]

"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