why is Vne a TAS?

[private]

Hello Todd,

In a recent thread on R.A..S.(interesting question about stall and airspeed)
that morphed into a discussion about coffin corner, Todd explained,

"Flutter limits high true airspeed operations, and it is possible to get to
the coffin corner where stall speed and Vne approach each other."

and Jim suggested we "consider Vne as a TAS".

Can I have the groups wisdom and further explanation of the reasons for this
and of how Vne changes with altitude.

Should we be doing preflight calculations of Vne IAS reductions when
planning aerobatics at 9000 ft. or soaring or high speed flight at 15-18,000
ft?

Thanks in advance

[[email protected]]

VNE can be set for a variety of reasons.

Structural strength. This one is probably limited by IAS/CAS

Flutter or other dynamics problems. This is releated to the dynamic
behavior of the airflow and is most likely a TAS limit.

So to really answer the question you need to know what the limiting
VNE factor was for your aircraft.




..




On Sun, 11 Sep 2005 04:46:58 GMT, "private" >
wrote:

>Hello Todd,
>
>In a recent thread on R.A..S.(interesting question about stall and airspeed)
>that morphed into a discussion about coffin corner, Todd explained,
>
>"Flutter limits high true airspeed operations, and it is possible to get to
>the coffin corner where stall speed and Vne approach each other."
>
>and Jim suggested we "consider Vne as a TAS".
>
>Can I have the groups wisdom and further explanation of the reasons for this
>and of how Vne changes with altitude.
>
>Should we be doing preflight calculations of Vne IAS reductions when
>planning aerobatics at 9000 ft. or soaring or high speed flight at 15-18,000
>ft?
>
>Thanks in advance
>

[Stefan]

private wrote:

> Should we be doing preflight calculations of Vne IAS reductions when
> planning aerobatics at 9000 ft. or soaring or high speed flight at 15-18,000
> ft?

Certainly. You'll find the numbers in the POH. (In real life, 9000 ft is
no issue. Soaring in wave up to 30'000 ft is.)

Stefan

[BTIZ]

> Should we be doing preflight calculations of Vne IAS reductions when
> planning aerobatics at 9000 ft. or soaring or high speed flight at
> 15-18,000
> ft?

Many gliders capable of Vne around 146 kias below 10,000 MSL actually have
lower IAS Vne charted for altitudes above 10,000ft MSL and in 3000ft blocks.
The DG1000 has the following limits.
Vne at 10,000ft is 146KIAS, 13,000ft is 138KIAS, 16,000ft is 131KIAS,
20,000ft is 114KIAS, 23,000ft is 117KIAS and 26,000ft is 111KIAS

Vne is based on IAS, TAS for the same IAS is higher at higher altitudes. But
remember, it's not just airspeed, or airspeed indicator errors associated
with altitude changes, but also other dynamic "q" factors on the airframe at
higher "effective" airspeeds.

Many jets have a Maximum operating Mach limit. Vmo would not change with
altitude, but the IAS limit associated with that Mach limit will continually
decrease as altitude increases. Eventually that Vmo (IAS) gets down to
Vstall, and that is the "coffin corner" in a previous post.

BT

[Ron Rosenfeld]

On Sun, 11 Sep 2005 08:02:49 -0700, wrote:

>VNE can be set for a variety of reasons.
>
>Structural strength. This one is probably limited by IAS/CAS
>
>Flutter or other dynamics problems. This is releated to the dynamic
>behavior of the airflow and is most likely a TAS limit.
>
>So to really answer the question you need to know what the limiting
>VNE factor was for your aircraft.
>
>
>
>

My understanding is that Vne, which is marked by a red line on the ASI, is
an *Indicated* Air speed; but that if it varies with altitude, there needs
to be some method of indicating to the pilot the appropriate limitations
thoughout the operating altitude range.

In my a/c this is done by a placard.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

[Dave S]

I remember the thread, and I have yet to have the notion that VNE should
be TAS explained satisfactorily.

Dave

private wrote:

> Hello Todd,
>
> In a recent thread on R.A..S.(interesting question about stall and airspeed)
> that morphed into a discussion about coffin corner, Todd explained,
>
> "Flutter limits high true airspeed operations, and it is possible to get to
> the coffin corner where stall speed and Vne approach each other."
>
> and Jim suggested we "consider Vne as a TAS".
>
> Can I have the groups wisdom and further explanation of the reasons for this
> and of how Vne changes with altitude.
>
> Should we be doing preflight calculations of Vne IAS reductions when
> planning aerobatics at 9000 ft. or soaring or high speed flight at 15-18,000
> ft?
>
> Thanks in advance
>
>

[Bob Moore]

"private" > wrote
> "Flutter limits high true airspeed operations, and it is possible to
> get to the coffin corner where stall speed and Vne approach each
> other."
> Can I have the groups wisdom and further explanation of the reasons
> for this and of how Vne changes with altitude.


It is probably not "flutter" that sets the high speed limit, but
rather "critical Mach bufett". Yes, even a glider has a Mmo but
no mach meter to indicate it, so an equivalent TAS becomes the limit.

Copied from a web site:

"The critical Mach number is the free stream Mach number at which sonic
flow first appears on the airfoil/wing.

Buffeting is the structural response to excitation produced by the shock-
induced flow separation, and is ultimtely related to the vortex formation
and breakdown. Buffeting may affect different parts of the airframe, but
the buffeting on the wing is the most important.

As the speed increases the wing is affected by shock stall, whose
appearance sets an upper limit to the speed. This limit is a function of
the flight altitude. Its envelope is called buffeting boundary."

Bob Moore

[BTIZ]

poor choice of wording on "Vne is based on TAS" ....
Vne is actually based on other structral limits, flutter, drag buffets,
control etc.. and is equated to a value in speed. Not all GA aircraft have
Mach Meters or "barber pole" indications, most only have IAS so that is what
is used. One can watch the barber pole move with control changes, altitude
changes or changes in wing sweep position.

BT

"BTIZ" > wrote in message
news:yN_Ue.14282$ct5.10523@fed1read04...
>> Should we be doing preflight calculations of Vne IAS reductions when
>> planning aerobatics at 9000 ft. or soaring or high speed flight at
>> 15-18,000
>> ft?
>
> Many gliders capable of Vne around 146 kias below 10,000 MSL actually have
> lower IAS Vne charted for altitudes above 10,000ft MSL and in 3000ft
> blocks. The DG1000 has the following limits.
> Vne at 10,000ft is 146KIAS, 13,000ft is 138KIAS, 16,000ft is 131KIAS,
> 20,000ft is 114KIAS, 23,000ft is 117KIAS and 26,000ft is 111KIAS
>
> Vne is based on IAS, TAS for the same IAS is higher at higher altitudes.
> But remember, it's not just airspeed, or airspeed indicator errors
> associated with altitude changes, but also other dynamic "q" factors on
> the airframe at higher "effective" airspeeds.
>
> Many jets have a Maximum operating Mach limit. Vmo would not change with
> altitude, but the IAS limit associated with that Mach limit will
> continually decrease as altitude increases. Eventually that Vmo (IAS) gets
> down to Vstall, and that is the "coffin corner" in a previous post.
>
> BT
>

[Stefan]

Bob Moore wrote:

> It is probably not "flutter" that sets the high speed limit, but
> rather "critical Mach bufett". Yes, even a glider has a Mmo but
> no mach meter to indicate it, so an equivalent TAS becomes the limit.

The typical Vne for gliders is around 150 knots. Pretty low for mach
buffets.

If you want to see what flutter is, look here:
http://www.dg-download.de/Videos/dg-300-flatterversuch.mpg

Stefan

[Mike Rapoport]

The limiting airspeed, Vne is the speed where the airplane structure still
meets the certification requirements. For light aircraft it is normally set
at 90% of Vd (demonstrated dive speed) The limit could be where the
maximium design gust equals the structural limit of the airplane or it could
be the highest speed where a reasonable flutter margin is present. As
others have said, it could be related to critical Mach number although this
is unlikely to be the limiting factor for slow, light airplanes. If the
limit is for gust loading, then it is based on CAS. If it is based on
flutter or Mach limit, it is based on TAS. Even if it is based on TAS, it
is given in IAS in airplanes without a Mach meter. or barber pole. The IAS
limit is based on the ceiling of the aircraft.. The same thing can be
accomplished by placards (reduce Vmo (or Vne) by x knots for every thousand
feet above xx,xxx' altitude.

The "coffin corner" is where stall and Mmo (not Vne) come together. I don't
think that there is an aiplane that can reach the coffin corner that even
has a Vne.

There is some discussion of this topic in one of Barry Schiff's books.

Mike

"private" > wrote in message
news:6FOUe.465897$s54.126911@pd7tw2no...
> Hello Todd,
>
> In a recent thread on R.A..S.(interesting question about stall and
> airspeed)
> that morphed into a discussion about coffin corner, Todd explained,
>
> "Flutter limits high true airspeed operations, and it is possible to get
> to
> the coffin corner where stall speed and Vne approach each other."
>
> and Jim suggested we "consider Vne as a TAS".
>
> Can I have the groups wisdom and further explanation of the reasons for
> this
> and of how Vne changes with altitude.
>
> Should we be doing preflight calculations of Vne IAS reductions when
> planning aerobatics at 9000 ft. or soaring or high speed flight at
> 15-18,000
> ft?
>
> Thanks in advance
>
>

[Stefan]

Mike Rapoport wrote:

> I don't think that there is an aiplane that can reach the coffin
> corner that even has a Vne.

Airplanes probably not. Aircraft, yes.

Stefan

[Mike Rapoport]

Which ones?

Mike
MU-2


"Stefan" > wrote in message
...
> Mike Rapoport wrote:
>
>> I don't think that there is an aiplane that can reach the coffin corner
>> that even has a Vne.
>
> Airplanes probably not. Aircraft, yes.
>
> Stefan

[Stefan]

Mike Rapoport wrote:

> Which ones?

I had thought of some older gliders which have pretty low Vne. After I
had hit the send button, it dawned to me that they also have a pretty
low stall speed, though...

Stefan

[Mike Rapoport]

"Stefan" > wrote in message
...
> Mike Rapoport wrote:
>
>> Which ones?
>
> I had thought of some older gliders which have pretty low Vne. After I had
> hit the send button, it dawned to me that they also have a pretty low
> stall speed, though...
>
> Stefan

I was thinking of sailplanes too. Hard to imagine a sailplane with a
critical Mach number equating to a IAS of 45kts or so even at FL500.

The key to this issue (IMHO) of Vne is that it may be based on any one of a
number of limiting factors, some of which are related to CAS and some to
TAS. The airplanes where Mach number is an issue (that I am familiar with)
have a Vmo and Mmo but don't use a Vne.

Mike
MU-2

[Stefan]

Morgans wrote:

>> If you want to see what flutter is, look here:
>> http://www.dg-download.de/Videos/dg-300-flatterversuch.mpg

> Ooooohh, THAT looks VERY scary! I had never seen such large amplitude
> flutter before. I had always seen small buzzing types of flutter. I hope I
> never see either kind, except in a video.

It was a test flight with a specially reinforced glider. Any production
glider would have lost its wings pretty quickly.

Stefan

[Montblack]

("Morgans" wrote)
>> If you want to see what flutter is, look here:
>> <http://www.dg-download.de/Videos/dg-300-flatterversuch.mpg>

> Ooooohh, THAT looks VERY scary! I had never seen such large amplitude
> flutter before. I had always seen small buzzing types of flutter. I hope
> I
> never see either kind, except in a video.


....when those new piezoelectric wings short circuit. <g>

BTW, if flutter happens what do you glider pilots do:
Nose up?
Nose down?
Start speaking German?


Montblack

[Stefan]

Montblack wrote:

> BTW, if flutter happens what do you glider pilots do:
> Nose up?
> Nose down?
> Start speaking German?

Use the chute.

Stefan

[Morgans]

"Stefan" > wrote
>
> If you want to see what flutter is, look here:
> http://www.dg-download.de/Videos/dg-300-flatterversuch.mpg

Ooooohh, THAT looks VERY scary! I had never seen such large amplitude
flutter before. I had always seen small buzzing types of flutter. I hope I
never see either kind, except in a video.
--
Jim in NC

[john smith]

> BTW, if flutter happens what do you glider pilots do:
> Nose up?
> Nose down?
> Start speaking German?

They say the same thing we do, only in German it's one word instead of
two and two syllables instead of one.

[Orval Fairbairn]

In article >,
"Montblack" > wrote:

> ("Morgans" wrote)
> >> If you want to see what flutter is, look here:
> >> <http://www.dg-download.de/Videos/dg-300-flatterversuch.mpg>
>
> > Ooooohh, THAT looks VERY scary! I had never seen such large amplitude
> > flutter before. I had always seen small buzzing types of flutter. I hope
> > I
> > never see either kind, except in a video.
>
>
> ...when those new piezoelectric wings short circuit. <g>
>
> BTW, if flutter happens what do you glider pilots do:
> Nose up?
> Nose down?
> Start speaking German?
>
>
> Montblack

Bail out! Catastrophic flutter can occur and destroy the structure in
less than 2 seconds.

[Orval Fairbairn]

In article >,
john smith > wrote:

> > BTW, if flutter happens what do you glider pilots do:
> > Nose up?
> > Nose down?
> > Start speaking German?
>
> They say the same thing we do, only in German it's one word instead of
> two and two syllables instead of one.

The Norwegian term is "Uff Dah!"

[private]

"BTIZ" > wrote in message
news:yN_Ue.14282$ct5.10523@fed1read04...
snip
> Vne is based on IAS, TAS for the same IAS is higher at higher altitudes.
But
> remember, it's not just airspeed, or airspeed indicator errors associated
> with altitude changes, but also other dynamic "q" factors on the airframe
at
> higher "effective" airspeeds.
>
Thank you for the informative reply. Please define "q"

[private]

snip
> On Sun, 11 Sep 2005 04:46:58 GMT, "private" >
> wrote:
snip
> >Can I have the groups wisdom and further explanation of the reasons for
this
> >and of how Vne changes with altitude.


Thank you all for the informative discussion.

Happy landings

[private]

"T o d d P a t t i s t" > wrote in message
...
snip
> ... Just checked Google ... at 100,000' they expect the sea
> level stall speed of 88 km/hr to increase to 740 km/hr TAS
> and Mach .66!

Got me thinking about Joe Kittinger and what the sound barrier must feel
like in freefall.

Happy landings

[private]

"T o d d P a t t i s t" > wrote in message
...
> "private" > wrote:
>
>
> >> with altitude changes, but also other dynamic "q" factors on the
airframe
> >at
> >> higher "effective" airspeeds.
> >>
> >Thank you for the informative reply. Please define "q"
>
> Q is the term aerodynamicists use to refer to "dynamic
> pressure." Dynamic pressure is equal to 1/2 x rho x V**2.
>
> It's the pressure your airspeed reads - the difference
> between pitot pressure and static pressure. It's a pressure
> solely due to airspeed.
>
> If you listen to a space shuttle launch you will hear a
> reference to "max-q." That's the point of max aerodynamic
> stress on the shuttle. As the shuttle accelerates, airspeed
> increases, which increases q, but as it climbs, rho (air
> density) decreases, which decreases q. After max-q the
> decreasing air density decreases q faster than the
> increasing speed increases it.
>

thanx, I should have remembered that, but then I would not have learned
about "max q"

Happy landings