Why does Vne vary with Altitude?
MarcArnold
with altitude. At sea level, the Vne is 146kts. At 30,000 ft, the Vne is
placarded at 97kts. Why is this so?
It doesn't seem to be Mmo related, but rather flutter related. If so, can
anyone explain why lower reynolds numbers affect the flutter
characteristics? I would assume that KIAS would be a valid indication at
all altitudes for flutter phenomena (discounting mach issues).
Thank you in advance.
********************************************
*** Marc Arnold "Onward and Upward" ***
*** 200 South Brentwood Boulevard, #21B ***
*** St. Louis MO 63105 ***
*** ***
*** (314)721-5801 (314)726-5114 fax ***
********************************************
J Bojack
one must consider true vs. indicated airspeed. Flutter is related to
true airspeed, and one must abide by those speeds if one wishes to avoid
flutter.
The only indication of airspeed we have when flying is our indicator,
which is appropriately named indicated airspeed. The conversion must be
made.
I have asked this question of many flying instructors and expert types,
who did not know the answer in practical terms.... it is not easily
understood.
John Bojack (J4)
Jeffry Stetson
>
>In response to my question about Vne decreasing with altitude, Frank says:
>
>>Better go back to the basics of Indicated Air Speed vs. True Air Speed.
>
>If it's that basic, Frank, why does my Cessna 421 have the same Vne at
>30,000 feet as at sea level (240 KIAS)? I'd appreciate a serious answer,
>if you have the time. Thanks!
Yes, I'd appreciate a serious answer too! My M20E has the Vne given in CAS,
calibrated air speed, which is IAS corrected for instrument errors.
The only "serious" answer I've seen to date on this was a letter to the editor
in Soaring, July 1990, page 4, by a professional airframe dynamicist, Mark
Morton.
A couple of quotes from that article:
"The critical (flutter) condition is ordinarily at sea level, unless the
aircraft has a large increase of maximum speed with altitude."
"typically, critical flutter speed increases proportionally with (air) density.
Indicated airspeeds are not corrected for density. Thus, indicated airspeeds
corresponding to stall and red line remain constant as a function of altitude,
although true airspeed has increased. .... Since typically flutter speeds
increase proportionately with density, fly indicated airspeeds to avoid a
flutter condition."
Appropriate mathematics and data plots are included in the article.
The counter-arguments of the "TAS" group I've seen to date fall into one of
the following lines:
1) "I'm louder and ruder than you are, therefore TAS."
2) "I learned it this way while you were in diapers, therefore TAS."
3) "I heard or read this from a well-known pilot/author, therefore TAS."
I have yet to see reference to a technical article by any aero-pro coming down on
the "fly TAS" side of the question. If there are such references, I'd like to
see the citations!
--
Jeffry Stetson ... Comm ASEL, Pvt SES & Glider, Instrument Airplane
EAA, SSA, AOPA, IAC, MAPA
Mooney M20E "Superduper 21"
Salto H-101 "Shiva - The Cosmic Dancer"
Stephane Martignoni
air speed. The Vne is related to a change of vertical velocity in the wind
over a given distance so that doesn't change.
Keith
Stephen Bell
: >with altitude. At sea level, the Vne is 146kts. At 30,000 ft, the Vne
: is
: >placarded at 97kts. Why is this so?
: Marc,
: Better go back to the basics of Indicated Air Speed vs. True Air Speed.
: Frank
Uh O, here we go.
The real answer is that flutter charactistics for most gliders are not
evaluated throughout the full altitude/airspeed range during flight testing.
Aerodynamic forces are proportional to indicated airspeed but this is not
the only factor to consider. Traditionally lowering the IAS VNE at altitude
to give a TAS corresponding to TAS for VNE at sea level is considered safe
practise. This is what your sailplane manufacturer has done.
Recent OSTIV papers on flutter speeds at high altitude have appeared in
"Technical Soaring", well worth looking up for anyone interested in the
flying high and fast ( essential reading for wave flyers ).
A suggestion for using a VNE of the mean value between equivalent airspeed
(almost the same as IAS) and TAS was offered in one of these articles.
TAS (T+E)/2 EAS
.
30k \ |
\ . |
\ |
20k \ . |
\ |
\ . |
10k \ |
\ . |
\ |
\ .|
sea level \ |
------------------------------------------------------------------
VNE (IAS) speed --->
a reduced IAS for VNE at altitude is still used but the reduction is not
as great as the reduction required for keeping TAS below sea level VNE.
The absence of flutter when using these speeds is not guaranteed, infact
the same applies to the "keep TAS below sea level VNE" speeds.
Speed and altitude are not the only factors. Add OAT,wing loading, flap
setting to the list for starters.
Steve
------------------------------------------------------------------------------
-----
Stephen Bell |
Lincoln University, \--------------------(*)--------------------/
Canterbury,
New Zealand. NIMBUS II - Driver XX
E-Mail: S.B...@ono.lincoln.ac.nz
Geoff Brown
>
>I don't understand why the Vne of my self-launching sailplane decreases
>with altitude. At sea level, the Vne is 146kts. At 30,000 ft, the Vne is
>placarded at 97kts. Why is this so?
Vne is a fixed True Air Speed (TAS), it remains constant at all altitudes.
Your ASI gives you your Indicated Air Speed (IAS). The ASI is not
really measuring speed, it is measuring the pressure of the air in front of
the aircraft. The higher you go, the lower the ambient pressure and hence
the lower the 'pressure' (or IAS) that your ASI reads for a given TAS.
When manufactures specify a Vne for an airframe, they do it for an
IAS at a specific altitude pressure. For gliders I think this is
about 10,000'. So at any height below that Vne is a fixed IAS, whilst
above that height it is a reducing IAS. I don't know what the pressure
altitude Vne is quoted for a Cessna.
--
Geoff Brown
G.B.D...@bnr.co.uk
BNR Europe Ltd., London Road, Harlow, Essex,U.K.
Frank Smith
writes:
>
>I don't understand why the Vne of my self-launching sailplane decreases
>with altitude. At sea level, the Vne is 146kts. At 30,000 ft, the Vne
is
>placarded at 97kts. Why is this so?
>
>It doesn't seem to be Mmo related, but rather flutter related. If so,
can
>anyone explain why lower reynolds numbers affect the flutter
>characteristics? I would assume that KIAS would be a valid indication
at
>all altitudes for flutter phenomena (discounting mach issues).
>
>Thank you in advance.
>********************************************
>*** Marc Arnold "Onward and Upward" ***
>*** 200 South Brentwood Boulevard, #21B ***
>*** St. Louis MO 63105 ***
>*** ***
>*** (314)721-5801 (314)726-5114 fax ***
>********************************************
>
MarcArnold
>Better go back to the basics of Indicated Air Speed vs. True Air Speed.
If it's that basic, Frank, why does my Cessna 421 have the same Vne at
30,000 feet as at sea level (240 KIAS)? I'd appreciate a serious answer,
if you have the time. Thanks!
Axel Morgenstjerne
> In response to my question about Vne decreasing with altitude, Frank says:
>
>
>>Better go back to the basics of Indicated Air Speed vs. True Air Speed.
>>
>
> If it's that basic, Frank, why does my Cessna 421 have the same Vne at
> 30,000 feet as at sea level (240 KIAS)? I'd appreciate a serious answer,
> if you have the time. Thanks!
As far as I know, flutter depends on true airspeed while the dynamical
forces that the aircraft have to resist depends on indicated airspeed - so
if your plane is flutter-limited your Vne depends on a fixed true airspeed,
i.e. the indicated airspeed never exceeded goes down with increasing
altitude. On the other hand, if your plane is limited by the dynamical
forces your Vne depends on a fixed indicated airspeed - which means Vne
rated in indicated airspeed will not change with altitude. Rated in true
airspeed it will grow with altitude. This also means, that a plane limited
by dynamic forces at low altitudes can be limited by flutter at high
altitudes.
It is quite obvious why indicated airspeed and the dynamic forces are
related - you actually measure the indicated airspeed by a dynamic effect
(the difference in dynamic presure and static presure).
The reason why flutter is related to true airspeed is because flutter
actually is not a dynamical phenomena - you can look at it as a resonance
phenomena instead. The resonance (flutter) occurs when the air passes over
the plane with a certain true speed, while the density of the air plays a
minor or no role.
Maybe flutter can be compaired to an emty bottle - when you blow across the
top of the bottle, you'll hear a sound when you hit the right airspeed -
it's a resonance phenomena too.
Well, at least that's how I understand it.
Axel Morgenstjerne, AEF, bygn.322, DTU, DK-2800 Lyngby, Danmark
tel:+45 4525 3450, fax:+45 4588 7133, privat:+45 4593 5510
Jeffry Stetson
>
[snip]
>The reason why flutter is related to true airspeed is because flutter
>actually is not a dynamical phenomena - you can look at it as a resonance
>phenomena instead. The resonance (flutter) occurs when the air passes over
>the plane with a certain true speed, while the density of the air plays a
>minor or no role.
Absolutely incorrect! Resonance is a type of dynamic phenomena. Dynamics
is the study of things *in motion*. You a correct in stating that Vne is
often set by things other than flutter. For example, too much pressure on
a windscreen can be a limit. This pressure comes about from the movement of
air, but as far as the analysis of the physical system is concerned, it is
a problem in *statics*, not dynamics.
For a given mechanical system, the flutter speed depends directly on air
density! The air is what supplies the restoring force in the harmonic
ocsillator, so how could it not depend on the air density? Would you
worry about flutter if you were flying your glider in space? The 'trick'
is that while the flutter speed depends on air density, the IAS also
depends on air density in about the same way, so the critical flutter
speed occurs at essentially the same IAS, (but different TAS).
Other factors may come into play, though, such as the effect of temperature
on the stiffness of the mechanical structure.
[snip]
Phil Woodruffe
use:
Most of your glider operating limits such as stalling speed and
'g' limits are functions of dynamic pressure. Since your ASI is
a dynamic pressure gauge, it is appropriate to use IAS to define
these limits.
Flutter is different in that it is a resonance effect caused by
the speed of air flowing across the wing, not the pressure. In
this case it is the speed of the air which is important, not the
amount of air. Consequently flutter is a function of TAS.
All the glider manuals I have ever seen quote VNE as an IAS at
Sea Level. Charts are then provided so you can fly at an
appropriately slower speed higher up.
Phil Woodruffe
--
Phil
Robert Goubitz
the limiting factor at altitude. How come that redline (Vmo) in the
airplanes I fly produces a TAS of say 400 KTS at 10,000 ft. while
redline around 28,000 ft produces 500 KTS TAS. I do understand that my
redline above 28,100 ft comes down in order to limit the Mach (Vmmo) and
that TAS pretty much remains constant at that decreasing indicated
redline except for variations due to non-standard temperatures. (The
barberpole on the airspeed indicator automatically decreases with higher
altitudes.) But that is an aerodynamic limit rather than a structural
limit such as the Vne that we're talking about here. I'm not trying to
juice this subject up but would like an aerodynamicist to explain Vne.
-
Bob Goubitz
Frank Smith
writes:
>
>In response to my question about Vne decreasing with altitude, Frank
says:
>
>>Better go back to the basics of Indicated Air Speed vs. True Air
Speed.
>
>If it's that basic, Frank, why does my Cessna 421 have the same Vne at
>30,000 feet as at sea level (240 KIAS)? I'd appreciate a serious
answer,
>if you have the time. Thanks!
>********************************************
>*** Marc Arnold "Onward and Upward" ***
>*** 200 South Brentwood Boulevard, #21B ***
>*** St. Louis MO 63105 ***
>*** ***
>*** (314)721-5801 (314)726-5114 fax ***
>********************************************
>
Well Marc,
You sure have stirred up a nest of responses. About your 421, never
flown one, but are you sure the instrument is IAS and not corrected for
a TAS indication? I still stick to my guns about IAS vs TAS is the
issue, but there are many guys out there that are more of the
aeronautical engineering types than me. If my answer seemed abrupt, it
was not meant that way and I hope you find the real answer in all the
responses to your valid question.
Frank
Philip Hawker
editor
>in Soaring, July 1990, page 4, by a professional airframe dynamicist,
Mark
>Morton.
density.
>Indicated airspeeds are not corrected for density. Thus, indicated
airspeeds
>corresponding to stall and red line remain constant as a function of
altitude,
>although true airspeed has increased. .... Since typically flutter
speeds
>increase proportionately with density, fly indicated airspeeds to avoid
a
>flutter condition."
expert, but I thought air density *decreases* with altitiude. Thus if
"critical flutter speed increases proportionately with density" then
(true) flutter speed is also decreasing with altitude - and if IAS is
less then TAS with altitude then you have the exact opposite of what you
say - you need to subtract a safety margin below flying TAS! To be able
to use IAS Vne then either "critical flutter speed must *decrease*
proportionately with (air) density" or it must increase inversely to it.
Or it's too late after a long day at work......
Phil Hawker
Swindon, UK
Peter Reading
Indicated Air Speed (IAS)
The answer is neither.
Vne is placarded to prevent the onset of flutter. Flutter is a
vibration which is caused by three forces acting:
The mass of the wing (and it's distribution)
The elastic forces i.e. how stii the wing is
The aerodynamic forces
The first two don't vary with altitude or speed, the aerodynamic
forces vary with IAS. The net effect is a Vne that does not vary
according to IAS or TAS.
Reference: Aircraft Structures for Engineering Students
Author: T H G Megson
Publisher: Arnold
ISBN: 0 7131 3393 7
There is a simplistic explanation in Chapter 13, there may be
more definitive texts.
Peter Reading
MarcArnold
>Well Marc,
>You sure have stirred up a nest of responses. About your 421, never
>flown one, but are you sure the instrument is IAS and not corrected for
>a TAS indication? I still stick to my guns about IAS vs TAS is the
>issue, but there are many guys out there that are more of the
>aeronautical engineering types than me. If my answer seemed abrupt, it
>was not meant that way and I hope you find the real answer in all the
>responses to your valid question.
Frank,
Thanks for the follow-up message. No hard feelings.
The 421's airspeed indicator gives two readings: The inner scale is
corrected for TAS, the outer is IAS. The POH provides corrections
to get CAS.
My thanks to the many informative postings about the relationship
between Vne and altitude. I've learned a lot and am now looking
forward to getting a copy of the relevant Technical Soaring article.
Thanks to everyone,
Marc
Judah Milgram
Jeffry Stetson <ste...@nscl.msu.edu> wrote:
>The counter-arguments of the "TAS" group I've seen to date fall into one of
>the following lines:
> 1) "I'm louder and ruder than you are, therefore TAS."
> 2) "I learned it this way while you were in diapers, therefore TAS."
> 3) "I heard or read this from a well-known pilot/author, therefore TAS.
Perhaps you also meant to mention
4) Because sometimes the manufacturer recommends it
Something that bothers me when this question comes up is that there seems
to be an underlying assumption that the engineers who certified the aircraft
didn't bother to think about how Vne could/should vary with altitude and
it's therefore up to pilots to come up with their own rules.
Vne is based on Vd, which is defined in the certification regs as an EAS
(close to CAS), not TAS. No science, just a definition. Sometimes
manufacturers do specify that Vne decreases with altitude. Sometimes they
don't. If you're concerned that the manufacturer is expecting you to reduce
Vne on your own initiative, the best place to ask is the manufacturer.
As for science, the flutter onset speed is neither a constant TAS nor
a constant EAS. Flutter analyses are typically performed at specified
values of TAS since it makes it easier to calculate the motion-dependent
aerodynamic forces. But the ambient air density (thus, EAS) also enters
the picture.
Judah Milgram
mil...@glue.umd.edu
Axel Morgenstjerne
ste...@nscl.msu.edu (Jeffry Stetson) writes:
> In article <19950411141...@iris.aef.dtu.dk>,
> ax...@iris.aef.dtu.dk says...
>>
>>
>
> [snip]
>
>
>>The reason why flutter is related to true airspeed is because flutter
>> actually is not a dynamical phenomena - you can look at it as a
>> resonance phenomena instead. The resonance (flutter) occurs when the
>> air passes over the plane with a certain true speed, while the density
>> of the air plays a minor or no role.
>>
>
> Absolutely incorrect! Resonance is a type of dynamic phenomena. Dynamics
> is the study of things *in motion*. You a correct in stating that Vne is
> often set by things other than flutter. For example, too much pressure on
> a windscreen can be a limit. This pressure comes about from the movement
> of air, but as far as the analysis of the physical system is concerned,
> it is a problem in *statics*, not dynamics.
I confess, you got me here - I should have written 'flutter is not a simple
dynamical phenomena' - . The simple aerodynamical limitations I had in mind
when I wrote my first reply, was things as the torsion of the wing as the
center of pressure moves backward with increasing velocity - an effect I
agree depends directly on IAS. The windscreen limit you mention I agree will
depend on the dynamical pressure (and pressure inside the cockpit). Feel
free to call it a static problem!
>
> For a given mechanical system, the flutter speed depends directly on air
> density! The air is what supplies the restoring force in the harmonic
> ocsillator, so how could it not depend on the air density? Would you
> worry about flutter if you were flying your glider in space? The 'trick'
> is that while the flutter speed depends on air density, the IAS also
> depends on air density in about the same way, so the critical flutter
> speed occurs at essentially the same IAS, (but different TAS).
Well, I don't really trust you here - it may be correct for some modes of
flutter, but in general I have my doubts. However, you're right the presence
of air is needed to induce flutter, so TAS cannot be fully correct. -
Anyway, the initial question I intended to answer was Vne/Altitude - Vne is
a velocity it is stated you should not exceed - two limiting factores are
the reason: 1) Deformation of the plane due to aerodynamic forces. 2)
Different kind of flutter. As for 1) the answer is in general simple: IAS,
but for 2) it's more complicated, but some phase coherence between the
movements of the part of the plane in question and the movements of the air
is required (otherwise the flutter mode can not pick up energy from the
air). This phase coherence is related to TAS, not IAS. However, this SIMPLE
MODEL is not the full truth, as it is dynamical forces (dependent on IAS)
that transfers the energy to the mode. Anyway, TAS are considered to be a
conservative evaluation that will keep you away from flutter problems as
you climb, so sometime you'll find it used that way!
[snap]
J. Milgram
>The counter-arguments of the "TAS" group I've seen to date fall into one of
>the following lines:
> 1) "I'm louder and ruder than you are, therefore TAS."
> 2) "I learned it this way while you were in diapers, therefore TAS."
> 3) "I heard or read this from a well-known pilot/author, therefore TAS."
Agreed, these are very irritating attitudes. I've experienced them too!
And "because the manual says so" isn't such a great explanation either
(was mine, sorry). So, just a bit more on why flutter doesn't necessarily
occur at fixed TAS or fixed EAS.
Some aerodynamic forces can indeed be written as a function of EAS only,
independent of density altitude. But not all. Example: the sectional
lift on a wing which is bending up and down. Imagine a local 2D section
with a vertical velocity hdot, thus a time varying incremental change in
angle of attack of approx. hdot/TAS; this is just a result of the
kinematics of the motion. The resulting force is proportional to
rho*TAS*hdot (i.e. rho*TAS**2*hdot/TAS). The rho*TAS term varies with
altitude whether you fly fixed TAS or fixed EAS.
Also, as the frequency of the bending motion increases, the oscillatory
lift decreases in amplitude and lags the motion slightly. This is a
result of vorticity shed by the airfoil convecting downwind. The effect
can be important. The governing parameter turns out to be a nondimensional
frequency, i.e. the frequency of vibration normalized to TAS (expressed
in profile semichords per unit time.)
There are even "apparent mass" terms arising due to acceleration of the
structure which don't depend on airspeed at all (in incompressible flow,
at any rate).
In cases where manufacturers reduce Vne with altitude, I don't know if it's
because they actually expect flutter or because (as someone here
suggested) they flight tested at only one altitude and don't trust their
analysis enough to clear the aircraft at the higher density altitudes. In
the latter case "fixed TAS" might be a good simple way to adjust Vne, but
I'd be curious to know how accurate it really is. It sure seems to be one
of those "well-known facts!"
- Judah
ma...@aslvx1.sugar-land.anadrill.slb.com
..
There is an interesting paper that was published in part
in the proceedings of the OSTIV congress at Borlange,
on the subject of flutter speed vs. altitude.
The author calculated flutter modes for the various Polish
sailplanes, SZD-55,56,PW-5 both as a function of indicated
airspeed and altitude.
..
The graphs clearly demonstate that no single statement can
be made that covers all sailplanes. The critical flutter
speed for each sailplane varied differently with altitude
and indicated airspeed. In one case (the rudder flutter mode
where motion of the fuselage was combined with wing bending),
the flutter would only occur in a narrow range of speeds around
90 km/hr and only at altitudes above 6 km.
What is apparent that following the 'true airspeed rule' is
overly conservative. On the other hand the flutter speed
for most modes decreases as a function of altitude when
following the airspeed indicator in the cockpit - but
not as fast as you might expect from computing TAS.
There is a lot of research to be done on this issue,
particularly for older gliders. The current sentiment
is that since the computational power is available now,
it should be required of new gliders that flutter
characteristics be better known than before.
Regards,
Peter Masak
sch...@hpbbn.bbn.hp.com
the following:
To maintain fluttering or any kind of oscilating you need the necessary
oscilator (certain flow of air, certain masses etc) and you need a certain
amount of energy to maintain the flutter against the mechanical resistance
(bending, hinges etc) and the air resistance. The proper attributes to
start the oscillating can happen at various speeds. The available energy
to maintain flutter increases with the square of the speed and linear with
the density of the air. The IAS or more correct the CAS is a direct indication
of the available energy. At the same TAS the available energy decreases
at higher altitudes as the density decreases. The energy to maintain
flutter is the sum of the energy required to overcome the mechanical resistance
and the air resistance. The mechanical resistance should be more or less
independant of the altitude (it may change with the temperature) but the air
resistance decreases with the altitude. Therefor also the required energy
to maintain flutter decreases with the altitude. If VNE is linked to the TAS
we can be sure that the available energy at higher altitudes will decrease
at least at the same rate as the required energy for maintaining flutter.
If we would fly at indicated VNE (IAS) at high altitudes we would be
maintaining the same energy in the airflow around us as at sea level at VNE
but as the required energy for flutter is decreasing the risc for flutter
would increase.
At very high altitudes the air resistance factor becomes less important
and we still keep a more or less constant mechanical resistance therefor
the space shuttle has no flutter problems in space (as argued in a previous
response).
In any case stick to your flight operation manual and stay away from VNE.
I have seen a glider after a testflight intentionaly exceeding VNE.
The test pilot left by parachute and one wing was split in 2 halve.
--
Frank-Peter Schmidt-Lademann
CSO Technical Marketing, CSO-Europe Boeblingen, West Germany
Internet: sch...@bbn.hp.com Hewlett-Packard GmbH
HPDESK: HPB600/UX c/o F-P Schmidt-Lademann
Phone: off:+49-7031-14-3985 Fax: x4196 Herrenberger Str. 130 W4
pri:+49-7056-8065 D-71034 Boeblingen
Judah Milgram
<ma...@aslvx1.sugar-land.anadrill.slb.com> wrote:
>There is an interesting paper that was published in part
>in the proceedings of the OSTIV congress at Borlange,
>on the subject of flutter speed vs. altitude.
> ...
>What is apparent that following the 'true airspeed rule' is
>overly conservative.
I haven't seen this paper yet, but have seen similar. I think
to be specific you'd have to say that the "TAS rule" is
*predicted* to be overly conservative. I don't know of any
published sailplane flight test data which would tend to confirm
this. If anyone knows of any ...
> The current sentiment is that since the computational power
> is available now, it should be required of new gliders that
> flutter characteristics be better known than before.
Computational power is of course good, but flutter analyses can
be limited by other factors - assumptions in the structural model;
shortcomings in the ground vibration test data; limitations of the
aerodynamic model; etc. etc.
The point being, no matter how detailed the analysis is, sooner or
later you flight test the sailplane; I think the certification
authorities require it, even though according to the regs an analysis
should suffice.
This applies to altitude effects too. I'd really like to see some
measured results for a single sailplane at different altitudes. Maybe
in practice "TAS" is overly conservative; maybe it's not as conservative
as thought; maybe it's very dependent on the particular sailplane.
regards,
Judah