Vne at altitude
Andy Sanderson
I have a question about Vne. I remember in one of my sadly rare wave
flights (hence my ignorance) coming down fairly fast from 20,000 feet
and not being quite sure at the time what indicated airspeed I could
go up to before I went past Vne. I chose 95 knots (Vne true airspeed
on my Vega is 135 knots), and survived, but when I got hold of the ISA
densities at various heights and plotted out a curve of Vne as an
*indicated* airspeed against altitude I found I was indeed
uncomfortably close to the line (only about 3 knots below it).
My question is this: Is the placarded Vne appropriate to sea level?
10,000 feet? Some other height? I remember being told to subtract 2
knots per thousand feet above 10,000 feet. The 2 knots per thousand
feet agrees fairly well with my plotted curve, but if the curve's
"origin" (i.e. the point at which the Vne is what's written on the
placard) is at 10,000 feet it would imply that you could (in theory at
least) exceed the placarded Vne by about 20 knots at sea level, and
still be OK, not that I'd try it.
Or, as with most things, does it vary from manufacturer to
manufacturer, or is it based on various country-specific airworthiness
requirements, and how can I find out? My flight manual makes no
mention of the height that the Vne(indicated) = Vne(true). It might
seem trivial to most thermal-bound pilots until you think that if
placarded Vne is true at sea level (i.e. the worst case), at a modest
flying height of 5,000 feet, indicated Vne has already been eroded by
perhaps 10 knots.
Nobody that I have spoken in our club to seems to have a complete
answer to this.
Any ideas?
Cheers,
Andy
Andy Sanderson
Essex & Suffolk Gliding Club
www.esgc.co.uk
André Somers
as@webdynix___nospam___.co.uk says...
> My question is this: Is the placarded Vne appropriate to sea level?
The Vne is a speed at which the aerodynamic forces become to great for
the construction. The reason the indicated airspeed is lower at attitude
is because of the lower pressure, resulting in lower dynamic forces in
your speedometer. But... the same goes for the forces on the airplane as
a whole! Because of these lower forces, the Vne is higher at altitude
than at sealevel. The effects of this higher Vne and the lower indicated
airspeed cancel. So: as long as you keep under Vne on your speedometer,
you're safe.
I think you should talk to an instructor. He might be able to explain
the situation better than I can.
André
--
Replies to my email are not appreciated unless explicitly asked for.
Bill H.
Vne goes down with altitude. Our club's Grob 103 manual includes a table
which lists the Vne at various altitudes. The Vne of 135 kias is good to
6500' (marked on airspeed indicator), then it goes down to 109 kias at
19,000'. As you mentioned, this is a factor in wave and high thermal
flying. Although dynamic pressure is certainly lower at 109 KIAS, true
airspeed is still high and this is the concern with regards to flutter and
other limiting factors. Bill H.
Jean Richard
>
> The Vne is a speed at which the aerodynamic forces become to great for
> the construction.
Not exactly. Vne is related to airframe capability to tamper oscillating
phenomenas due to airflow. For this reason, Vne is function of true
airspeed and not indicated airspeed (aerodynamic forces are function of
indicated airspeed).
The main hazard when exceeding Vne is not aerodynamic force, but flutter,
which is an uncontroled oscillation of airframe.
Staffan Ek
for dimensioning Vne. The aerodymanic forces depends on the dynamic pressure
which is what the airspeed indicatior messures (with some error but accurate
enough).
Flutter on the other hand depend mainly on TAS. Many fiberglass airplanes
are quite close to flutter when flying at Vne. This is why many of them
(like the G103 mentioned below) have speed restrictions on higher altitudes.
Not all glider types have these restrictions in their manuals but I would
rather be safe than sorry.
Staffan
Bill H. <bho...@msn.com> wrote in message
news:Ohzn2y7kBHA.448@cpimsnntpa04...
Ian Strachan
<as@webdynix___nospam___.co.uk> writes
>I have a question about Vne.
One definition of Vne in a UK Ministry of Defence publication (JSP 318B)
is: "Never Exceed (NE) Limits - Limitations which are close to the
limit of the tested or design flight envelope, beyond which there is no
guarantee of safety, airworthiness, resistance to flutter or structural
integrity."
>My question is this: Is the placarded Vne appropriate to sea level?
>10,000 feet? Some other height?
The answer is not necessarily easy, although the table or graph of IAS
against altitude in your particular glider flight manual should tell
all.
Vne in IAS terms should decrease above, say, above 6000ft / 2000m,
certainly above 10,000ft / 3000m, otherwise it is not correctly
calculated and you could literally come apart at high altitude if you
insist on using sea level Vne in terms of IAS. Some older gliders had
no such Vne scale with altitude in their flight manual, but that does
not mean that they are immune from the laws of nature and aerodynamics.
If you either disagree with these statements or want to fly a glider at
high altitude, it may be worthwhile reading on ....
IAS is an indication of aerodynamic pressure and this in itself will be
limiting for a given design. Such as in the stressing of forward facing
parts of an airframe so that they do not collapse at high IAS. In
calculation terms, that is the easy bit, "half ro vee squared" and all
that, and a bit of proof testing on the ground ...
However, oscillations are a different matter. The tendency of a
flexible airframe to oscillate is due to the interplay of aerodynamic
forces, how any oscillations are damped, and the inertia of the masses
of the parts of the airframe as they move. High damping and low inertia
will tend to suppress oscillation (the so called "dead beat" return to
the original position), low damping and high inertia will allow
oscillation not only to occur but eventually (as speed and/or altitude
increases) to become divergent, one triggered. In aircraft, nasty
things happen such as so-called "divergence" of aerodynamic surfaces.
That is, your wings or tail twist off!
The low damping/high inertia case is unfortunately exactly the condition
met at high altitudes.
To take an extreme case, at 40,000 ft the TAS/IAS ratio is about 2.
That is, an airliner flying at 225 knots indicated will have a true
airspeed of about 450 knots. That is why airliners cruise at such
altitudes. Drag, and therefore fuel used, is proportional to 200 knots
but speed through the air is twice that! Ingenious! But airliners are
designed for this environment.
A quick look at oscillation: If part of an airframe is disturbed, such
as through turbulence or control activity, the restoring force due to
natural aerodynamic stability, being aerodynamic, is proportional to IAS
(dynamic pressure), but the inertia of the disturbance is proportional
to TAS which is the actual movement through the air. So what is
sometimes called "aerodynamic damping" is about halved at 40,000ft. The
danger of unstable airframe oscillation or flutter, is obvious.
I pick 40,000ft not because gliders fly there, but because of the 2:1
ratio of TAS/IAS, very easy to remember and the effects are clearly
demonstrated. At other altitudes you can calculate TAS/IAS ratio by
taking one over the square root of the density with respect to sea level
(do not worry, a couple of tables are shown below for glider speeds).
Flight testing for divergent airframe or control surface oscillations is
dangerous and not carried to extremes, for obvious reasons. Therefore
"safe" protocols have been developed which are designed to keep most
types of airframes out of trouble. Flutter testing of high speed
aircraft, fighters and transports, employs exotic methods such as so-
called "bangers" attached to critical parts of the airframe to trigger
oscillations, brave crews fly the test and scientists pore over the
results to look for indications of oscillations and to fix a "safe" Vne
(and Mne) schedule with height.
For gliders, such expensive testing is not done but the "safe protocol"
method is generally used instead.
The normal German (LBA) protocol used for the glider Vne schedule with
altitude is "constant IAS from Sea Level to 2000 metres, then constant
TAS". I have this on an excel spreadsheet, from which some results are
tabulated below for the ICAO International Standard Atmosphere:
Table 1 is for a Sea level Vne of 100 knots, which may be low for modern
gliders but is easy to multiply up to your glider's sea level Vne and in
proportions for the other altitudes:
Alt IAS TAS Mach Alt
Feet Metres
0 100 100 0.15 0
1 100 101.5 0.15 305
2 100 103.0 0.16 610
3 100 104.5 0.16 914
4 100 106.1 0.16 1219
5 100 107.7 0.17 1524
6 100 109.4 0.17 1829
6.5620 100 109.9 0.17 2000
7 99.3 110.3 0.17 2134
8 97.8 110.3 0.17 2438
9 96.3 110.3 0.17 2743
10 94.8 110.3 0.17 3048
11 93.3 110.3 0.17 3353
12 91.9 110.3 0.17 3658
13 90.4 110.3 0.17 3962
14 88.9 110.3 0.18 4267
15 87.5 110.3 0.18 4572
16 86.1 110.3 0.18 4877
17 84.7 110.3 0.18 5182
18 83.3 110.3 0.18 5486
19 81.9 110.3 0.18 5791
20 80.5 110.3 0.18 6096
25 73.8 110.3 0.18 7620
30 67.5 110.3 0.19 9144
35 61.4 110.3 0.19 10668
11 km 60.1 110.3 0.19 11000
40 54.7 110.3 0.19 12192
50 43.0 110.3 0.19 15240
60 33.9 110.3 0.19 18288
Table 2 for a Sea Level Vne of 150 knots,
typical of many gliders including mine:
Alt IAS TAS Mach Alt
Feet Metres
0 150 150 0.23 0
1 150 152.2 0.23 305
2 150 154.5 0.24 610
3 150 156.8 0.24 914
4 150 159.2 0.24 1219
5 150 161.6 0.25 1524
6 150 164.1 0.25 1829
6.5620 150 164.9 0.26 2000
7 149.0 165.5 0.26 2134
8 146.7 165.5 0.26 2438
9 144.4 165.5 0.26 2743
10 142.2 165.5 0.26 3048
11 140.0 165.5 0.26 3353
12 137.8 165.5 0.26 3658
13 135.6 165.5 0.26 3962
14 133.4 165.5 0.26 4267
15 131.3 165.5 0.26 4572
16 129.1 165.5 0.27 4877
17 127.0 165.5 0.27 5182
18 124.9 165.5 0.27 5486
19 122.8 165.5 0.27 5791
20 120.8 165.5 0.27 6096
25 110.8 165.5 0.27 7620
30 101.2 165.5 0.28 9144
35 92.1 165.5 0.29 10668
11 km 90.2 165.5 0.29 11000
40 82.1 165.5 0.29 12192
50 64.6 165.5 0.29 15240
60 50.8 165.5 0.29 18288
--
Ian Strachan
André Somers
a.t.s...@student.utwente.nl says...
the problem more difficult to answer. I'd contact the manufacturer of
your glider if you want to be sure, and stay on the safe site until you
are (and maybe even then).
John Giddy
there must also be a placard, of reducing Vne (indicted)
with altitude, fixed in view of the pilot.
Cheers, John G.
"Ian Strachan" <I...@ukiws.demon.co.uk> wrote in message
news:HkcNwzA$m3M8Ewa$@ukiws.demon.co.uk...
XcNick
>25 73.8 110.3 0.18 7620
>30 67.5 110.3 0.19 9144
>35 61.4 110.3 0.19 10668
Holy Moly! Thank you for the math. The FAA may not need a placard for this, but
I am going to add one. I don't think so well at 25,000 with an O2 failure and I
will be going down at the fastest rate possible. Now I know what that is. How
embarasing.
BPattonsoa
>but
>I am going to add one. I don't think so well at 25,000 with an O2 failure
You don't need a chart in the airplane. I have a "true airspeed" airspeed in
my RV-6A. It has a knob that you set at the alititude and outside temperature
and this moves a True airspeed scale alongside the indicated markings (above 90
knots to the end, 250) so the needle points at both indicated and true.
Have never seen one in a glider, or even know if one is made in the speed range
we fly at. Works great in the RV, generally ground speed and adjusted airspeed
are right on, assuming no winds.
I came back from the Sports Nats this year and looked at all my traces. Was
amazed to see how fast I was flying between thermals, until I realized that it
was true airspeed, (groundspeed) that I was seeing. Sure spent a bunch of time
above yellow (120 knots in my HP-18) when I thought I was going 90-95.
Bruce Patton
596S
Armando R. Pucci
Concerning Vne and altitude, would you please inform Mr Andy Sanderson
that if a
glider complies and is certified to JAR (Joint Airworthness
Requirements) 22, the following
paragraphs are applicable and must have been verified by the
certification authority:
JAR 22.1545 - Air speed indicator markings
For Vne a radial line. If Vne varies with altitude, there must be a
means to indicate to
the pilot the appropriate limitations throughout the altitude range.
JAR 22.629 - Flutter
The saiplane must be free from flutter, aerofoil divergence, and
control reversals in each
configuration and at each apprpriate speed up to at least VD.
JAR 22.1505(a) Air-speed limitations
The never exceed speed, Vne, must not exceed 0.9 times the maximum
speed
demonstrated in flight tests Vdf.
The Vdf must not exceed the design maximum speed, VD and must not be
less than
0.9 times the design maximum speed (choosen by the applicant, but
above certain
design minimum values given in JAR 22.335 according to saiplane
category)
Have a Happy Epiphany
Eng. Francisco Leme Galvao
Sao Paulo / Brazil
--------------------------------------------------------------------------------
Newsgroup: rec.aviation.soaring
Subject: Re: Vne at altitude
Author: mailto:Ian Strachan <i...@ukiws.demon.co.uk
Date/Time: 06:00 03 January 2002
--------------------------------------------------------------------------------
In article , Andy Sanderson
writes
>I have a question about Vne.
One definition of Vne in a UK Ministry of Defence publication (JSP
318B)
is: "Never Exceed (NE) Limits - Limitations which are close to the
limit of the tested or design flight envelope, beyond which there is
no
guarantee of safety, airworthiness, resistance to flutter or
structural
.......................................................................
Table 1 is for a Sea level Vne of 100 knots, which may be low for
modern
gliders but is easy to multiply up to your glider's sea level Vne and
in
proportions for the other altitudes:
60 50.8 165.5 0.29 18288
-
Ian Strachan
XcNick
Staffan Ek
The TAS/flutter problem gives an idea about how Vne (as indicated on
indicator) should be reduced with altitude. But what about the manouver
speed?
My own idea is that the manouver speed is totally dependent of aerodynamic
forces and thus connected with dynamic pressure. The IAS for manouver speed
shouldn't vary with altitude. Is that a correct assumption?
And what about older airplanes, like my own H-301, that have limits for
rough air. Is this limited mainly by TAS or dynamic pressure? And what about
manouver speed if it isn't anything about it in the manual? Is the manouver
speed and rough air speed the same or can I do full control deflections at
Vne?
Staffan
BPattonsoa
>
>>I have a "true airspeed" airspeed in
>>my RV-6A.
>
>
>
I got mine from Vans, already marked for the RV-6. Great company, run by a
serious glider pilot, Van, recently, if things worked out, is flying a new
Ventus CM after bashing around in a DG-600M for the last few years. .
Aircraft Spruce, their 2000-2001 version has true airspeeds on page 326. There
are some in the 40-180 knot range. Don't indicate how high they will adjust
to. I think mine in the -6 will only go up to 18,000 feet.
Bruce Patton
XcNick
going to look into this because if it will go 50,000 it will make my bird apear
less of a trainer and more "wave machine". (shut up Al)
Somebody give me an excuse to put a G-meter in there for high altitude flight.
nospam
Va means design maneuvering speed. It comes from the V-g diagram and is
based on IAS
Vb means design speed for maximum gust intensity. It also comes from the V-g
diagram and is also based on IAS
If both speeds are published for an aircraft, Va will be less than Vb. Va
assumes the pilot inputs maximum control deflection while Vb is based on a
wind guest intensity. For this reason, whenever operating above Va you must
limit your control inputs to generally no more than 1/3 their travel.
These speeds are not adjusted for altitude/TAS - unless Vne is reduced below
their value - but they ARE reduced for weight. Va and Vb are based on
maximum takeoff weight and should be reduced for lower weights: SQRT(actual
weight / gross weight) * V speed
And as I am sure you know, the V-g diagram is the basis for Vne from a
structural standpoint. As noted in other replies it must be reduced to avoid
flutter at higher TAS. And although not relevant to Gliders, it must also be
reduced to ensure the Maximum Mach number is not exceed (which for straight
wing aircraft can be below Mach 0.7) and for Bird Strike protection (this is
why jets usually must fly slower when below 14,000').
Ivan
"Staffan Ek" <staff...@hem-pc.bip.net> wrote in message
news:3c35c...@d2o940.telia.com...
Eric Greenwell
bpatt...@aol.com says...
> I got mine from Vans, already marked for the RV-6. Great company, run by a
> serious glider pilot, Van, recently, if things worked out, is flying a new
> Ventus CM after bashing around in a DG-600M for the last few years. .
Make that a Ventus 2 CM and a DG 400.
--
Delete the REMOVE from my e-mail address to reply directly
Eric Greenwell
Richland, WA (USA)
XcNick
Does this mean I have to burn my T-shirt from Van that says: Friends don't let
friends fly plastic airplanes?
Eric Greenwell
xcn...@aol.com says...
> >Make that a Ventus 2 CM and a DG 400.
>
> Does this mean I have to burn my T-shirt from Van that says: Friends don't let
> friends fly plastic airplanes?
No Way! The Ventus and DG aren't airplanes, remember. It's OK to fly
plastic gliders!
We're fortunate up here in WA and OR to fly gliders with Dick
occasionally, and it was a couple years before I even knew he was
Van's Aircraft.
Andy Sanderson
does not comply with modern practices like putting a chart of Vne at
various altitudes into the flight manual. Someone has since shown me
their modern (German) manual that indicates a constant Vne(IAS) up to
2000 metres, then a decrease above that, and my guess is that as the
flight testing of my glider type was probably done at about 6000 feet
I will be (fairly) safe if I think of mine in much the same way - in
fact, for ease of calculation, I will treat Vne as the placarded 135
knots up to 5000 feet and take off 2 knots for every 1000 feet above
that. Even so, I shall still attempt to stay well clear of this
limit.
Andy
On Wed, 2 Jan 2002 17:20:06 +0000 (UTC), as@webdynix___nospam___.co.uk
(Andy Sanderson) wrote:
>
>I have a question about Vne
<snip>