On Dec 31 2007, 6:03 am, "
wrote:

VNE at various altitudes

I have looked at a number of posts on this thread. May I attempt a
sort of summary?

At 40,000 ft, the ratio of True Airspeed to Indicated Airspeed is
approximately two (using the
ICAO ISA). This is why an airliner can fly at, say, 200 knots "on the
clock" and yet travel over
the ground at 400 knots. Since drag and therefore fuel consumption is
approximately
proportional to IAS, this is great for jet airliners. However, it's
less great for gliders.

Looking at flutter and stability, aerodynamic stability is
approximately proportional to IAS
whereas inertia is approximately proportional to TAS. Aerodynamic
stability provides restoring
moment(s) after a disturbance, that is what things like dihedral and
the vertical and horizontal tail
surfaces are for. Also, movements like pitch and roll produce changes
of angle of attack that in
themselves are (slightly) stabilising. The overall effect should be
that the aircraft is stable to
disturbances such as due to turbulence. The other side of the coin is
that "Inertia" implies that
any divergence will tend to continue, and needs to be damped out by
restoring moments before
an unstable situation develops such as what is commonly called
"flutter" that has claimed the lives
of several pilots over the years. So Vne in terms of IAS is clearly
not constant with altitude, if
divergent flutter is to be avoided. The only question is, by how much
it should reduce?

In many gliders, the protocol for Vne with altitude is one which I
understand was originally
formulated by the German certification authority, the Luftfahrt
BundesAmpt (LBA). It is also
used in JAR 22, the European airworthiness requirement for gliders
(now operated under the new
European Airworthiness and Safety Authority, EASA).

The protocol says: "For gliders, Vne IAS is assumed to be constant
from Sea Level to altitude
2000m, then decreases with altitude at constant TAS".

The idea was that this was a reasonable assumption, maybe a bit on the
cautious side, that could
be accepted by the LBA and JAR22 in the absence of real flutter
testing at high altitudes. If you
look at your flight manual (if you fly an European-produced glider),
you may well find that the
Vne table with altitude uses this protocol. Mine certainly does.

In numerical terms, to take an easy figure, if your glider Vne was 100
knots at sea level (where
IAS and TAS are the same), it would still be 100 kt IAS at 6562 ft
(2000m). Using the ICAO
ISA, IAS then reduces at altitude such that at:
10,000 ft it is 94.8, then:
15kft = 87.5
20kft = 80.5
25kft = 73.8
30kft = 67.5
35kft = 61.4
40kft = 54.7

For a real glider, multiply these figures by the ratio of your Sea
Level Vne to 100 knots. If 130
knots at SL, assuming above protocol, at 20kft the Vne IAS will be
104.7knots. I have a MS
Excel spreadsheet that does this, you enter your glider's Sea Level
Vne and the table with altitude
is shown (if you want a copy, email me).

However, the "official position" is that you must still use what it
says in your own glider's
Flight Manual, in case it is different to the above.

The manuals for older gliders may not allow for this reduction in Vne
IAS with altitude, and
advice should then be sought. In the USA I guess that this would be
from the SSA and/or the
FAA, in the UK is would be the BGA because we have delegated technical
powers from our CAA
and the matter would be dealt with in the first instance by the BGA
Technical Committee.

Background to Flutter Testing.

Unlike expensive powered aircraft, glider testing does not include
real flutter testing at altitude.
In powered aircraft, vibration is provoked in flutter testing by small
"bangers" or "kickers" that
artificially produce motion in wings, tail surfaces and so forth, so
that damped motion can be
proved from the sensors in the test aircraft. In my earlier
incarnation as a military test pilot, I have
been involved in high level flutter testing for a number of
aircraft.

This is not done in gliders for obvious reasons of cost and the
difficulty of taking prototype gliders
up to altitude with the correct "kickers" and instrumentation
sensors. Therefore, the above
protocol was produced.

I am sorry that this post is long. But it may review some of the
ground ...

Ian Strachan
Lasham Gliding Centre, UK