Flutter speed (the normal cause of the break-up you
are concerned about) varies with altitude but not so
much as does the IAS. As an example, if your flutter
speed is 180 knots at sea level, meaning you can fly
up to 179 knots IAS (and TAS) at sea level, if you
climb to an altitude where 180 knots TAS is 100 knots
IAS, you will probably be able to go up to abouit 140
knots IAS before flutter occurs.

Don't take that as definitive for your glider, it varies
from type to type.

Your 1.5% error per thousand feet only holds good over
a very limited altitude range, space does not start
at 67,000 feet.

At 06:06 31 December 2007, wrote:
Hi Gang
Last week Minden had some excellent wave soaring.
Off tow at 7.7k
msl and within minutes up to 18k. I decided to go up
north skirting
Reno Intl. I had the transponder on and was monitoring
Reno Approach.
My goal was to try and fly as fast as possible maintaining
an altitude
very close to 18k by speeding up in lift and slowing
down in light
lift or sink. This worked well for the first 35 miles
where my IAS
(indicated air speed) ranged between 50 knots and
110 knots flying
never less than 17k. Then I got into some real lift
and pointed the
nose down and noticed my Becker transponder registering
18.2k which
is, of course, a no no. I then glanced at the IAS.
It read 138 knots
which is 165 knots TAS. (Every 1000 feet of altitude
above sea level
results in an error of 1.5% in IAS.) So what to do?
If I pulled the
spoilers at that speed, the shock might destroy them
or the glider. So
I gently pulled the stick back and translated speed
into altitude
going above 19k. At about 70 knots I pulled the spoilers
and got
myself down to below 18k. I wonder if ATC caught that?
Why was this so
bad? Well the VNE at sea level for the SparrowHawk
is 123 knots and it
has been demonstrated that at 171 knots the wings come
off. This
really gave me cause for concern. How quickly one can
get into trouble
by not paying attention. In the future I will fly slower
and use the
spoilers to compensate for excessive lift so as to
maintain altitude.
This story raises some questions about VNE at various
altitudes
which should be of interest to all of us glider pilots.
I Googled
combinations of words such as 'flutter altitude', 'VNE
altitude' and
'aircraft breakup altitude' to try and come up with
information on
whether the flutter/breakup characteristics of an aircraft
are less at
altitude than sea level at the same TAS. Intuitively
it would seem so
but intuition may not work here. I found nothing useful.
I know that
the World's ultimate high altitude motor glider the
U2, which was
designed in the 50s, had much study done on it with
regard to its
operating speed window of about 20mph (stall to breakup)at
80k feet
msl. There should now be declassified documents on
those studies which
might answer my questions. I would appreciate any pointers
anyone. If
I find anything useful I will summarize it on RAS.
Flying is often unforgiving of errors and I will
definitely be more
vigilant after this wake up call.
Dave