All the characteristic speeds remain the same, as indicated on your
instruments, whatever altitude you are at. That is assuming the
instruments work from pressure, or changes of pressure. So if your
sailplane stalls at 37 kts indicated at sea level, it will stall at 37 kts
indicated at 20,000 ft. Maybe not absolutely accurate, but near enough to
be practical. An analogy would be putting in ballast, for the same glide
angle, speed and sink are higher.
The reason for a reduction in one characteristic speed, VNE, as you go
higher, is flutter in elastic structures, which is a function of true
airspeed, not indicated airspeed. Eventually you get to"coffin corner"
where the increasing true stall speed meets the reducing true VNE, and you
are stuck, you can't go higher, and you can't go faster.
The "free lunch" is your ground speed, as you go higher, for the same
indicated airspeed, your ground speed increases.

Dave


At 14:26 01 January 2009, wrote:
On Jan 1, 5:45=A0am, " wrote:
Having done most of my flying at lower altitudes, I have wondered
about the contradicton between my unscientific observations when
flying at high altitude and what I would expect from my somewhat
limited knowledge of physics and aerodymanics. =A0I certainly believe
that true airspeed increases with altitude. =A0I use a rule of thumb
of
about 2 percent per thousand. =A0So (at 17,000 feet) a Indicated
airspeed of 42 knots becomes 56 knots true airspeed. =A0An indicated
airspeed of 70 knots becomes 94 knots true airspeed. =A0It just does
not
feel like or the instruments don't seem to indicate sink rates (I
have
made no careful observations) one would expect for the higher true air
speeds. =A0Is there no free lunch?

If you are flying a modern sailplane at 70 kts IAS the theoretical
difference in sink rate between sea level and 17,000' would be
something like 40 feet per minute - from 150 fpm to 190 fpm. That
should be observable if the airmass isn't going up/down much, but if
there is the kind of lift that can get you to 17,000' you may find a
lot of noise in the readings. There is no free lunch - you are right.
The question is, do you think your casual observation is well
calibrated enough to pick up the 40 fpm difference in sink rate?

9B