On Jan 1, 5:45*am, " 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. *I certainly believe
that true airspeed increases with altitude. *I use a rule of thumb of
about 2 percent per thousand. *So (at 17,000 feet) a Indicated
airspeed of 42 knots becomes 56 knots true airspeed. *An indicated
airspeed of 70 knots becomes 94 knots true airspeed. *It 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. *Is 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