On Jun 18, 1:39*pm, Hellman wrote:
I've been flying gliders for years, but the following question only
occurred to me recently: In mechanically compensated varios, the TE
pressure is supposed to be as much below static as pitot is above
that. That makes sense since "ram air pressure" increases as v^2, as
does the kinetic energy of the ship. But pitot pressure is
proportional to IAS, whereas the kinetic energy of the ship would seem
to need TAS. If I haven't made a mistake (and please correct me if I
have), TE compensation will only be correct at one altitude. At higher
altitudes it will be under compensated, and at lower altitudes over
compensated. Comments please!

Of course you are correct that at a given TAS the suction generated in
the TE probe will decrease proportional to the air density at that
altitude.

However don't forget that the rate of air density at altitude does not
decrease linearly.

The air pressure at altitudes we fly at is pretty much 1013 / 2^
(altitude/18000). If you work out the derivative of that you'll find
the rate of change of pressure at a particular altitude is
proportional to the pressure at that height.

That means that the change in potential energy from a 1m change in
height will *also* be misread proportional to the air density at that
height.

So the two effects cancel each other out.