Vapour trails

On Mon, 13 Dec 2004 at 03:20:38 in message
, Capt.Doug
wrote:

No, less air, because the density of the ambient air is less as altitude
rises. Less air in the front means less air out the back (though the
pressure ratio can be the same). Jet engines produce less thrust at
altitude. There is less cooling air which means that maximum exhaust
temperature is reached at a lower thrust. The efficiency gains come from the
forward speed of the engine (sort of a ram effect) and the lower aerodynamic
drag at altitude (higher true airspeed).

This interests me as it is often said, the idea of less drag at
altitude presumably comes from the idea that drag depends on air
density? Which of course it does. However if you fly for maximum range
than you fly close to maximum lift/drag ratio which depends only on
getting the correct alpha (ignoring compressibility effects).

So since lift = weight, drag depends on weight and it reduces as fuel is
burned. The aircraft flies faster to create the lift at altitude but the
drag is presumably almost the same?

Am I wrong?
--
David CL Francis