"David CL Francis" wrote in message
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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?

Yes.

The drag is actually less. The indicated airspeed is a good way of seeing
how the airframe is currently being affected by the ambient air at whatever
density it is. Regardless of the air's actual density, the 1G stall speed
is always the same, and for constant engine power, cruise speed remains
remarkably constant (I'm not sure whether it is actually constant, but
having flown a turbocharged engine at altitudes up to 18,000' and noting an
airspeed drop only higher than 16,000', the turbocharger's "critical
altitude", I am confident in saying that, when measured by indicated
airspeed, there's practically no change as long as power is kept constant).

As altitude goes up and indicated airspeed remains constant, TRUE airspeed,
on the other hand, goes up. Same lift (equal to weight, as you note), but
you're going faster for the same power. Obviously thrust didn't increase
(and in fact, decreased, since you get less thrust from the prop due to the
less dense air...though with a constant speed prop, much if not all of the
lost thrust can be regained using coarser prop pitch), so the only way to go
faster is for drag to have decreased.

Since lift is constant, maximum lift/drag ratio still occurs at the
particular angle of attack where drag is minimized. But the ratio is
higher, because drag is lower. It's the angle of attack that's constant,
not the ratio itself.

Pete