On Wed, 15 Dec 2004 at 16:52:25 in message
, Peter Duniho
wrote:
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.

That is true I agree but lift and drag both depend on TAS. Do you claim
that that lift does, but drag does not depend on TAS?

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.

But if drag is lower then lift should also be lower.

Interesting Peter but you have not yet convinced me. If you do a test on
an airfoil you will determine its lift and drag coefficients. Apart from
effects due to Reynolds number and compressibility those figures apply
to all conditions. The plot of CL against CD remains the same - why
should it change? You are effectively saying that an aircraft with an
L/D of say 12 has a much higher maximum lift drag ratio at high altitude
- how much 25 or more?

Does a high performance glider with a normal Lift/drag max of 50 have an
even higher one at high altitude?

Since lift and drag both depend directly on indicated airspeed (which
is merely a correction due to air density) for given IAS the ratio of
lift and drag will be produced at the same AoA at all altitudes. There
is a substantial difference in kinematic viscosity from sea level to
high altitude and that will change the Reynolds Number characteristics
although the higher true speeds also change the Reynolds number at least
partly in compensation - how much I am not sure but it will not change
lift and drag separately AFAIK.

Don't forget that both lift and drag depend on true airspeed and on air
density. It is the concept of dynamic pressure (0.5 *density *
velocity^2) which gives us the convenience of IAS.

I would like to get this a bit straighter in my mind. My theoretical
studies are so long ago that things may have changed and so may I!
--
David CL Francis