"Marc J. Zeitlin" wrote in message
...
Jim Carter wrote:

During a recent discussion about calculating glide ratio, I began
to wonder what the effects of pressure altitude were on the glide
ratio of an aircraft. Since air is 50% as dense at FL180 as at sea
level, would the glide ratio increase (glide further) as altitude
decreases?

To a first order approximation, the maximum glide ratio (L/D) does not
change with altitude. Since what's changing is the density of the
air, and both the lift and the drag are linearly related to the air
density, the effect cancels out.

Since the max. L/D occurs at a given IAS for any particular weight and
CG position, the TAS will change as the altitude changes, but the L/D
will not (again, to a first order approximation).

--
Marc J. Zeitlin
http://www.cozybuilders.org/
Copyright (c) 2007

I was not sure the relationship between lift and drag was linear, so thanks
for clearing that point up. You've also answered the question very well.
Thanks.

--
Jim Carter
Rogers, Arkansas