Yes and no. If you're measuring efficiency in terms of range, once you've
found your best cruising airspeed and engine speed, altitude won't affect
range much, though you'll cover the distance faster the higher you go. If
your best range is at 55% power, then you'll probably do best to climb as
high as 55% will allow (assuming still air) and lean for best economy. See
http://142.26.194.131/aerodynamics1/...nce/Page7.html

That said, the differences between airframe and prop designs are
significant. Light weight, better L/D and a constant speed prop mean better
range at all altitudes. A Mooney is more efficient than a 172 with the same
engine.


wrote in message
oups.com...

hi: I have a simple question for the piloting physics majors. we all
know that planes have less air resistance to overcome at higher
altitudes, but that normally aspirated planes have less power at higher
altitudes. presumably, both are proportions of what happens at sea
level, and are hopefully not too plane dependent. That is, I would
guess that a 160hp engine would lose about the same proportion of power
as a 320hp engine. for lycomings, at 10,000', this proportion is about
50%. something similar [proportional reduction] may also happen to air
resistance, regardless of whether the plane is a cub or a lancair.

this leads me to a very simple question: on a standard day, without
any winds, what would be the optimal altitude for [cruise] speed in a
normally aspirated airplane? is this best altitude dependent on
aircraft to a first-order, or is it fairly constant across airplanes?

sincerely,

/ivo welch