Re: Flying High ?

Date:         03 Mar 98 03:12:51 
From: (Merlin Dorfman)
Organization: Netcom On-Line Services
References:   1 2
Next article
View raw article
  or MIME structure

RobertS975 ( wrote:
: >        Can somebody please explain to me why planes (jet aircraft) fly
: >at very high altitudes on long range flights, and how does this conserve
: >fuel and get maximum distance ? How does this conserve fuel and save
: >time if you can only go half the speed you normally can go at low
: >altitude and you have to use twice the amount of power to get too half
: >the power you get at lower altitudes ?

: Some of your premises in your question are wrong.  Fuel consumption in
: jet aircraft is far lower at altitude mainly because the air resistence
: is so much lower at higher altitudes.  Because of the thinner air, the
: indicated airspeed will be lower, but the true airspeed corrected for
: the less dense air and colder temps will be higher.   Jet engines are
: notorious fuel hogs at lower altitudes.  Hope this answers your
: questions.

     I think the part about getting better fuel efficiency at high
altitudes is correct, but I'm not so sure about the part about lower
drag.  The wings have to generate enough lift to carry the  airplane,
and the lift-to-drag ratio does not vary much with air density at
subsonic speeds, so the drag is independent of altitude, i.e., you
have to fly faster or at a higher angle of attack in thinner air so
the drag goes up.
     Piston engine planes flew at lower altitudes because their fuel
efficiency was not improved by flying higher.  Jets do better at high
altitude.  Obviously there is some design freedom--the engine can be
designed with some optimum altitude--but overall IIRC the best
economy is obtained if it is optimized for high altitude.
                                     Merlin Dorfman