Re: aircraft performance in descent

Date:         04 Jan 97 03:55:50 
From:         drela@athena.mit.edu (Mark Drela)
Organization: Massachusetts Institute of Technology
References:   1
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In article <airliners.1997.40@ohare.Chicago.COM>, pmacdou367@aol.com (PMacdou367) writes:
|> Could anyone with a background in aircraft performance please give me a
|> good explanation of this "fact".
|>
|> 2 identical planes are at FL350, however 1 weighs 200k and the other 300k.
|> IF they both conduct a flight idle, same IAS, descent, the heavier
|> airplane will have a lower rate of descent, contrary to what one's
|> intuition would tell them. Therefore if you are plannning a descent then a
|> heavier airplane must start a descent earlier because of the lower rate.

In steady flight, the descent angle is the same as the Net_Drag/Lift ratio,
where  Net_Drag = Drag-Thrust.  Assuming small angles this can be written as

 descent angle  =  sink_rate/TAS  =  (Drag - Thrust)/Weight

For a given TAS and thrust setting, the sink rate will vary as 1/Weight.

Note that this assumes that Drag is the same, which is only true if the
profile drag dominates the induced drag sufficiently (i.e. if the descent
is done at low CL, or high IAS for the current weight).  For lower IAS
descents, the sink rate of the heavier airplane may well be higher due
to its higher induced drag.

  Mark Drela                          First Law of Aviation:
  MIT Aero & Astro          "Takeoff is optional, landing is compulsory"