aircraft performance in descent

Date:         03 Jan 97 04:36:35 
From: (PMacdou367)
Organization: AOL,
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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.

I have some performance manuals that suggest the above is corrrect, can
anyone explaint the aerodynamics? I suspect it has to do with angle of
attack but I'm out of my depth there.