Re: aircraft performance in descent

Date:         04 Jan 97 03:55:50 
From:         ifly <ifly@indigo.ie>
Organization: Indigo
References:   1
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pmacdou367@aol.com (PMacdou367) wrote:
>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.

This is true if both of the airplanes are flying at or near the heavier
airplane`s best LD speed in the descent, but the lighter airplane would
glide (only slightly) better if both aircraft were flown at their
respective best L/D speeds. It is partly to do with AoA, partly inertial.
Most airliners descend at relatively high speeds, 300 kts or thereabouts
until 10,000 feet, anyway, and these speeds favour a heavy airplane in
regards a good L/D. At higher altitudes, this is further compounded by
the effects of high mach numbers on the wing,which favors high speeds,
but lower weights.
Clear as mud, now!???

Jeff Morris