Re: aircraft performance in descent

Date:         05 Jan 97 03:22:35 
From:         boardsailot@earthlink.net (Gabe Sanders)
Organization: Earthlink Network, Inc.
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On 03 Jan 97 04:36:35 , 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.

Yes, this is definitely true with a constant airspeed.  (Many
experienced pilots are unaware of this.)  The easy explanation is,
that the heavier airplane has much more potential energy to dissipate
in its descent than the lighter one.  So at the same speed, it will
take longer to get down.

The more politically correct explanation;  At the same airspeed a
heavier airplane will be much closer to L/D (lift over drag) max.  The
closer to L/D max the better the driftdown.  For example, a heavy
airplane l/d max may be about 240 kts. while the same airplane with
less weight may have a 220 kt. l/d max.   So if both airplanes were to
descent at 300 kts., the heavy airplane would be producing more lift
to drag and result in a lower descent rate.  If both airplanes flew at
l/d max both airplanes would have the same descent rate, but the
heavier airplane would fly farther because it's flying faster.

Hope this is understandable.

Gabe

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