Yes, agreed, Bristow's "check the momentum" explanation is next to useless. However, I know this effect is true. The machine I fly is very sensitive to this; you have to start a barber pole descent much earlier if you're heavier.
I've perused and searched the Tech Log, and have carefully poured through about 4 aerodynamics texts (Kermode, Dole, Naval Av, etc.), and have not found a definitive answer. I found a plausible explanation in an old (borrowed) internal Air Canada document on fuel efficiency: "higher landing weights will increase the descent distance for the SAME descent speed since it takes longer to dissipate the greater potential energy."
Also, lots of confusing discussion at:
http://www.pprune.org/forums/showthr...hreadid=129617
where the concensus seems to be that that the heavier a/c would be flying closer to Vmd (min drag speed).
And, a somewhat believable explanation from Bellerophon at:
http://www.pprune.org/forums/showthr...hreadid=125401
I'd be really happy if someone could point me to a definitive reference on this.
A heavier a/c will descent faster if you DON'T fix the airspeed. I.e. fix the AOA, and the heavier aircraft will descent at a higher airspeed, leading a higher descent rate.
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Also, a link to a picture re. the excess thrust discussion above:
http://history.nasa.gov/SP-367/f119.htm