Originally Posted by
ReverseFlight
Thanks for all your answers. I guess the max RoD is achieved at max speed (MMO?VMO) rather than min speed (Vls), although I have no reference for this. My A320 FCTM just says :
The explanation is based on the balance of forces in the dive.
At constant speed, the drag of the aircraft is balanced by the thrust and the gravity component along the flight path. The gravity component increases as the dive steepens, so for the steepest dive angle you want the most drag and the least thrust. Drag being proportional to speed squared, the faster you go the steeper the dive angle can be.
Furthermore, what counts is not dive angle but rate of descent. The faster you go for a given dive angle, the higher the ROD will be.
Therefore, for a given aircraft configuration, the faster you go, the higher the ROD will be, and its probably something like a cube factor on the speed. So speed is a very powerful determinant of ROD.
The only slight caveat is the statement "for a given aircraft configuration". Obviously if you change the configuration to achieve more drag, you can achieve a steeped angle. BUT if you give up speed limits in order to achieve a steeper angle, you may well (and in many cases do) lose out on ROD.
Going up the back side of the drag curve doesn't work because you get a fair amount of drag, but even if you could get the same dive angle as at Vmo, your rate of descent, which depends on both speed AND dive angle, will be anaemic compared to the VMO case - perhaps only 1/3 the ROD.