Rho is fixed in this scenario because you're comparing like-for-like. That is, at any instant, what speed corresponds to the AoA for best L/D ratio. Imagine it this way: You're in a glide at a certain weight and the correct speed for the AoA to achieve max L/d. A few skyivers land on you. At the very instant the weight increased when they er...boarded the aircraft, what speed is needed to maintain the AoA.
Conversely, while in the same glide, all the occupants exited or you dumped stuff overboard instantly (dropping bombs would work). What speed is now needed for the best AoA?
In both these scenarios, the weight is considered to have changed with negligible change to the other factors. Another way to get to grips with the problem is in this thought experiment: Imagine an aircraft able to have multiple glides (eg with a climb to altitude between each one) at different weights. The aircraft would then experience the same rho at some point in each descent.
Last edited by Tinstaafl; 2nd Aug 2009 at 00:41.