If both aircraft are gliding at the same IAS then they cannot both be at their best lift to drag ratio speed (Vmd). If we assume that the heavier aircraft is at its Vmd, then the lighter aircraft is flying faster than its Vmd. As stated by Captain Fathom, this higher speed will create higher drag.
The aircraft are gliding so this increased drag must be matched by the component of weight acting down the glidepath. In order to increase this component the nose must be pushed further down to make the glidepath steeper.
So if two aircraft glide at the same IAS but at different weights, the lighter aircraft must use a greater nose down attitude and a steeper glidepath. With both aircraft at the same IAS, the lighter one, with the steeper glidepath will have the greatest ROD and the smallest glide range.
This appears to be contrary to the conventional wisdom that "weight does not affect glide range", but that statement is based on the assumption that the aircraft in question is gliding at the correct Vmd for its weight. In the situation described above only one of the two aircraft can be at its Vmd.
Last edited by Keith.Williams.; 9th Oct 2004 at 14:50.