This reminds me of the thread on max drag/thrust thread some time ago. All contributors know the stuff pretty well but no one answers the question. Eventhough it is the precise wording of it that only allow us to answer properly.
Typical JAR ATPL catch-up.
So, two aircraft (or one a/c on two occasions), same speed, different weights. ROD?
Can not be answered. It is possibe that the lighter will have lesser ROD. It is also possible that the heavier will have a higher ROD. It is also possible that they will have the same rate of descent.
The real perfomance is L/D, Height/Distance or glide ratio whatever it is - airframe characteristic. No matter the weight, as described in previous postings. Yet, for every
given weight the optimal speed must be selected in order to achieve this. Not the case of our example.
To assess this properly, one would need to know the optimal speed for the two weights and then be wise enough to judge how close each of the planes would be. (I am not!)
Now, imagine a side-view of the two, descending.
Once again, these two are able to glide at same angle, but need different speeds for it. The question in place does not allow them.
Let's assume the selected speed is the optimal glide speed of the lighter aircraft computed as to include residual thrust/weight effect. This light aircraft would then fly the best glide, longest distance. The heavier one tough, is off its speed to achieve this optimal ratio, so it performs worse. They still fly side-by side, but the heavier one loses more height.
Higer ROD. In pure theory it also falls behind some. Do the vectors.
Let's assume the selected speed is the optimal glide speed of the heavier aircraft.
... blah, blah, blah ... It is higher ROD for the lighter one now.
There's really no need to elaborate on the third option I've put forward.
Cheers,
FD.
PS: For those still wondering, yes "aircraft flying at max speed demonstrates both max thrust and max drag at the same time".