Originally Posted by
Vessbot
Starting from the very basics. Straight and level, unaccelerated flight. Lift opposes Weight, Thrust opposes Drag. Everything is 90 or 180 degrees to everything else.
Then, think of a climb or descent. Weight no longer opposes Lift, but is slightly off (by the amount of the climb or descent angle). Due to this off-ness of angle, Weight becomes partially aligned with either Drag (in a climb) or Thrust (in a descent.) In the picture a few posts up, this is represented by the green forward-pointing vector in the bottom triangle. This is called the Forward (if descending, or Rearward if climbing) Component of Weight. Put more simply, going up hill, weight is pulling you back, and if going downhill, weight is pulling you forward. This is the same mechanic as what happens in a car going uphill or downhill. (Literally the same, it’s not an analogy or any other type of mental trick). I bet you already understood this in a common-sense gut level, if not by drawing triangles. This is why you tend to slow down uphill and speed up downhill.
Now, consider two planes descending at the same angle. If everything about them is the same, then their Weight is the same, and their Forward Component of Weight is the same, and their Thrust and Drag are the same, and their speed is the same. Everything. Now, imagine one is heavier. If we keep everything else the same (descent angle, Thrust, Drag) then the one with greater Weight will also have a greater Forward Component of Weight. Therefore it will try to go faster.
But since we want to keep to the same speed as the lighter one (it’s ATC-assigned speed, or it’s Vmo/Mmo, or whatever) and we can’t reduce drag since we’re already at idle, then the only option for the heavier plane is to reduce the descent angle. Once that’s reduced, then the Forward Component of Weight is reduced (and matches the lighter plane again) not because Weight is reduced, but because the descent angle is.
TLDR: Higher weight pulls the aircraft forward harder, and a reduced slope counteracts this effect.
TLDR in math: (gamma, γ, is descent angle) Longitudinal forces in a descent are T + W sin γ forward, balanced by D backward. If speed is constrained to be the same, then D is the same. T is zero (idle) therefore W sin γ must be kept constant. For that, higher W requires lower sin γ, which means lower γ.