as i understood,for a prop-piston aricraft :
during a gliding, through the equation:
tanγ= D/W≈ 1/K
sink rate≈ VD/W≈ V/K
by which we get(theoretically):
Vx(Vmp) is the speed at which we get minimum sink rate
and Vy(Vmd) match up with minimum γ
but when it comes to a powered descent:
sink rate≈ V△D/W= -△P/W
then because at Vx(Vmp) the prop aircraft gets minimum excess drag gets minimum γ
at Vy(Vmd) gets minimum -excess power gets minimum sink rate
which does not add up, can't even make a graphic for powered descent.
what is wrong with that?