Originally Posted by
Just This Once...
Again, it was suggested to me to watch the g-meter, which was irrelevant and hence my flippant answer.
I'm very comfy with the simple stuff and the difficult. But this is simple, transitioning from equilibrium of 4 forces by lowering the nose with constant power will produce an acceleration in 2 axis - vertical and horizontal. If the desire is for a constant airspeed then either thrust has to be reduced (less power) or more drag added. Your lift vector is now pointing away from the vertical due to your descent angle (or glide angle) but weight remains the same. Down you go, with the lift vector sum less than the force of gravity acting on your weight, initially in in non-equilibrium / accelerated flight. If/when equilibrium is achieved in the descent with both the vertical and horizontal acceleration at zero the descent will continue as the weight vector remains in the same vertical plane but the total lift vector sum is reduced as it is angled towards the flight path, working (partly) in opposition to drag.
The vectors you talk of are forces which when acting on a mass produce acceleration (Newton's 2nd law of motion.) The situation you describe would produce an ever increasing vertical speed as the mass accelerates vertically. To maintain a vertical speed there has to be no acceleration, so there no force imbalance in the vertical plane.
The lift from the wing is perpendicular to the flightpath and the drag vector, thus has no effect on it.
Next time you set yourself up in the descent at the same speed as level flight, note which way you trim and think why. (hint: you trim nose up to maintain a greater AoA that is needed to provide more lift perpendicular to the wing chord to provide the same vertical vector that you had before you commenced the descent.)
It is a lot simpler just to read a simple guide to aerodynamics, they usually cover Newton's laws and have pictures.