An aerodynamics question (for experts only)

[cwatters]

For the plane to be in S&L flight TWO things must be true...

1). The sum of all the TRANSLATIONAL (straight line) forces must be zero

AND

2). The sum of all the TORQUES/ROTATIONAL forces must be zero.

If you define LIFT(cog), THRUST(cog), WEIGHT(cog) and DRAG(cog) as the orthogonal (right angles) translational components of all the forces that act through the center of gravity then it's true that they balance to zero.

But then you have to look at the other rotational forces. If the true center of drag is a bit above the CoG then that causes a rotational pitch-up. A high thrust line creates a pitch down. The wings pitching moment causes a pitch down etc etc. All these rotational forces must also balance to zero or the plane will be rotating about it's CoG - as it flys along S&L.

Consider what happens when you put the gear down. The translational component of drag increases so you must apply more power to maintain speed. However the center of drag also moves downwards. This causes a rotational force (pitch down) which may require a trim change. The trim change adds up elevator restoring the balance between rotational forces.

The TWO types of force must balance or the plane will either accelerate in some direction or rotate.