optimal airplane propeller placement along the vertical axis?

Question

is there a guideline for optimizing flight based on where the engine/propeller is placed along the vertical axis? some planes have the propellers well below the wings, some on the wings, some above the wings.

i've been reading about center of mass, center of lift, center of pressure, and center of thrust, and none of these seem to address this.

is there any sort of "center of drag" concept? so like if i'm quickly pulling a plane horizontally through the air on a string, the point of string attachment where the wings will be level to the ground? eg if i had a plane with a very tall fuselage that was completely under the wings, and attached the propellers to the wings, i'm assuming the plane would have a tendency to nose-dive due all the drag pushing against the front of the fuselage under the propeller/(thrust vector). ?

namely, will the bottom 2 planes, below, have a tendency to lurch forward/downward because of this (unlike the top plane)? something that would have to be counteracted by the tail and be less efficient because of it?

Answer 1

The vertical placement of the thrust line (relative to the aircraft's CG) will determine whether the engines make a positive, neutral, or negative contribution to the pitching moment. This contribution to moment will vary with thrust - and potentially abruptly in the case of an engine failure.

The similar contribution of drag gets resolved into the overall pitching moment of the aircraft. But certainly, retracting the landing gear (mostly a drag change) can change this moment (for example).

Many aircraft are designed with a preference to place the thrust line running through the vertical location of the CG to neutralize this contribution to the moment. However, the moment arm of the thrust line (to the CG) is relatively short (say compared to the distance from the CG to the horizontal tail) and this contribution to moment is at best a second order effect.

Consequently, if aircraft have a good reason to place the engines high or low (as compared to the CG), they will probably do so for that good reason -- and will then work to resolve whatever penalties are caused by this contribution to moment.