So let's say you have a high-wing aircraft
Allright, I choose the Dornier Do-24 ATT, a very well behaved high wing airplane with excellent seaworthiness. Below I show it in frontal and side view with lift and weight forces symbolized by arrows. I decomposed the lift into its zero-lift moment and put the lift vector at the neutral point, where all angle-of-attack-dependent lift acts, so the lengthwise distance between weight and lift vector is proportional to the static longitudinal stability and is delineated by a pair of arrows. Of course, when you shift the lift vector into the center of pressure, it will be aligned with the weight vector and the zero-lift moment will disappear.
This is my baseline high wing design, and next I raise wing and tail by a grotesque amount. No less will do to make the differences obvious.
Raise the wing vertically to make the high-wing even higher. What does this do to the airplane's flight characteristics?
If we neglect the additional vertical surface at the back, what has changed are inertias in all axes, which have grown by the increased distance between the horizontal surfaces and the fuselage. Also, the center of gravity has moved up a bit.
What are the consequences for the flight characteristics?
- Maneuvering will be more sluggish and will require larger control surface deflections.
- Rolling will cause a sideways shift at the pilot's station which might be a bit confusing, but one can get used to this.
- Stability hasn't changed. Static stability is identical to the baseline and since the tail lever arm is still the same, pitch damping is also the same. The increased fore-back motion of the wing during a pitch oscillation will mostly make itself felt in inertial changes, aerodynamically this is of minor consequence.
- The higher engine placement means higher pitching moment contributions from the engines. More downforce on the horizontal tail is needed to compensate the pitch-down moment from the high engines, and throttle changes will require high pitch trim changes.
Now what about lateral stability? And what about pitch changes away from level flight at cruise speed? For this we need another sketch. The baseline first:
On the left, the airplane flies a coordinated turn with a bank angle of 45° and on the right it is in level flight close to stall at 15° angle of attack. Turning adds a centrifugal force which acts at the center of gravity and needs a bank angle and lift increase so lift is sufficient to balance the resulting mass forces (denoted as R here). Since both forces act along the centerline, no imbalance or instability comes with the high wing arrangement. However, the side sketch now reveals a larger distance between the mass and lift forces, which means that the airplane becomes longitudinally more stable at low speed. The zero-lift moment has to become larger to trim this high pitch angle by incasing the negative elevator deflection. Note that the green circle has grown in size and weight to reflect this. The increased elevator travel helps to keep stick forces at low dynamic pressure up and requires more elevator travel for stalling than in a low wing configuration.
And now we do the same for the high wing version:
Again, not much has changed, only that the stabilizing effect of a high pitch attitude is now even more pronounced and the zero-lift moment needs to become even larger than before. This will require a bigger tail, or only a small range of angles of attack can be trimmed. Turning feels the same, apart from the need to overcome the higher inertia with more forceful commands, and again no instability can be seen.
But a low wing ... a low wing will still show the same results. Angle of attack changes mean less change in static longitudinal stability and lateral stability is unaffected. Only the effect of the fuselage on the yaw-induced rolling moment will require a low wing to have more dihedral which makes it ever so slightly less efficient. But mayhem and carnage fail to manifest themselves.
And what does an uncoordinated turn do? Now the airplane will sideslip and the weight vector will not be aligned with the centerline in the frontal view. But the lift vector, still orthogonal to the wing, will, so no rolling moment from weight or wing lift develops. Only the side force of the vertical tail and the fuselage might cause a very small roll contribution which can easily be balanced with a bit of aileron. Again, mayhem and carnage fail to manifest themselves.
The central fallacy here is the comparison with the broom, balanced on a fingertip. Airplanes (and drones or rockets, for that measure) are different. Conservation of momentum dictates that all rotations take place around the center of gravity, and lift, being the result of surface pressures, is always orthogonal to the surface of the wing. In consequence, regardless of wing position, a bank angle will not cause a destabilizing rolling moment.