Originally Posted by
Fortissimo
I think the larger effect is not so much any centre of pressure movement (which does contribute) but the fact that your overall lift is a V-squared function. When you accelerate you have to reduce AoA to maintain the same lift (for level flight), so lower nose attitude and more AND trim.
section pitching moment, CsubM tends to increase on symmetric and cambered sections with increasing AoA. A section with a reflex camber may maintain low moments, which is why they are used occasionally on rotor sections, where they reduce pitch link loads. As Mach increases, pitching moment tends to increase. At very high Mach, (above MMO, approaching MD,), CsubL for a given AoA reduces, when a normal shock develops on the underside of the section. As washout results in a higher AoA inboard, and section T/C is greater normally inboard, the shock development is most pronounced inboard, and to maintain a constant total lift, the tips become more loaded, resulting in a shift rear wards of the total lift relative to longitudinal station, with increased lift distribution outboard. Separate to all that, the aircraft is required to have a nose down trim needed for increasing speed above trim speed (AoA) and vice versa, the result of inflow to the tail, tail volume and section moment design. A change in speed alters the normal force from the section as a function of the differences of the squares of the speeds, therefore level flight requires a lower AoA which is achieved by a lowering of attitude. To achieve that outcome the slope of CsubL to AoA, the a-slope need to be equal or the tail has to have a lower slope than the wing, when the inflow. Volume and section coefficients are considered. You can fly a plane with neutral static stability, they are fun to fly, they just are higher workload on instrument flying for extended periods, ask any Helo pilot ( helos are mildly statically stable, but are dynamically unstable in pitch in forward flight and hover and also in yaw in hover)