Originally Posted by
LEOCh
Agreed with the rest of your post, but as a counter-example to the statement above, consider a 737 NG loaded to it's absolute rear limit (36%MAC), and trimmed to fly at high but nonstalling AoA. It's CP will be relatively forward and close to but not reaching the wing aerodynamic centre of 25%MAC (will be approximately 32%MAC). CP is hence ahead of the CG and the tail is lifting slightly to maintain trim. However, the aircraft is still longitudinally stable, as although the tail is lifting it's local AoA is less than the wing. Of course this is an edge case for stability, more often CP is behind the CG in all flight regimes and the tail is always in downforce.
nope.
25% is a convention for measurement of moments, so a wing with zero pitching moment will have a Cp at 25% chord. Refer to Abbot and VonDoenhoff to look at the moments that occur on a section. Next time walking around your brand A or B plane have a look at the section of the stab, it is an inverted cambered section, which means it has a zero lift line, ZLL, that is considerably beyond a zero stab LE up limit, which is usually around 2 to 3 degrees up dependent on flavour. The stab resides in an area of down wash on standard tails, less so for T tails or cruciform tails. The stab on an A or B brand does not get to a point in normal use of trimming to an up force. The elevators of course may result in a change above the stab limit, but would be untrimmed. For your vanilla flavoured brand, the neutral point is way, way further back, around a center of mass aft of 40% for a plane with an aft envelope limit of 32%. Long time back we looked at a B744 that achieved 43.5% in flight... vs a 32% envelope. The plane was marginally statically stable (being generous), and appeared to be slightly dynamically unstable, the autopilot coped with the mis load, but dang if the elevators weren’t working their passage, they oscillated for the whole flight. To achieve that level of load error, the nose wheel was not on the ground at 80kts, and on landing, the plane sat with the nose wheels off the ground, unable to steer for taxi. Even at that case, the tail was producing a slight down force.