Why it doesn't matter if tail produce lift or downforce for neutral point position?

I am really strugle what neutral point represent and what it depends on..

In this link we can see how neutral point is calculated.Thay use wing and tail areas and distance between aerodynamic center (L) to determine neutral point position (D).Calculation is set like tail produce lift..(So calculation is the same like you must find fulcrum at see saw..)

What if tail produce downforce(what is often case).If this tail in my example produce downforce,neutral point will move to the left,completly different position.

Does neutral point dependt at change of aerodynamics forces at wing and tail,for example when you change tail incidence,etc?

If I put aircraft in vertical position to eliminate torque by gravity,connect with joint so he can rotate, now must find place at plane which will not rotate(moment is zero) when is subject to airflow.

Is this place neutral point?

The neutral point is the lengthwise location of the moment reference point where the pitching moment will not change with a change in angle of attack. It can be defined for a wing alone or for the whole airplane.

Of course will a change in the incidence of the tail also change the pitching moment around the neutral point. But that is besides the point: The special characteristic of the neutral point is its neutrality regarding to changes in angle of attack.

Mathematically speaking: If the slope of the pitching moment over angle of attack is zero, the reference point for this moment is the neutral point. If you pick a different point ahead of the neutral point, the slope will become negative (which characterizes a stable configuration) and vice versa. The neutral point depends on the derivative of the pitching moment over angle of attack, so the absolute value of it doesn't matter. It's the change with angle of attack that counts.

If the center of gravity coincides with the neutral point, the airplane has neutral longitudinal stability. Hence the name.

This all is valid only in the range of attached flow. If there is massive separation, all bets are off.

• Neutral point must change if tail produce downforce or lift?.Calculation in my post use wing and tail planform area and distances between AC to find neutral point position.It is same calculation if two people are on see saw and you must find where is fulcrum located.
– user53913
Jan 17, 2021 at 11:19
• @EBV821, no, because it does not depend on what lift is produced at any specific angle of attack, only how it changes when the angle of attack changes. Jan 17, 2021 at 11:33
• @JanHudec So neutral point is not place where moment caused by all aerodynamics forces is zero?So if I hold plane at neutral point and negelct torque by gravity, plane will rotate about that place?(look at my "test" at bottom of post)
– user53913
Jan 17, 2021 at 11:41
• @EBV821 most planes would rotate, yes. Jan 17, 2021 at 11:52
• @EBV821 for the range of AoA where lift grows linearly with AoA, i.e. approximately before onset of stall on either surface. Jan 17, 2021 at 11:57