While using XFLR v5 for a basic analysis I am facing this issue. For a fixed configuration I varied the tail incidence angle and XFLR is displaying a change in the Neutral point location. But as per theory neutral point location is not dependent on the tail incidence angle at all. I was hoping if someone could help me with this issue.
I have done some homework and I will explain why the Neutral point does not vary with the tail incidence angle.
We can see that the Neutral point is a function of the following factors only,
Tail volume (Horizontal tail sizing and Arm length)
Tail efficiency factor (Ratio of dynamic pressure at the tail and at the wing),
Lift curve slope for the wing and HT (CLalpha - which is fixed and not dependent on the tail incidence),
Fuselage moment contribution
So none of these factors are dependent on the tail incidence angle and hence there musnt be a change at all. What shocked me the most is the fact that errors in variable quantites can be accounted for but errors in quantities that shouldnt vary is very confusing. It is a very standard statement that '' Static margin or ultimately the Cmalpha shouldnt get affected by the change in tail incidence'' This is the information I could get about it.
As you suggested, neutral point does not change with changes in decalage (tail incidence).
For more explanation, see these links from a model airplane forum--
https://www.rcgroups.com/forums/showpost.php?p=24692807&postcount=3 https://www.rcgroups.com/forums/showpost.php?p=24694083&postcount=7 https://www.rcgroups.com/forums/showpost.php?p=24694082&postcount=6 https://www.rcgroups.com/forums/showpost.php?p=24694515&postcount=9
The links are more focussed on "static margin" than "neutral point", but implicit in the idea that the "static margin" is not changing if the decalage is changed, is the idea that the "neutral point" is not changing either.
Maybe someone can add to this answer to address the issue you are having with XFLR v5.
I'm not that familiar with XFLR, so I'll answer with the general knowledge.
The neutral point (aka aerodynamic centre, AC) does change with the tail incidence. Why shouldn't it?
The peculiarity (and intention) of AC is that it doesn't move with the angle of attack (AoA) only. If you balance your airplane at AC, by locating CG there or by hinging (and weight balancing) the model in a wind tunnel, the airplane will be neutral with respect to AoA changes. It won't 'weathervane'.
But this doesn't mean AC won't move with configuration. If you pull the elevator/stab up, this will redistribute the moments such that the neutral point moved farther back. Think this way: AC, by definition, is the point about which the added moments due to increase of AoA from the wing, stab and everything else, balance each other out. Or, in other words, where we can think the added lift occurs. When you reduce the stab incidence (with respect to the wing), you make it to contribute more to the moment balance (again, the balance due to changes of AoA and nothing else, it's boring to repeat but it's important to understand. We are not talking about the absolute balance).
But what can you do with that? Your stab now produces a pitch-up moment. Yes, the airplane actually became a bit more stable, but it will start doing loops. If we want to restore neutrality, we'd need to move CG back, to the new AC - which would only make things worse. The only way we can balance the situation(*) is to move CG forward, until it compensates the added pitch-up. Which again increases static stability. (In practice, of course, the opposite happens: when CG is forward, we make the tail to provide pitch-up).
So, in reality, we can't simply choose tail incidence as an independent variable. It is needed to trim the airplane as much as to provide stability. But, indeed, any aerodynamic configuration changes may shift the AC and affect stability.
(*) We can also tilt the thrust to provide the required moment, but this would be double waste: the tail works against the engine. Nevertheless, technically, this way it is possible to make a statically neutral airplane with any desired tail incidence.
This may be a good time to dig into the formulas your software uses and try doing them manually.
It seems XFLR v5 is doing a few steps at a time. Changing tail incidence indirectly affects the static stability nuetral point by assuming CG will move to new center (of lift) pressure.
Moving CG changes the torque arms of the stabilizing and destabilizing forces, which will change the static stability of the aircraft.
For example, is one reduces downforce of tail, center of pressure (of entire plane) moves back, matching CG moves back, now there is more area ahead of the CG. One cannot ignore the CG when considering the effects of aerodynamic forces on an aircraft.
If one defines "nuetral point" as the aerodynamic center of the entire aircraft, where a change in pitch does not create a pitching moment, then a change in tail incidence should only slightly affect its location. This net aerodynamic center still needs to be behind the center of gravity for positive stability.
More good info in this answer.