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.

  • This really boils down to where would your CG be at neutral trim. But the tail must also set the wing at optimal AOA. For best fuel efficiency, 1 wing is better than 2, which is why airliners can trim the H stab. To paraphrase the great Peter Kampf, "weight issues become vanishingly small". At higher speeds (cruiseing), it is about aerodynamics. – Robert DiGiovanni Oct 9 at 17:31
up vote 1 down vote accepted

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.

The calculation of the neutral point of an aircraft is about moments, not aerodynamics. The tail incidence angle has nothing to do with the neutral point, just as it also has nothing to do with the calculation of the CG.

  • But moments arise because of aerodynamics! Stability is about reaction to disturbances; with most disturbances, airflow changes, which leads to change of moments, which provide that reaction. You can't separate them. – Zeus Oct 9 at 4:20
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    The above answer is correct. In the past I've seen a very clear explanation of this on model airplane forum. I will add a link to that discussion when I come across it. – quiet flyer Oct 9 at 15:33
  • @Zeus CG does not take into consideration anything but weights and arms while the aircraft is not in motion. Neutral point is a measurement relative to the CG for the purposes of determining positive or negative stability. That is why you do not take into consideration those things you mentioned. – Juan Jimenez Oct 10 at 8:00
  • You said "calculation... is about moments, not aerodynamics". This is "not even wrong": most moments are aerodynamic. That's what I was referring to. – Zeus Oct 10 at 23:16
  • @zeus You still don't understand. CG and neutral point calculations are static calculations. No "dynamics.' Have you ever actually done a CG or neutral point calculation? Here, this explains both in very simple terms. Notice the diagrams: angle of incidence is not present in any of the explanations. instructables.com/id/Introduction-65 – Juan Jimenez Oct 12 at 7:41

I have done some homework and I will explain why the Neutral point does not vary with the tail incidence angle.

Neutral Point location fundamental equation The above equation is the fundamental equation defining the location of the neutral point.

We can see that the Neutral point is a function of the following factors only,

  • Aerodynamic centre

  • 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),

  • Downwash effect

  • 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.

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.

  • Why shouldn't it? Because as long as the lift is proportional to angle of attack (almost correct unless approaching stall) and the centre of pressure of each airfoil (wing and stabiliser) does not move with angle of attack (slightly incorrect), the same change in angle of attack will cause the same increase in force at the same two points independent of how you shift the zero lift line, and consequently the neutral point around which there is no moment will be the same. In practice the centres of pressure do move a little, so there is slight variation. – Jan Hudec Oct 10 at 5:03
  • Thanks for defining "neutral point" as the center of all lifts. In this respect, yes, it will not change with pitch or change of velocity. Zeus was correct there. Interestingly, a change in decalage, or moving elevator, will move the center of lifts. Zeus again correct here. But moving elevator up, pushing tail down, moves center of lift FORWARD. Easier to see if elevator down, more lift in back. Solving by moving weight only works at one speed, requiring constant trimming. Once again, contributions from all appreciated. – Robert DiGiovanni Oct 10 at 12:35
  • @RobertDiGiovanni, Neutral point is not center of all lifts! That would be overall centre of pressure. Neutral point is a point around which the pitching moment does not change with angle of attack (a.k.a aerodynamic centre; neutral point is usually used of the whole aircraft, aerodynamic centre of individual airfoils). In all practical aircraft, the neutral point is significantly behind the overall centre of pressure, which must coincide with centre of gravity in straight flight. – Jan Hudec Oct 10 at 17:38
  • This would seem to relate to a comparison of torque moments of wing and tail areas and would be desirable to avoid an uncommanded pitch up and stall. In other words, lift from the tail compensates for forward shift of wing in pitch up? I will make a further effort to fully understand it. Thanks! – Robert DiGiovanni Oct 10 at 22:47

One way of thinking of the neutral point is "where would I put my CG so the trim of the airplane would have neither negative or postive pitch stability" when given the "dive test" (behavior with increase in velocity). Forward CG will nose up because the elevator pitching moment becomes stronger while the weight moment is constant. If you move your weight back, elevator up trim is less, reducing nose up with increased speed. Further back, and plane noses down with increased speed (unstable in pitch) due to down trim.

So if you change your tail incidence (same effect as moving the elevator) your center of all lifts will also change, there for where you put your CG also changes to a new neutral point.

Easier said than done. To be truly neutral, there must not be any aerodynamic asymmetry. Can't fix it by moving weight around, especially if the center of lift on the wing(s) changes with speed or pitch.

What many designers do is go for positive pitch stability as it is generally safer, aerobatic planes may prefer it to be closer to nuetral.

  • This explanation is misleading. Airspeed has nothing to do with longitudinal stability per se; speed stability is an indirect consequence of it. Aerodynamic changes with speed and with pitch are fundamentally different. Please see footnote to this answer. (AC and neutral point are the same thing). – Zeus Oct 9 at 3:43
  • Zeus, sorry, if you increase tail incidence and move weight forward the aircraft will still do loops. You are making a "one speed wonder". The key is to aerodynamicly trim so there is neither up or down tendency. You dive test to prove, weight is set at center of all lifts in the neutral trim configuration. Build models and you will see! – Robert DiGiovanni Oct 9 at 11:24
  • You probably wanted to comment to my answer? There I mentioned 'loops', but in the context of reduced tail incidence (or just elevator up). You can compensate it with a more forward CG (although usually it's the opposite: you compensate forward CG with elevator up). My comment here related to the explanation of stability via airspeed changes; this is a common misconception. In practice, one can 'prove' stability with 'dive test' because the effects are linked, but in the context of the question (and generally) it creates false understanding how pitch stability works. – Zeus Oct 10 at 0:41
  • Interesting how CG and elevator are related. When I started scratch builds and read about the dive test, I thought it was backwards until I realized my throws were decelerating and the dive test is accelerating. Depends how we look at things. Adjusted weight on glider because elevator was glued. Nose heavy will drop ballisticly, tail heavy flies straight out and stalls. But what seemed opposite clicked, and I added it to my "tool box". Cheers – Robert DiGiovanni Oct 10 at 3:00

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