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We are in the conceptual stage of design and, using Raymer & Nicolai as references, I am doing a rough calculation of "power off" neutral point (N.P.) to find the optimum c.g. and a.c. positions.

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As seen in this photo, which is also reflected in Nicolai, when doing a static longitudinal stability analysis, the moments are summed assuming the c.g. is aft of the wing/body a.c. and the tail provides a negative moment (nose down) while the wing provides a positive moment. It assumes both tail and wing provide positive lift. I understand that in this configuration, the positive tail lift would shift the overall a.c. (neutral point) aft and the c.g. would just need to be in front of the neutral point for static stability.

However, what is confusing for me is that conventional stability usually involves placing the c.g. in front of the a.c. and n.p. so that the wing provides a negative moment (pitch down) and the tail stabilizes this by providing a negative lift (down force) to pitch the nose up. Why assume the c.g. is aft of the a.c. and the tail is providing positive lift? Why not sum the moments assuming the c.g. is in front of the a.c. for calculating neutral point?

Any help would be much appreciated, thanks!

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  • $\begingroup$ Look here: aviation.stackexchange.com/questions/47306/…. Does this help? $\endgroup$
    – Peter Kämpf
    Mar 15, 2018 at 23:56
  • $\begingroup$ Kind of. I am still confused on why static stability in two respected textbooks is analyzed assuming a positive vertical tail lift and the c.g. behind the a.c. of the wing. I understand that theoretically that configuration can be stable, but why not analyze the moments and assume the c.g. is in front of the wing a.c. like in most conventional designs? $\endgroup$
    – Jason
    Mar 16, 2018 at 21:50

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Why assume the c.g. is aft of the a.c. and the tail is providing positive lift?

Maybe the two authors were sick and tired of that old meme that positive longitudinal stability requires negative lift on the tail. This is simply not true and never was, still it is believed like some religion.

What is true is that more static stability needs less tail lift, and many aircraft have so much stability built in that their tail lift is always negative. But with modern flight control systems this negative lift means more drag than necessary. Back in the old days, when the pilot wanted to let go of the stick or yoke in order to read a map or take a leak, a tail downforce was the best way to make sure the aircraft would behave. Still, low and sometimes missing roll stability meant that the pilot could never take his eyes off the horizon (real or artificial) for more than a short while.

Why not sum the moments assuming the c.g. is in front of the a.c. for calculating neutral point?

The position of the center of gravity relative to the wing's aerodynamic center has no significance for stability — all what counts is the center of gravity position relative to the overall aerodynamic center, fuselage and tail effects included. A smaller tail means that for the same stability the tail lift has to be lower, and most aircraft have the smallest tail surfaces that their designers could fit.

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  • $\begingroup$ Thanks for the clarification! This is along the lines of what I suspected. So the authors' method will give you the farthest aft c.g. point to optimize drag? I guess if you really needed the design to be more stable, you could calculate N.P. assuming a negative tail lift but it would reduce maneuverability and increase drag. $\endgroup$
    – Jason
    Mar 19, 2018 at 15:02

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