I understand that if the center of gravity is in front of neutral point, it is stable. If the center of gravity is behind the neutral point, it is unstable. Based on the diagram, is it implied that h (center of gravity) cannot be greater than 1? When h = 1, there will only be c (mean aerodynamic chord). What does it mean if h>1? Is it off the wing basically?

enter image description here

hnwb = 0.25 (typically)

When I solved a problem, my h value was greater than 1. Isn't that incorrect because it is off the wing?

  • 1
    $\begingroup$ Welcome back to Stack Exchange! What do you mean by "value for the center of gravity"? Isn't the center of gravity a point on the aircraft, not a number? I can only guess what you're trying to ask. $\endgroup$ – Terran Swett Aug 2 '19 at 21:59
  • $\begingroup$ You can calculate the center of gravity in terms of mean aerodynamic chord. $\endgroup$ – M.D. White Aug 2 '19 at 22:09
  • $\begingroup$ I cannot find hnwb, h and hn in the diagram. $\endgroup$ – Koyovis Aug 2 '19 at 22:11
  • $\begingroup$ I've changed the picture, sorry. $\endgroup$ – M.D. White Aug 2 '19 at 22:17
  • $\begingroup$ So it sounds like you're saying that for the location of the center of gravity, 0 means the leading edge of the wing and 1 means the trailing edge? Am I understanding that right? $\endgroup$ – Terran Swett Aug 2 '19 at 23:55

If I understand your question correctly, h is a fraction of mean aerodynamic chord c. That fraction can be >1. It could be that the aircraft is stable with a huge download on the tail.

The mean aerodynamic chord is an aid, to simplify calculations. It is not a hard limit for the equilibrium points. Whether the value is reasonable or not is hard to say without having seen the full problem to be solved.

  • $\begingroup$ Never seen an NP off the MAC. But maybe with tandem wing? $\endgroup$ – JZYL Sep 2 '19 at 21:36
  • $\begingroup$ @Jimmy no I haven’t either, but it is not wrong by definition. $\endgroup$ – Koyovis Sep 2 '19 at 23:28

The reasonable value for center of gravity compared to the center of lift is a 2 part process in horizontal stabilizer design. Set static stability aside for now. For most efficient flight you want the main wing only generating lift. Ideally, the horizontal stabilizer should be at 0 AOA, holding the wing in place at optimal Lift/Drag AOA. Center of gravity is directly under Center of Lift. Job 1 complete.

Now we want to introduce static stability, also known as speed stability. This is extremely important when you are "low and slow" to avoid stalling. It is a terrific safety feature even with computer assisted control. So you move your TRIM tab to give a little bit of tail downforce and move your CG slightly forward as needed for adequate static stability. Job 2 done.

The 3rd function of the horizontal stabilizer is to pitch the nose down if the plane sinks, in order to avoid ever increasing AOA and unrecoverable "deep stall". Here, within reasonable limits, the more area, the better. Wedge or delta shaped designs are popular with birds.

So there is a difference between a horizontal stabilizer and a trim tab. Champions of tiny Hstabs should be aware of this (although a longer fuselage does also increase pitching torque).


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