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Probably the question will be a little bit confusing but I'll try my best to make it clear.

Due the position of the CP with respect of the AC, symmetrical airfoils don't produce pitching moments, but cambered airfoil having the CP behind the AC produce a nose down pitching moment.

If we consider now the CG effect, a CG forward the AC will cause a nose down pitching moment, an aft CG the opposite.

Most aircraft with variable incidence tailplane have the possibility to trim much more degrees for nose up than for nose down.

enter image description here
Airbus A330; source: airlinercafe.com

Is this mainly due to the position (and range) of the CG, delianeated during the design phase and normal flight operation?

Is the pitching moment of the wing (only), much less relevant?

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  • $\begingroup$ I don't think I've ever seen a moveable stab with a larger range above neutral than below it. Any examples? $\endgroup$
    – John K
    Commented Jan 24, 2021 at 22:50
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    $\begingroup$ @JohnK: I think the OP is trying to say that, starting from the neutral position, the stabiliser can go a lot further in the aircraft-nose-up (stabiliser-leading-edge-down) direction than in the aircraft-nose-down (stabiliser-leading-edge-up) direction. $\endgroup$
    – Vikki
    Commented Jan 25, 2021 at 0:02
  • $\begingroup$ Oops you're right I was careless reading it. I thought he was asking why the stab travel would have different travel ranges on some airplane vs others. $\endgroup$
    – John K
    Commented Jan 25, 2021 at 1:14
  • $\begingroup$ You do understand that there isn't a Nose Up/Nose Down "neutral position" per se right? It's just a range for low AOA/fast trim and high AOA/slow trim with the slow end having to account for the highest pitching moment worst case and the high end having to account for the lowest pitching moment worst case. NU/ND just describes the pitch tendency that is apparent to the pilot when a change is made. For the pilot, the trim control is best thought of as a "hands free" speed control dial with ND=fast(er) and NU = slow(er). $\endgroup$
    – John K
    Commented Jan 25, 2021 at 17:44
  • $\begingroup$ The larger nose-down range is for operations with flaps extended. The clean configuration will always operate near ground set. $\endgroup$
    – Peter Kämpf
    Commented Jun 24, 2021 at 19:52

1 Answer 1

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The wide negative trim range is for operation with lowered flaps.

Fowler flaps shift the center of lift backwards both by adding wing area at the trailing edge and by increasing camber. Both effects require more downforce on the tail to shift the sum of all lift forces such that it remains at the center of gravity.

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