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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
    Jan 24 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
    Jan 25 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
    Jan 25 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
    Jan 25 at 17:44
  • $\begingroup$ The larger nose-down range is for operations with flaps extended. The clean configuration will always operate near ground set. $\endgroup$ Jun 24 at 19:52
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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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The root cause is the nature and theory of aerofoils. The design mission is to use the wing in an optimal way and finally the wing + body combination in the most optimal way to achieve the design goals. Most of the design goals answer to the following questions:

  • do you want to go fast (speed), or
  • do you want to lift more weight (payload),

For a given amount of fuel

  • do you want to go as far as possible (range), or
  • do you want to stay in the air for the longest period (endurance), OR, combinations such as
  • do you want to carry a specific weight a specific distance?

There will be an optimal aerofoil for each of these, and new designs can be attempted if there isn't one that fits the requirement or it is felt that there's room for improvement.

There are some aerofoil properties that tend to be common for the bulk of the commercial jet designs, as most of them are chasing similar goals:

  • generally best efficiency (in terms of L/D) is around 4˚ to 6˚ AoA. This angle is achieved by combining a level flight pitch attitude of 2˚ to 3˚ with the wing mounted at a 2˚ to 3˚ Angle of Incidence**.

  • Stalling AoA in clean configuration somewhere around 15˚+, and

  • most of these aerofoils even produce lift till 1 or 2 degrees below 0˚ AoA.

  • With high lift devices extended, the typical AoA could vary between 7˚ and 10˚depending on slat/flap configuration.

(** Angle of Incidence: wings are mounted with the leading edge higher than the trailing edge, so the angle between the average chord line and the longitudinal axis of the airplane is fixed and it is called the Angle of Incidence. Thus an airplane rolling down a runway already presents an AoA of a few degrees and inflight, we don't have to cruise at an uncomfortable pitch attitudes of 5 to 6˚ nose up )

The physics of aerofoils and aerodynamics, as mentioned above, dictate that, the airplanes are mostly flown at different nose high attitudes of upto 20˚ including the slat/flap extended regime of flight, and climb phase. Much lesser part of the flight is spent at negative pitch attitudes of 0 to -4˚ for descents. Thus the 'lopsided' stabilizer range.

The Stabiliser settings for take-off depend on the CG, GW (Gross Weight), Flap setting, and T/O Thrust for the flight so as to present a 'standard' stick force and control effectiveness for the pilot and it must be understood that though weight acts through the CG, the GW is actually acknowledging the Lift being created by the wing to lift that weight.

BTW, a symmetric aerofoil produces no pitching moment only at zero Angle of Attack. Mount it at a small angle of incidence and it will produce a lift component acting through CP. It's just that there are other aerofoil shapes that have fewer compromises affecting the typical design goals.

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  • $\begingroup$ the airplanes are mostly flown at different nose high attitudes of upto 20˚ including the slat/flap extended regime of flight, and climb phase. Much lesser part of the flight is spent at negative pitch attitudes of 0 to -4˚ for descents. Thus the 'lopsided' stabilizer range. This is complete nonsense. Please try again! $\endgroup$ Jun 24 at 22:17
  • $\begingroup$ I admit, the wording is putrid, I shall change it shortly. The point was simply that the max nose down trim requirement is to maintain 4 deg nose down compared to nose up requirements as high as 20 deg nose up. Not that there's a linear relationship, either. The highest N Up seting that I notice is for the trim bias setting during auto land. $\endgroup$
    – skipper44
    Jun 25 at 0:36
  • $\begingroup$ The 20° nose up is during climb when the whole airplane is flying on an inclined flight path. Following you logic, a vertically climbing F-15 would need -90° stab trim which clearly is not true. $\endgroup$ Jun 25 at 18:31
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I guess the altitude of the airport (density and temperature) will have great influence on the trim down range more also the CG.

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  • $\begingroup$ Really? Guess again. $\endgroup$
    – Koyovis
    Jul 28 at 23:58

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