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Within the POH of my aircraft, in a part titled "Control Surfaces Travel Limits", the manufacturer states:

Stabilator: Up 18° --- Down 3°

Why stabilator has a lower travel limit for down movements?

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    $\begingroup$ Maybe because you'll never have to do a landing flare upside-down? $\endgroup$ Feb 17 at 13:59
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    $\begingroup$ what is the aircraft model? $\endgroup$ Feb 17 at 17:37

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Most aircraft can handle higher positive G loading than negative, which allows more "up" stabilator than down.

Also, most aircraft center of gravity limits are such that the stabilator has positive trim (for static stability).

The third reason is it is usually not a good idea to steer aggressively towards terrain. The aircraft natural longitudinal stability (tail feathers) will help pitch down if airspeed slows to a point where insufficient lift is created.

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    $\begingroup$ +1 for the phrase "steer aggressively towards terrain". $\endgroup$ Feb 17 at 18:19
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    $\begingroup$ Not only can aircraft handle more +g's than -g's, the same is true for humans. Fighter pilots who can take +9g's with a g-suit, training, physique, and technique will red out at -2 to -3g's. $\endgroup$ Feb 18 at 5:37
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Stabilator angle determines angle-of-attack of the main wing, since it trims the balance of forces in pitch to set the wing AOA that the static stability forces of the "system" (the wing and tailplane opposing moment interaction) will seek.

The low AOA trim point (high speed) will be with the stabilator chord line somewhere closer to parallel with the main wing chord line, that is, minimal decalage. The high AOA trim point (low speed) will be with a significant decalage (to generate the maximum trimming down force to hold the nose up).

So you have a range of motion of the stabilator surface, 21ᵒ in this case. Leading edge all the way up is maximum speed, all the way down is minimum speed. Some point in that range needs to be set as a datum for rigging purposes.

The datum is normally the airplane's horizontal axis or close to it (typically the upper fuselage longeron), and this will be referenced as zero degrees for rigging purposes. The actual range of movement above and below this is determined by the aerodynamic requirements.

In your particular case, with the stabilator set parallel to the longitudinal axis, designated as "zero degrees" as a rigging reference, the range of motion for maneuvering and trim is leading edge 18ᵒ below that datum for minimum speed, and 3ᵒ above that datum for maximum speed.

So basically, you have that odd asymmetrical up-down range, because the neutral point datum is offset to one side because that's where the rigging neutral is set for convenience.

enter image description here

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The purpose of these design limits is to prevent the pilot from inadvertently overcontrolling and overstressing the aircraft during normal maneuvers. The quote is from a FAA document

The limits are designed in the process of getting the type certificate. Type certificates referr to different categories. A plane in the Acrobatic category will have different (larger G-forces) limits as compared to a Normal category plane.

My guess is that the manufacturer has designed the plane for low negative G forces but higher positive, and hence the difference in control authority in the pitch axis. Always remember that the design process has to make compromises, say between strenght of aircraft parts vs weight or cost of the parts.

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