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The Beechcraft 1900, in addition to the usual wings and vertical and horizontal stabilizers, and the extra vertical tail surfaces added for improved directional stability, has a pair of horizontal tail surfaces, called "stabilons", on the sides of the rear fuselage (not to be confused with the 1900's actual horizontal stabilizers, which are mounted atop the vertical stabilizer in a T-tail configuration):

stabilons on two 1900Ds

(The stabilons of these two 1900Ds are circled in green to avoid confusion with the shadows of their horizontal stabilizers on the ground. Image by ERIC SALARD at Flickr, via russavia at Wikimedia Commons, cropped by Marc Lacoste at Wikimedia Commons, annotated by me.)

How, exactly, do the stabilons operate? Are they fixed structures? Do they move up and down to trim the aircraft? Do they move along with the elevators when the pilot commands a pitch input?

Not a dupe of this question or this question, neither of which discusses how the stabilons operate.

Vikki
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2 Answers2

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This Flight Safety Training Manual describes them as improving "deep stall" characteristics and C of G range.

The stabilons are fixed surfaces that add effective stabilizer area to improve the center of gravity range, and also, by their location, add that additional surface area in a low location that mitigates the tail blanking problem you get with T tails.

So they would have just increased the area of the existing stabilizer, but that didn't fix the stall issues which are inherent to the T tail. You could say that they added extra area as needed, and "de-T-ified" the configuration somewhat in the process by putting extra surfaces down low to provide a more positive pitch recovery than is provided by the existing T tail. A clever application of a band-aid solution that fixed two separate problems at the same time.

John K
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    For a deep stall the fuselage is too long - deep stall prone configurations have much less distance between wing and tail. But the low position of that additional tail surface is structurally advantageous, sitting at a point where its additional lift and mass can be carried by the existing structure. – Peter Kämpf Jan 01 '22 at 08:12
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    The "deep stall" bit comes from the Flight Safety training manual that I linked to as my source., and I assume the writer got it from the OEM's engineering organization. In any case made some edits. – John K Jan 01 '22 at 17:11
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    Thanks for pointing me to the manual! On the same page they say that the T-tail puts the tail in undisturbed air - so the stabilons are in disturbed air? Reads a bit like written by marketing, not engineers. Still, they sure help to make a deep stall impossible. – Peter Kämpf Jan 01 '22 at 22:43
  • Wait, I thought horizontal stabilizers on the aft end subtracted from lift, providing downforce. Forward canards provide lift (and aren't blanked by the wings). – Harper - Reinstate Monica Jan 02 '22 at 00:20
  • The stabilizer/elevator system is generally subtracting lift FOR TRIM; that is, it will provide a net downforce to oppose an overall nose down pitching moment, as part of a balance of force system that drives the entire thing to want to align itself into wind at a specific angle of attack. For static stability purposes, this basic vertical weathervaning tendency, lift can be in either direction, depending on whether the "weathervane", the airplane's aerodynamic footprint rotating about its C of G, is on one side or the other side of its target alignment at any given instant. – John K Jan 02 '22 at 02:09
  • @PeterKämpf: All T-tailled designs are susceptible to deep stall - ones with a longer wing-tail distance will be more susceptible, since they don't need as high of an angle of attack to blank the horizontal tail. As for the disturbed-air bit, in normal flight, the stabilons are in the downwash of the wing (which also decreases their effective AoA and increases the nose-up trim force they provide) while the T is in undisturbed air; in a highly-stalled condition, the T is blanked out by the wings' wake, but the stabilons are in undisturbed air and provide a restoring pitch-down moment. – Vikki Jan 02 '22 at 15:36
  • @Harper-ReinstateMonica: Usually yes, but horizontal tails do provide positive lift when trimmed for a CoM near the aft limit, especially at high airspeed (the range of stabilizer motion generally extends slightly into the stabilizer-nose-up zone when at near-maximum nose-down trim, and both aft CoM and high IAS tend to make a longitudinally-stable aircraft pitch up, requiring nose-down trim to counter). Negative tailplane lift is not required; the tailplane's wing loading merely needs to be less positive than that of the main wings. – Vikki Jan 02 '22 at 15:39
  • @Vikki Maybe you read this next. Having the tail in the wake is not enough - the pitching moment must have another equilibrium point, and that requires a much higher AoA than what a Beech 1900 is capable of. Also, both the lower and the upper tail are in the downwash of the wing. Please do not confuse wake with downwash. – Peter Kämpf Jan 02 '22 at 18:14
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They're fixed.

  1. The maintenance manual shows the rigging of the pitch/roll moving only the conventional elevators/ailerons.

  2. An Air Force Magazine issue—since it's also operated by the USAF under the designation C-12J—spells it out:

... auxiliary fixed horizontal tail surface (stabilon)...

For the general principle, see: How does an aircraft tailplane work? With the stabilon in the downwash of the main wing, this helps reduce the total lift per area of the combined tailplanes for more stability.

  • Is it in the downwash from the wing or upwash from the props? As these were made for cargo too (and also not podded jets), combined with the endcaps on the upper tail, the design indicates a need to avoid a stall due to rear CG placement. (but good references). – Robert DiGiovanni Jan 01 '22 at 01:51