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What are the advantages of a Trimmable Horizontal Stabilizer (THS)? For example, a THS is used on some Airbus and Embraer aircraft. What is the most important advantage, and why don't some other successful airliners have this type of horizontal stabilizer?

ERJ-170 THS

Image used under CC BY-SA 3.0 (Source). Depicting the right side THS on an ERJ-170


Note: A Trimmable Horizontal Stabilizer (THS) differs both from a Stabilator and a horizontal stabilizer with a trimmable Elevator

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    $\begingroup$ I can't think of any airliner that doesn't have a trimmable horizontal stabilizer. Most aircraft in general have it, right down to small GA planes. The method for trimming may be different, but it's still there. Are you asking about this particular style of trim, where the entire stabilizer is moved instead of just the control surface or a trim tab? $\endgroup$ – Ron Beyer Feb 5 '16 at 2:45
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    $\begingroup$ Yes. The entire horizontal stabilizer is moved. And with regards to what you said about the airliners that doesn't have a THS , the dc-7 was a very succesful airliner but with fixed stabilizers, the Lockheed L-1049 Super Constellation also had fixed stabilizers. So... $\endgroup$ – kepler22b Feb 5 '16 at 2:52
  • $\begingroup$ The trim tabs can be seen in this picture for the DC7... Another view $\endgroup$ – Ron Beyer Feb 5 '16 at 3:02
  • $\begingroup$ L 1049 trim tabs visible in this picture $\endgroup$ – Ron Beyer Feb 5 '16 at 3:07
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    $\begingroup$ @RonBeyer: kepler22b is right - the rim tabs are for zeroing stick forces, which is not needed in hydraulically operated elevators. Both the L-1049 and the DC-7 had a fixed stabilizer, and that is what the question is about. $\endgroup$ – Peter Kämpf Feb 5 '16 at 8:37
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The main advantage is smaller elevator deflection angles. This comes handy in two cases:

  • When high-lift devices are deployed, the center of pressure on the wing shifts backwards by up to a third of wing chord. Fowler flaps add wing area aft of the trailing edge, and slotted flaps are able to generate high suction peaks. The result is a massive change in trim, and the empennage now has to generate generous downforce. Changing the lift on the empennage by elevator deflection alone will exceed the maximum practicable deflection angle and leave no margin for control. By adjusting the stabilizer incidence, the elevator can be held near its neutral position and has reserves for control.
  • In transsonic flight the elevator might not always have a linear characteristic. The contour break due to an elevator deflection induces shocks which in turn lead to flow separation which reduces the control effectivity and can even reverse the control characteristic. Since the transition from subsonic to supersonic flight shifts the center of lift backwards, the empennage needs to add downforce when the aircraft accelerates in the transsonic speed range. An elevator deflection might not be able to produce the desired lift change, and only adjusting the stabilizer such that the elevator can be held neutral can restore trim and control effectiveness.

Older airliners from the propeller age had lower wing loadings and less powerful flaps. The center of lift on the wing changed less with flaps, so a fixed stabilizer was sufficient. But once the wing loading goes up to jet levels, and the wing is fitted with slotted fowler flaps, a moveable stabilizer is unavoidable.

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    $\begingroup$ Now remains the question why keep the horizontal stabilizer with separate elevator. Supersonic aircraft generally don't and move the whole surface, but all airliners keep it split. $\endgroup$ – Jan Hudec Feb 5 '16 at 9:47
  • $\begingroup$ @JanHudec the answer is "actuator dynamic range" (or bandwidth, if you prefer) $\endgroup$ – Federico Feb 5 '16 at 13:21
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    $\begingroup$ @JanHudec: Adding some camber by deflecting the elevator makes the stabilizer-elevator combination more effective. The whole tail surface can be made a bit smaller without compromising control power. $\endgroup$ – Peter Kämpf Feb 5 '16 at 19:48
  • $\begingroup$ @JanHudec -- the TriStar used a full-blown stabilator -- its just that the THS is much more well-entrenched in the halls at A, B, that other B, and E ;) $\endgroup$ – UnrecognizedFallingObject Feb 6 '16 at 1:00
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Trimmable horizontal stabilizers (THS) are found in majority of airliners and large transport aircraft (like C-17 globemaster III, for example). They are usually part of the trimming system of the aircraft, unlike the elevators, which are controlled by conventional pilot inputs (like yoke etc.). The THS offers some important advantages like:

  • The required elevator deflection angles are smaller in case of trimmed aircraft and the system has full elevator deflection angles at extreme trim angles.

  • If the stabilizer is not trimmed, the (human or auto)pilot has to continuously adjust the controls to prevent the aircraft from pitching up or down more than required.

  • Aligning the elevator with the stabilizer reduces drag.

  • It allows for a wider range of c.g. movement compared to the elevator-trim tab system.

The decision to use a THS (or not) depends on the design. For example, the BAe 146 had a fixed tail plane in order to reduce complexity (achieved in part by the elimination of leading edge slats). The THS does add more complexity to the system and has been involved in a few accidents, like the Alaska Airlines Flight 261 and China Airlines Flight 140.

The stabilator or all moving tailplane is entirely different (with no elevators) and is mostly used in supersonic aircraft. It is used mainly to overcome the problem where elevator becomes unusable due to shockwaves produced by tailplane and the problem of Mach tuck. Combat aircraft to use them as they create a large pitching moment for lesser control effort. Also, they are used for roll control via differential movement.

In civil aircraft, Lockheed L-1011 Tristar used stabilators.

L-1011 stabilator

Image from tristar500.net

It is also used in some GA aircraft (like Piper Cherokee); the strong control response of the stabillator can result in overcorrection- this is overcome in part by using an anti-servo tab.

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    $\begingroup$ It is difficult to tell in that image, but it looks to me like the stabilator has more curvature in the lower (deflected) image than it does in the upper image. Is that true, or is it just an effect of the viewing angle? $\endgroup$ – FreeMan Feb 5 '16 at 16:56
  • $\begingroup$ @FreeMan, the difference in curvature is real and is due to the deflection of a mechanically-linked segment of the stabilator. $\endgroup$ – Porcupine911 Feb 5 '16 at 17:55
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    $\begingroup$ Yay me! :) Thanks, @Porcupine911 $\endgroup$ – FreeMan Feb 5 '16 at 18:35
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There are 3 reasons for the existence of a THS.

  1. The large speed range of jet airplanes.
  2. The large trim changes with changes in the wing configuration, slats flaps etc.
  3. The large range of CENTER of GRAVITY that is possible.
  4. to reduce the drag produced by the tailplane especially during cruise, thus improving range significantly.
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    $\begingroup$ This does not really seem to add anything to the other answers already posted. Please consider elaborating and adding relevant sources to your answer so that it can actually function an an independant answer to the original question. $\endgroup$ – J. Hougaard Aug 27 '16 at 5:22
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Some EASA questions for ATPL mention the fact that stabilizer trim is less sensitive to flutter when compared to elevator trim

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