What are the advantages of a Trimmable Horizontal Stabilizer?

Question

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?

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

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

Answer 1

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.

Answer 2

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.

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.

Answer 3

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.

Answer 4

Some EASA questions for ATPL mention the fact that stabilizer trim is less sensitive to flutter when compared to elevator trim

Answer 5

There are two basic advantages to a trimmable stabilizer:

• greater trim range than other methods.
• less drag than other methods.

These advantages are most significant in large airliners, supersonic planes and those rare tailless wings with washout which "put the tail at the ends of the wings". Other answers expand on some of these applications.

The downside is increased structural weight, as the structure supporting the whole stabilizer is very small yet must be strong and rigid in two axes while pivoting about the third. So airliners that can live without it are usually happy to do so.