Engineering the Tail: Structure and Function of the Empennage

Introduction

In most aircraft, the sole function of the tail unit is to provide the required stability and control. Stability is the tendency of the aircraft to return to its original attitude by itself.

Since an aircraft flies in three-dimensional space, stability and control are required in three directions. These axes are lateral (left and right), vertical (up and down) and longitudinal (fore and aft). For aircraft turns, three manoeuvre cases are used.

For pitch, which is rotation about the lateral axis, the horizontal tail with elevators is used. 

For yaw, which is rotation about the vertical axis, the vertical fin with rudder is used. 

For roll, which is rotation about the longitudinal axis, the ailerons are used.

The fin provides stability in yaw. When the aircraft is required to yaw, the rudder is deflected. The tailplane provides stability in pitch. When the aircraft is required to climb or descend, the elevators are deflected.

If the position of the centre of gravity varies, or if the aircraft speed is changed, the elevator position necessary to maintain level flight will change. Therefore a small extra control surface is added to each main surface to allow the pilot to trim the aircraft.

This tends to be very similar in many respects to that of the wing structure, though is generally of a lower aspect ratio and therefore subjected to smaller bending moments. 

Several designs are possible for both the vertical and horizontal tails, including:

• A single spar layout with spar caps taking the bending loads, very popular for light aircraft.
• A single spar with a root pivot operating either a full slab tail or a taileron, used on many high-speed (supersonic) fighters.
• A standard two spar construction.
• A multi-spar layout, especially on thin section tails.
• Full-depth honeycomb construction.

There are two main categories used for transport aircraft. The smaller ones generally use a standard built-up or integral layout but with simplifications due to the lack of any stringers

Smaller Aircraft

Larger Aircraft

Larger transports are even more conventional and have a classical configuration comprising spars, skin, ribs and stringers.

Fighter planes

Many fighters use full depth honeycomb (FDH) construction  due to the required section thinness.

Vertical stabilizer (fin)

Regarding the vertical stabilizer (fin), it is common practice to combine the fin integrally with the rear fuselage section. Using this method, the fin spars are extended to form the rear fuselage frames or bulkheads. The fin “root rib” is made to coincide with the upper surface of the rear fuselage. The rear fuselage bulkhead is often inclined to continue the line of the rear spar. The fin root shear and bending loads are all transmitted into the fuselage skin via the root rib.

For smaller and lighter aircraft, the fin is often designed as a completely separate detachable component.

For transport aircraft the layout is usually similar to that of the conventional wing or horizontal tail.

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