AIRCRAFT TAIL DESIGN

The empennage or tail assembly provides stability and control for the aircraft. The empennage is composed of two main parts: the vertical stabilizer (fin) to which the rudder is attached; and the horizontal stabilizer to which the elevators are attached.

The major difference between the tail and wing is that, the wing is designed to carry substantial amount of lift ,and tail is designed for provide stability and control to the aircraft.

Tail assembly generally having low aspect ratio than wing to delay the stall at tail and which make aircraft under control while after wing stall.

Tail assembly (specially horizontal stabilizer) should always placed above or below the plane of wing to avoid the effect of downwash on tail.  high tail assembly is most favorable .

TYPES OF TAIL CONFIGURATION

There are many different forms an aircraft tail can take in meeting these dual requirements of stability and control, that are stated below :-

1)  Conventional Tail 

a)   The conventional tail design is the most common form.

b)   It has one vertical stabilizer placed at the tapered tail section of the  fuselage and one horizontal stabilizer divided into two parts, one on each side of the vertical stabilizer .

c)  For many airplanes, the conventional arrangement provides adequate stability and control with the lowest structural weight.

2) T- TAIL

a) T-tail is inherently heavier than a conventional tail because the vertical tail must be strengthened to support the horizontal tail.

b)   due to end plate effect, the T-tail allow smaller vertical tail. The T-tail lifts the horizontal tail clear of the wing wake (downwash) and propwash, which make it more efficient and hence allow reducing its size and also allows high performance aerodynamics and excellent glide ratio as the horizontal tail empennage is less affected by wing slipstream. This also reduce the buffet on the horizontal tail, which reduce fatigue for both the structure and the pilot.

c) The disadvantages of this arrangement include higher vertical fin loads, potential flutter difficulties, and problems associated with deep-stall.

Here question may arise that why T-tail prone to suffer dangerous deep stall condition?

,the region is, At the stall, lift is significantly reduced, drag is significantly increased and the airflow across, and behind, the wing becomes turbulent. , this turbulent air in the wake of a stalled mainplane can affect the horizontal stabilizer, substantially reducing the effectiveness of the elevators and potentially negating the ability to recover from the stall by using pitch controls to reduce the mainplane angle of attack.

3) CRUCIFORM TAIL

a) cruciform tail is compromise between the T-tail and conventional tail arrangement.

b) The cruciform tail gives the benefit of clearing the aerodynamics of the tail away from the wake of the engine and wing’s wake, while not requiring the same amount of strengthening of the vertical tail section in comparison with a T-tail design.

4) H-TAIL OR TWIN TAIL

 

a)   H-tails use the vertical surfaces as endplates for the horizontal tail, increasing its effective aspect ratio.

b)   H-tail is heavier than conventional tail but its end plate effect allow a smaller horizontal tail.

c)    On multi-engine propeller designs H-tails are sometimes used to reduce the yawing moment associated with propeller slipstream impingment on the vertical tail. .

d)   A special case of H- tail is twin boom tail or double tail where the aft airframe consists of two separate fuselages, “tail booms”, which each have a rudder but are usually connected by a single horizontal stabilizer.

e)   Disadvantages of H-tail includes complex control linkages and reduced ground clearance.

5) V- TAIL

a)   V-tails combine functions of horizontal and vertical tails. They are sometimes chosen because of their increased ground clearance, reduced number of surface intersections.

b)    the V-tail is lighter, has less wetted surface area.

c)    Sometimes called ruddervators, combine the tasks of the elevators and rudder.

d)   V- Tail offer reduced interference drag but at some penalty in control actuation complexity ,as the rudder and elevator control inputs must be blended in a mixer to provide the proper movement of V-Tail.

6) Y –TAIL

 

a)   Y tail is similar to V-tail, except that the dihedral angle is reduced and a third surface is mounted vertically beneath the V. this third surface contains the rudder whereas the V surface only provide pitch control.

b)   This tail arrangement reduce the complexity of ruddervators while reducing interference drag when compared to a conventional tail.

TAIL SURFACE SIZING

Horizontal tail

The Neutral Point location x_np is primarily controlled by size of the horizontal tail and its moment .

arm from the CG. A measure of this tail effectiveness is the horizontal tail volume coefficient:

V_h =(S_t*L_t /S_w*mac)

A well-behaved aircraft typically has a Vh which falls in the following range:

0.30………0.60

If Vh is too small, the aircraft’s pitch behavior will be very sensitive to the CG location. It will also show poor tendency to resist gusts or other upsets, and generally “wander” in pitch attitude, making precise pitch control difficult.

Vertical fin

The primary role of the vertical tail is to provide yaw damping, which is the tendency of yaw oscillations of the aircraft to subside. The vertical tail also provides yaw stability, although this will be almost certainly ensured if the yaw damping is sufficient. One measure of the vertical tail’s effectiveness is the vertical tail volume coefficient:

V_F=(S_F*L_F/b_*S_W)

Most well-behaved aircraft typically have a Vf which falls in the following range:

0.02………..0.05

If Vf is too small, the aircraft will tend to oscillate or “wallow” in yaw as the pilot gives rudder or aileron inputs . and   also give poor rudder roll authority in an aircraft which uses only the rudder to turn.