Empennage Group (Civil Aircraft)
Geometrical definitions and sizing of the empennage group (i. e., H-tail and V-tail) are provided in Section 3.22. These are the lifting surfaces that stabilize and control an aircraft. Because they are lifting surfaces, they follow the same rule of wing shaping. Chart 4.2 systematically tabulates the types of empennage configuration options available. Other types of empennage configuration are possible. An aircraft with pitch stability and control surface in the front is known as the canard configuration. The canard surface can share some lift (in civil aircraft designs) with the wing. The H-tail can have either a dihedral or anhedral angle. The twin V-tail can be straight or inclined either way.
Most civil aircraft designs have two surfaces almost orthogonal to each other like the V-tail and H-tail. The V-tail is always symmetrical to the aircraft centerline (there are exceptions). The H-tail can have either a dihedral or anhedral angle. The H-tail can be positioned low at the fuselage (with dihedral), at the top as a tail (with anhedral), or anywhere in between as a midtail configuration (Figure 4.24). Any combination of the scheme is feasible, but it ultimately is decided from the
(Figures 4.29 and 4.30)
(the H-tail can be given dihedral or anhedral. Twin V-tail can be straight or inclined either way)
(d) Twin V-tail on twin boom (e) Twin V-tail on fuselage (f) Three V-tail (Constellation)
Figure 4.25. Other types of civil aircraft empennage design options
various aerodynamic, stability, and control considerations, which are discussed in Chapters 3,4,6, and 11.
The civil aircraft empennage layout is relatively simpler; however, there are also unconventional types. Some interesting empennage arrangements are shown in Figure 4.30. The Beechcraft Bonanza 35 has a V-shaped tail (Figure 4.25a); in some designs, it can be inverted to a V-tail. One of the early Lear designs had a Y-shaped empennage (Figure 4.25b) – that is, a V-tail with a vertical fin extending below the fuselage. In the past, a circular-duct empennage has appeared (Figure 4.25c). The merits of the unconventional empennage are its applicability, but most designs have horizontal and vertical surfaces. V-tail, Y-tail, and circular-tail designs are more complex but follow the same routine as conventional designs – that is, resolving the forces on the surface into vertical and horizontal directions. This book discusses only the conventional designs.
If the V-tail size is large due to a short tail arm, the area could be split into two or three V-tails (Figure 4.25d, e, and f) from the structural and aerodynamic considerations. The twin V-tail can be straight or inclined either way.
The H-tail position relative to the V-tail is a significant consideration; the options available are shown in Figure 4.26. It can be from the lowest position through the fuselage to the other extreme, on the top as a T-tail. Any position in between is considered the midtail position.
Designers must ensure that the H-tail does not shield the V-tail. The wake (i. e., dashed lines in Figure 4.26) from the H-tail should not cover more than 50% of the V-tail surface and should also have more than 50% of the rudder area free from its wake to maintain control effectiveness, especially during spin and stall recoveries. Shifting the V-tail aft with the rudder extending below the fuselage will bring the fin and rudder adequately outside the wake. A dorsal and ventral fin can bring out more fin surface outside the wake, but the rudder must be larger to retain effectiveness. Lowering the H-tail would move the wake aft; however, if it is too low, it may hit the ground at rotation – especially if the aircraft experienced a sudden bank due to wind
gusts. Chapter 6 discusses the H-tail position relative to the wing; that is, at a high angle of attack, the wing wake should avoid the H-tail in the near-stall condition so that the pitch control remains adequate.
Care must be taken so that the H-tail is not within the entrainment effects of the jet exhaust situated at the aft end, which is typical for aft-mounted or within – fuselage jet engines. A military aircraft engine is inside the fuselage, which may require a pen-nib-type extension to shield the jet-efflux effect on the H-tail. In that case, the H-tail is moved up to either the midlevel or the T-tail.