Typically, for civil aircraft, the H-tail planform area is from one fifth to one fourth of the wing planform size. Figure 12.11 shows a cluster of H-tail designs with a tail volume coefficient of 0.7. As in wing design, the H-tail can have a sweep and a dihedral (a twist is not required). Sweeping of the H-tail would effectively increase the tail arm LHt, which is an important consideration when sizing the H-tail. For a T-tail configuration, the tail arm further increases.
6.6.1 Vertical Tail
Typically, for civil aircraft, the V-tail planform area is about 12 to 20% of the wing reference area. For propeller-driven aircraft, the V-tail could be kept slightly skewed (less than 1 deg) to offset a swirled-slipstream effect and gyroscopic torque of rotating engines and propellers. The V-tail design is critical to takeoff – especially in tackling yawed ground speed resulting from a crosswind and/or asymmetric power of a multiengine aircraft. A large V-tail can cause snaking of the flight path at low speed, which can be resolved easily by introducing a “yaw-damper” (a matter of aircraft control analysis). At cruise, a relatively large V-tail is not a major concern.
From the statistics given in Figure 12.11, it can be seen that there is a cluster of V-tail designs with a tail volume coefficient of 0.07. For the T-tail configuration, the tail volume coefficient could be reduced to 0.06 because the T-tail acts as an endplate at the tip of the V-tail. As in wing design, the V-tail can have a sweep, but the dihedral and anhedral angles and the twist are meaningless because the V-tail needs to be symmetric about the fuselage centerline. Sweeping of the V-tail would effectively increase the tail arm LVT, an important dimension in sizing the V-tail. It is important to ensure that the V-tail, especially the rudder, is not shielded by the H-tail to retain effectiveness, especially during spin recovery. With a T-tail, there is no shielding.
The empennage design has considerable similarity to the wing design. Section 4.9 describes various types of empennage; here, only the conventional design with an H-tail and a V-tail are considered. Following is a stepwise approach to empennage design:
Step1: Decide the aerofoil section.
In general, the V-tail aerofoil section is symmetrical but the H-tail has an inverted section with some (negative) camber. The t/c ratio of the empennage is close to the wing-aerofoil considerations. A compromise is selected based on the aircraft design Mach number and the wing sweep chosen.
Step 2: Establish the H-tail and V-tail reference areas.
Initially, during the conceptual study, the H-tail and V-tail reference areas are established from the statistical data of the tail volume coefficients (see Section 12.5). The positions of the H-tail and V-tail relative to the fuselage and the wing are decided by considering the aerodynamic, stability, control, and structural considerations.
Step 3: Establish the empennage aspect ratio, sweep, taper ratio, and dihedral. The empennage planform is generally but not restricted to a trapezoidal shape. A strake-like surface could be extended to serve the same aerodynamic gains as for the wing. The choices for the empennage aspect ratio, wing sweep, and taper ratio are interlinked and follow the same approach as for the wing design. The empennage aspect ratio is considerably lower than that of the wing. All these parameters are decided from stability considerations and eventually fine – tuned through CFD analysis and wind-tunnel testing, with the hope that flight-test results will not require further tweaking.
Step 4: Establish the control surfaces.
Initially, the control areas and dimensions of the elevator and the fin are earmarked from statistics and semi-empirical data. At this stage of study, the control surfaces can be postponed until more details are available to accurately size the control areas. In this book, the control surfaces are not sized. Subsequently, in the next design phase, when the finalized aircraft geometry is available, the empennage dimensions are established by formal stability analysis. A worked-out example follows in the next section.