Figure 3.46. Horizontal tail and vertical tail

3.Establish the wing sweep for the Mach number of operations.

4.Establish the wing span from the previous three steps. For commercial

transport aircraft, the wing span is currently restricted to a maximum of

80 m.

5.Establish the wing dihedral and anhedral angles; it is generally within 1 to 5 deg for the dihedral.

6.Establish the wing twist; it is usually within 1 to 2 deg (generally downwash).

At the conceptual stage, the twist, dihedral, and anhedral are taken from experience. Subsequently, CFD analyses can fine-tune all related parameters for the best compromise. Ultimately, wind-tunnel tests are required to substantiate the design.

3.22 Empennage

Typically, the empennage consists of horizontal and vertical tails for aircraft stability and control. Various types of empennage configurations are described in Chapter 4. The dominant type has a vertical tail (V-tail; U.K. terms are fin and rudder) in the plane of symmetry with a symmetrical aerofoil. A horizontal tail (H-tail; U.K. terms are stabilizer and elevator) is like a small wing at the tail (i.e., the aft end of the fuselage). The last two decades have seen the return of aerodynamic surfaces placed in front of the wing (see Figure 3.48); these are called canards and are discussed in subsequent chapters. This section addresses the definitions associated with the empennage and canard as well as the tail volume coefficients (see Chapter 12).

The V-tail of a single-engine, propeller-driven aircraft may have an offset of 1 to 2 deg to counter the effects of rotating propeller slipstream.

3.22.1 H-Tail

The H-tail consists of the stabilizer (fixed or moving) and the elevator (moving) for handling the pitch degree of freedom (Figure 3.46a). The H-tail can be positioned low through the fuselage, in the middle cutting through the V-tail, or at the top of the V-tail to form a T-tail (see Figure 3.33).

Military aircraft can have all moving H-tails with emergency splitting in case there is failure, and there are several choices for positioning it (see Chapter 4). Figure 3.46a shows the geometrical definition of conventional-type H-tail surfaces. Like the wing planform definition, the H-tail reference area, SH, is the planform area including the portion buried inside the fuselage or V-tail for a lowor mid-tail location, respectively. The T-tail position at the top has a fully exposed planform.

Figure 3.47. Geometric parameters for the tail volume coefficients

3.22.2 V-Tail

The V-tail consists of a fin (fixed) and a rudder (moving) to control the roll and yaw degrees of freedom (see Figure 3.46). The figure shows the geometrical definition of a conventional-type V-tail surface reference area, SV. The projected trapezoidal/rectangular area of the V-tail up to this line is considered the reference area, SV. Depending on the closure angle of the aft fuselage, the root end of the V-tail is fixed arbitrarily through a line drawn parallel to the fuselage centerline, passing through the point where the midchord of the V-tail intersects the line.

3.22.3 Tail Volume Coefficients

Tail volume coefficients are used to determine the empennage reference areas, SH and SV. The definition of the tail volume coefficients is derived from the aircraft stability equations provided herein. The CG position (see Chapter 8) is shown in Figure 3.47. The distances from the CG to the aerodynamic center at the MAC of the V-tail and H-tail (i.e., MACVT and MACHT) are designated LHT and LVT, respectively, as shown in Figure 3.47. The ac is taken at the quarter-chord of the MAC.

H-Tail Volume Coefficient, CHT

From the pitching-moment equation (see Chapter 12) for steady-state (i.e., equilibrium) level flight, the H-tail volume coefficient is given as the H-tail plane reference area:

where CHT is the H-tail volume coefficient, 0.5< CHT <1.2; a good value is 0.8. LHT is the H-tail arm = distance between the aircraft CG to the ac of MACHT. In general, the area ratio SHT/SW ≈ 0.25 to 0.35.

V-Tail Volume Coefficient, CVT

From the yawing-moment equation (see Chapter 12) for steady-state (i.e., equilibrium) level flight, the V-tail volume coefficient is given as the V-tail plane reference area:

Figure 3.48. Three-surface canard configuration (Piaggio P180 Avanti)

where CVT is the V-tail volume coefficient, 0.05< CVT <0.1; a good value is 0.07. LVT is the H-tail arm = distance between the aircraft CG to the ac of MACVT. In general, the area ratio SVT/SW ≈ 0.15 to 0.25.

Chapter 6 describes how to estimate the empennage areas; a number of design iterations are necessary. Figures 12.15 and 12.16 give statistical values of tail volume coefficients.

Canard Configuration

Canard is French for “goose,” which in flight stretches out its long neck with its bulbous head in front. When a horizontal surface is placed in front of the aircraft, it presents a similar configuration; hence, this surface is sometimes called a canard.

The Wright Brothers’ Flyer had a control surface at the front (with a destabilizing effect), which resulted in a sensitive control surface. Military aircraft use a canard to enhance pitch control. However, the use of a canard in civil aircraft applications serves a different purpose (Figure 3.48).

In general, the inherent nose-down moment (unless a reflex trailing edge is employed) of a wing requires a downward force by the H-tail to maintain level flight. This is known as trimming force, which contributes to trim drag. For an extreme CG shift (which can happen as fuel is consumed), high trim drag can exist in a large portion of the cruise sector. The incorporation of a canard surface can reduce trim drag as well as the H-tail area, SH. However, it adds to the manufacturing cost and, until recently, the benefit from the canard application in large transport aircraft has not been marketable.

Many small civil aircraft have a canard design (e.g., Rutan designs). A successful Bizjet design is the Piaggio P180 Avanti shown in Figure 3.48. It has achieved a very high speed for its class of aircraft through careful design considerations embracing not only superior aerodynamics but also the use of composite materials to reduce weight.