Considerations in Configuring the Empennage
Following are general considerations important for configuring the empennage (see also Section 6.6):
The descriptions and definitions of the empennage are in Sections 3.22 and 4.9. The dominant civil aircraft empennage consists of the H-tail and V-tail placed symmetrically about the fuselage axis. The H-tail could be positioned anywhere (see Figure 4.24), going through the aft fuselage to the tip of the V-tail forming a T-tail. Some aircraft have twin booms, where the empennage has the same function; the V-tail is split over two booms.
It is important that the V-tail remains effective for the full flight envelope. Shielding of the V-tail, especially the control areas, may prove to be dangerous. A designer must ensure that the V-tail keeps at least 50% of the rudder unshielded (see Figure 4.26) at a high angle of attack. (The canard configuration is not worked out in this book). Also, at a high angle of attack, the H-tail should not remain within the wing wake; otherwise, it must be enlarged to be effective.
If a FBW control system is incorporated, the empennage sizes can be reduced because the aircraft would be able to fly safely under relaxed stabilities. However, this book is not concerned with control laws as design input in an introductory course. The FBW concept is introduced in Chapter 12 but not analyzed. It will not be long until tailless aircraft such as the B2 bomber appear in civil aircraft designs, especially for BWB aircraft.