Configuring a Civil Aircraft Empennage: Positioning and Layout
The function of the empennage is to provide a force/moment for stability and control. The fuselage length, wing reference area (SV), and tail arms LHT and LVT are the main parameters governing the empennage size. Semi-empirical relations given in the definition of tail volume coefficient (see Section 12.5) provide the statistical empennage size required (see Figure 12.11).
The H-tail is placed as a T-tail on a swept-back V-tail that would provide an increased tail arm, LHT and LVT, which would save weight by not having a longer fuselage. Smaller aircraft would benefit from a T-tail; however, to support the T-tail load, the V-tail must be made stronger with a small increase in its weight. Care must be taken to ensure that the T-tail does not enter the wing wake at a high angle of attack. This can be achieved by positioning it high above the wing wake at near stall or having a larger H-tail and/or an all-moving H-tail acting as an elevator. (Earlier aircraft encountered these problems; in a deep stall, there was insufficient elevator power in the low-energy wing wake for the aircraft to recover in the pitch plane before crashing.)
Selection of the empennage aerofoil and planform follows the same logic as for the wing design. V-tail designs have symmetrical aerofoil sections. The H-tail camber is influenced by the aircraft’s CG position. In general, negative camber is used to counter a nose-down moment of the wing. H-tail and V-tail designs are discussed separately in the following subsections. The current design tendency indicates a little higher tail volume coefficient as compared to the historical design trend (see Figure 12.11).