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Chapter 14
Empennage
The aircraft can have one or more tail surfaces, or no tail surface. Each tail is designated as
horizontal or vertical, affecting some parameter definitions.
14–1 Geometry
The tail is described by planform area S, span b, chord c = S/b, and aspect ratio AR = b 2 /S. The
tail volume can be referenced to rotor radius and disk area, V = Sℓ/RA; to wing area and chord for
horizontal tails, V = Sℓ/Swcw; or to wing area and span for vertical tails, V = Sℓ/Swbw. Here the tail
length is ℓ = |xht − xcg| or ℓ = |xvt − xcg| for horizontal tail or vertical tail, respectively. The geometry
is specified in terms of S or V ; and b, orAR, orc. The elevator or rudder is described by the ratio of
control-surface chord to tail chord, cf /c; and the ratio of control-surface span to tail span, bf /b.
The tail contribution to the aircraft operating length is xtail +0.25c (forward), xtail − 0.75c (aft), and
ytail ± (b/2)C (lateral)- where C = cos φ for a horizontal tail and C = cos(φ − 90) for a vertical tail.
14–2 Control and Loads
The tail is at position z F , where the aerodynamic forces act. The scaled input for tail position can
be referenced to the fuselage length or the rotor radius.
The horizontal tail can have a cant angle φ (positive tilt to left, becomes vertical tail for φ =90deg).
Thus the component axes are given by C BF = X−φ. The control variables are elevator δe and incidence
i.
The convention for nominal orientation of the vertical tail is positive lift to the left, so aircraft
sideslip (positive to right) generates positive tail angle-of-attack and positive tail lift. The vertical tail
can have a cant angle φ (positive tilt to right, becomes horizontal tail for φ =90), so the component axes
are given by C BF = X−90+φ. The control variables are rudder δr and incidence i.
14–3 Aerodynamics
The aerodynamic velocity of the tail relative to the air, including interference, is calculated in
component axes, v B . The angle-of-attack αtail (hence C BA ) and dynamic pressure q are calculated from
v B . The reference area for the tail aerodynamic coefficients is the planform area, S. The wetted area
contribution is Swet =2S. From the elevator or rudder deflection and geometry, the increments in lift
coefficient, maximum-lift angle, and drag coefficient are evaluated: ΔCLf, Δαmaxf, ΔCDf.