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Aircraft 57
forward
axes for description of
aircraft geometry
(arbitrary reference)
WL
BL
SL
port
starboard
Figure 7-1. Aircraft geometry.
x
aircraft coordinate
system
(origin at CG)
right), and waterline (WL, positive up), measured relative to some arbitrary origin (fig. 7-1). The x-y-z
axes are parallel to the SL-BL-WL directions. One or more locations are defined for each component
of the aircraft. Each component will at least have a location that is the point where component forces
and moments act on the aircraft. Each location is input in fixed or scaled form. The fixed-form input
is SL/BL/WL (dimensional). The scaled-form input is x/L (positive aft), y/L (positive right), and
z/L (positive up), based on a reference length L, from a reference point. The reference length is the
rotor radius or wing span of a designated component, or the fuselage length. The reference point can
optionally be input, or the location (hub) of a designated rotor, or the location (center of action) of a
designated wing component, or the location (center of action) of the fuselage, or the location of the
center of gravity. Fixed input can be used for the entire aircraft, or just for certain components.
From this fixed or scaled input and the current aircraft size, the actual geometry (x, y, z) can be
calculated for each location. There are also options to calculate geometry from other parameters (such
as tiltrotor span from rotor radius and clearance). This calculated geometry has the sign convention of
the aircraft axes (x positive forward, y positive right, z positive down), but has the origin at the reference
point (which may or may not be the center of gravity). All input uses the same sign convention; all
internal calculations use the same sign conventions. Table 7-1 summarizes the conventions.
Table 7-1. Geometry conventions.
layout scaled input calculated motion and loads
origin arbitrary reference point reference point center of gravity
x SL (+ aft) x/L (+ aft) x (+ forward) x (+ forward)
y BL (+ right) y/L (+ right) y (+ right) y (+ right)
z WL (+ up) z/L (+ up) z (+ down) z (+ down)
z
y
aft