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Definition of axes and angles
The aim of this topic is to explain the commonly used axes for aircraft simulation and aerodynamic analysis.
The aerodynamic forces and moments on an aircraft are produced by the relative motion with respect to the air and depend on the orientation of the aircraft with respect to the airflow. Two orientation angles (with respect to the relative wind
) are needed to specify the aerodynamic forces and moments. These angles are the angle of attack
and the sideslip angle
. The image below shows the definition of
with respect to the body-fixed coordinate system. The origin of the body-fixed coordinate system coincides with the aircraft's center of gravity and its
-axis is parallel to the fuselage reference line and its
-axis in the (conventional) aircraft plane of symmetry.
Body-fixed coordinate system and aerodynamic angles
The commonly adopted convention is to label rotations about the
. The angles are referred to as the roll angle, the pitch angle, and the the yaw angle. Rotation rates about the axes are labeled by
and are usually measured in radians per second. Transnational velocities along the
-axes are labelled by
The force and moments acting on the aircraft are commonly defined in terms of dimensionless aerodynamic coefficients. The coefficients are functions of the two aerodynamic angles, the Mach number, the Reynolds number, the control surface deflection, and the aircraft thrust. The thrust generated by the aircraft engines can affect the aerodynamic coefficients.
Other factors that affect the aerodynamic coefficient are geometry changes such as deployment of landing gears, addition of external fuel tanks, ground proximity effect, etc. The definition of the coefficients with respect to the body-fixed coordinate system is:
In the above equations
is the aircraft reference area,
is the aircraft reference span,
is the aircraft reference chord (mean aerodynamic chord), and
is the dynamic pressure (
The Aviumtechnologies Panel Method uses the above definition of the aerodynamic coefficients.