Dynamics of Aircraft Flight

2Aerodynamic Forces and Moments and their Derivatives

The components of aerodynamic forces and moments are written as dimensionless coefficients called aerodynamic coefficients.

2.1. Definitions

The aerodynamic pressure is defined by the following equation:

with

• – P: the density of the air undisturbed by the aircraft;
• – V: the relative aircraft speed with regard to the air speed, air undisturbed by the aircraft;
• – S_ref: the reference surface, generally the surface of the maximum cross-section;
• – L_ref: the reference length, generally the diameter of the body of the aircraft.

The aircraft’s drag force (R_x) is positive when directed from the center of gravity to the rear of the aircraft.

The aircraft’s lateral lift force (R_y) is positive when directed to the right.

The vertical lift force of the aircraft (R_z) is positive when it is directed along the descending vertical.

The torque (GM_x) is positive when it tends to give positive roll, i.e. this torque turns the aircraft to the right (if we look at the base of the aircraft).

The torque (GM_y) is positive when it tends to give nose-up pitch torque (positive nose-up with rising aircraft).

The torque (GM_z) is positive when it tends to turn the aircraft to the right in the yaw plane (positive yaw to the right of the aircraft).

2.2. Aerodynamic forces

2.2.1. Drag expression

The drag force is positive when it is directed from the ...