Showing content mapped to EASA Part-66. Change authority in the header.
The atmosphere
ISA (International Standard Atmosphere): sea level pressure 1013.25 hPa, temperature 15 °C, temperature lapse 1.98 °C per 1000 ft up to the tropopause (36 090 ft). Density decreases with altitude — this reduces lift and thrust.
Four forces
In steady level flight lift = weight and thrust = drag. Lift acts perpendicular to relative airflow, drag parallel to it. Weight acts through the C of G toward the earth's centre.
Lift equation
L = ½ · ρ · V² · S · CL, where ρ is air density, V is TAS, S is wing area and CL is the lift coefficient (depends on angle of attack and aerofoil shape). Doubling airspeed quadruples lift.
Angle of attack & stall
AoA is the angle between the chord line and the relative airflow. CL rises with AoA until the critical angle (~15° on most aerofoils), where airflow separates from the upper surface and lift collapses — the stall. A stall is caused by AoA, not airspeed.
Stability
Longitudinal stability (pitch) is provided by the horizontal stabiliser and CG position. Lateral (roll) stability comes from dihedral, sweepback and keel effect. Directional (yaw) stability comes from the vertical fin.