CPL Aerodynamics key terms

The angle between the wing chord line and the relative airflow approaching the wing. Increasing it raises lift up to the critical angle, after which the airflow separates and lift falls.

The direction of the air stream passing the aircraft as seen from the aircraft, combining the aircraft's forward motion with any wind. All aerodynamic angles are measured against this direction.

A straight reference line drawn from the leading edge to the trailing edge of an aerofoil section. Angles such as angle of attack and angle of incidence are measured from it.

The curvature of an aerofoil, measured as the maximum distance between the mean camber line and the chord line. More camber generally produces more lift at a given angle of attack.

The fixed angle between the wing chord line and the longitudinal axis of the aircraft, set by the designer. It is distinct from angle of attack, which varies in flight.

A dimensionless number describing how much lift an aerofoil generates for its size and the dynamic pressure of the airflow. It depends mainly on angle of attack and aerofoil shape.

The pressure created by air motion, equal to half the air density multiplied by the square of true airspeed. It is the pressure that actually generates lift and drag on the airframe.

The point on an aerofoil where the total aerodynamic force effectively acts. As angle of attack increases, it usually moves forward, creating a pitching moment about the centre of gravity.

Drag caused by the generation of lift, arising from the wingtip vortices and downwash that tilt the lift vector rearward. It increases with lift coefficient and decreases with speed.

Drag from form, skin friction and interference that does not contribute to lift. It rises with the square of airspeed, unlike induced drag, which falls as speed increases.

Air deflected downward behind a wing as it generates lift. It reduces the effective angle of attack and tilts the lift vector rearward, producing induced drag.

Rotating masses of air trailing from the wingtips where higher-pressure air beneath the wing spills around to the lower-pressure upper surface. They are strongest when heavy, slow and clean.

The thin layer of air next to a surface where velocity is slowed by friction. Its behaviour determines skin-friction drag and whether the airflow separates from the wing.

The point on an aerofoil where the airflow comes to rest and pressure reaches its maximum. Its position moves along the lower surface as angle of attack increases.

A relationship stating that where air flows faster its static pressure falls, provided energy is conserved. It explains part of the pressure difference producing lift over a wing.

The sum of induced drag and parasite drag. It is lowest at the speed where the two components are equal, which is the speed for maximum lift-to-drag ratio.

The ratio of lift produced to drag incurred at a given condition. Best glide performance occurs at the maximum value, often called best L over D.

The horizontal distance an aircraft travels for each unit of height lost in a power-off glide. It is numerically equal to the lift-to-drag ratio at the glide speed.

The airspeed giving the greatest horizontal distance per unit of altitude lost. It is normally close to the speed for maximum lift-to-drag ratio and is used for engine-failure glides.

The ratio of total lift to aircraft weight, expressed in g. It increases in turns and manoeuvres, raising stall speed and structural loading on the airframe.

The maximum speed at which full deflection of a single flight control can be applied without exceeding the design load factor. Above it, controls must be used gently to avoid structural damage.

The maximum airspeed at which the aircraft may be operated in any condition. Exceeding it risks structural failure or flutter and is prohibited.

Indicated airspeed corrected for position and compressibility error. It reflects the dynamic pressure acting on the airframe and relates directly to lift and structural loads.

Indicated airspeed corrected for position and instrument installation error. It is the airspeed used when referring to published performance figures for an aircraft type.

The speed shown on the airspeed indicator, derived from the pressure difference between the pitot and static sources. It is uncorrected for position error, density or compressibility.

The point through which the whole weight of the aircraft acts. Its position relative to the aerodynamic centre determines longitudinal stability and control effectiveness.

The distance between the centre of gravity and the aerodynamic centre, usually expressed as a fraction of mean chord. A larger positive margin gives stronger, more stable pitch behaviour.

The tendency of an aircraft to return to its trimmed pitch attitude after a disturbance. It depends on the relationship between the centre of gravity and the aerodynamic centre.

The tendency of an aircraft to resist rolling disturbances and return to level flight. It is provided mainly by wing dihedral, sweepback and high-wing geometry.

The tendency of an aircraft to point into the relative airflow after a yaw disturbance. It is provided mainly by the vertical fin and rear fuselage area.

The rolling moment produced by sideslip, caused mainly by wing dihedral angle. It gives lateral stability but can cause Dutch roll when coupled with strong directional stability.

A coupled oscillation in yaw and roll, usually with the roll lagging the yaw. It occurs when directional stability is strong relative to lateral stability, and is damped by yaw dampers.

The tendency of an aircraft to yaw away from the direction of a roll, caused by increased induced drag on the down-going aileron wing. It is countered with coordinated rudder input.

The angle of attack at which the airflow separates from the wing and lift reaches its maximum. Exceeding it causes a stall at any airspeed, weight or attitude.

A condition in which the airflow separates from the wing because the critical angle of attack is exceeded, causing a loss of lift and an increase in drag.

The lowest airspeed at which an aircraft can maintain level flight at a given weight and load factor. It increases with weight, load factor and altitude.

A stall occurring at a higher-than-normal airspeed because increased load factor raises the angle of attack. It commonly happens in steep turns or abrupt pull-ups.

An aggravated stall in which the aircraft autorotates in a helical descent, with one wing more deeply stalled than the other. Recovery requires reducing angle of attack and stopping rotation.

The ratio of true airspeed to the local speed of sound. As it approaches one, shock waves and compressibility effects alter lift, drag and control response.

The free-stream Mach number at which airflow somewhere over the aircraft first reaches the speed of sound. Beyond it, shock waves begin to form and drag rises rapidly.

A nose-down pitching tendency at high Mach number caused by shock-induced airflow separation and rearward movement of the centre of pressure. It reduces elevator effectiveness and requires prompt correction.

A dangerous high-speed vibration in which aerodynamic forces and structural flexibility reinforce each other. It can destroy an aircraft rapidly and is avoided by staying below the never-exceed speed.