Brunei DCA CPL(A) Principles of Flight key terms

A defined model of the atmosphere assuming sea-level pressure 1013.25 hPa, temperature 15 °C and a standard lapse rate, used as a common reference so aircraft and engine performance can be compared.

The altitude in the standard atmosphere at which air density equals the actual density at the aircraft's position. High density altitude reduces lift and engine power, lengthening takeoff and climb.

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

The direction from which air meets the aerofoil as a result of the aircraft's motion and the air mass movement. All aerodynamic forces are referenced to this direction.

The single force obtained by combining all pressure and shear distributions over a body. It is normally resolved into lift perpendicular to the relative airflow and drag parallel to it.

A relationship stating that in steady, incompressible flow along a streamline, an increase in fluid speed is accompanied by a decrease in static pressure and vice versa, helping explain pressure distribution over a wing.

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

The straight line joining the leading edge to the trailing edge of an aerofoil. It provides the datum from which angle of attack and camber are measured.

The curvature of an aerofoil, measured as the maximum distance between the mean camber line and the chord line. Positive camber lets a wing produce lift at low or zero angle of attack.

The point on a wing where the resultant aerodynamic force effectively acts. On a cambered aerofoil it moves forward as angle of attack increases, which affects pitching moments.

The point on an aerofoil about which the pitching moment remains essentially constant as angle of attack changes. For most subsonic aerofoils it lies near 25 percent chord.

A dimensionless number expressing how effectively an aerofoil generates lift at a given angle of attack. It rises roughly linearly until the critical angle, then falls sharply at the stall.

Lift equals one half times air density times true airspeed squared times wing area times lift coefficient. It shows lift varies with the square of speed and directly with density.

The thin layer of air next to a surface where velocity changes from zero at the skin to free-stream value. Its behaviour determines skin friction and whether the flow separates.

The detachment of the boundary layer from the upper surface when the adverse pressure gradient becomes too strong. It destroys lift and produces the buffet and sink associated with a stall.

Drag that does not contribute to lift, comprising form drag and skin friction. It increases with the square of airspeed and is unaffected by angle of attack at a given speed.

Drag created as a by-product of lift, mainly from wingtip vortices. It is greatest at high angle of attack and low speed, and varies inversely with the square of airspeed.

Drag caused by the shearing action of air passing over the aircraft's surfaces within the boundary layer. Smoothing and laminar flow reduce it, and it grows with wetted area.

The ratio of lift produced to drag incurred at a given condition. It peaks at the best glide ratio, and the maximum value indicates aerodynamic efficiency.

A graph of total drag against airspeed, obtained by adding parasite and induced drag. It reaches a minimum at the speed where the two components are equal, giving best endurance.

The power needed to overcome drag at a given speed, equal to drag multiplied by true airspeed. Its curve is lowest at a speed different from that for minimum drag.

The power the engine and propeller can deliver at a given altitude and configuration, reduced by propeller inefficiency. Excess over power required gives climb performance.

The initial tendency of an aircraft, after a disturbance, to return toward its original condition. Positive static stability is a prerequisite for any useful dynamic stability.

The behaviour of an aircraft over time after a disturbance, described as damped, undamped or divergent oscillation. It depends on static stability and the damping available.

Static stability about the lateral axis, governing pitch. It requires the centre of gravity to lie forward of the neutral point, with the tailplane and fuselage contributing.

The centre of gravity position at which the aircraft has zero longitudinal static stability. Loading the centre of gravity aft of it makes the aircraft unstable in pitch.

The distance between the centre of gravity and the neutral point, usually expressed as a fraction of mean chord. A larger margin means stronger pitch stability but heavier control forces.

The tendency of wing dihedral to produce a rolling moment when the aircraft is yawed, opposing sideslip. It contributes to lateral stability and interacts with directional stability.

A coupled lateral-directional oscillation in which rolling and yawing motions alternate, typical of strong dihedral effect with weak directional stability. It is normally damped by yaw dampers.

The adjustment of a control surface or tab so that the aircraft can be held in a desired attitude without continuous control force. Trim changes with speed, power and configuration.

The ability of a control surface to produce a moment, depending on deflection, area, moment arm and dynamic pressure. Effectiveness falls at low speed and in thinner air.

The angle of attack at which lift reaches its maximum before airflow separates. It is a fixed value for a given wing and is reached at any speed, weight or attitude.

A condition in which the wing exceeds its critical angle of attack and lift decreases sharply. Recovery requires reducing angle of attack, not simply adding power.

An autorotative stall in which one wing is more deeply stalled than the other, producing yaw and roll. Recovery requires reducing angle of attack, then applying anti-spin rudder.

The ratio of total lift to weight, expressed in g. It increases in turns and pull-ups and determines the structural stress on the airframe.

The speed above which full control deflection could overstress the airframe before it stalls. Below it, the wing stalls before the limit load factor is exceeded.

The free-stream Mach number at which airflow somewhere on the aircraft first reaches the speed of sound, creating shock waves and drag rise. Sweep and thin aerofoils raise it.

A nose-down pitching tendency at high Mach number caused by aft movement of the centre of pressure and shock-induced flow changes. It can worsen if the aircraft accelerates.

The proportion of engine power converted into useful thrust power by the propeller. It varies with airspeed and blade angle and is optimised by correct pitch selection.

The engine whose failure produces the greatest adverse yaw and control difficulty, determined by propeller rotation direction and asymmetric thrust effects. It governs minimum control speeds.

The yawing moment produced when thrust is greater on one side of an aircraft, as after an engine failure. It must be countered by rudder, causing drag and reduced performance.

The lowest airspeed at which the aircraft can be controlled with a specified engine failed and maximum rudder applied. Certification defines it for ground and airborne cases.