EASA ATPL 081 Principles of Flight key terms

The angle between the wing chord line and the relative airflow approaching the aerofoil. Lift increases with angle of attack up to the critical value, beyond which the boundary layer separates and the wing stalls.

The angle of attack at which maximum lift coefficient is reached and airflow begins to separate over the upper surface. A wing always stalls at this angle regardless of weight, speed, or bank angle.

The thin layer of air next to a surface where velocity rises from zero at the skin to free-stream value. It may be laminar or turbulent, and its behaviour determines friction drag and stall characteristics.

Detachment of the boundary layer from the surface when the adverse pressure gradient becomes too strong for the air to follow the contour. It destroys lift and greatly increases pressure drag.

In steady, incompressible flow along a streamline, an increase in velocity is accompanied by a decrease in static pressure and vice versa, provided total pressure remains constant.

For steady flow through a streamtube, mass flow rate is constant, so the product of density, cross-sectional area, and velocity is unchanged along the tube.

Where a streamtube narrows, velocity increases and static pressure falls. This principle explains pressure changes over a wing and the behaviour of air through a carburettor venturi.

The point on an aerofoil, usually near the leading edge, where airflow is brought to rest and dynamic pressure converts fully into static pressure, producing the highest local pressure.

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

The point on an aerofoil about which the pitching moment coefficient remains essentially constant as angle of attack changes, typically near 25 per cent chord.

Drag that accompanies lift, caused by downwash and wingtip vortices tilting the lift vector rearward. It increases with lift coefficient squared and decreases with higher speed and greater aspect ratio.

The sum of skin friction and form drag, independent of lift. It rises with the square of airspeed and dominates total drag at high speed.

The ratio of wingspan squared to wing area. High aspect ratio reduces induced drag and improves climb and glide performance, while low aspect ratio favours roll rate and structural strength.

Rotating air masses shed from wingtips where higher-pressure air beneath the wing spills around to the lower-pressure upper surface. They create induced drag and dangerous wake turbulence.

Rearward inclination of the wing leading edge, used to delay the onset of shock waves at high Mach numbers. It reduces lift-curve slope and worsens tip stall tendencies at low speed.

The tendency of wings set at an upward angle to generate a rolling moment away from a sideslip. It provides lateral static stability but too much can cause Dutch roll.

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

The minimum airspeed at which the wing can produce the lift required in unaccelerated flight. It increases with the square root of load factor and with weight.

The tendency of an aircraft, after a pitch disturbance, to return to its original angle of attack. It requires the centre of gravity to lie ahead of the neutral point.

The centre-of-gravity position at which the aircraft has neutral longitudinal static stability. Moving the centre of gravity aft toward this point reduces stability and stick force.

A coupled oscillation in which the aircraft alternately yaws and rolls, usually with yaw leading roll. It arises from strong dihedral effect combined with weak directional stability.

The tendency of an aircraft to yaw back into the relative airflow after a sideslip, provided mainly by the vertical fin and fuselage area behind the centre of gravity.

A trailing-edge high-lift device extended to increase camber and wing area, raising lift coefficient at a given speed. It lowers stall speed but increases drag and pitching moment.

A leading-edge device that opens a slot allowing high-energy air to flow over the upper surface, delaying separation to a higher angle of attack and increasing maximum lift.

A surface that disrupts airflow over the wing to reduce lift and increase drag. Used in flight to steepen descent or increase rate of descent, and on the ground to dump lift.

A small hinged surface on a primary control that produces a hinge moment to hold the control in a chosen position, relieving the pilot of sustained control force.

A control system in which pilot inputs are converted to electrical signals processed by computers that command hydraulic actuators, enabling envelope protection and artificial stability.

An autorotative stall in which the aircraft descends in a helical path 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. It determines compressibility effects, and the local value over the wing can exceed the free-stream value significantly.

The free-stream Mach number at which airflow somewhere over the aircraft first reaches Mach 1, creating a shock wave and the onset of drag rise and buffet.

Airframe vibration caused by shock-induced separation or shock oscillation at high Mach number, warning of impending loss of control effectiveness and possible upset.

Separation of the boundary layer behind a shock wave, causing loss of lift, buffet, and control problems. It occurs when the aircraft exceeds its critical Mach number.

The difference between the geometric pitch distance the propeller would advance in one revolution and the actual distance advanced, expressed as a percentage or ratio.

The angle between the propeller blade chord and the plane of rotation. It is set by the constant-speed unit to maintain an efficient angle of attack as speed and power change.

A propeller whose blade angle is automatically adjusted by a governor to absorb engine power at a selected RPM, maintaining efficiency across a wide range of speeds.

The tendency of a propeller-driven aircraft to yaw when the propeller axis is inclined to the airflow, because the descending blade takes a greater angle of attack and produces more thrust.

The maximum speed at which full, abrupt control deflection can be applied without exceeding the design load factor. Above it, controls must be used with care to avoid structural damage.

The additional load factor imposed when the aircraft encounters an updraught or downdraught. Its effect depends on gust velocity, true airspeed, wing loading, and lift-curve slope.

Disturbed air left behind an aircraft, dominated by wingtip vortices. It is strongest when the generating aircraft is heavy, slow, and clean, and it sinks and drifts with the wind.

A sudden change in wind speed or direction over a short distance, which can cause large, rapid changes in airspeed and lift, potentially leading to an unrecoverable low-altitude upset.