NCAA CPL Principles of Flight key terms

The angle between the wing chord line and the relative wind. It directly controls lift coefficient and is the primary factor determining whether the wing stalls.

The direction of airflow relative to the aircraft, opposite to the flight path. All aerodynamic angles and forces are measured against this reference direction.

A relationship stating that where fluid velocity increases, static pressure decreases. It explains part of the pressure difference that produces lift on a wing.

For every action there is an equal and opposite reaction. A wing deflects air downward, and the air pushes the wing upward, contributing to total lift.

The point on the leading edge where airflow comes to rest and divides over the upper and lower surfaces. Pressure is highest at this location.

A thin layer of air next to the wing surface where velocity rises from zero at the skin to free-stream speed. Its behavior governs skin friction and separation.

Smooth, orderly airflow in which streamlines move parallel without mixing. It produces low skin friction but is easily disturbed and separates readily.

Chaotic, mixing airflow with higher skin friction but more energy near the surface. It resists separation longer than laminar flow, which can delay a stall.

The detachment of the boundary layer from the wing surface when the pressure gradient becomes too steep. It reduces lift and increases drag, leading to a stall.

The speed shown on the airspeed indicator, derived from dynamic pressure. It reflects aerodynamic performance directly and is the reference for most certification limits.

Indicated airspeed corrected for position and installation error. It is closer to true aerodynamic speed and is the basis for published performance and stall figures.

Calibrated airspeed corrected for compressibility effects in the pitot-static system. It equals true airspeed at sea level in standard conditions.

The actual speed of the aircraft through the surrounding air mass. It rises with altitude for a given indicated speed because air density decreases.

The ratio of true airspeed to the local speed of sound. It quantifies compressibility effects and determines shock wave formation on the airframe.

The rate at which pressure disturbances travel through air, depending mainly on temperature. It decreases as temperature drops, so it falls with increasing altitude.

The free-stream Mach number at which airflow somewhere on the airframe first reaches Mach 1.0. Exceeding it begins shock wave formation and drag rise.

The wingspan squared divided by wing area. High aspect ratio wings produce less induced drag for a given lift and are typical of efficient cruising aircraft.

Aircraft weight divided by wing area. Higher wing loading means higher stall speeds, less maneuverability, and a rougher ride in turbulence.

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

The downward deflection of air behind a wing, tilting the lift vector rearward. The rearward component is induced drag and grows as angle of attack increases.

Drag caused by lift production, arising from downwash and wingtip vortices. It decreases with higher speed and increases sharply at high angle of attack.

Drag unrelated to lift production, split into form drag and skin friction. It increases with the square of airspeed and dominates at high speed.

Lift divided by drag at a given condition. The maximum value defines best glide angle and occurs at the speed where induced and parasite drag are equal.

Trailing-edge devices extended to increase camber and wing area. They raise lift and drag, lowering stall speed and steepening approach descent.

Leading-edge devices that extend forward to energize the boundary layer over the upper surface. They delay stall and allow higher angles of attack.

Upper-surface panels raised to disrupt lift and add drag. They assist roll control, steepen descent, and help dump lift after touchdown.

The initial tendency of an aircraft to return to equilibrium after a disturbance. Positive static stability means it starts back toward the original condition.

The behavior of an aircraft over time after a disturbance, described by oscillatory motion. It determines whether oscillations damp out, persist, or grow.

A long-period, low-damping oscillation in which the aircraft trades altitude for speed while angle of attack stays nearly constant. It is usually easily corrected.

The point where the aircraft's total weight acts. Its position relative to the aerodynamic center strongly affects longitudinal stability and control forces.

The angle at which airflow separates from the wing and lift begins to decrease. The wing stalls at this angle regardless of speed, weight, or attitude.

The lowest speed at which the wing can produce enough lift to support the aircraft in level flight. It increases with weight and load factor.

An aggravated stall in which one wing is more deeply stalled than the other, producing autorotation and a descending helical path. Recovery requires reducing angle of attack.

Unequal thrust from multi-engine aircraft when one engine fails, creating yaw toward the failed engine. It requires rudder input and increases minimum control speed.

The minimum control speed with the critical engine inoperative, at which rudder authority can still maintain control. It is a certification limit shown as a red radial line.

The engine whose failure most adversely affects performance and control, typically the one with the greater yawing moment. It is usually the left engine on conventional twins.

The ratio of lift to weight, expressed in g units. It multiplies stall speed by its square root and defines maneuvering and structural limits.

A chart plotting airspeed against load factor that defines the structural flight envelope. It shows limit loads, ultimate loads, and corner speed.

The maximum speed at which full control deflection will stall the wing before exceeding the limit load factor. It decreases as aircraft weight decreases.

The difference between theoretical and actual advance per revolution. It reflects the energy lost to accelerating air and producing thrust.

Thrust horsepower divided by shaft horsepower, expressing how effectively engine power becomes useful thrust. It varies with airspeed and blade angle.

The angle between a propeller blade's chord and its plane of rotation. It is set by the pitch control to keep angle of attack efficient across speeds.

A nose-down pitch tendency at high Mach numbers as shock-induced separation moves the center of pressure rearward. It can lead to loss of control if uncorrected.

Airframe vibration caused by shock waves and separated airflow at high Mach number. It warns of approaching buffet boundaries and reduced control effectiveness.

A dimensionless number relating lift to dynamic pressure and wing area. It rises with angle of attack until the critical angle is reached.