NCAA PPL Principles of Flight key terms

The acute angle between an airfoil's chord line and the direction of the relative wind approaching it. Increasing this angle raises lift up to the critical value, beyond which the wing stalls.

An imaginary straight line drawn from the leading edge to the trailing edge of an airfoil. It serves as the reference for measuring angle of attack and is fixed to the wing.

A line drawn midway between the upper and lower surfaces of an airfoil, equidistant from both. Its curvature describes the wing's camber and how lift is generated.

The curvature of an airfoil, 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 airflow striking the aircraft, opposite in direction to the aircraft's flight path. Its angle relative to the chord line defines angle of attack and it changes as the flight path changes.

The aerodynamic force generated perpendicular to the relative wind that supports the aircraft in flight. It increases with airspeed, air density, wing area, and angle of attack up to the stall.

The aerodynamic force acting parallel to the relative wind and opposing the aircraft's forward motion. It has two main components: parasite drag and induced drag.

Drag caused by friction and form resistance of the aircraft's surfaces moving through the air, plus interference between components. It increases with the square of airspeed and is unrelated to lift production.

Drag created as a by-product of lift, largely from wingtip vortices and downwash. It is greatest at high angles of attack and low airspeed, and decreases as airspeed increases.

A component of parasite drag arising from the shape of the aircraft and the pressure differential between its front and rear surfaces. Streamlining reduces it.

A component of parasite drag caused by air clinging to and rubbing along the aircraft's surfaces. Smooth, clean surfaces and proper finishing reduce it.

The ratio of lift produced to drag produced at a given angle of attack. Maximum value occurs at the best glide angle and determines glide performance and range.

A sudden loss of lift occurring when the wing exceeds its critical angle of attack. It can happen at any airspeed, attitude, or weight, and recovery requires reducing the angle of attack.

The angle of attack at which airflow separates from the wing and lift begins to decrease sharply. It is a fixed value for a given wing and marks the stall point.

An aggravated stall resulting in autorotation about the aircraft's vertical axis, with one wing more stalled than the other. Recovery requires reducing angle of attack, then applying rudder opposite the rotation.

The thin layer of air next to a wing's surface where airflow is slowed by friction. Its behavior determines whether flow stays attached or separates, which affects lift and stall.

The downward deflection of air behind a wing as it generates lift. It contributes to induced drag and, near the ground, is altered in a way that reduces drag.

The reduction in induced drag and change in lift that occurs when flying very close to the ground, roughly within one wingspan. It can cause float during landing or premature liftoff.

The point at which the aircraft's total weight is considered concentrated. Its position affects stability, stall recovery, and control effectiveness, and must stay within approved limits.

The point on an airfoil where the total aerodynamic force is considered to act. It shifts forward as angle of attack increases and moves aft as angle of attack decreases.

The aircraft's resistance to pitching disturbances about its lateral axis. It depends on the relationship between the center of gravity and the horizontal stabilizer's downforce.

The aircraft's resistance to rolling disturbances about its longitudinal axis. Dihedral, sweepback, and keel effect contribute to it.

The aircraft's resistance to yawing disturbances about its vertical axis. The vertical stabilizer and fuselage side area provide it, tending to align the aircraft with the relative wind.

The upward angle of a wing from root to tip. It creates a rolling moment that levels the wings after a sideslip, contributing to lateral stability.

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

The tendency of an aircraft, over time, to return to equilibrium after a disturbance. It describes whether oscillations damp out, remain constant, or grow.

The ratio of the total load on the aircraft's structure to its actual weight. It increases in turns and maneuvers, raising stall speed and structural stress.

The maximum speed at which full, abrupt control deflection will not overstress the aircraft. It decreases as weight decreases, so it must be adjusted for actual weight.

The minimum airspeed at which the aircraft can maintain level flight before exceeding the critical angle of attack. It increases with weight, load factor, and forward center of gravity.

The airspeed that produces the greatest glide distance for a given altitude loss, occurring at the maximum lift-to-drag ratio. It varies with weight.

The tendency of the aircraft to yaw away from the direction of a turn when ailerons are applied. It results from increased induced drag on the descending aileron's wing and is corrected with rudder.

The reaction force that tends to roll the aircraft opposite the direction of propeller rotation. It is one of the four left-turning tendencies in single-engine propeller aircraft.

Asymmetric propeller thrust occurring at high angles of attack, where the descending blade takes a larger bite of air. It produces a yawing tendency, most noticeable at slow speed and high power.

The tendency of a spinning propeller to react to an applied force as though that force acted 90 degrees later in the direction of rotation. It causes yaw or pitch during attitude changes.

A long-period longitudinal oscillation in which the aircraft alternately climbs and descends while nearly maintaining airspeed. It is a form of dynamic instability that may require pilot correction.

The actual trajectory of the aircraft through the air, which may differ from where the nose points. The relative wind is always opposite to the direction of the flight path.

The total force produced by an airfoil as air flows around it, combining lift and drag into a single resultant vector. Its point of application is the center of pressure.