CPL Aerodynamics flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Critical angle of attack

The angle of attack at which airflow separates from the wing and lift begins to decrease. It is fixed for a given wing and configuration, so stall always occurs at this angle regardless of speed, weight, or attitude.

Load factor in a level turn

The ratio of lift to weight, equal to 1/cos(bank angle) in a level turn. At 60 degrees of bank it is 2, meaning the wing carries twice the aircraft weight and stall speed rises by the square root of 2.

Induced drag and aspect ratio

Drag caused by lift generation, proportional to lift squared and inversely proportional to the square of airspeed, span efficiency, and aspect ratio. High aspect ratio wings such as glider wings produce less induced drag for the same lift.

Best glide speed

The speed giving maximum lift-to-drag ratio and therefore maximum still-air range. It is roughly the minimum power required speed, increases with weight, and is raised by a headwind and lowered by a tailwind.

Density altitude

Pressure altitude corrected for temperature and humidity. High density altitude reduces air density, lengthening takeoff roll, reducing climb performance, and raising true airspeed for a given indicated airspeed.

Static margin

The distance between the centre of gravity and the neutral point, expressed as a percentage of mean aerodynamic chord. A positive static margin means the centre of gravity is ahead of the neutral point and the aircraft is longitudinally stable.

Neutral point

The centre of gravity position at which the aircraft has zero longitudinal static stability. Moving the centre of gravity aft toward the neutral point reduces stability and control forces; beyond it the aircraft becomes unstable.

Adverse yaw

Yaw in the direction opposite to the intended turn, caused by the descending aileron producing more induced drag than the rising one. Coordinated rudder into the turn counters it and keeps the turn balanced.

Critical Mach number

The free-stream Mach number at which airflow somewhere on the aircraft first reaches Mach 1. It decreases with thicker wings and increases with sweepback, so swept wings delay shock formation and buffet.

Spin recovery sequence

Reduce power, neutralise ailerons, apply full rudder opposite the direction of rotation, then ease the elevator forward to break the stall. Centralise rudder as rotation stops and recover from the resulting dive.

Speed of sound in air

Depends only on temperature, not pressure or density. It is approximately 39 times the square root of the absolute temperature in kelvin, about 661 knots at sea level in the standard atmosphere.

V_X versus V_Y

V_X gives the best angle of climb, the speed for maximum excess thrust. V_Y gives the best rate of climb, the speed for maximum excess power. V_Y is normally the higher of the two.

Dihedral effect

Roll produced by sideslip. When the aircraft slips, the lower wing meets the airflow at a higher angle of attack, creating a rolling moment that tends to level the wings and provides positive lateral stability.

Centre of pressure

The point on a wing where the resultant aerodynamic force acts. As angle of attack increases, it moves forward on a cambered wing, producing a pitching moment that affects longitudinal trim and stability.

Parasite drag

Drag from skin friction and form drag that does not contribute to lift. It increases with the square of airspeed, so it dominates at high speed, while induced drag dominates at low speed.

Manoeuvring speed V_A

The maximum speed at which full control deflection cannot overstress the structure. It decreases with weight, and above it abrupt or full control inputs risk structural damage from excessive load factor.

Weathercock stability

Directional static stability provided mainly by the vertical fin. In a sideslip the fin generates a restoring yaw moment that turns the aircraft back into the relative wind, keeping it aligned with its flight path.

Dutch roll

A coupled oscillation in which the aircraft alternately yaws and rolls. It occurs when directional stability is strong relative to dihedral effect, and is damped by a yaw damper on larger aircraft.

Mach tuck

A nose-down pitching tendency at high Mach number caused by shock-induced flow separation and rearward movement of the centre of pressure on the wing. It reduces elevator effectiveness and can lead to a steep dive.

Shock stall

Loss of lift and onset of buffet caused by airflow separation behind a shock wave on the wing. It occurs at high subsonic speeds before the aircraft itself exceeds Mach 1.

Aspect ratio

Wing span squared divided by wing area. Higher aspect ratio reduces induced drag for a given lift coefficient, improving climb and glide performance but increasing structural bending loads.

Angle of attack

The angle between the wing chord line and the relative airflow. It determines lift coefficient and is independent of pitch attitude, so an aircraft can stall in a dive or at high speed.

Takeoff ground roll and weight

Ground roll increases approximately with the square of weight for the same conditions. A 10 percent increase in weight raises the ground roll by about 20 percent and also increases the liftoff speed.

Excess power

The difference between power available and power required at a given speed. Dividing excess power by weight gives rate of climb, so the speed for maximum excess power is the best rate of climb speed.

Excess thrust

The difference between thrust available and thrust required. Dividing excess thrust by weight gives climb gradient, so the speed for maximum excess thrust is the best angle of climb speed.

True airspeed approximation

True airspeed exceeds indicated airspeed by roughly 2 percent per 1000 feet of altitude. It is used to estimate TAS from IAS, but it becomes less accurate at high altitude and high speed.

Static stability

The initial tendency of an aircraft to return to equilibrium after a disturbance. Positive static stability combined with positive dynamic stability produces a stable aircraft; static stability alone does not guarantee damping.

Trim tab

A small movable surface on a control surface that holds it in a desired position without continuous pilot force. It works by creating a local aerodynamic moment that balances the control surface hinge moment.

Stall speed in a turn

The stall speed in a level turn equals the straight-and-level stall speed multiplied by the square root of load factor. At 60 degrees of bank, load factor is 2 and stall speed is 41 percent higher.

Autorotation

The self-sustaining roll and yaw of a stalled aircraft, in which the more deeply stalled wing descends and produces less lift, driving rotation. It is the mechanism that sustains a spin.

Mach number

The ratio of true airspeed to the local speed of sound. For a given indicated airspeed, Mach number increases with altitude because temperature and the speed of sound decrease.

Induced drag versus speed

Induced drag varies inversely with the square of airspeed, so it dominates at low speed and high angle of attack. Parasite drag varies with the square of airspeed, so total drag is minimum where the two are equal.