Basic Rotary Wing Aerodynamics40 cards

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Newton's First Law of Motion

A body at rest will stay at rest, and a body in motion will remain in motion at the same speed and in the same direction until acted on by an unbalanced external force.



An objects resistance to change.


Newton's Second Law of Motion

The force required to produce a change in the linear motion of a body is directly proportional to the product of its mass and acceleration.



A change in the magnitude or direction of the velocity vector with respect to time. In this case, velocity refers to the direction and rate of linear motion.


Newton's Third Law of Motion

For every action there is an equal and opposite reaction.



An object that produces a useful aerodynamic reaction when moved through the air. An airfoil is the shape of a cross-section of a wing.


Aerodynamic Center

The point on the chord line about which the moment coefficient is constant with changes in angle of attack. Also, the point where all changes to lift effectively occur.


Angle of Attack

The angle between the chord line and the resultant relative wind.


Angle of Incidence

The angle between the chord line and the path of rotation. The physical angle of the airfoil.


Bernoulli's Law

In a flow of incompressible fluid, the sum of the static and dynamic pressures remains constant if gravity and friction are disregarded.


Boundary Layer

The slow moving or stagnant air next to the surface of an airfoil.


Center of Pressure

The point along the chord line through which all aerodynamic forces act.


Chord Line

A straight line that connects the leading and trailing edges of an airfoil.



The force that opposes the motion of an airfoil. It is parallel to, but opposite of, the relative wind.


Flight Path Velocity (FPV)

The speed and direction of an airfoil as it passes through the air.


Induced Velocity (Flow)

The air vector perpendicular to the path of rotation produced because the wing or rotor is generating lift.



The force perpendicular to the relative wind.


Relative Wind

Air in motion with respect to an airfoil. It is equal to and opposite the FPV.


Resultant Relative Wind

Airflow from rotation that is modified by the induced velocity.


Rotational Relative Wind

Relative wind produced by the rotation of the rotor blades of an aircraft.


Total Aerodynamic Force (TAF)

The combination of the pressure differential about an airfoil producing lift and profile drag is known as the total aerodynamic force.


Profile Drag

Due to the skin friction of the rotor blades. it remains relatively constant with airspeed until treating blade stall or advancing blade compressibility effects begin to appear.


Induced Drag

Results from the rearward tilt of the lift vectors due to the downward velocities imparted to the air by the rotor as it produces lift. It is greatest at a hover and decreases with airspeed.


Parasite Drag

Due to the drag of the fuselage, landing gear, rotor hubs, and other non-lifting parts of the aircraft.


Blade Twist

The blade is twisted so that at the tip the angle of incidence is lower than at the root. This tends to distribute the lift more evenly and produces better hover performance and delay in retreating blade stall.


Phase Lag

A rotor acts like a gyroscope. If an force is applied to the rotating body it will manifest itself 90 degrees from the point of application in the direction of rotation.


Lead & Lag

When hinged blades are allowed to move fore and aft in the plane of rotation independent of the other blades.



A mechanical change in the angle of incidence of a rotor blade. Also defined as the rotation of the blade about its spanwise axis.


Blade Flapping

The up and down movement of a rotor blade about a flapping hinge during rotation. The horizontal flapping hinge allows the blade to move up and down in response to changing flight conditions or pilot control inputs.



An upward bending of a rotor system when the aircraft is producing lift. Coning increases as gross weight and G-forces increase while RPM is held constant.



The helicopter fuselage will rotate in the opposite direction of the main rotor rotational direction.


Translating Tendency

The thrust produced by the anti-torque device, i.e. tail rotor, tends to push the aircraft sideways in the direction of thrust.


Ground Effect

A condition of improved performance encountered when operating near the ground. It is due to the interference of the surface with the airflow pattern of the rotor system and more pronounced closer to the surface.


Dissymmetry of Lift

The difference in lift between the advancing half and the retreating half of the rotor disc.


Translational Lift

Improved efficiency of the rotor system due to reduced induced velocity, and subsequent increase in angle of attack, as the aircraft transitions from a hover to forward flight.


Transverse Flow Effect

The difference in lift between the forward and aft section of the rotor disc. This is due to the differences in the angle of attack caused by coning and induced velocity between the fore and aft sections of the rotor disc.


Retreating Blade Stall

A stall that starts at or near the tip of a rotor blade cause by the high angle of attack required to compensate for dissymmetry of lift.


Settling with Power

A condition of powered flight where the aircraft settles in its own downwash; near vertical decent (300-1000 fpm rate of decent)


Dynamic Rollover

A rollover that occurs when the aircraft is in contact with the ground and rolls far enough to put the aerodynamic center of gravity past the pivot point. Three conditions must occur: a pivot point, a rolling motion and exceeding the critical angle.


Ground Resonance

A whirling unbalanced centrifugal force that gets in phase with the natural frequency of the aircraft rocking it on its landing gear. This is caused by a bunching up of the rotor blades on one side of the rotor disc in helicopters with lead-lag hinges.