In the

aerodynamics Aerodynamics, from grc, ἀήρ ''aero'' (air) + grc, δυναμική (dynamics), is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dyn ...

rotorcraft A rotorcraft or rotary-wing aircraft is a heavier-than-air aircraft with rotary wings or rotor blades, which generate lift by rotating around a vertical mast. Several rotor blades mounted on a single mast are referred to as a rotor. The Inter ...

helicopter A helicopter is a type of rotorcraft in which lift and thrust are supplied by horizontally spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attribut ...

s, phase lag refers to the angular difference between the point at which a control input to a rotor blade occurs and the point of maximum displacement of the blade in response to that control input. This displacement occurs in the direction of rotor rotation. Phase lag may vary depending on rotor tilt rate, ratio of aerodynamic damping to blade inertial forces ( Lock number), offset of flapping hinge from axis of rotation (e/R ratio), and coupling of blade flap, drag, and feather motions, and often results in ''cross-coupling'' between the aircraft control axes. Phase lag is a property of all rotating systems acted upon by a periodic force. Because of phase-lag, rolling a rotorcraft to the left or right would theoretically require a forward or backward cyclic if there were no mechanical correction. The rotor control system is angularly shifted as much as necessary to compensate for phase-lag and provide helicopter response that matches movement of the cyclic stick. The amount of phase lag depends on the distance of the flapping hinge from the rotor hub. If the hinges are only slightly offset, phase lag will be 80-90 degrees, however a semi-rigid rotorhead will typically have phase lag of 75-80 degrees. Phase lag is not caused by

gyroscopic precession Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In oth ...

, which always has a lag of 90 degrees.{{cite book , last1=Croucher , first1=Phil , title=Professional Helicopter Pilot Studies , date=2007 , isbn=9780978026905 , page=2-24 , access-date=18 January 2022 , url=https://www.google.co.uk/books/edition/Professional_Helicopter_Pilot_Studies/AovdKRWSqJAC The

transverse flow effect Transverse flow effect is an aerodynamic effect encountered when a helicopter moves horizontally (typically forward) through the air, which causes the rotor disc to roll to the side. It is also known as transverse roll or inflow roll. Transverse f ...

results in a roll due to phase lag. Phase lag is not to be confused with ''advance angle'', which refers to the mechanically fixed angle between the pitch link attachments at the blade and the swashplate. Advance angle is fixed and cannot vary.

References

Rotorcraft Helicopter aerodynamics

See also

References