Relative Wind

Relative wind is the angle of airflow as it impacts an airfoil.

Movement of an airfoil through the air creates relative wind.  The direction of airfoil in relation to the air changes the angle of the airflow, or relative wind.  Relative wind is parallel but in the opposite direction of the airfoil’s direction.

With fixed-wing aircraft, this is a simple concept.  With rotary wing aircraft, there are several other factors impacting the relative wind, mainly rotation of the rotor blades, and the induced flow from lift production.  With helicopters, when someone refers to relative wind, they are usually referring to resultant relative wind.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-8
Principles of Helicopter Flight, 2nd Edition, pg. 47
FM 3-04.203-2007 Fundamentals of Flight pg. 1-8

Other Helicopter Aerodynamic Principles

Rotational relative wind is the relative wind produced by the rotation of the rotor blades.

Rotational relative wind is parallel to the physical flight path of the rotor blades.  The rotational relative wind is at its maximum velocity at the rotor tips.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-8
Principles of Helicopter Flight, 2nd Edition, pg. 47
FM 3-04.203-2007 Fundamentals of Flight pg. 1-9

Other Helicopter Aerodynamic Principles

Resultant relative wind is angle airflow at the rotor blades considering rotational relative wind and induced flow.

When at a hover in a calm, no-wind condition, resultant relative wind is the combination of rotational relative wind and induced flow.  However, the movement of the helicopter and wind velocity also affect the angle of the airflow at the rotor blades.  Resultant relative wind is a factor used in determining or describing many aerodynamic factors.  Often, relative wind and resultant relative wind are used simultaneously.Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-8
Principles of Helicopter Flight, 2nd Edition, pg. 47
FM 3-04.203-2007 Fundamentals of Flight pg. 1-10

Other Helicopter Aerodynamic Principles