Define angle of attack.

Define angle of attack.

• A. The angle between the chord of an airfoil and the resultant relative wind
• B. The angle between the rotor blade and the flight path
• C. The angle between the chord line and the rotor plane
• D. The angle between the leading edge and the wind direction

Answer: (A)

Angle of attack is the angle between the airfoil’s chord line and the relative wind flowing past the airfoil. This angle determines how much lift the airfoil can produce at a given speed. For a helicopter rotor blade, the relative wind is the combination of the blade’s motion through the air and the helicopter’s forward airspeed (plus any induced flow). The orientation of the chord line to that resulting wind defines the angle of attack, which in turn governs the lift coefficient and stall tendency. The other descriptions don’t capture this definition: the angle between the blade and the flight path describes blade attitude rather than how the airfoil faces the incoming air; the angle between the chord line and the rotor plane is not the same as the relative wind; and the angle between the leading edge and wind direction misrepresents the reference line used for AOA, which is the chord line.

Angle of attack is the angle between the airfoil’s chord line and the relative wind flowing past the airfoil. This angle determines how much lift the airfoil can produce at a given speed. For a helicopter rotor blade, the relative wind is the combination of the blade’s motion through the air and the helicopter’s forward airspeed (plus any induced flow). The orientation of the chord line to that resulting wind defines the angle of attack, which in turn governs the lift coefficient and stall tendency.

The other descriptions don’t capture this definition: the angle between the blade and the flight path describes blade attitude rather than how the airfoil faces the incoming air; the angle between the chord line and the rotor plane is not the same as the relative wind; and the angle between the leading edge and wind direction misrepresents the reference line used for AOA, which is the chord line.