What is a disadvantage of a nonsymmetrical airfoil in rotor aerodynamics?

• A. Center Of Pressure Movement Causes Twisting Force And Stress On The Airfoil
• B. Increased Stall Resistance
• C. Lower Angle Of Attack Requirements
• D. Reduced Drag During High-Speed Flight

Answer: (A)

The key idea here is how a nonsymmetric (cambered) airfoil behaves differently from a symmetric one as the angle of attack changes, and what that means for a rotor blade. A cambered airfoil has lift that tends to be accompanied by a nonzero pitching moment, and the point along the chord where the aerodynamic lift effectively acts— the center of pressure—shifts with the angle of attack. On a rotor blade, this shifting center of pressure creates a twisting tendency about the blade’s longitudinal axis. In other words, as the blade moves through different AoA during rotation, gusts, or blade-flap motion, the lift doesn’t just push the blade up or down; it tends to twist it, producing torsional loads that the blade must resist. This torsional loading increases structural stress and can complicate blade design and pitch control, making cambered airfoils less favorable for rotor systems in terms of structural loads.

The other statements don’t describe the rotor-specific drawback of cambered airfoils as accurately. Cambered profiles don’t inherently provide more stall resistance; in many cases they stall earlier or more abruptly. They do not always require lower angles of attack to generate lift; cambered airfoils actually produce lift at smaller AoA, which is advantageous in other contexts but contributes to the twisting moment in rotation. And cambered airfoils do not typically reduce drag during high-speed flight; their drag characteristics at high speed aren’t inherently better than symmetric counterparts and can be worse due to the camber.