A mechanic explains that most main rotor blades use a symmetrical airfoil rather than a highly cambered (unsymmetrical) airfoil. What is the primary aerodynamic advantage of a symmetrical airfoil for rotor blade design?
- AIt produces more lift at a given angle of attack than a cambered airfoil
- BIts center of pressure stays relatively constant as angle of attack changes, reducing blade twisting moments
- CIt completely eliminates the need for main rotor blade twist
- DIt increases the critical Mach number encountered at the blade root
Show answer & explanationAnswer & explanation
Correct answer: B. Its center of pressure stays relatively constant as angle of attack changes, reducing blade twisting moments
A symmetrical airfoil has identical curvature on the upper and lower surfaces, so its center of pressure remains nearly stationary throughout the range of angles of attack the blade encounters in flight. This minimizes torsional (twisting) loads on the blade structure, which is critical since rotor blades experience rapidly changing angles of attack every revolution.
Why the other options are wrong
- A. Cambered airfoils generally produce more lift at a given AOA, not symmetrical ones.
- C. Blade twist is still used to compensate for varying blade section speeds, independent of airfoil symmetry.
- D. Critical Mach number concerns relate to blade tip speed and thickness, not the reason for choosing symmetry.
Symmetrical Airfoil
An airfoil shape with identical upper and lower surface curvature, chosen for main rotor blades because its center of pressure stays nearly fixed across changing angles of attack.
- Upper and lower surfaces are mirror images
- Center of pressure travel is minimal with AOA changes
- Reduces blade twisting (torsional) loads
- Cambered airfoils shift CP more and produce more lift but more twisting moment
Memory trick: Symmetrical = balanced twin, CP doesn't wander, blade doesn't twist.