A helicopter's main rotor blades exhibit a designed decrease in pitch angle from root to tip, known as blade twist. What is the primary aerodynamic purpose of this design feature?
- ATo increase the blade's natural lead-lag frequency for smoother operation
- BTo reduce blade weight and simplify manufacturing
- CTo prevent retreating blade stall entirely at all airspeeds
- DTo distribute lift more evenly along the blade span by compensating for the difference in rotational velocity between root and tip
Show answer & explanationAnswer & explanation
Correct answer: D. To distribute lift more evenly along the blade span by compensating for the difference in rotational velocity between root and tip
Because blade tip speed is much faster than blade root speed (velocity increases with radius during rotation), an untwisted blade would produce far more lift near the tip than the root. Blade twist reduces pitch angle toward the tip, equalizing the lift distribution along the span for more efficient rotor performance and reduced blade stress.
Why the other options are wrong
- A. Lead-lag frequency relates to blade dynamics in the plane of rotation, not to twist's aerodynamic lift-distribution purpose.
- B. Blade twist is an aerodynamic design feature affecting lift distribution, not primarily a weight or manufacturing simplification.
- C. Blade twist reduces but does not entirely prevent retreating blade stall, which can still occur at high airspeeds or load factors.
Blade Twist
A built-in reduction in pitch angle from root to tip of a rotor blade, designed to more evenly distribute lift along the blade span by compensating for the higher rotational velocity at the tip.
- Blade tip travels faster than blade root due to rotational geometry
- Without twist, tip would produce disproportionately more lift than root
- Twist improves rotor efficiency and reduces structural loads
Memory trick: Blade twist: less pitch at the fast tip keeps lift even, like dimming a bright spot.