Rotor Blade Design Overview

Rotor Blade Design

• Symmetrical vs. Asymmetrical Blades: Symmetrical blades have identical upper and lower surfaces, while asymmetrical blades have a higher camber on the upper surface and a flatter surface on the bottom. Symmetrical blades are better for aerodynamic pitching, while asymmetrical blades produce more lift for the same area. Engineers achieve comparable pitching characteristics on asymmetrical blades by sweeping the trailing edge upward by 3 degrees.

• Common Blade Design: Symmetrical design is more common, although manufacturers are increasingly using asymmetrical designs.

• Planform Types: Two main planform types exist:

  • Parallel/Uniform: The leading and trailing edges run parallel to each other, and the chord (width) is consistent from root to tip. Benefits include cost-effectiveness due to simplified manufacturing (reuse of ribs).
  • Tapered: The leading and trailing edges converge towards the tip, and the chord length decreases towards the tip. Produces lift more evenly across the blade without corrective measures, though lift production is unequal across the blade at different rotational speeds.

• Compensation for Uneven Lift: Uniform planform blades produce more lift at the tip than the root. Engineers compensate by adding negative twist to the blade. This adjusts the angle of attack (AOA) at different points along the blade to maintain equal lift production at differing speeds. Tapered plans compensate for speed variation with variable chord lengths, ensuring a more even lift distribution.

Rotor Blade Materials and Construction

• Leading Edge Material: All rotor blades have stainless steel caps for protection against erosion from dust particles during flight. These materials improve durability and lifespan.

• Rotor Blade Painting: The bottom of rotor blades is painted black to prevent glare and its effect on the pilot's visibility.

• Static Discharge on Composite Blades: Composite blades require reinforcement against static electricity by extending bonding strips from tip to root to redirect any static build-up to the airframe.

• Blade Material Options: Rotor blades can be made from metal, composite materials, or wood.

• Inspection Method: Tap testing is a primary method for inspecting composite rotor blades.

• Contruction Types: Bonded construction is preferred for its strength over alternative mechanical fastening methods. Single-pocket design is easier, quicker, and less complex for manufacturers to build. Multi-pocket designs require precise jigs for alignment and are more complex to produce initially, but are easier and less expensive to repair compared to single-pocket designs.

Rotor Blade Balance and Rigging

• Balance Weights: Rotor blades have spanwise, chordwise, and tracking mass balance weights.

  • Spanwise balance weights are located center to tip.
  • Chordwise balance weights are on the leading edge, typically between 30% and 50% of the chord length.
  • Tracking balance weights are located on the leading and trailing edges of the tip portions.

• Servo Tabs: These trailing edge tabs control blade angle of attack, making the blades smoother to control and increasing both efficiency and ease of control. They also reduce and stabilize vibrations.

• Rigging Procedures: Rigging starts with the collective system and is followed by the correlator (or auto-throttle system), if the aircraft has one. The aircraft is rigged with the lowest possible weight to ensure the rotor's speed is adequate for the angle of attack.