Rotor Blade Maintenance and Design

Rotary Wing Aerodynamics

• Rotor blades are designed as symmetrical airfoils to create a stable aerodynamic pitching characteristic.
• Aerodynamic stability is achieved when the center of gravity, center of pressure, and blade-feathering axis are aligned at the same point.
• This alignment ensures the blade remains stable during pitch changes.

Rotor Blades - Design and Construction

• Rotor blade design and construction vary among manufacturers, though all aim to create the most efficient and economical lifting device.
• Helicopter design requirements influence rotor blade design and construction.

Rotor Blades - Materials

• Common materials for rotor blades include aluminum, steel, brass, and fiberglass.
• Early rotor blades were often made of wood.
• Metal blades followed, then composite blades using various materials as the newest blade design.

Wooden Rotor Blades

• First production rotor blades were laminated wood (birch, spruce, pine, and balsa).
• A steel core was placed near the leading edge of the blade for increased strength.
• The exterior surface was covered with resin-impregnated fiberglass cloth.
• Two-thirds of the blade's outboard leading edge portion is often protected with stainless steel.
• Blades are typically manufactured in matched pairs.

Metal Rotor Blades (Aluminum)

• Metal blades have been in production for over 40 years.
• Construction varies widely due to manufacturing expense and different manufacturers.
• A typical metal blade has a hollow, extruded aluminum spar that forms the leading edge.
• Quality control is a key advantage of this construction.

Metal Blades - Design Features

• Single blades can be exchanged without needing matched sets.
• Aluminum pockets on the trailing edge of the spar improve the blade's streamlining.
• An aluminum tip cap, secured via screws, improves streamlining and aids in bonding.
• Like other metals, blade life is limited due to stress during flight.

Composite Rotor Blades

• Fiberglass or composite blades are load-bearing components, either using fiberglass or metal spars.
• Roving's, which are strips of glass, make up the spar and are impregnated with epoxy resin.
• The roving materials are wrapped around a foam core, and fiberglass cloth creates the blade's skin.
• Root reinforcement plates are frequently added to the blade.
• The leading edge is often covered with a stainless steel strip.

Rotor Blades - Internal Structural Components

• Rotor blades typically have ribs, I-beams, and spanwise channels to support the blade skin.
• Aluminum honeycomb foil is another common supporting structural element.

Blade Nomenclature

• The Blade Planform is the overall shape viewed from above, either uniform or tapered.
• Uniform planforms are more common as they are simpler to manufacture.

Blade Construction- Single Pocket/Fairing

• Single-pocket blades have a one-piece skin on both top and bottom that extends across the entire span and chord, positioned behind the spar.
• This design is relatively simple and easier to construct because it has a minimum number of pockets.

Blade Construction-Multiple Pockets/Fairings

• Multiple-pocket blades are more costly, but pockets are simpler to replace if damaged, making repeated use possible.
• This construction approach is more flexible overall
• Blade pockets or fairings, coated with fiberglass, provide support

Blade Construction - Bonds and Bonding

• Bonding joins multiple parts using adhesive compounds.
• Using bonding is less expensive than other construction types, such as using rivets or screws.
• Manufacturers avoid drilling holes, except at blade ends, to aid in overall structural strength.

Rotor Blade Preservation and Storage

• Condemn and dispose of damaged blades.
• Repair or seal any holes to prevent moisture and corrosion.
• Remove all foreign matter with soap and water.
• Protect any bare or exposed metal with corrosion-resistant coating.
• Secure blades within shock-mounted supports when packaging and storing them.
• Record required information on the container, such as the blade's NSN, model, and serial number.

Blade Balance

• Balancing methods include chordwise, spanwise, or tracking weights.
• These are often added to the leading edge to ensure proper balance.

Blade Construction - Trim Tabs

• These tab adjustments allow controllers to position the rotor blades on a similar plane for flight.

Blade Construction- Tail Rotor Blades

• Tail rotor blades primarily provide directional control—not lift.
• Metal or fiberglass, and they are generally made using a similar method to main rotor blades, extrusions, and sheet channels.

Blade Construction -Fiberglass Blades, Metal Blades

• The blade skin for metal blades is formed around the spar and is bonded to the inside.
• Larger blades often have internal supporting structures (aluminum honeycomb and ribs) for reinforced support.
• Smaller blades may not have internal reinforcement.
• Fiberglass blade skins are formed around and bonded to H-shaped titanium spars, further reinforced by aluminum honeycomb.

Blade Construction - Blade Construction (Figures 3-10, 3-16)

• The mast is station zero, which is referred to in diagrams for balance points during manufacturing.

Blade Balance-Spanwise, Trammeling

• Adding or subtracting washers on the blade tip allows for spanwise balance.
• Other models add the corresponding weights in the cuff attachment bolts.
• Also, weights can be added or removed at the spanwise balance bolts
• Fully articulated tail rotor systems must be trammeled before balancing.
• Trammeling involves aligning the tail rotor blades equally, leading to a 2º lead angle.

Blade Construction (Figures 3-18, 3-20, 3-21, 3-22)

• The repairer must understand that changing weights changes the center of gravity.
• Adding/subtracting weight has the effect of moving the center of gravity.
• Spanwise weights are at the blade tip for attaching to the spar.
• Weights applied at different locations impact the rotor's stability.

Blade Construction - Blade Construction (Figures 3-23, 3-24)

• Attaching weights in front of and behind a blade's feathering axis will help the blade track evenly.
• Tracking weights can be shifted to adjust the orientation of the blade in the plane of rotation.
• Trim tabs allow for the blade to be moved relative to its initial position, and to alter the leading edge's flying position up or down relative to the baseline.

Rotor Blade Construction - General

• All blades require a specific build and tracking orientation in the plane of rotation.
• Failure to meet this specification leads to various issues and problems, both structurally and practically affecting flight.