Rotor Blades - AVIA-1035

AVIA-1035 1  Page 3-5 FM1-514-1991 Rotor Blades  The design and construction of a rotor blade vary with the manufacturer, although they all strive to manufacture the most efficient and economical lifting device.  The pa...

AVIA-1035 1  Page 3-5 FM1-514-1991 Rotor Blades  The design and construction of a rotor blade vary with the manufacturer, although they all strive to manufacture the most efficient and economical lifting device.  The particular helicopter design places certain requirements on the main rotor blades, which influence their design and construction. 2  Page 3-5 FM1-514-1991 Rotor Blades  Most rotor blades are designed as symmetrical airfoils to produce a stable aerodynamic pitching characteristic.  Aerodynamic stability is achieved when the center of gravity, center of pressure, and blade-feathering axis all act at the same point.  The blade is more stable in flight because these forces continue to act at almost the same point as the blade changes pitch. 3  Page 3-5 FM1-514-1991 Rotor Blades  Assymetrical airfoils are not as common but are becoming more popular.  This unsymmetrical airfoil blade is capable of producing greater lift than a symmetrical airfoil blade of similar dimensions.  Aerodynamic stability is achieved by building a 3° upward angle into the trailing edge section of the blade.  This prevents excessive center-of pressure travel when the rotor blade angle of attack is changed. 4  Page 3-6 FM1-514-1991 Rotor Blades  A variety of materials are used in the construction of rotor blades; aluminum, steel, brass, and fiberglass are most common.  The first rotors were made of wood and used on several early models. Many are still in use today.  Next came metal blades followed by composite blades utilizing several types of materials. This is the newest type of blade. Page 3-6 FM1-514-1991 5 Rotor Blades 6  Page 3-6 FM1-514-1991 Rotor Blades - Wood  Wooden Rotor Blades  First production rotor blades were laminated wood.  Various types of wood; birch, spruce, pine and balsa, in combinations to obtain the strength and aerodynamic shape necessary for rotor construction  Steel Core was placed within the wood lamination near the leading edge of the blade.  Exterior surface covered with a resin impregnated fiberglass cloth Page 3-6 FM1-514-1991 7  Page 3-6 FM1-514-1991 Rotor Blades - Wood  Wooden Rotor Blades 8  Page 3-6 FM1-514-1991 Rotor Blades - Wood  Approximately 2/3’s of the outboard portion of the leading edge of the blade is protected with a stainless steel cap for abrasion purposes.  Due to variations of the wood, most blades are in matched pairs, meaning that one blade cannot be changed by itself.  During construction each blade is matched to a master blade and then matched to the pair.  A disadvantage is the effect of moisture on the blade. This situation can be corrected with a short run-up of the helicopter. 9  Page 3-6 FM1-514-1991 Rotor Blades - Metal  Metal (Aluminum)  Metal Blades have been in production for over 40 years. Because of construction expense and the various manufactures involved, the construction varies considerably.  A typical metal blade has a hollow, extruded aluminum spar which forms the leading edge of the blade (Figure 3-6).  A distinct advantage to the metal blade is the quality control during construction.  10  Page 3-6 FM1-514-1991 Rotor Blades - Metal  Single blades may be changed without the use of matched sets.  Aluminum pockets bonded to the trailing edge of the spar assembly provide streamlining.  An aluminum tip cap is fastened with screws to the spar and tip pocket.  Like other metal dynamic components exposed to stresses induced in flight, the metal blade is life limited. 11  Page 3-7 FM1-514-1991 Rotor Blades  Figure 3-6 12  Page 3-6 FM1-514-1991 Rotor Blades - Metal  A steel cuff bolted to the root end of the spar provides a means of attaching the blade to the rotor head.  A stainless steel abrasion strip is adhesive-bonded to the leading edge.  One item that all blades have in common is a bonded type of construction.  This is done by a heat and pressure process and has some advantages 13  Page 3-6 FM1-514-1991 Rotor Blades - Composite  Fiberglass or Composite Blades  The main load-carrying member of a fiberglass blade is made of either a Fiberglass Spar or a Metal Spar. ( Fig 3-7)  A typical fiberglass spar blade makes use of a procured roving spar. Roving's are strings or strips of glass material.  This material is impregnated with epoxy resin and wound around a foam core.  The skins of the blade are made of fiberglass cloth. 14  Page 3-6 FM1-514-1991 Rotor Blades - Composite  In addition, root reinforcement plates are added  Trailing edge foam filler and with the trailing edge is another roving strip.  The fairing or pockets are fiberglass covered bonded over either aluminum ribs or aluminum foil honeycomb.  The leading edge of the blade is protected with a stainless steel strip covering the span of the leading edge.  Balancing weight is added to the tip by the manufacture 15  Page 3-7 FM1-514-1991 Rotor Blades  Figure 3-7 Composite Blade – Aluminum Hollow Spar 16  Page 3-7 FM1-514-1991 Rotor Blades  Figure 3-7 Composite Rovings 17  Page 3-6 FM1-514-1991 Rotor Blades  Additional protection is added with a rubber erosion strips bonded to the lower surface of the blade  A steel socket threaded to the blade spar shank provides an attaching point to the rotor head.  A stainless steel tip cap is fastened by screws to the blade spar and blade tip pocket.  Internal grounding strips are bonded in place to transfer the static electricity to the aircraft. 18  Page 3-6 FM1-514-1991 Rotor Blades  Blade Nomenclature  Planform  The blade planform is the shape of the rotor blade when viewed from above (Figure 3-8).  It can be uniform (parallel) or tapered.  Uniform planforms are most often selected by the manufacturer because, with all the ribs and other internal blade parts the same size, they are easier to make. 19  Page 3-7 FM1-514-1991 Rotor Blades  Figure 3-8 20  Page 3-7 FM1-514-1991 Rotor Blades  Old video of man hit by Helicopter Blades (:43) 21  Page 3-6 FM1-514-1991 Rotor Blades  The uniform blade requires only one stamping die for all ribs, which reduces blade cost.  This design has a large blade surface area at the tip; it must therefore incorporate negative tip twists to make a more uniform lift along the blade span.  If the blade angle is the same for the length of the blade, the blade will produce more lift toward the tip because it moves at a higher speed than the blade root.  This unequal lift will cause the blade to cone too much or bend up on the end. 22  Page 3-6 FM1-514-1991 Rotor Blades  The tapered planform blade makes a more uniform lift throughout its length.  Few blade manufacturers use it, however, because the manufacturing cost is too high due to the many different-shaped parts required to fit the tapered airfoil interior. 23  Page 3-6 FM1-514-1991 Rotor Blades  Twist  The blade-element theory applies to a rotor blade as well as to a propeller.  Therefore, most rotor blades are twisted negatively from root to tip to get more even distribution of lift. 24  Page 3-6 FM1-514-1991 Rotor Blades  The SKIN may be fiberglass or aluminum and may consist of single or multiple layers.  The thin skin can easily be damaged by careless handling on the ground.  Most main rotor blades are of either single-pocket or multiple-pocket construction. 25  Page 3-7 FM1-514-1991 Rotor Blades  Root  The blade root is the section nearest the center of rotation that provides a means of attachment to the rotor head (Figure 3-9).  It is heavier and thicker than the rest of the blade to resist centrifugal forces. 26  Page 3-7 FM1-514-1991 Rotor Blades  Root 27  Page 3-7 FM1-514-1991 Rotor Blades  Figure 3-9 Blade Root 28  Page 3-7 FM1-514-1991 Rotor Blades  Tip  The tip is located furthest from the center of rotation and travels at the highest speed during operation (Figure 3-10).  The blade tip cap also has a means for attaching balance weights. 29  Page 3-8 FM1-514-1991 Rotor Blades  Figure 3-10 30  Page 3-8 FM1-514-1991 Rotor Blades  Leading Edge  The part of the blade that meets the air first is the leading edge (Figure 3-11).  For the edge to work efficiently, airfoils must have a leading edge that is thicker than the trailing edge.  The leading edge of all blades is covered with a hard, abrasion-resistant cap or coating to protect against erosion caused by sand and dust. 31  Page 3-8 FM1-514-1991 Rotor Blades  Figure 3-11 32  Page 3-8 FM1-514-1991 Rotor Blades  Trailing Edge  Trailing edge is that part of the blade that follows or trails the leading edge and is the thinnest section of the airfoil (Figure 3-12).  The trailing edge is strengthened to resist damage, which most often happens during ground handling. 33  Page 3-8 FM1-514-1991 Rotor Blades  Figure 3-12 34  Page 3-8 FM1-514-1991 Rotor Blades  Span and Span Line  The span of a rotor blade is its length from root to tip (Figure 3-13).  The span line is an imaginary line running parallel to the leading edge from the root of the blade to the tip.  Span line is important to the blade repairer because damages are often located and classified according to their relation to it. 35  Page 3-8 FM1-514-1991 Rotor Blades  Figure 3-13 36  Page 3-8 FM1-514-1991 Rotor Blades  Defects paralleling the span line are usually less serious because stress lines move parallel to the span line and would therefore pass the damage without interruption. Chordwise damage interrupts lines of stress. 37  Page 3-8 FM1-514-1991 Rotor Blades  Chord and Chord Line  The chord of a rotor blade is its width measured at the widest point (Figure 3-14).  The chord line of a rotor blade is an imaginary line from the leading edge to the trailing edge, perpendicular to the span line.  Blade chord line is used as a reference line to make angular measurements. 38  Page 3-8 FM1-514-1991 Rotor Blades  Figure 3-14 39  Page 3-8 FM1-514-1991 Rotor Blades  Spar  The main supporting part of a rotor blade is the spar (Figure 3-15).  Spars are usually made of aluminum, steel, or fiberglass; they always extend along the span line of the blade.  Often the spar is D-shaped and forms the leading edge of the airfoil.  Spars are of different shapes, depending on the blade material and on how they fit into the blade airfoil. Page 3-8 FM1-514-1991 40  FM1-514-1991 Rotor Blades  Figure 3-15 41  Page 3-9 FM1-514-1991 Rotor Blades  Doublers  Doublers are flat plates that are bonded to both sides of the root end of some rotor blades to provide more strength.  Not all blades use doublers since some spars are made thick enough to provide the needed strength at the root end. 42  Page 3-9 FM1-514-1991 Rotor Blades  Top  The low-pressure side of the blade is the top. The top is the blade surface which is viewed from above the helicopter. It is usually painted olive drab, flat grey or flat black when the blade skin is plastic or metal. 43  Page 3-9 FM1-514-1991 Rotor Blades  Bottom  The high-pressure side of the blade is the bottom.  The bottom is the blade surface which is viewed from the ground.  It is always painted a lusterless (Flat-no gloss) black to prevent glare from reflecting off the blade and into crew compartments during flight. 44  Page 3-9 FM1-514-1991 Rotor Blades  Blade Stations  The mast is usually station zero, and station numbers move outward to the blade tip (Figure 3-16). 45  Page 3-9 FM1-514-1991 Rotor Blades  Figure 3-16 46  Page 3-9 FM1-514-1991 Blade Construction  Single Pocket or Fairing  The single-pocket or fairing blade is made with a one-piece skin on top and bottom (Figure 3-17).  Each skin extends across the entire span and chord, behind the spar.  This style is simple and easy to make because of the minimum number of pockets or fairings that need positioning and clamping during the bonding process. 47  Page 3-10 FM1-514-1991 Blade Construction  However, minor damage to the skin often results in the blade being thrown away since replacing the skin costs more than replacing the blade. 48  Page 3-10 FM1-514-1991 Blade Construction  Multiple Pockets or Fairings  Most large rotor blades built with the multiple-pocket or fairing shape behind the spar are costly (Figure 3-18).  This type of blade is selected since damage to the skin cover requires that only the pocket (or fairing) be replaced.  The high-cost blade can then be used over and over.  This type of blade is more flexible across the span, which cuts down on blade vibrations. 49  Page 3-10 FM1-514-1991 Blade Construction  Figure 3-18 50  Page 3-10 FM1-514-1991 Blade Construction  Internal Structural Components  Rotor blades have internal structural parts that help to support the blade skin – ribs, I- beams, spanwise channels, and aluminum honeycomb foil. 51  Page 3-10 FM1-514-1991 Blade Construction  Bonds and Bonding  Bonding is a method of putting two or more parts together with an adhesive compound.  Bonding helps reduce the use of hardware like bolts, rivets, and screws that need holes and therefore weaken the strength of the bond.  To ensure full strength, manufacturers never drill holes in load-carrying parts of the blade except at the inboard and outboard ends. 52  Page 3-10 FM1-514-1991 Blade Construction  However, bonds react to the chemical action of paint thinners and many cleaning solvents.  Careless use of these solvents will dissolve bonded joints.  The surface area where two objects are bonded together is known as the faying surface (Figure 3-19). 53  Page 3-11 FM1-514-1991 Blade Construction  Figure 3-19  Composite Blade Construction (3:31) 54  Page 3-9 FM1-514-1991 Blade Construction  Blade Balance  Three types of weights to balance the blade are mass chordwise, spanwise, and tracking (Figure 3-20).  Mass balance weights (bars) are placed into the leading edge of a blade while the blade is being made(Figure 3-21).  This is to ensure that correct chordwise balance is about 25 percent of chord.  The type of metal and its shape and location vary with the manufacturer. 55  Page 3-11 FM1-514-1991 Blade Construction  Figure 3-20 56  Page 3-11 FM1-514-1991 Blade Construction  Figure 3-21 57  Page 3-11 FM1-514-1991 Blade Construction  The repairer is not allowed to move the weights in most helicopter blades.  When moving of weights is allowed, however, the repairer must remember that changing weights will move the center of gravity forward or backward.  Spanwise balance weights are at the tip of the blade, usually where they can be attached securely to the spar (Figure 3-22).  They are normally installed in the blade during manufacture. 58  Page 3-12 FM1-514-1991 Blade Construction  Figure 3-22 59  Page 3-11 FM1-514-1991 Blade Construction  When movement is necessary, the repairer should always remember that adding spanwise weight moves the center of gravity outward.  Subtracting weight moves the center of gravity inward.  When moving the span-wise weight is permitted, the weight change is computed by the repairer mathematically after the blade has been weighed. 60  Page 3-11 FM1-514-1991 Blade Construction  To be efficient and vibration-free, all rotating blades should track on about the same level or plane of rotation. Failure of blades to track correctly causes vibrations which can —  Damage parts of the helicopter.  Reduce riding comfort.  Cause a loss in blade performance due to air turbulence made by the rotating blades. 61  Page 3-11 FM1-514-1991 Blade Construction  One way of retaining track is to attach tracking weights in front of and behind the feathering axis at the blade tips (Figure 3-23).  By adding removing or shifting tracking weights, the repairer can move a blade track up or down to match the track of the other blade or blades.  This causes all blades to move in the same tip path plane. 62  Page 3-12 FM1-514-1991 Blade Construction  Figure 3-23 63  Page 3-11 FM1-514-1991 Blade Construction  Trim Tabs  Another method used to align the rotor blade on the same plane of rotation is the use of trim tabs (Figure 3-24).  Using tracking weights adds to building costs, but the same results may be achieved by cheaper methods; for example, putting a sheet metal trim tab on the trailing edge of the blade.  The trim tab is usually located near the tip of the blade where the speed is great enough to get the needed aerodynamic reaction. 64  Page 3-12 FM1-514-1991 Blade Construction  Figure 3-24 65  Page 3-12 FM1-514-1991 Blade Construction  Figure 3-24 66  Page 3-11 FM1-514-1991 Blade Construction  In tracking operations the trim tab is bent up to make the leading edge of the rotor blade fly higher in the plane of rotation.  Or it is bent down to make it fly lower.  The trim tabs are adjusted until the rotor blades are all flying in the same plane of rotation. 67  Page 3-11 FM1-514-1991 Blade Construction  Tail Rotor Blades  Tail rotor blades are used to provide directional control only.  Made of metal or fiberglass, they are built similarly to main rotor blades.  Metal tail rotor blades are made of aluminum; the spars are made of solid aluminum extrusions, hollow aluminum extrusions, and aluminum sheet channels. 68  Page 3-12 FM1-514-1991 Blade Construction  Fiberglass tail rotor blades are made of fiberglass sheets; the spars are made of solid titanium extrusions. Refer to Figure 3-25. 69  Page 3-12 FM1-514-1991 Blade Construction  Figure 3-25 70  Page 3-12 FM1-514-1991 Blade Construction  Metal Blades  The blade skins are formed around and bonded to the spars, which in most cases form the leading edge of the blades.  Metal blade skins are supported from the inside with aluminum honeycomb and ribs. Some smaller blades have no bracing or support inside them. 71  Page 3-12 FM1-514-1991 Blade Construction  Fiberglass Blades  The blade skins are formed around and bonded to H-shaped titanium spars.  The blade skins are supported inside with aluminum honeycomb.  The space around the spar is filled with foam plastic. 72  Page 3-13 FM1-514-1991 Blade Construction  Blade Balance  Spanwise  On some models spanwise balance is accomplished by adding or subtracting washers on the blade tip.  On others the washers are added to the blade-cuff attaching bolts.  On some models blades are balanced chordwise by adding weights to the tips behind the spanwise balance screw. 73  Page 3-13 FM1-514-1991 Blade Construction  Other models are balanced by adding weights to the trailing edge of the blades near the cuff end.  Trammeling  Fully articulated tail rotor systems must be trammeled before they are balanced.  Trammeling consists of aligning the tail rotor blades an equal distance to one another with a 2° angle of lead to the blades. 74  Airframe 2-34 Rotor Blade Preservation and Storage  Accomplish the following requirements for rotor blade preservation and storage:  Condemn, and dispose of locally, any blade which has incurred nonrepairable damage.  Tape all holes in the blade, such as tree damage, or foreign object damage (FOD) to protect the interior of the blade from moisture and corrosion.  Thoroughly remove foreign matter from the entire exterior surface of blade with mild soap and water. 75  Airframe 2-34 Rotor Blade Preservation and Storage  Protect blade outboard eroded surfaces with a light coating of corrosion preventive or primer coating.  Protect blade main bolt hole bushing, drag brace retention bolt hole bushing, and any exposed bare metal (i.e., grip and drag pads) with a light coating of corrosion preventive.  When packaging, secure blade in a shock-mounted support and secure the container lid. 76  Airframe 2-35 Rotor Blade Preservation and Storage  Place copy of manufacturer’s blade records, containing information required by CARs, and any other blade records in a waterproof bag and insert into container record tube.  Eliminate old markings from the container that pertained to the previous shipment or to the original item it contained. 77  Airframe 2-35 Rotor Blade Preservation and Storage  Stencil the blade National Stock Number (NSN), model, and serial number, as applicable, on the outside of the container.  Modern Rotor Blades (2:57) 78