Helicopter rotor blade design process


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Helicopter rotor blade design process

  1. 1. Helicopter Rotor Blade 1 Running Head: HELICOPTER ROTOR BLADE DESIGN PROCESS Helicopter Rotor Blade Design Process Mersie A. Melke Embry-Riddle Aeronautical University Daytona Beach, Florida Department of Distance Learning Instructor: James MacKay September 30, 2008
  2. 2. Helicopter Rotor Blade 2 Abstract Rotor blades of helicopters have to possess stiffness and strength that keep structural stresses within functional limits (Edwards, Davenport, 2004). The geometry should produce aerodynamic forces. The aerodynamic and structural loads developed by the rotation of the blades fluctuate throughout the rotational cycle, inducing fatigue (Edwards, Davenport, 2004). In addition, environmental effects need consideration (Edwards, Davenport, 2004). Consequently, using an appropriate material for helicopter rotor blades constitutes the study of variable requirements. The design process of helicopter rotor blades regarding strength analysis and material selection is the topic of discussion of this paper. Additionally, this paper shows historical and current rotor blade material compositions to have a practical perspective of helicopter rotor blade design.
  3. 3. Helicopter Rotor Blade 3 Structural Loading of a Rotor Blade The rotor blades of a helicopter are airfoils that provide aerodynamic forces when exposed to a relative motion of air across their surface. The rotational motion of the rotor hub initiated by the helicopter engine develops this relative motion, as well as forward, sideward and backward flight. While developing aerodynamic lift and drag forces, structural loads occur on the blades along their span and across their chord. In order to discuss these loads, one must first visualise the three dimensional geometry of the blades. Structural loads that act on rotor blades are three-dimensional. A combination of these structural loads is also possible. Due to their length, bending moments that vary along the span of the rotor occur on the blades. Because of these bending moments, the top section of the blades is compressed and tension forces act on the bottom during rotation. These forces are directly proportional to the lift generated by the blades and the radial distance from the rotor hub. Bending forces vary during the rotational cycle of the blade. This is because when a rotor blade advances in to an incoming flow of air and when the same blade retreats from the relative flow of air, the lift load produced is not equal. As a result, a variation in bending forces on the blade develops. Due to the above dynamic manifestations, the rotor blade flaps about the horizontal plane of its rotation and cyclic loads also occur at the same time. Another load is the force added during the feathering motion of the blade. Feathering refers to the twisting movement of the blade about its span wise axis. Twisting of the blades by the collective control in order to change rotor blade angle of attack is intentional. There is also a tendency for the blade rotation to force the blade to twist to a pitch angle of zero due to rotational inertia (Watkinson, 2004). During autorotation, a non-powered vertical decent, the rotor blade has three aerodynamic regions, which are the driven region, the driving region, and the stall
  4. 4. Helicopter Rotor Blade 4 region. Force vectors are not equal in each region because rotational relative wind is slower at the blade root and increases continually toward the blade tip (FAA, 2000). The above-mentioned twisting forces affect the strength of the blade. The rotor blades on a parked helicopter droop down along their span. A deflection of the blades is possible by lifting the tip.. Rotating blades are not in static equilibrium and they should accelerate towards the hub if they are to follow a circular path. This requires an inward or centripetal force. Appendix A shows that if the blade bends upwards, the downward component of the centripetal force will balance the lift at some coning angle, and the resultant will be a horizontal force only allowing no further bending to takes place. Appendix A also depicts the reaction of the blades at the rotor head. The force from the coning blade has an upward component, which is the lift, and an outward component due to the rotation. In hovering conditions, if the blades are properly balanced and all have the same coning angle, the horizontal forces cancel in the rotor head and only lift results. Adjusting all of the blades to the same angle needs a process called tracking which ensures that the collective control applies the same pitch to each blade. Unbalanced or untracked blades result in vibration. “Accurate blade balancing and tracking is important” (Watkinson, 2004). Available Materials for Rotor Blade Manufacturing From a structural point of view, a blade is essentially a continuous load-bearing member with clearly defined principal stress directions (Edwards, Davenport, 2004). In order to bear the types of loads described above, the materials used must carry these loads. At the same time, the material should exhibit a resilient character that would with stand the cyclic nature of the loads. Wood was a raw material for making blades. This turned out to be a poor choice of materials due to two reasons. One reason was that wood had a character to ingest moisture when exposed to an
  5. 5. Helicopter Rotor Blade 5 environment laden with water vapour. Consequently, the rotor blade made of wood lost their rotational stiffness, which induced vibration. Another reason was that after twisting, wooden blades permanently changed their cross sectional geometry affecting aerodynamic loading scenarios. This meant that wooden blades are not appropriate for cyclic loads. For this reason, metals replaced wood as a possible candidate for rotor blade manufacturing. Consider appendix B, which depicts the physical properties of the candidate materials able to make rotor blades. Aluminum, stainless steel and titanium are limited to specific strengths of 0.17–0.23 Mega Pascal (Mpa) and specific stiffness of around 0.025 Giga Pascal (Gpa). “The metal with the best mechanical properties is titanium but its use is controlled because of its cost and availability” (Edwards, Davenport, 2004). However, composite materials, for instance; glass fibre and epoxy, aramid fibre and epoxy and carbon fibre and epoxy have specific strengths of 0.22–0.94 MPa and specific stiffness of 0.01–0.11 GPa. Another reason for choosing composites to make rotor blades is the weight advantage available as compared to metal blades. Composite materials also have the advantage of making variable cross sections across the span of the blade, which will accommodate the variable aerodynamic loading discussed earlier. Thus, metals as compared to composites are not a viable choice. Corrosion is a problem with any metal aircraft structure especially for stressed components such as helicopter blades. Metal blades are susceptible to stress and crevice corrosion (Edwards, Davenport, 2004). The problem is particularly severe when operating near the sea. Salt water is corrosive to aluminum. Therefore, stainless steel and titanium are used. “However, these materials are difficult to process and fabricate, especially titanium, which has a high melting point” (Edwards, Davenport, 2004). Composite materials are not prone to corrosion
  6. 6. Helicopter Rotor Blade 6 problems, although prolonged exposure to moisture demands special resins for preventive maintenance activities. Composites when exposed to fatigue loads do not fail immediately. This means a crack in the matrix that makes up the composite material will lead to a loss of stiffness of the blade. This character helps operators to notice and take preventive action for the wear and tear of blades by appropriate inspections and maintenance procedures. “The Civil Aviation Authority (CAA) in the United Kingdom (UK) and Federal Aviation Authority (FAA) in the United States of America (USA) approved operating life for metal helicopter blades are around 1500–3000 hours. The CAA or FAA approved operating life for composite blades is unlimited, although most operators inspect their blades at regular intervals, typically after 10,000 hours” (Edwards, Davenport, 2004). Although composite materials have the aforementioned advantages over metals, they do have disadvantages of their own. One disadvantage is that composite blades are susceptible to erosion. Erosion, particularly in the tip region is a major problem in composite blades. However, metals do not have such kind of eroding behaviour. As a result, in areas of the rotor blade where erosion occurs like the leading edge, using metals as a cover for a composite blade helps in bringing out the advantages of both materials. “Titanium is the favourite material for the inboard region of the blade but for the more complex shaped tip region, more advanced metallic materials are employed such as electro-deposited nickel”(Edwards, Davenport, 2004). Material Content of Sample Helicopters Rotor Blades In this section the topic covered, are rotor blades of contemporary and non-contemporary helicopters and the material or materials used to make them. From this, one would be able to identify a trend in the usage of material types by helicopter manufacturers.
  7. 7. Helicopter Rotor Blade 7 Discussion of rotor blades installed on Bell Textron Helicopters and that of Boeing helicopters ensues, to get a practical perspective. Bell’s first all composite rotor blade was initiated in 1972. The 214B and 214ST helicopters installed derivatives of this design. This blade had a D spar arrangement as shown in appendix C. This D spar arrangement had axially wound fiberglass spar caps wound around a main attachment pin on the inboard end (Dobyns, Rousseau, Minguet, 2000). As a result, the fiberglass functioned as a load path for primary loads on the blade such as bending. Aerodynamic contours of these blades were the same as the ones for the preceding metal blades with part number M214A. Consequently, in 1975, full use of the design flexibility of composite blades employed, developed a blade with non-linear variation in airfoil section, thickness, chord and twist (Dobyns et al.). The M412 helicopters were the once that used this blades. Boeing started using composite rotor blades in the mid 1960’s. The rotor blades on the tandem helicopters CH-46 and CH-47 and their later variants were the initial composite material applications by Boeing. These blades mostly employed fibreglass with epoxy cured at 250 degree Fahrenheit (Dobyns et al.). The main load bearing structure is a “D” spar similar to the one shown in appendix C that transitions into a pin wrap type design at the root end. The trailing edge is a minimum core over a nomex core. The Boeing-Sikorsky built RAH-66 which is a reconnaissance and attack helicopter employs a rotor blade with a carbon and epoxy tube spar with leading and trailing edges formed from honeycomb with fibreglass face sheets for the outer surface (Dobyns et al.). Rotor Blade Design Considerations Until now, the topic was loads acting on rotor blades. In addition, examples of practical rotor blades installed on helicopters elaborated on the material of choice for rotor blade
  8. 8. Helicopter Rotor Blade 8 manufacturing. As a trend, one can see the use of composite materials on rotor blades as becoming functionally acceptable. Manufacturing advantages of composite materials over metals is one of the reasons for choosing composite materials over metals. The life of the blades also has important implications on operating cost and must be maximised to ensure economic viability. The dominant effect on the life of blades results from fatigue but in-service conditions such as corrosion and erosion also have damaging secondary effects. Improvements in these areas lead to increased reliability and reduced maintenance. However, in order to understand the design process of rotor blades, one must integrate load analysis of both static and cyclic loads, material choice based on the preceding load analysis and aerodynamic performance analysis of a rotor blade. The following paragraphs will outline design function requirements of rotor blades. “A combination of high specific strength and controllable specific stiffness is desirable in main rotor blade” (Edwards, Davenport, 2004). This is necessary in order to separate the forced vibration of the rotating blades from the natural frequencies in the airframe. A summed frequency of the rotor blade vibration and the natural frequency of the airframe will cause instability and as a result uncontrollability of the helicopter. In addition, as a supplement to the stiffness consideration of rotor blades appropriate vibration dampers and isolators will be part of the rotor head design. Because of the cyclic loads experienced by the blades, fatigue strength coupled with enhanced strain properties is desirable in a rotor blade material (Edwards, Davenport, 2004). Operating conditions of the helicopter is another design consideration of a rotor blade. Whether it’s a dusty environment or an icy one, the rotor blade should be able to withstand and develop the necessary lift force for the type of maneuver done with in the performance limit of the helicopter. The blades have to be resistant to foreign body impact. The tip of the blade, which
  9. 9. Helicopter Rotor Blade 9 is at the maximum radius, travels at the maximum achievable rotational speed or angular speed. Because of particles in the air, the tip is prone to abrasion. The erosion effect is particularly intense at take off and landing when the down draft from the rotor picks up loose materials from the ground (Edwards, Davenport, 2004). Abrasive resistant materials are therefore necessary for all the leading edges of the blades. Another environment related design consideration should be the ability to conduct lightning strikes with out structural damages. Rotor blades should be fitted with electrical conductors along their span to properly discharge sudden build-up of electrical power through them. De-icing of blades is a design consideration for helicopters operating in icy weather. Heating or de-icing of leading edge of the rotor blades prevents the formation of ice and as a result damage of helicopter structure and engine by ice thrown off the blades. Still another design consideration for rotor blades is the cost. “The main requirement is to achieve a functional design at minimum cost” (Edwards, Davenport, 2004). In order to comprehend the cost involved, considering the cost of ownership is essential. The term cost of ownership encompasses the engineering hours that go in to the design of the blade, the manufacturing hours cost, the raw material purchasing cost, the machinery lease for manufacturing cost and the maintenance necessary to keep the blade in functional condition when the blades are working. In order to optimise the cost of ownership rotor blade designers and manufacturers must ensure the decrease of initial acquisition costs by mechanizing or automating the production process (Edwards, Davenport, 2004). Conclusions Rotor blade design is an interdisciplinary function, which involves the compromise and consideration of the variables that build the design process. While planning for the production of
  10. 10. Helicopter Rotor Blade 10 the blades the factors discussed above, need consideration. These factors could supplement each other and may contradict each other. For instance to design and manufacture a rotor blade with 10,000 flight hours life time, it would require compromising between two factors which are the acquisition cost of the raw material to be used and the intended life time of the blade. The conciliation of these factors will always be with in the boundaries of the regulations dictated by governing civil aviation authorities like the FAA. In addition, rotor blade design is also an evolving area of research. Blades installed on helicopters are also data source to improve the efficiency, environmental effects and durability of the blades. The tapered tip rotor blade shown in appendix D minimizes noise, while the same is true of the British Engineered Rotor Planform (BERP) tip (Conlisk, 2001). These kinds of rotor blade design are products of research and development processes, which involve repetitive prototype advancement tested both in the laboratory and on actual helicopters. The governing factor in this particular scenario is the availability of the market that would absorb and raise profit for the designers of the blade. In summary, one can say that in order to develop a rotor blade design process the analysis of loads and stresses alone is not adequate. Similarly, separate analysis of cost of raw materials, manufacturing processes, market study or environmental effects will not suffice. A simultaneous analysis of all the variables mentioned above is a necessity. Compromise of design factors within the governing civil aviation regulations is also required. Therefore, simultaneous analysis of relevant factors and compromise within governing regulations constitute the appropriate tools to develop a rotor blade design process.
  11. 11. Helicopter Rotor Blade 11 References Conlisk, A.T. (2001). Modern helicopter rotor aerodynamics. Progress in aerospace sciences, 37, 419-476. Retrieved September 15, 2008, from http://www.sciencedirect.com/science/article/B6V3V-43W08J4-1/2/dcfa5d8e1c812922b 9bead91f47596de Dobyns, A., Rousseau, C.Q., Minguet, P. (2000). Comprehensive composite materials. Helicopters application and design, 6, 223-242. Retrieved September 15, 2008, from http://www.sciencedirect.com/science/article/B7589-49H1FC0-5X/2/57117dc3e9f7e081c ab321e3e89ed081 Edwards, K.L. and Davenport, C. (2006). Materials for rotationally dynamic components: rationale for higher performance rotor-blade design. Materials and Design, 27, 31-35. Retrieved September 11, 2008, from http://www.sciencedirect.com/science/article/B6TX5-4DPGXKG-4/2/9c20e8b7a620e9e1 9f7fc796d01526bb Federal Aviation Administration (2000). Rotorcraft Flying Handbook. (FAA-H-8083-21) Washington, DC: Author. Watkinson, J. (2004). The Art of the Helicopter. Burlington MA: Elsevier Butterworth- Heinemann
  12. 12. Helicopter Rotor Blade 12 Appendix A Inward acceleration of the blades makes them rotate. At (a) the blades cone upwards until the resultant of the lift and the blade tension is perfectly horizontal. At (b), depicted is the force balance at the rotor head. The tension in the upwardly coned blades cancels in the horizontal direction leaving only a vertical component to balance the weight of the machine.
  13. 13. Helicopter Rotor Blade 13 Appendix B Candidate material properties Young’s Specific Specific Ultimate tensile Density Material modulus stiffness strength strength (MPa) (Kg/m3) (GPa) (GPa/Kg m-3) (MPa/kg m-3) Aluminum 70 480 2770 0.025 0.173 Stainless steel 200 1240 8030 0.025 0.154 Titanium 110 1035 4430 0.025 0.234 Unidirectional composite (60% volume fraction) Carbon – high 180 1000 1600 0.113 0.625 modulus Carbon – high 140 1500 1600 0.088 0.938 strength Aramid 75 1300 1400 0.054 0.929 Glass 40 1000 1900 0.021 0.526 Woven composite (50% volume fraction) Carbon – high 85 350 1600 0.053 0.219 modulus Carbon – high 70 600 1600 0.044 0.375 strength Aramid 30 480 1400 0.021 0.343 Glass 25 440 1900 0.013 0.232
  14. 14. Helicopter Rotor Blade 14 Appendix C Cross-section through a typical composite rotor blade
  15. 15. Helicopter Rotor Blade 15 Appendix D Several rotor blades: (a) UH-60; (b) rotor blade with a tapered tip; and (c) a British Engineered Rotor Planform (BERP).