The Future of Rotary-Wing Aircraft

RICHARD M. CARLSON

James Burke, in his celebrated Public Broadcasting System television series, ''Connections'', insists that "we cannot know where we are going unless we know where we have been." In the case of rotating wings, the history dates back to the early Chinese top, Leonardo da Vinci's classic sketches, and more recently to Sir George Cayley's helicopter models. But it is generally accepted that the rotorcraft equivalent to the Wright Brothers' era started in 1923 with Juan de la Cierva, at Getafe, Spain. Cierva was attracted to the unpowered, autorotating, wing because of its lifting potential at very low forward speeds. He was motivated, for safety reasons, to provide fixed-wing airplanes with a simple device that would eliminate stall and provide the capability to land safely in the event of engine failure. Cierva called his original configuration a gyroplane, which later became known as the autogiro. The whole of Cierva's effort can be appreciated by the fact that between 1920 and 1930 he developed, and in most cases flew, some 30 different experimental autogiro types. This effort resulted in production licenses to a number of foreign countries and the production of some 480 autogiros between 1924 and 1944, 185 of which were the final model C-30.



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The Future of Aerospace The Future of Rotary-Wing Aircraft RICHARD M. CARLSON James Burke, in his celebrated Public Broadcasting System television series, ''Connections'', insists that "we cannot know where we are going unless we know where we have been." In the case of rotating wings, the history dates back to the early Chinese top, Leonardo da Vinci's classic sketches, and more recently to Sir George Cayley's helicopter models. But it is generally accepted that the rotorcraft equivalent to the Wright Brothers' era started in 1923 with Juan de la Cierva, at Getafe, Spain. Cierva was attracted to the unpowered, autorotating, wing because of its lifting potential at very low forward speeds. He was motivated, for safety reasons, to provide fixed-wing airplanes with a simple device that would eliminate stall and provide the capability to land safely in the event of engine failure. Cierva called his original configuration a gyroplane, which later became known as the autogiro. The whole of Cierva's effort can be appreciated by the fact that between 1920 and 1930 he developed, and in most cases flew, some 30 different experimental autogiro types. This effort resulted in production licenses to a number of foreign countries and the production of some 480 autogiros between 1924 and 1944, 185 of which were the final model C-30.

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The Future of Aerospace Meanwhile the vertical takeoff and landing (VTOL) enthusiasts had been hard at work developing a powered rotor, engine, and flight-control system that would accomplish manned VTOL flight. This was no simple task since the engines and speed reduction transmissions were very heavy and the powered rotors were required to provide lift, propulsion, and control of the aircraft about all three axes. Further, the powered rotor configurations were inherently unstable. It is generally acknowledged that the first successful helicopter flight was accomplished by the co-axial Breguet-Doran on June 26, 1935. This was followed one year later by the Fock-Achgelis, side-by-side rotor, model FW-61, flown by Hanah Reich in Berlin. This helicopter activity, especially the FW-61 in Nazi Germany, created great concern in the U.S. Congress, which in 1938 appropriated $2 million for rotary-wing R&D. This national interest stimulated Igor Sikorsky and the United Aircraft Corporation to develop and fly in December 1941 the VS-300, the first helicopter with a single lifting main rotor and a single tail rotor. This configuration was then applied to a U.S. Army/Air Force design requirement, resulting in the Sikorsky Model R-4 with a larger rotor and engine. The model R-4, as shown in Figure 1, was subsequently placed in production in 1942. During World War II, some 385 R-4, R-5, and R-6 helicopters were produced for use by the U.S. Army. At the conclusion of the war, a number of other pioneering FIGURE 1 Sikorsky Model R-4, the first production helicopter.

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The Future of Aerospace FIGURE 2 Piasecki XHRP, the first tandem helicopter. efforts were under way in the helicopter development arena, one of which was evolving at Piasecki Aircraft, now Boeing Helicopter Company. Piasecki had built and flown a small single-rotor helicopter similar to the R-4B and was in the process of developing the world's first tandem helicopter. It was at that time, in 1945, that Alexander Flax joined the Piasecki group to assist in designing and flying tandem helicopters like the XHRP shown in Figure 2. It did not take Al Flax long to discover that rotary-wing aircraft are different from fixed-wing airplanes. For one thing, 40 percent of the empty weight of a helicopter rotates and the lifting rotor in forward flight experiences in one revolution, as broad an aerodynamic, structural dynamic, and aeroelastic environment as a fixed wing sees in its entire lifetime. Al Flax absorbed all of this challenging physics and decided to document some of it. In 1947 he published the classic expository paper on "The Bending of Rotor Blades" in the Journal of the Aeronautical Sciences. Four years later, with coauthor Len Goland, he published a second classic paper dealing with the dynamics of rotor blades. This first paper was my introduction to the rotary-wing community, 42 years ago, when I reported to the Hiller Helicopter Company for work as a stress analyst. The chief engineer, Wayne Wiesner, handed me a copy of the paper and said "read this, Dick, and you will be the Hiller rotor blade structural expert." Twenty years later I met Al when he was at the Institute for Defense Analyses and was serving on the "blue ribbon'' review board for the Lockheed AH-56 Cheyenne helicopter. During World War II the U.S. and British governments purchased some 400 Sikorsky R-4, R-5, and R-6 helicopters, which

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The Future of Aerospace were used for observation and reconnaissance. This experience provided the catalyst for commercial and military exploitation of the helicopter's unique VTOL and hovering capabilities. Today, logistic supply, air assault, antiarmor, medical evacuation, air-sea rescue and antisubmarine applications in the military forces are as common as traffic report, disaster coverage, emergency medical service, offshore oil support, and executive transportation applications are in the commercial sector. Today 10 countries produce small, medium-size, and large helicopters for both commercial and military applications, and a number of other producer countries are coming up fast. The first R-4B helicopter could hover at about 2,000 feet on a standard day in-ground-effect (IGE), carry a crew of two, and fly at 70 miles per: hour for two hours with a 50-pound payload (the parachutes). Needless to say, the performance of today's helicopters has improved dramatically, which accounts for their current broad acceptance and application. To illustrate both the magnitude and the source of this improvement in performance, this paper follows a format used by George Schairer in a recent paper entitled "On the Design of Early Large Swept Wing Aircraft." He traced the evolution of the three basic elements of the Breguet Range Equation (Figure 3), drag efficiency (L/D), fuel efficiency (ηp/c), and weight efficiency (W1/W0), and reviewed the advances in technology that provided for the introduction of the Boeing 707 jet transport. Expanding on this format, Figure 3 portrays the mission segments that must be considered, in addition to range, to exploit VTOL capability. Vertical takeoff and landing must be available over a broad range of altitudes and ambient temperatures. The installed power (power loading, lp0) required to achieve this broad range is a function of rotor aerodynamic efficiency (figure of merit, Mf), rotor disk loading (w, a design parameter) and engine lapse rate characteristics. Many VTOL missions (e.g., military, rescue, logging) require that significant time be spent in hovering flight. As with power loading, the fuel required for this flight mode is a function of rotor aerodynamic efficiency and disk loading and, as in the range segment, a function of fuel and weight efficiency. For purposes of this presentation, weight efficiency is expressed in terms of фE and

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The Future of Aerospace FIGURE 3 Altitude, range, and hover, mixed-mode missions. фp, which are the empty weight (WE) and payload (Wp) fractions of gross weight (WG). Of course, advances in many technologies are responsible for the enhanced status of the helicopter today. It is important to acknowledge contributions from the fields of dynamics, flight control, acoustics, safety, simulation, and above all mathematical modeling and computational methods. But, in this paper, let us address only those technologies associated with the three mission segments presented in Figure 3, which are more easily quantified in terms of vehicle performance (i.e., propulsion, aerodynamics, and materials/structures). ADVANCES IN PROPULSION TECHNOLOGIES Since the introduction of the R-4B, the most significant technical event during the past 50 years has been the arrival of the turboshaft engine. The impact of changing from the reciprocating engine to the gas turbine is illustrated in Figure 4 for three powerplants originally designed to deliver 800 shaft horsepower. The R-1300 powered the Sikorsky H-34, the T-53 is in the Bell UH-1 and AH-1, and the ATDE demonstration engine provided

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The Future of Aerospace the technology for the T-800 gas turbine, which will power the new Army Comanche helicopter. The initial reduction in size and complexity provided by the gas turbine is due to the application of turbomachinery concepts to the Brayton thermodynamic cycle, resulting in much higher mass flow per unit volume in the turbine than in the reciprocating engine. Figure 5 presents historical trends in specific fuel consumption and specific weight for reciprocating and gas turbine engines used in production helicopters. The impact of the change over to gas turbines can be clearly seen. Although the specific weight dropped dramatically, the specific fuel consumption increased because materials limit the peak temperature of the Brayton thermodynamic cycle. This cycle is also more sensitive than the reciprocating engine cycles to inlet air temperature. It is interesting to note the continued improvement in specific fuel consumption and specific weight of the gas turbine. This progress is primarily due to a combination of increased efficiency of turbomachine components, increased compression ratios, and materials that allow for higher operating temperatures. FIGURE 4 Size comparison of helicopter engines rated at 800 shaft horsepower.

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The Future of Aerospace FIGURE 5 Trends in specific fuel consumption and specific weight. IMPROVEMENT IN ROTARY-WING AERODYNAMICS For many years, with a few exceptions, the subject of rotorcraft drag efficiency, or L/D ratio, has been a secondary design consideration. This has been due to the fact that forward speed performance was generally limited by the aerodynamics of the main rotor, which produces airframe vibration, and by a preponderance of low-speed mission requirements. Nevertheless, there have been improvements in rotorcraft drag efficiency, as indicated in Figure 6. These improvements have resulted from "cleaner" power plant installations and enhanced understanding of rotor-airframe interference drag. The majority of rotorcraft aerodynamics research has been directed to the main rotor. The early rotor blades (see Figure 7) were aerodynamically constrained by manufacturing, cost, and control load considerations. The early rotor blades used symmetrical airfoil sections of constant thickness-to-chord ratio with linear twist variation along the blade radius. These blades were primarily focused on improving lift efficiency in hover flight, with forward flight performance as a "fall-out." With acceptance of hydraulic "booster" controls and some im-

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The Future of Aerospace FIGURE 6 Trends in rotorcraft drag efficiency. provement in manufacturing techniques, unsymmetrical (cambered) airfoils and variable thickness/platform tip sections were incorporated, which improved both hover efficiency and forward flight capability. A third generation of main and tail rotors started to emerge in the late 1960s with the advent of reliable fibrous composite materials, initially S-glass and later graphite and Kevlar. Application of these materials and their flexible manufacturing techniques has allowed the rotor-craft aerodynamicist to develop new airfoil sections and variations in blade geometry that improve high-speed as well as hover performance. Figure 8 presents trend data for improvements FIGURE 7 Advances in rotor geometry.

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The Future of Aerospace FIGURE 8 Improvement in rotor figure of merit, Mf. in rotor figure of merit, Mf, which plays a significant role in two of three rotorcraft mixed-mode mission segments. NEW MATERIALS AND STRUCTURES Traditionally, we think of the application of new aerospace engineering materials in terms of saving weight, that is, reducing empty weight fraction. However, for rotorcraft and the application of fibrous composites, it was the promise of improved aerodynamic efficiency that proved to be the catalyst. In fact, application of these materials to rotor blades has not reduced rotor weight, since a rotor system mass moment of inertia requirement effectively eliminates any weight benefits. Notwithstanding the absence of rotor blade weight reduction, a second significant benefit has resulted from the use of fibrous composites in rotor blades. Whereas the replacement lives of metal components were notoriously low, the benign fatigue failure characteristics of composite materials have virtually eliminated the need for periodic replacement of rotor system components and, in addition, have enhanced the ballistic survivability of these components. Virtually all modern

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The Future of Aerospace FIGURE 9 Composite materials R&D programs. helicopters and many older models today employ fibrous composite blades. Having been successful with rotor blades, the rotorcraft structures engineers turned their attention to the airframe, since these composite materials still offered significant potential weight and cost savings. In the 1970s (see Figure 9) the U.S. Army and NASA initiated a number of rotary-wing composite air-frame component R&D projects to establish the feasibility of 10–35 percent reductions in weight and cost. These reductions were so attractive that, in 1982, the U.S. Army initiated a major composite airframe program (ACAP) to integrate the results of the component projects into a complete rotocraft airframe to validate the weight and cost predictions. Contracts were awarded to Sikorsky Aircraft and Bell Helicopter to design, fabricate, and flight test a helicopter with a complete composite airframe. The two major goals were to demonstrate a 22 percent reduction in weight and an 18 percent reduction in cost relative to an "all metal" baseline. Both contractors met or exceeded these goals. These two programs provided the technology base and confidence level for the use of composites in the new Army RAH-66 Comanche helicopter. Figure 10 presents the rotor-

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The Future of Aerospace craft trends in empty weight fraction, фE, and clearly shows the impact of the gas turbine. The engine weight fraction, фe, was reduced from approximately 0.12 to 0.04, or a change of 0.08, while the total empty weight fraction was reduced from approximately 0.70 to 0.55, or a change of 0.15. This reduction in empty weight fraction represents a 100 percent increase in useful load fraction for modern helicopters relative to the R-4B. It is interesting to note that, while further reductions in gas turbine specific weight (see Figure 5) have occurred since their introduction, they do not appear in the engine weight fraction trends. This is consistent with the power and disk loading trends shown in Figure 11, since decreased power loading increases empty weight and increased disk loading decreases empty weight. These trends indicate that the certificating agencies (Department of Defense and Federal Aviation Administration) and the user community have preferred to use any net weight reductions that have occurred to improve altitude and temperature performance. In any event, the empty weight trend has remained essentially level for the past 25 years (see Figure 10), and the potential empty weight fraction reductions due FIGURE 10 Trends in empty weight fraction, фE, and engine weight fraction, фe.

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The Future of Aerospace FIGURE 11 Trends in rotor craft disk loading and power loading. the application of composites to rotorcraft airframes are yet to be realized in production vehicles. Returning to Figure 3, we have now quantified the vehicle parameters that affect rotorcraft performance in the altitude-range-hover time segments of VTOL mixed-mode missions. Combining the trend changes in rotor efficiency (Mf), drag efficiency (L/D), fuel efficiency (η/c)H and (η/c)p, and weight efficiency, фE, we may now summarize the status of today's modern rotorcraft relative to the 50-year-old R-4B. This comparison is presented in Figure 12, which indicates a substantial increase in altitude vertical takeoff performance from 2,000 to 10,000 feet and a typical range-payload design point with 3 times the range and 2.75 times the payload. The modern helicopter hover time capability is preserved only by virtue of the greatly increased payload weight fraction, the total hover time rotor efficiency of the modem helicopter being less than the R-4B because of its larger disk loading.

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The Future of Aerospace FIGURE 12 Altitude, range, and hover performance comparison.

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The Future of Aerospace THE ECONOMIC OUTLOOK FOR ROTORCRAFT What does the future of rotorcraft, and its attendant technology, offer as an economic motive for those involved in the development and operation of VTOL aircraft? First, one must consider the fact that since the U.S. Army's initial purchase of 385 helicopters, its inventory grew to 8,454 vehicles in 1990, and U.S. Department of Commerce statistics indicate that since 1950 the U.S. helicopter industry has produced some 19,000 helicopters for the civilian market. The Soviet armed forces helicopter inventory stands at 4,000 today, and European production since 1950 totals approximately 15,000 vehicles, half of which have been produced by Aeroepatiale in France. Clearly a competitive VTOL market has been established, and new helicopter projects are now emerging in the United States, France, Germany, Great Britain, Japan, China, Singapore, and India. These projects are all providing increased forward speed, as indicated by the trends in Figure 13. Increased cruise speed in turn relates to increased productivity, which is generally defined as the product of payload and speed divided by empty weight (see Figure 14). FIGURE 13 Trends in helicopter cruise speed.

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The Future of Aerospace FIGURE 14 Trends in helicopter productivity. But what, if any, are the future commercial and military markets, and what is the rotorcraft industry response to these markets? The most challenging new market emerging from the commercial sector is coming from the Pacific Rim. The Japanese government has begun a program to establish an island network of heliports as an integral part of that nation's planned transportation system. Japan refers to VTOL as the Fourth Revolution, after rail, auto, and airplane. Figure 15, reproduced from the report of a study conducted by Japan Heli Network Co. Ltd., shows the projected extent of the heliport network by the year 2020. Work has begun on the first 10 heliports, and golf courses appear to have higher priority than cities. One can extend Japan's planning process south to Indonesia, Malaysia, and Singapore, and it is clear that the entire western portion of the Pacific Rim is a latent market for high-productivity VTOL aircraft. The outcome of recent events in Southwest Asia will most certainly support similar high-performance rotorcraft developments. The Desert Storm experience was the first real-life test of the U.S. Army Airborne logistics, assault, and antiarmor assets in a combined arms role. Figure 16 presents a dramatic

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The Future of Aerospace FIGURE 15 Japan's projected heliport network. view of what organic airborne capability can produce. The heavy dashed lines indicate the positions of multinational forces 12 hours and 48 hours after combat operations began. Desert Storm has been referred to as ''a high-technology test bed'' and a validation of emerging army doctrine called ALO, for air-land-operations. This new doctrine envisions changing the Army Corps area of operations from a European defensive scenario with dimensions of a 50-kilometer front and a 180-kilometer depth to a more global capability with Corp dimensions of 100 kilometers by 450 kilometers. Again, it is clear that this dramatic change invites the development of new high-performance VTOL aircraft. Since the 1950s the rotary-wing community has been actively pursuing alternate VTOL configurations that produce performance improvements beyond those of the helicopter, and these efforts are now showing signs of success. Tilt-wing performance increases were clearly demonstrated in the 1960s with the XC-142, and more recently tilt-rotor developments such as the XV-15 and the V-22 have confirmed (see Figure 17) sub-

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The Future of Aerospace stantial performance improvements. Recent NASA high-speed rotorcraft studies, based on past experimental flight tests and many wind tunnel tests, clearly indicate these configurations are capable of producing substantially greater productivity and range than the conventional helicopter. New commercial projects have been initiated to match the capabilities of these advanced rotorcraft configurations to new emerging markets. Three of these projects are the European EUROFAR tilt rotor, the Japanese Ishida tilt wing, and the U.S. National Civil Tilt Rotor initiative (Bell-Boeing), which is led by the FAA Vertical Flight Program with strong support by NASA and Congress (see Figure 18). Fifty years of research and development in the rotary-wing field have produced numerous improvements since the first flight of the R-4B. The helicopter today is no longer an oddity but an integral part of our daily life. The desire to take off and land vertically and fly in the earth's boundary layer has per- FIGURE 16 Positions of multinational forces in Operation Desert Storm.

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The Future of Aerospace FIGURE 17 Tilt-rotor developments XV-15 (left) and the V-22 (right). FIGURE 18 New tilt-rotor and tilt-wing developments: (left) Eurofar; (center) Ishida; (right) Bell-Boeing.

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The Future of Aerospace sisted, and technology and dedication have accommodated that desire. We have seen that the advent of the turbine engines, application of composite materials, and advances in rotary-wing aerodynamics have had a profound impact on the utility and acceptance of the helicopter. The future of the helicopter is clearly assured, and advanced rotary-wing aircraft, with an additional degree of rotation, are ready to extend the performance and utility of the helicopter.

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