Sikorsky Utility Helicopters, A Hard Beginning
One of Igor Sikorsky’s most oft quoted words of wisdom is about the value of individual creativity when he observed that “The work of the individual still remains the spark that moves mankind ahead even more than teamwork.” His personal aeronautical achievements along with those of the company that he founded were the direct result of individual creativity often nourished and brought to life by team efforts. Creativity has always been the foundation of success in every field of human endeavor and that has been especially true at the company that Igor Sikorsky founded. His experimental VS-300, first flown in 1939, led directly to the R-4 model which was the first production helicopter in the world creating a whole new industry.
Although it was only a light two-passenger observation machine, the R-4 and its successors provided a small glimpse into how the helicopter might serve the human scene in war and in peace. Most memorable among those glimpses was the unique capability to rescue people in distress from places that were inaccessible by other means. The first recorded military rescue was in April 1944 in the jungles of Burma when an R-4 rescued one pilot and three wounded soldiers in Japanese held territory, making three trips to do so. The next year, in November 1945, an R-5 performed the first civilian rescue by lifting two crewmen from a barge being blown toward the rocks at Penfield Reef during a raging northeaster on Long Island Sound.
Those and other early helicopter exploits ignited the imagination of aeronautical designers and potential helicopter operators across the world. Their imagination and creativity made this extraordinary machine both commonplace and indispensable. Achieving that stature required continued innovation and that was true at Sikorsky Aircraft particularly when the existence of this company became in serious question immediately following the end of the Second World War.
The R-4 design used the now widely accepted single tail rotor configuration that Igor Sikorsky settled on from his experimentation with various tail rotor arrangements on his VS-300. The R-4 was a larger helicopter with a 38 ft. main rotor compared to the VS-300 30 ft. Its engine was a 200 HP Warner R-550. Gross weight at 2540 lb was more than double that of the VS-300. Empty weight was 2011 lb. weight was 2011 lb.
Over 400 units of the R-4 and its follow-on R-5 and R-6 models were produced during the wartime period from 1942 through 1945 with production rates reaching ten units per month.
The end of WWII resulted in cancellation of many R-5 orders for completed helicopters lined up at Sikorsky’s flight field in Bridgeport CT.
During the early post WWII years, the Army began to envision helicopters able to transport troops and cargo while the Navy was developing new requirements for personnel rescue and for anti-submarine missions. These new missions required substantially greater payloads than helicopters produced to that time were capable of carrying. Good experience with the small Sikorsky observation type helicopters encouraged military planners to envision more demanding missions, particularly naval missions involving shipboard operation. Nearly all of these missions were for helicopters that fit within what is now considered to be the utility or intermediate size category, effectively creating the beginning of the utility transport market.
Design innovation and creativity were needed for the helicopter to reach its true potential in the years following the emergence of the R-4 models because military missions began to push the limits of the early helicopter capabilities. That was especially true at Sikorsky Aircraft during the post WWII years when its future began to look dismal as military orders for its products ceased to exist. The euphoria of the R-4 series production records soon turned to despair when the company realized that the successful R-4 design could not be scaled up to larger sizes nor could that configuration be competitive with emerging new designs offered by other companies. Sikorsky Aircraft desperately needed design and technology innovations to create a new configuration able to compete in the new post-war marketplace. Fortunately, during years of declining business, the company was able to develop the right combinations of new technologies and innovative designs that propelled it to great success. This is the story of the individuals and their “sparks” that helped build the foundation for Sikorsky Aircraft’s preeminence in the helicopter industry and more recently in the utility helicopter market in particular.
Sikorsky’s initial entry into this emerging utility market was anything but a success. In fact it was a total failure. That failure was especially serious because it was caused by the company’s basic helicopter configuration, as it existed in that era. The company found that it could not produce a successful large helicopter simply by scaling up the configuration that proved to be so successful for small helicopters. That configuration, embodied in Sikorsky’s light observation models had a basic flaw that limited its application to small helicopters. Sikorsky recognized this problem after losing the very first competition that it entered for a utility size helicopter. That competition, begun in 1945, was for a U.S. Navy search and rescue helicopter whose payload and range exceeded the capabilities of Sikorsky’s existing models.
Sikorsky entered this Navy competition with a new design that it called the S-53 that the Navy identified as the XHJS-1. The S-53 was a relatively large helicopter at the time with a gross weight of 5500 pounds and powered by a Continental R-795-34 engine rated at 525 HP. But the basic problem of the S-53 was that it was patterned after earlier configurations and its larger size simply magnified the shortcomings inherent in those earlier models.
The engine location in the S-53, directly below the main rotor, followed the practice of all prior Sikorsky models such that all disposable payload had to be placed forward of the rotor rotational axis. Similar to past practice, the fully articulated main rotor had zero flapping hinge offset which greatly limited the control moments that the rotor could produce which in turn severely limited excursion of the center of gravity created by payload variation
The final competitors for this new Navy mission were Sikorsky and Piasecki, who proposed a tandem rotor design. Both companies were awarded contracts to build prototypes for the Navy’s competitive fly-off evaluation. Sikorsky built three of the XHJS-1 models and Piasecki built three XHJP-1 models. Both models were designed to the same mission requirements and used the same engine as specified by the Navy. That was the first direct competition between Sikorsky Aircraft and the predecessor company of Boeing Vertol for a new utility helicopter. A quarter century later in 1972 the same two companies would again be in a head-to-head competition for a new generation helicopter, this time for a U.S. Army utility mission called the Utility Tactical Aircraft Transport System (UTTAS) that later evolved into the UH-60 Black Hawk. As in the earlier Navy competition, the competing UTTAS designs would also be based on the same specification and the same engine. Innovations and creativity would again decide the outcome.
The XHJS-1 was flown for the first time in September 1947 by veteran Sikorsky pilot Bob Decker and, after less than 18 months of design and development, the prototypes were delivered to the Navy for evaluation. Flight tests, conducted by Navy pilots at NATC Patuxent River revealed that the Sikorsky design had serious deficiencies. It was severely limited in its allowable center-of-gravity (CG) range as well as in control of aircraft pitch and roll attitudes and rates. The XHJS-1 was unable to handle the excursions in CG caused by loading or unloading a rescued person to the extent that it was eliminated from the Navy’s competitive evaluation.
The Navy selected the Piasecki tandem rotor design for its first search and rescue helicopter. Later, that design evolved into the Piasecki HUP series of which hundreds were built for the U.S. Army and the U.S., Canadian, and French Navies.
Soon after the search and rescue competition, Sikorsky again lost a U.S. Navy competition for an anti-submarine warfare and mine sweeping helicopter. The winner was the Bell Helicopter Company who entered the utility market with a tandem rotor helicopter configuration, the Bell HSL. That design, which was Bellrotor machine, proved to be unsuccessful and the program was terminated.’s first and last tandem
After losing that competition to Bell, coupled with the earlier loss to Piasecki, the future of Sikorsky’s single rotor configuration became uncertain and the company saw little if any new business opportunities. Sikorsky Aircraft’s future looked bleak indeed faced with essentially no market for observation helicopters and unable to penetrate the new and potentially larger utility market. In his unpublished history of Sikorsky Aircraft, Igor Sikorsky’s cousin, I.A. “Prof” Sikorsky states on page 183: “After Sikorsky lost out, the company was again virtually without business and few prospects seemed available for it”.
Basic design flaws
The primary reason for Sikorsky’s difficulty in its ability to scale up its earlier configuration was because of the location of its engine, which was placed directly under the main rotor. The engine was located under the rotor in order to simplify the transmission gearing, however that location forced the payload to be carried in a much less desirable location. That location was just behind the cockpit and well forward of the main rotor where the helicopter’s center-of-gravity, CG, is positioned by design. Because of that forward location, the loading and off-loading of payload caused large fore and aft excursions of CG position, which in turn adversely affected the helicopters pitch attitude and flying qualities. However, the small earlier helicopters, with their limited payload capacity, could cope with small CG movement by using ballast that could be moved before flight. That ballast, however, reduced the already small payload capacity. But for larger helicopters, payload would need to be carried near or under the main rotor and a new location would have to be found for the engine.
The second problem was with the design of the main rotor head and, in particular, with location of the blade flap hinge. Sikorsky’s main rotors have historically been of the fully articulated design in which the blades are essentially free to flap up and down as well as to lead or lag in response to aerodynamic and inertia forces. This freedom of motion was provided by what were called flap hinges and lag hinges. Early Sikorsky rotors were designed with the flap axis located on the rotor’s rotational axis with the intent of reducing vibratory loads in the rotor head and airframe. This configuration, referred to as a zero offset rotor, was unable to generate significant control moments. When the rotor thrust vector was tilted through application of cyclic pitch, the control moment available to change aircraft pitch and roll attitudes was simply the rotor thrust multiplied by the thrust vector’s distance from the aircraft’s center-of-gravity. This control power limitation is similar to that of a teetering two-blade rotor and is insufficient to accommodate large center-of-gravity excursions or to provide a high level of maneuver control power.
The search for a workable solution
The emergence of the tandem rotor design encouraged Sikorsky pioneers to consider adding a horizontal tail rotor to their single main rotor configuration as a means of providing greater pitching moments to compensate for center of gravity excursion. This new approach was embodied in the S-54, which was strictly an experimental model not intended for production. Its purpose was to explore the effects on performance and flying qualities of the single rotor arrangement modified to have a small horizontal rotor mounted at the tail along with a conventional tail rotor to counter torque as well as to provide yaw control. Because this horizontal tail rotor produced lift, which added to that of the main rotor, this configuration was called a sesqui-tandem design.
The S-54 was built by modifying an existing R-4B that was borrowed from the U.S. Air Force. It made its first flight on December 20, 1948 and was retired in early 1949 after having been flown 4 hours and 20 minutes. This S-54 configuration was found to produce the desired results however it was too complex and ungainly to be considered as the right solution.
The experimental S-54 was an R-4B model with a 13 foot diameter, three-bladed rotor mounted in the horizontal plane several feet above the 5 foot diameter tail rotor.
Viewed from the rear, the S-54’s horizontal lifting tail rotor with its cyclic pitch control system and supporting truss structure along with the original R-4 tail rotor are clearly seen.
The search for a better solution continued and the breakthrough for Sikorsky was achieved by two major design innovations introduced during the late 1940s. Together, these innovations permitted the design of larger single rotor helicopters having large cabin volume and unprecedented control power. Both innovations were implemented in an extraordinary short time and flown on a new Sikorsky design, the S-55, which made its first flight in November 1949 less than a year from go-ahead. This all-new model was able to demonstrate its capabilities in time to rescue Sikorsky’s position in the industry. These design innovations paved the way for Sikorsky to enter the utility transport market with a configuration able to challenge Frank Piasecki’s successful tandem rotor design.
Sikorsky’s chief of advanced design and later chief engineer, Edward F. Katzenberger, created the first major innovation. His idea was to relocate the reciprocating piston engine from under the rotor to a location forward of the cabin and
below the cockpit. This was a dramatic configuration change that opened up a spacious cabin compartment directly below the main rotor. As a result, payload could then be carried on or near the aircraft’s center of gravity and essentially in line with the rotor’s rotational axis. This relocation of the engine also made the engine more accessible and maintenance easier by virtue of large clamshell doors that opened to expose the engine at ground level.
The S-55 was produced for every service of the US military as well as for many foreign governments and its configuration was copied as well.
The innovative engine placement in the nose of the helicopter provided excellent engine access and, more importantly, permitted a large cabin directly beneath the main rotor. The engine installation angle was chosen to permit its drive shaft to travel up between the pilots and enter the main transmission at the optimum location. Access to the cockpit was through the cabin and up a short stairway.
The second innovation was the creation of the so-called offset flapping hinge concept that was first flown on Sikorsky’s S-52 designed by Sikorsky pioneer Ralph Alex, who was one of the founders of the American Helicopter Society. He relocated the flapping hinge outboard of the rotational axis by 7.56 inches with the intention of simplifying the rotor hub design. What was found during flight-testing was that not only were the vibratory forces found to be tolerable but aircraft control and maneuverability was greatly enhanced from earlier models.
The blades’ flapping motion, which produces a once-per-revolution vertical shear at the flap hinge, produces a steady moment at the rotor head for rotors with three or more blades. As a result, the main rotor became capable of generating large control moments able to compensate for variations in center-of-gravity movements and, equally important, significantly improved the helicopter’s trim and maneuverability. It was because of this control power, that it was possible for the S-52 to become the first helicopter to be able to perform a 360-degree loop maneuver as well as to demonstrate unprecedented agility. Unlike zero offset rotors or two-blade teetering rotors wherein control power is directly dependent on thrust, the offset rotor retains significant control power even when its thrust is lowed as during push over, low g maneuvers.
The coincident lag and flap hinges of this S-55 rotor head were offset 9 inches from the axis of rotation. Connected to the flap hinge was the sleeve/spindle assembly that provided blade pitch change motion. All hinges were grease lubricated which dominated the maintenance burden of early fully articulated rotors.
Because of the very significant benefits provided by the relocated engine and the offset flapping hinge rotor, as well as by its hydraulically boosted flight control system discussed below, the S-55 quickly captured a major share of the utility market. Its great success completely reversed Sikorsky’s business prospects for the immediate future.
The H-19 version of the S-55 became an early troop carrier for the U.S. Army during the Korean conflict. It set the stage for creation of the Air Mobility Doctrine that dramatically changed battlefield operations. That change in doctrine enormously expanded the role of the helicopter and the size of its industry.
Two other developments by Sikorsky engineers added to the rapid expansion of the utility market. The first was the development of irreversible hydraulic servos to assist the pilot in controlling helicopter flight. The effort required to control rotor blade pitch was becoming unmanageable because of the rapidly increasing magnitude of control system loads as helicopter size and speed increased. Early Sikorsky models like the S-51 employed irreversible mechanical jackscrews to try to isolate rotor blade control loads from the pilot’s cyclic and collective control sticks, but that approach was not practical for larger machines and even in moderate sized aircraft the friction and aerodynamic forces severely limited the pilot’s ability to achieve precise control.
It was a pioneer in the field of dynamics at Sikorsky, Walter Gerstenberger, who proposed the idea of using hydraulically powered controls to overcome this problem. While other helicopter innovators attacked this problem with mechanical stabilizer bars and servo tabs, Gerstenberger took the brute force approach, which was just beginning to be exploited by the fixed wing industry. But he went well beyond the fixed wing practice of using only booster servos by proposing the installation of irreversible hydraulic servo on all helicopter control axes.
In the late 1940s, hydraulic servos were test flown on a modified S-51 with very enthusiastic acceptance by the pilot, who no longer had to “fight” the controls. Irreversible hydraulic servo controls became standard for all subsequent Sikorsky models with the S-55 being the first production model to incorporate them. In this and later models, the main rotor servos were positioned vertically, connected to the stationary swash plate at their upper end and to the transmission at their lower end to react control loads. The only exception to this servo arrangement is the UH-60 Black Hawk, whose servos were positioned horizontally, side-by-side, just ahead of the transmission and protected by airframe structure. This arrangement was selected to provide greater ballistic protection against ground fire as well as to provide a lower rotor profile for air transport requirements, but at the expense of increased system weight.
The second major development was that of the electronic flight stabilization system that has become an essential element of nearly all helicopter flight control systems. It happened that Gerstenberger was among the earliest engineers to recognize the need for such a stabilization system when Sikorsky’s S-58 was being adapted for Anti-Submarine Warfare (ASW) missions as the HSS-1 for the US Navy. The problem faced by pilots in trying to hover an unstable aircraft at night and during bad weather, while managing four independent degrees of freedom, seemed to be insurmountable without assistance. However, the approach being pursued at the time by other innovators was based on using a conventional fixed-wing autopilot together with special instrumentation. Gerstenberger conceived the idea of using a limited authority control stick steering concept to introduce the feedback control inputs.
Another Sikorsky engineer, Ted Carter, reduced this revolutionary concept to practice. Carter led a three-man team comprised of electronic engineer Harold Oakes, hydraulics specialist Henry Angel, and test pilot Jack Stultz to design and debug the final design. The United Aircraft Corporation recognized Ted Carter and his team for their contributions to achieving successful helicopter instrument flight capability in 1958 by awarding him the George Meade Gold Medal and his team silver medals for outstanding engineering achievement. This unique approach to automatic stabilization system design was found to be essential for anti-submarine warfare missions and has been incorporated in all subsequent Sikorsky models for all missions.
Changing Leadership in the Utility Market
These four critical and creative innovations led to a succession of very successful Sikorsky utility helicopter models. These innovations were great examples of the works of individuals creating the sparks that helped move the helicopter industry forward.
In the 1950s, Sikorsky began producing the S-55 (H-19) and later the S-58 (H-34) utility transport models both of which helped to expand the worldwide utility market.
The S-58 was configured much like its predecessor except that its landing gear was changed from the S-55’s quad gear design to a main/tail wheel arrangement that years later was to influence selection of that same configuration for the UH-60 Black Hawk design. The S-58 also had a four-blade rotor with a flapping hinge offset of 12 inches. Its rotor diameter was 56 ft. compared with 53 ft. for the S-55 and its gross weight was 10,500 lbs. compared with 7100 lbs. of the S-55.
Their large cabins, excellent rotor control power, irreversible hydraulic control servos, and automatic stability augmentation systems contributed to their wide acceptance by U.S. and foreign military organizations. Both models set production records with the S-55 first reaching 1281 units and the S-58 later reaching 1821 units including foreign licensee production. Their substantial build rates drove Sikorsky’s large increase in helicopter production during the 1950s. These two models were the first true utility helicopters and became the first troop carrying helicopters for both the Army and the Marine Corp.
Both the S-55 and S-58 were in their obsolescence stages when turbine engines became available to replace reciprocating engines. Although the S-58 in particular benefited from life extension through installation of turbine engine retrofit kits, it could never compete with helicopters designed at the start to capture the many advantages of turbine engines. Turbine engines completely solved the engine placement problem for newly designed helicopters as their smaller size permitted engines to be located near the rotor and above the cabin for single rotor helicopters.
This design flexibility set the stage for the emergence of Bell’s UH-1 Huey with its zero offset teetering rotor powered by a single Lycoming T-53 turbine engine. The Huey, designed originally for aeromedical evacuation, developed into a major success that dominated the utility market for several decades. Its operational longevity reflected a sound basic design that has been continuously improved with its original teetering two-blade rotor eventually giving way to semi-rigid, multi-blade rotors. Derivative models of the Huey found their way into all US military services as well as many foreign military organizations making it the utility model of choice worldwide during the 1960 and 1970 decades.
During that period, Sikorsky’s efforts focused on the development of heavy lift helicopters that included the turbine-powered S-64 Skycrane (CH-54) and S-65 heavy transport (CH-53) along with advanced research projects including the S-61F high speed compound helicopter, the S-69 Advancing Blade Concept, and the S-72 Rotor Systems Research Aircraft. The company also worked to adapt its medium size amphibious S-61 helicopter, the SH-3D designed for anti-submarine warfare, to a broad spectrum of military and commercial missions, civilian transport, and industrial applications. However, when measured in the number of helicopters delivered per year, Sikorsky’s output suffered significant decline from the late 1950s through the late 1970s essentially because it had lost the utility market. Employment suffered as well through most of that long period.
The good news was that while waiting for the opportunity to re-enter the continuously enlarging utility market, Sikorsky’s research and development business reached new highs. During the late 1960 and 1970 period, its contract research and development business exceeded the combined R&D business of the rest of the U.S. helicopter industry. Achievement of that R&D domination was made possible in large measure by the dedicated efforts of Wesley A. Kuhrt who was Sikorsky’s president during that period. As a result of his focus, new technologies and design innovations, primarily related to rotor systems, were created and demonstrated. These advancements included high performance cambered airfoils, swept blade tip shapes, titanium blade spars, elastomeric main rotors, and bearingless and canted tail rotors. Each was flight demonstrated at large scale using Sikorsky CH-53D, SH-3D and H-34 models. By the early 1970s, these innovations were ready and waiting for the opportunity to become the foundation of a new helicopter design.
That new design opportunity came with the emergence of the Army’s Utility Tactical Transport Aircraft System (UTTAS) whose design specification, issued to industry in 1972, took full advantage of the operational lessons learned from the Viet Nam war. The intense industry competition to win the UTTAS prototype award, followed 52 months later by an even more intense competition to win the Black Hawk production award, is well documented. Also well documented are the works of the individuals whose creations produced the great success of the H-60 Black Hawk/Seahawk series and all of their many derivative models serving in the worldwide utility market.
The UH-60M is the most current of the H-60 series that have been in continuous production for over 35 years for the U.S. Army. Approximately 1000 of the UH-60A and 1000 of its follow-on UH-60L models have been delivered to the U.S. Army. Over 1200 UH-60M models in both the utility transport and the medical evacuation versions are expected to be delivered. In total, over 3000 UH-60 models have been delivered to foreign and U.S military operators as of early 2012
The SH-60A Seahawk that was the first derivative of the original UH-60A itself spawned a long line of variants for the U.S. Navy as well as for international naval organizations. This picture is of the U.S Navy’s latest MH-60R, which together with the MH-60S, are replacing seven different helicopter models in U.S. Navy inventory. Their missions include inner and outer zone ASW protection, anti-surface ship warfare, search and rescue, vertical replenishment, and vertical onboard delivery
The Black Hawk and Seahawk models will continue to be improved both in the air vehicle and in mission systems during the early decades of the twenty first century. However, they will eventually be overtaken by new technologies as well as by the evolving characteristics and requirements of land and sea battlefields. The utility market will without doubt continue to be a major driver for the helicopter industry and replacements for the Black Hawk/Seahawk will again become the most sought after and most intense competition imaginable. Who are those individuals whose sparks will move the technology and rotary wing industry forward? What will their creative innovations be that will create the technical foundation as well as shape the configuration of the next generation utility helicopter? Can conventional wisdom predict the answers
Ray Leoni, June 2012
LAST UPDATE 6-24-2012
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