In the beginning
The XH-40 to the UH-1A
The extraordinar y usefulness of the Bell 47 Sioux during the Korean War, especially in the role of medical evacuation transport, had been a clear pointer to the future of battlefield utility helicopters. The US Army were particularly interested in developing such a vehicle, an interest that would lead to the introduction of a legend.
The development of the UH-1 and its eventual use in an astounding variety of roles began with the lessons learned by the US Army during the Korean War, which had introduced a number of new weapons and vehicles to the United Nations forces fighting in the country. The Bell 47 Sioux helicopter had fulfilled a number of roles and is best remembered for the evacuation of wounded personnel to the Mobile Army Surgical Hospitals just behind the front line. Around 18,000 casualties were evacuated by air, saving many lives because of the speed with which the wounded reached medical aid. This lesson was not lost on the US Army, nor on civilian medical organisations worldwide and would result in the development of the medical evacuation (medevac) and air ambulance services we know today. The Bell 47s had also operated in logistics support, taking ammunition and rations to the front line and supporting units cut off from other ground forces. The transport of supplies and personnel into the battlefield was also seen as a prime requirement for future helicopters and these two roles began to shape the Army’s thinking regarding new aircraft.
The experiences of the Korean War prompted Bell Helicopters to write a detailed report on the use of the helicopter on the battlefield, a report which also suggested the future improvements required in helicopter performance in order to efficiently fulfil these roles. This sat very well with a US Army Colonel, Jay Vanderpool, who had already been pressing for the development of high performance and armed helicopters for close battlefield support. The Key West Agreement of 1948 had rigidly defined areas of responsibility and resources between the US Army, Navy and Air Force. While this agreement gave the army the ability to develop and field reconnaissance and medevac air assets, there were a number of senior army personnel who believed that close air support should also be within the army’s purview. Against this background of growing support for the concept of a truly capable multirole battlefield support helicopter, by 1952 the US Army recognised a requirement for a medevac and transport aircraft which would also have to be capable of operating as a trainer for instrument flying. The helicopters then available were all considered but were found to either lack power and therefore load lifting capability, were too large and unmanoeuvrable, or were too mechanically complex to be easily maintained in the field. Reliability, manoeuvrability and excess power were all going to be vital if the new helicopter was to survive on a modern battlefield. The initial requirement was refined until it was issued in November 1953. Twenty companies responded to it with new designs. The specification was exacting for the time, an 8000lb (3629kg) payload had to be carried over a range of at least 100 nautical miles (185 km) at speeds above 100 kts (185kph). The new helicopter also had to have sufficient power to be able to hover at heights of up to 6000ft (1829m) and yet be able to be carried easily inside a Lockheed C-130 Hercules transport aircraft. Bell was one of the companies that responded to the requirement, and with Lycoming had been developing a novel approach to the power problem in helicopters, between them devising the airframe and engine which would provide the necessary performance. The Lycoming Turbine Engine Division at Stratford, Connecticut, had begun developing a new turboshaft engine in 1951 under the design leadership of Anselm Franz. Franz had been the chief designer at Junkers’ engine department in Germany during the Second World War and had produced the Jumo 004 turbojet that had powered the Messerschmitt Me 262 jet fighter. Franz’s design concept was to produce a small, light but tremendously powerful engine suitable for operation as a turboprop or to provide shaft drive for helicopters. A simple five stage axial compressor was mated to a sixth stage centrifugal compressor which fed an annular combustion chamber before the exhaust gasses exited over two impeller turbines which drove the compressors ahead of them. The main drive gearbox and ancillary drives for electrical generators and other aircraft systems were mounted forward of the first stage compressor, central to the air intake, resulting in a very small engine, only 58in long (1.48m) and 23in (0.58m) in diameter. The layout also allowed the airflow through the engine to keep the heat generated by the combustion chamber away from the gearbox for the propeller or rotor,
depending on how the engine was being used. The most impressive result of this engine development was its power to weight ratio, the first version of the LTC-1, as it was initially known, produced 700hp for a weight of just 655lb (297kg), a tremendous leap forward over the relatively low powered and heavy piston engines that had been fitted to helicopters up to this point. The new turbine had a number of other advantages in addition to its low weight. Its fuel consumption was much lower than a piston engine, meaning less fuel needed to be carried to achieve a given endurance or lift a given payload. The high power output bestowed high speed on the helicopter, and the simplicity of the engine’s design reduced the time required and therefore the costs of maintaining the aircraft. In short, the turbine was exactly what the helicopter industry had been looking for to revolutionise the capabilities of its aircraft.
Bell Helicopters was well aware of the shortcomings of existing rotary winged aircraft and was certain that the new Lycoming turbine could solve the performance issues and meet or exceed the US Army’s requirement. The design team at Bell started with a blank sheet of paper to produce an aircraft that would take best advantage of the performance the engine offered. Yes, there were proven elements from their earlier designs in the new aircraft, but the new powerplant allowed the design team a degree of freedom hitherto unknown in helicopter construction. The helicopter they designed can be thought of as three separate sections. The section around the centre of gravity does all the work as far as the stresses in the helicopter are concerned. It consists of two main beams that run fore and aft from the rear of the centre section to the nose. These have lateral bulkheads linking them together, but at the centre of gravity they are joined by the lift beam, a vertical structure that has the transmission with its planetary type gearbox and rotor mounted at the top and the hook for slinging external loads at the bottom. The main bulkhead at this point forms the rear wall of the cabin. The lower space aft of this compartment is separated into three sections. The first, again under the centre of gravity, housed the port and starboard main fuel tanks, aft of which were the combustion heater unit then a crew baggage compartment which would later be used to house the weapons control electronics on the armed versions of the helicopter. A deck separated these three compartments from those above. Central above the fuel tank sat the main rotor transmission and rotor head, to either side of which were the oil and hydraulic tanks and aft of which was the engine. The air intake for the turbine was immediately behind the main rotor transmission and drew cooling air over the gearbox when the engine was in operation. The semi rigid rotor head design featured Arthur Young’s stabiliser bar, mounted above the main rotor in this case, a 44ft (13.41m) span two bladed rotor with a 14in (35.6 cm) chord to the blades.
Forward of the main bulkhead was the cabin structure. The two main floor beams and their interconnecting bulkheads formed the structure on which the cabin floor sat; beneath which were two under floor fuel tanks and the forward attachment point for the main undercarriage. This consisted of two curved transverse tubular legs, the rear one of which was just aft of the centre of gravity in the centre section, the legs attaching to the main skids at either end. The skids were long, almost the length of the cabin and cockpit and sat 8ft 4in (2.54m) apart, giving the helicopter tremendous stability on the ground. The undersides of the skids were fitted with replaceable metal shoes to prevent wear to the skids themselves. The cabin was intended to seat eight armed troops in two rows of four canvas seats, ahead of which the two pilots sat in a tandem cockpit with full dual controls. The cabin was 98.5in (2.5m) wide deliberately to accommodate standard stretchers across its width. The side cabin doors slid aft, fully opening to allow up to four of these stretchers to be loaded quickly and easily. Forward of these, the pilots each had a car type door on either side of the cockpit. The instrument panel and avionics bays were supported by the front ends of the two main floor beams, either side of which the lower nose was glazed to afford the pilots the best possible view while landing. The pilots’ doors were extensively glazed, in addition to which a large split windscreen ran the full width of the cockpit and a large window was set into the roof above each pilot’s head. The last section of the design bolted onto the rear end of the centre section structure with just four bolts, and comprised the semimonocoque tail boom, tail rotor and drive, the fin and elevators. The tail rotor was large, 102in (2.6m) in span and mounted on the top of the fin on the port side. The shaft drive to the tail rotor ran along the top of the tail boom under its own protective shroud then up the leading edge of the fin via a bevel gearbox at its base. The twin elevators were all moving flying surfaces mounted on either side of the tail boom underneath the base of the fin. These were linked to the pilots cyclic control column and provided pitch stability in the cruise as well as increasing the aircraft’s allowable centre of gravity range while loaded. A great many of these design features have become standard on helicopters, but it must be remembered that at the time Bell was at the cutting edge of rotary winged
development and this design would become the world’s first turbine powered helicopter to enter production. To learn more about the use of turbine engines in helicopters, Bell had taken a Model 47 and fitted it with a 425hp Continental CAE XT-51 turboshaft. This single example first flew on October 20, 1954, and was used to test components for the new design, eventually being designated as the XH-13F and used by the US Army as a turbine testbed from April 1955 onwards. Meanwhile, the Bell entry for the US Army’s medevac and utility helicopter competition had been selected as the winner on February 23, 1955, Bell being contracted to provide three prototypes of its Model 204 as it was known under the designation XH-40.
As mentioned earlier, Bell’s chief test pilot, Floyd Carlson made the first flight of the XH40 prototype on the day Larry Bell died, October 20, 1956. The helicopter had been built at Bell’s Hurst plant on the outskirts of Fort Worth and was powered by an early development model of the Lycoming LTC-1 turboshaft engine, designated YT53-L-1 in its military guise, which produced 700hp. Only 16 months had elapsed since the initial design work had begun and even before the first flight the US Army had ordered six more development prototypes for service testing, designated YH-40S. The urgent requirement for the type and the promise the design showed even from this early stage meant that the other two XH-40S joined the flight test programme in 1957 and all six YH40s were delivered by August 1958. The XH-40S had proved to have a maximum speed of 138mph (222kph) and a service ceiling of 17,500ft (5334m), but there were a number of changes the army required in the design. The cabin was too short to comfortably carry the envisaged load of troops or stretchers and had to be stretched by 1ft (30.48cm) in length. Fortunately, in basing the design on the two floor beams as the main structural element, such a stretch was easily accommodated. The main cabin sliding doors also had to be stretched by the same amount and the engine cowlings needed to be redesigned to improve cooling and ease of access for maintenance. The elevators were increased in span to 9ft 4in (2.84m) and moved forward on the tail boom to about two thirds of the way along its length. Lastly, the ground clearance was increased by 4in (10cm) by increasing the length of the two main skid support tubes on the undercarriage. This last change was to allow ground crew to more easily attach under-slung loads and to prevent the fuselage ‘bottoming’ on rough terrain. All of these changes were incorporated in the six YH-40S, which were also fitted with the first production version of the new engine, the Lycoming T53-L-1A producing 860hp, but derated to 770hp to extend engine life and reliability.
While the construction of these first batches of helicopters was going on, the army had introduced a new designation system in 1956, which meant the helicopter became the HU-1, the trials aircraft becoming the XHU-1 and YHU-1. The six YHU-1S were followed by a pre-production batch of nine HU-1S, which began to be delivered for service trials from June 30, 1959. The new designation quickly gave rise to the nickname the aircraft is known by the world over – Huey. So popular was this name that the casting of the rudder pedals was changed to incorporate the word into the face of them, a practice continued to this day.
The trials with the prototype aircraft were still ongoing when the US Army issued its first production order on March 13, 1959, these being designated HU-1AS to differentiate them from the pre-production airframes. A total of 173 HU-1AS were built, and were intended to give the US Army experience of operating the type as it was the first turbine powered helicopter in both production and in general service. Fourteen of these helicopters were delivered equipped for blind flying trainers as the TH-1A, entering service with the US Army Aviation School at Fort Rucker in Alabama and being used to teach instrument flying on Army helicopter pilot courses. The first operational units to receive HU1As were the 82nd Airborne Division at Fort Bragg in North Carolina and the 101st Airborne Division (AD) at Fort Campbell in Kentucky, along with the 57th Medical Detachment (MD) of the Brooke Army Medical Centre at Fort Sam Houston in Texas. The 57th MD was the first to use the aircraft in its intended medevac role, but the first to be deployed overseas were two HU1As from the 82nd AD who took part in an exercise in Panama in early 1960. The following year, the first HU-1AS to be permanently based outside the US were delivered to the 55th Aviation Company in Korea, from which time the HU-1A began to spread across the world, several European based units receiving the type from the end of 1961 onwards. With the introduction of the production variant came the official name, Iroquois, but in practice this has almost never been used as the nickname Huey was already both widespread and popular. A number of changes happened to Bell during this period. In July 1960, Bell Aircraft and Bell Helicopters were bought by Textron Incorporated of Rhode Island. The Bell Aerospace Corporation was founded to cover the experimental, fixed wing and rocket motor activities of the group, while the Bell Helicopter Corporation was renamed the Bell Helicopter Company. Things changed for their helicopter too, not least of which through the interest aroused by it setting six Fédération Aéronautique Internationale (FAI) recognised world records in 1960. These included climbing to 19,686ft (6000m) in just eight minutes 7.1 seconds and sustaining a speed of 142.22mph (228.9kph) over a 100km (62 miles) closed course. One final change was made as the Huey began to enter large scale service. In 1962 the US armed forces introduced a new standardised designation system which meant that the HU-1 became the UH-1, the designation we know it by today.
Despite the intention only to use the first production HU-1AS as evaluation aircraft, five of the aircraft of the 57th MD were deployed to Vietnam in April 1962, the first Hueys to arrive in theatre.
They were tasked with evacuating wounded Army of the Republic of Vietnam (ARVN) soldiers from battlefields, and for the next 10 years, the thudding of the Huey’s rotor was to be a familiar sound over the war torn country. The next group of UH-1AS to arrive in Vietnam were with a new trials and evaluation unit, the US Army’s Utility Tactical Transport Helicopter Company (UTTHCO) based at Tan Son Nhut airport. Fifteen UH-1AS had arrived on October 9, having been ferried from Okinawa via a workup period in Thailand, during which the helicopters had been extensively modified. It is worth explaining that by this time, the forces of other countries had found more military uses for the helicopter. A small number of Sioux were armed with machine guns by the French Army and provided fire support to ground forces on counter insurgency missions during the Algerian War between 1954 and 1962. These proved particularly useful in countering dispersed guerrilla forces in the mountain regions, and were soon joined by more heavily armed versions of other Sikorsky and Piasecki types fielded by the French Army and Navy. The US Navy and Royal Navy had already begun developing armed anti-submarine warfare helicopters to track and attack submarines with sonars and depth charges, while in the Soviet Union, Mil-8s had been fitted with rocket pods in an experiment to provide air support to ground forces during airborne assaults. These lessons were not lost on the US Army, who had begun to experience losses to ground fire among the transport helicopters deployed to Vietnam. The Piasecki CH-21C Shawnee twin rotor cargo helicopters based at Da Nang and Tan Son Nhut with the 8th, 33rd, 57th and 93rd Transportation Companies (Light Helicopter) had been flying troop transport and supply missions with ARVN units since February 1962. These transport helicopters were increasingly coming under attack by North Vietnamese Army and Vietcong units who were beginning to field more anti-aircraft weapons, particularly heavy machine guns, shooting down a CH-21 near the Laotian border in July and killing its four man crew.
In order to protect the transport helicopters, the UTTHCO was formed using 15 UH-1AS taken from the 53rd Aviation Detachment on Okinawa. The Hueys were extensively modified with weapons to enable them to act as an armed escort to the transports and to ensure the intended landing zone was clear of enemy activity and antiaircraft weapons. Experiments began with various free swinging door mounted machine guns aimed and fired by the crew in the cabin. The best way to mount these was found to be on bungee cord, elasticated rope slung from the cabin roof, which effectively stabilised the weapons prior to the introduction of fixed pintle door mountings later in the conflict. Mountings were also devised by the UTTHCO senior engineers to allow rocket packs to be carried on the skids, these containing seven or eight of the 2.75in Folding Fin Aerial Rockets (FFARS). Alongside these were a .30 cal M-37 or 7.62mm M-60 machine gun, again in improvised mounts on the skids, both these and the rockets being fired by the pilots. Two of the unit’s engineers, Chief Warrant Officers Clemuel Womack and Cleatus Heck were to be responsible for the development of a standardised mount for rocket pods which would be adopted throughout the Huey fleet. Back in the US, a single UH-1A, 58-2038, had been redesignated as the XH-1A and used for a variety of weapons trials, including the fitting of a grenade launcher in a turret on the nose. Meanwhile, using the improvised mounts, the UH-1AS of UTTHCO under the command of Major Robert Reuter began flying missions on October 16, 1962, their task being to protect the transport helicopters and ARVN forces engaged in Operation Morning Star, aimed at clearing Tây Ninh province of enemy personnel. From these small beginnings, the armed version of the UH-1 grew in complexity and capability and would eventually develop into an entirely new design, but that helicopter will be covered in another issue of Aviation Classics. The UH-1A had proved itself a quantum leap forward in performance over the earlier piston powered helicopters and was already proving incredibly adaptable despite still being under evaluation from its recent entry into service. The early experiences of Huey crews in Vietnam showed that the aircraft was more than rugged enough to take the stresses of operational flying and simple enough to easily maintain in the field. However, the US Army had also recognised that the performance of the UH-1 would have to be increased still further in future model in order to fully enable it to meet their needs in all environments, especially the hot and high conditions to be found in southeast Asia. These requirements had been put to Bell and an order placed for an improved version as early as July 1959 while the UH-1AS were still being delivered. This would lead to the UH-1B, and a plethora of sub-types as will be described next.
The Bell H-13 had proved the value of the helicopter in the medevac and battlefield logistics roles during the Korean War.
Three views of the prototype XH-40 on a test flight in front of the staff at the Bell plant. Note the doors, engine cowlings and tail rotor drive shaft cover have not been fitted for these flights.
The second prototype Bell XH-40 with the all flying elevators in the full down position. Note that a stabiliser bar has been added to the main rotor, but below the blades.
A rare colour shot of the prototype Bell XH-40. Note the original position of the all flying elevators at the end of the tail boom and the low mounted stabiliser bar on the main rotor.
The prototype Bell XH-40 with the engine cowling and other covers removed on a early test flight shows the arrangement of the engine, gearboxes and main rotor transmission clearly. It also shows how the aircraft was built in three sections, cabin, centre section and tailboom.
The first prototype Bell XH-40, 55-4459, during field trials with the US Army. Note how low on the ground the fuselage is sitting, which led to an undercarriage modification on the production aircraft.
A close up of the early Lycoming YT53-L-1 turboshaft engine installed in the prototype Bell Xh-40.this small, light engine produced 700hp.
Two views of the third prototype Bell XH-40, 55-4461, showing the increased span of the all flying elevators and their changed position, further forward on the tail boom and that the stabiliser bar has moved to above the main rotor.
The first turbine helicopter in production and service underwent extensive testing in all environments, such as this Bell HU-1A seen during cold weather trials. The 57th Medical Detachment (MD) of the Brooke Army Medical Centre at Fort Sam Houston in Texas were the first to receive the Bell HU-1A in the medevac role. An early Bell HU-1A at Fort Rucker, the Army Aviation School in Alabama.
The 57th MD was the first unit to deploy the Bell UH-1A, as it was now known, to Vietnam in medevac support to ARVN forces in April 1962.
The cabin of the Bell UH-1 was 98.5in (2.5m) wide deliberately to accommodate standard stretchers across its width.
The Piasecki CH-21C Shawnee was being used in air assault and logistics support missions in Vietnam from February 1962 onwards.
Fifteen UH-1AS were deployed to Vietnam in October 1962 with the US Army’s Utility Tactical Transport Helicopter Company (UTTHCO) based at Tan Son Nhut airport. These were the first UH-1S to be armed to act as escorts to the CH-21C transport helicopters.
Two .30 cal machine guns were mounted on the skids of the UTTHCO UH-1AS, the ammunition feed running from boxes in the cabin through slots cut in the floor and sides of the cabin.
Three views, including two rare colour shots, of the weapons package improvised on the skids of the UTTHCO UH-1AS. On each side of the aircraft are eight 2.75in Folding Fin Aerial Rockets (FFARS) and a .30 cal Browning M-37 machine gun.
Right: A Bell UH-1A of the US Army Aviation Board, the test and evaluation unit behind much of the development of the helicopter in service.
One of the innovative senior engineers responsible for successful fitting of weapons to the UTTHCO UH-1AS was Chief Warrant Officer Cleatus Heck who was killed during his second tour in Vietnam.
Major Robert Reuter was the commander of the UTTHCO in 1962 and took these rare shots of armed UH-1AS in flight in Vietnam from their base at Tan Son Nhut Airport. His photographs are now part of the US Army Aviation Museum’s archive.
Another of the US Army weapons trials on the Bell UH-1A was fitting six AGM-22 wire guided missiles.these were a licence built version of the French Nord S.11 anti-tank missile and were used in Vietnam from September 1965 onwards.
One of the US Army Test Board aircraft used in developing the armament systems of the UH-1, seen here with the M-21 weapons system of miniguns and rocket pods on the universal fuselage mounts and an M-5 grenade launcher in a turret in the nose.the M-5 was first tested on a UH-1A.