Able to operate as either a helicopter, compound helicopter, or fixed-wing aircraft, the Sikorsky S-72 was an ambitious attempt to create a helicopter-plane hybrid that could hover like a chopper and soar like a jet.
The product of a long lineage of older models dating back to the 1960s, although the NASA-funded S-72 X-Wing would become the most advanced iteration of the Sikorsky family of compound rotorcraft, the S-72 would certainly be the most explosive of the lot.
Origins – The S-61F and S-66
In 1964 Sikorsky Aircraft was granted a research contract by the US Army and Navy to develop their own variant of a compound rotorcraft that was anticipated to travel at speeds as high as 230 miles per hour.
The resulting S-61F was an overhauled SH-3A Sea King anti-submarine helicopter, furnished with dual Pratt & Whitney J60-P-2 engines on either side of the lower fuselage, and modified with a variety of anti-drag features including a rounded nose, a streamlined belly, a larger vertical tail fin, and a big horizontal stabilizer equipped with elevators ripped from a Cessna T-37 jet trainer.
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The S-61F also had wings which were 170 square feet in area and 32 feet in span in addition to a six-bladed rotor.
Perhaps the most unconventional aspect of the S-61F was that it was designed to operate in various configurations for research purposes, being able to fly with or without wings, with or without turbojets, and with either five or six-bladed main rotor.
Designated NH-3A by the military, the S-61F made its debut on May 21st 1965, attaining a speed of 187 miles per hour in the first of 113 flights that were completed by May 8th 1967. Overall the S-61F was an unqualified success clocking a maximum speed of 255 miles per hour, and in Sikorsky’s final report on March 20th 1969, it was recommended that the project should continue with a focus on making it even quicker.
However, despite the wealth of information about compound rotorcraft flight characteristics it had provided, shortly after Sikorsky’s final evaluation was released the US Army decided to cancel the program.
Proceeding alongside the S-61F was the development of another compound rotorcraft experiment referred to as the S-66.
The S-66 was to compete against Lockheed’s CL-840 as part of the Advanced Aerial Fire Support System (AAFSS) program set up by the US Army, which sought to find an advanced high-speed escort for troop-carrying helicopters or a direct fire support aircraft for use in Vietnam.
But the S-66 would never go beyond the design phase, losing out to the Lockheed CL-840 in November 1965. Although the S-66 would never fly, one of its key components, a tail rotor that could be swung 90 degrees to the rear to function as a pusher propellor to provide additional forward thrust called a Rotor-prop, would be installed on a standard SH-3A for evaluations.
Activated once the pilot had reached 80 miles per hour, the Rotor-prop worked surprisingly well and did exactly what it was meant to do before it was discontinued by Sikorsky.
In February 1972 Sikorsky announced that they were to take part in a competition known as the Advancing Blade Concept (ABC). The aim was to investigate a rotor system which, using two co-axial contra-rotating rotors, could theoretically take advantage of the aerodynamic lift potential of advancing blades.
Interestingly, an air vehicle set up with an ABC rotor system would not need a tail rotor, since the contra-rotating three-blade rotors cancelled out any torque.
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The result was the S-69, given the moniker XH-59A by the Army, ostensibly created to test the new ABC layout. Equipped with a fully retractable tricycle landing gear, the S-61 appeared more like a conventional plane than a helicopter and was made extremely fast thanks to a streamlined airframe and a panoply of drag-reducing components.
Following encouraging wind tunnel tests at the NASA Ames Research Center, the S-69 took to the skies for the first time on July 26th 1973. The project was soon marred by an accident that occurred a month later, resulting in serious damage to the aircraft, which compelled Sikorsky to totally revamp its modified control system. A second version of the S-69 flew on July 21st 1975 with considerably more success.
It was able to reach a top speed of 184 miles per hour in level flight and 224 miles per hour in a shallow dive, all the while maintaining an impressive performance that would continue for another 2 years of testing, and which would reveal the ABC to be a lot quieter and efficient than other rotor systems.
Up to this point, the S-69 had only been examined in its pure helicopter configuration, which was to change in March 1977 when compound testing was announced.
Fitted with two Pratt & Whitney J60-P-3A turbojets on either side of the fuselage in 1978, the next year high-speed tests went ahead at United Technologies Division’s Developmental Flight Test Center at West Palm Beach, Florida, where the S-69 reached 303 miles per hour, becoming the first rotorcraft to attain such a speed.
Despite its accomplishments however, Army financial considerations over the next few years would for the time being put a premature end to the ABC system, a concept that was rekindled decades later with Sikorsky’s announcement on June 1st 2005 that they were constructing the ABC-enhanced X-2.
S-72 and the X-Wing
Developed concurrently alongside the S-69, the S-72 was a Sikorsky-Bell joint project submitted to the US Army’s Rotor Systems Research Aircraft (RSRA) design competition, which was looking for a high-speed craft to test a wide array of different rotor systems and integrated propulsion systems.
Given a contract in January 1974 for the assembly of 2 prototypes, the first S-72 was rolled out on June 7th 1976 and had the unique ability to fly as a pure helicopter, a compound helicopter, or a fixed-wing aircraft.
The S-72 had a length of 70 feet and 7 inches, a height of 14 feet and 6 inches, an empty weight of 14,490 pounds, and borrowed heavily from its earlier incarnation, possessing five-bladed main and tail rotors from the S-61F as well as an identical airframe.
In addition, it had a pair of wings with conventional ailerons and flaps that had an area of 470 square feet and a span of 45 feet. Furthermore, the wings could be adjusted mid-flight from an angle of -9 degrees to 15 degrees, and they were also installed with air brakes to allow precise control of airspeed, while further flight stability was augmented by an advanced flight control system and trim in all axes.
Elsewhere, it was built with a low-set stabilizer of 88 square feet, a geared elevator situated on the tail boom, and a helicopter T-tail of 17 square feet. Performance-wise, the compound rotorcraft was powered by two General Electric TF34-GE-400A turbofans placed on either side of the fuselage similar to the Lockheed S-3 Viking.
On October 12th 1976 the S-72 hovered through the air for the first time in helicopter mode, going on to complete a further 21 flights by February 1977. After being transferred to NASA’s Wallops Island Flight Center in Virginia for the second phase in July, it was moved again to NASA Ames Research Center at Moffett Field, California, on February 11th 1979.
In a compound setting, the second S-72 travelled over the skies of Wallops Island for the first time on April 10th 1978 before, like its copter counterpart, being taken to Moffett Field for further analysis.
As the US Army’s RSRA test bed, a mind-boggling variety of rotor system types, including composite bearingless, variable-geometry, gimbaled, articulated, hingeless, circulation control, reverse velocity, and jet flap were attached and unattached to the S-72.
In order to get information on the lift and torque of each rotor, the main transmission of the S-72 was mounted on a specially designed balance that provided the desired data.
In the event of an emergency during rotor systems testing, the pilot of the S-72 could push a button that jettisoned the main rotor blades via explosive charges, transforming it instantaneously into fixed-wing aircraft.
Another novel safety measure was the Aviation Yankee Extraction System, the first ejection seats ever installed in a rotorcraft device. If activated, the explosive charges atop the craft were initiated to remove the hazardous rotor before a catapult rocket system fired the crew members upwards.
In 1984 Sikorsky was awarded a contract by NASA and the Defense Advanced Research Projects Agency (DARPA) to convert the helicopter-configured S-72 into a demonstrator for the company’s exciting new X-Wing System, which attempted to explore an efficient combination of vertical lift and high cruise speeds.
The X-Wing worked by using a four-blade main rotor to vertically fly upwards and then forward. Next, an X-shaped fixed wing was employed when enough forward thrust had been achieved.
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NASA envisioned that such a craft would have numerous military applications and could be versatile enough to carry out air-to-air and air-to-ground tactical operations, airborne early warning, electronic intelligence, antisubmarine warfare, and search and rescue missions.
The re-designated S-72X first flew in the fixed-wing configuration on December 2nd 1987, attaining a top speed of 301 miles per hour at Dryden Flight Research Centre. Yet despite showing mountains of promise, the project was shelved in 1988 after certain technological barriers proved too expensive to surpass.
- Crew: 3
- Length: 70 ft 7 in (21.51 m)
- Wingspan: 45 ft 1 in (13.74 m)
- Height: 14 ft 6 in (4.42 m) to top of rotor hub
- Empty weight: 21,022 lb (9,535 kg) (compound helicopter configuration)
- Gross weight: 26,200 lb (11,884 kg) (compound helicopter configuration)
- Powerplant: 2 × General Electric T58-GE-5 turboshaft, 1,400 shp (1,000 kW) each
- Powerplant: 2 × General Electric TF34-GE-400A turbofan, 9,275 lbf (41.26 kN) thrust each
- Main rotor diameter: 62 ft 0 in (18.9 m)
- Maximum speed: 300 kn (350 mph, 560 km/h)