Back in the 1970s and 1980s, a joint research program between NASA, DARPA, the US Navy, and Sikorsky would spawn one of the most interesting and unique aircraft designs of all time. Called the Sikorsky X-Wing, this aircraft was designed to combine the best of what both helicopters and fixed-wing aircraft had to offer.
Five years of research and development, and many successful tests and technical innovations, would ultimately see the program canceled just before the finishing line. A hybrid of the helicopter and fixed-wing aircraft, yet something of its own class, the aircraft would never take to air.
What was the Sikorsky X-Wing?
The Sikorsky X-Wing was a concept aircraft that was designed to combine the hovering capabilities of a helicopter with the speed of fixed-wing aircraft.
Based on the S-72 RSRA (Rotor Systems Research Aircraft), it was developed through a collaboration with the U.S. Navy, DARPA, NASA, and Sikorsky.
The basic concept behind the Sikorsky X-Wing was first developed by Professor Ian Cheeseman at the University of Southampton, England. His experiments into something called a "Flying Stovepipe" would ultimately lay the foundations for a working prototype of the craft.
Cheeseman based his experiments on the Coanda principle (or effect) to first create, and then modulate the lift of a cylindrical blade. According to this principle, when air moves at a tangent over a curved surface it will tend to stick to it and follow the curve of its surface until the curvature of the surface becomes too great and the air detaches once again.
If applied correctly, the principle could be used to create a kind of virtual flap on an airfoil or rotor. Cheeseman believed that such a rotor could, in theory, be stopped mid-flight and used more like a traditional fixed-wing on an airplane, allowing an aircraft to take off and land vertically using the rotor, but avoid advancing/retreating blade lift dissymmetry when moving into forwarding flight.
His design consisted of a cylindrical rotor blade whose lift was varied by means of blowing air through slots. Shutting off the blowing would allow the aircraft to be supported by the wings while the rotor slowed to a halt and stowed for cruise efficiency on a fixed-winged aircraft. The process would then be reversed for the landing phase.
Cheeseman tested the design by building a moving rig using a bus chassis with the rotor system fitted to an instrumented tower at one end. The rotor was turned using a transmission system while adjustable air scoops behind the rear wheels provided forward propulsion. The vehicle was able to race up and down a runway with very rapid acceleration and braking.
Inspired by Cheeseman's work, engineers at the U.S. Navy's David W. Taylor Naval Ship Research and Development Center (DTNSRDC) developed the concept further. They began to experiment with circulation control by blowing air tangentially onto a rounded trailing edge of a streamlined shape similar to a conventional airfoil.
This research eventually evolved into the concept of an X-Wing, which would use circulation control airfoils to produce lift in either direction and allow the rotor/wing to maintain lift and control when beginning and stopping rotation.
In the mid-1970s, DARPA would become interested in the concept and joined the Navy in the X-program. The main idea, according to NASA, was to " investigate ways to increase the speed of rotor aircraft, as well as their performance, reliability, and safety."
It was also hoped that such aircraft could reduce the noise, vibration, and maintenance costs typically associated with conventional helicopters of the day.
Over the next few years, a small test vehicle was developed and tested at NASA's Ames wind tunnel. During these trials, the prototype was put through its paces over a period of seven weeks. During testing, helicopter, fixed-wing, and conversion flight modes were examined.
More than 30 automatic rotor stop and start tests were conducted, at speed of 180 knots, and seemed to show great promise. At this point, NASA was also onboard, and a decision was made to attempt to build a larger-scale working prototype.
In the early 1980s, Sikorsky was contacted to aid in the development. As one of the foremost helicopter designers in the country, it was believed they were best-placed to help in furthering the program.
Not only that, but they had an "ace" up their sleeve -- the Sikorsky S-72 RSRA testbed helicopter. Specifically designed to test novel rotor blade systems, this aircraft seemed like the perfect testbed for furthering the X-Wing program.
The S-72 was a unique aircraft, with a variable incidence wing, whose role was to provide offload and overload for any tested helicopter engine's lifting requirements. These kinds of wings can be tilted at different angles for takeoff and landing.
The S-72 also had auxiliary propulsion and drag brakes to provide offload and overload a test rotor's horizontal thrust needs. These capabilities made it an ideal test vehicle for testing the concept of the X-wing's rotor starting and stopping in a predictable and stable manner without the need for the test rotor to support its full design lift during its development phase.
Despite the head start that the S-72 offered, the development of the X-Wing would be a very challenging enterprise for all concerned.
The working prototype S-72 RSRA had a gross weight of 10,887 kg (24,000 lbs), had a rotor diameter of 15.5 m (51 feet) with a blade cord of 1 m (3.39 feet). Its recorded disc loading was 17.86 kg/m2 (12 lbs/ft2). It was powered by two General Electric T58-GE-5 turboshaft engines mounted on its port and starboard fuselage.
Today, one X-Wing testbed, Sikorsky S-72 RSRA aircraft remains in storage at NASA's Ames facility (Dryden Flight Research Center), while another is said to be in bad shape at Fort Rucker in Alabama.
How did X-Wing aircraft work?
By combining the best of both worlds with regards to helicopters and fixed-wing craft, the X-Wing could theoretically break the trend of VTOL aircraft of the day where higher speed aircraft required higher hovering disc loading. This is essentially the ratio of the aircraft's weight divided by the area of the rotor disc (usually expressed as lbs per ft2).
X-Wing aircraft could also achieve fixed-wing flight without the need for dedicated fixed-wings on the aircraft. This promised a lower weight empty fraction (the ratio of its empty weight divided by its gross weight) when compared to helicopters or fixed-wing aircraft alone.
For X-wings to work in practice, the rotor blades would need to be very different from conventional helicopters. This is because they would need to carry more lift without the benefit of a traditional helicopter's use of centrifugal force/effect to stiffen the blades during rotation.
The X-Wing rotor blades also had to be very stiff indeed. The forwardmost two would, after all, act as forward-swept wings during horizontal flight. Blades also needed to be "double-ended" as the rearmost blades would have to fly "backward" once the rotor blade was stopped and fixed into place, effectively acting as forward-swept wings.
Because of this, the blades were made of composites to provide the required strength, while also limiting the weight as much as possible.
Thanks to the design's reliance on the Coanda effect, a conventional helicopter's mechanical swashplate (the device used to tilt the rotor blades in response to pilot commands) could be replaced with a specially designed valving system to feed compressed air to the rotor blades.
In helicopter mode, this worked by blowing air out of the trailing edge of the blades to create virtual flaps and simulate cyclic pitch. During conversion to fixed-wing mode, complications with the changing aerodynamics would require air to be blown from the leading and trailing edges simultaneously.
This system is far more technically complex than a helicopter's swashplate, and so a state-of-the-art computer-aided valve control system was also developed. As the system effectively relied on using compressed air to help generate lift, the prototype X-Wing also needed a large compressor to circulate and control airflow and provide a high-energy clutch and brake for starting and stopping the rotor blade.
The rotor hub would also need feedback control during rotor blade conversion modes. While a risky design, the payoffs could potentially be huge, if successful.
Another complication for engineers was the need to retain the S-72's safety function of pyrotechnically severable rotor blades, in case of emergency. During such events, the aircraft would then be able to fly perfectly well as a fixed-wing aircraft.
According to Sikorsky, "it was desired to keep this feature for X-Wing testing. Developing this system for the very thick composite flex beams of the X-Wing became another large development project and a great achievement for the team."
To this end, two teams were set up. One to focus on the technicalities of the rotor blade system, and the other to figure out how to integrate it into the S-72.
By 1985, a working prototype had been developed, and testing began in December of that year. Further testing in 1986 paved the way for the first test flight.
In September of the same year, the RSRA test aircraft was officially unveiled to the world. The same month the aircraft was transported, via NASA’s Super Guppy KC-97 cargo aircraft, to NASA’s Dryden Flight Test Research Facility at Edwards Air Force Base in California.
Test flights for the aircraft were initially envisaged to be a gradual build-up, starting with taxiing, and then flying without X-Wing blades to establish some baseline data. Since the S-72 testbed airframe was designed to fly perfectly well without the rotor blades, it could safely land if the rotor failed for any reason.
This was a milestone for Sikorsky as although the company started out designing fixed-wing aircraft in their early days, a Sikorsky-built fixed-wing aircraft had not flown in over 50 years.
After establishing a baseline, the next phase was to fly with just two of the rotor blades in place, followed by four, with the rotors stopped. By around mid-1987 the aircraft was scheduled to fly with the rotor/wing turning a full speed with the testing of mid-flight rotor/wing stoppage by the end of the year.
Sadly this would never happen. Governmental funding issues and a change in Defense priorities saw the program scrapped in 1987.
Where any demonstrator or production variants of X-Wing aircraft planned?
While the Sikorsky X-Wing was still under development, various designs were envisaged for a final working demonstrator aircraft using the same technology. Variants included the use of convertible engines to provide the craft's rotor with power and compressor, as well as, thrust for high-speed fixed (rather locked) wing flight mode.
Other versions envisaged the use of more conventional turboshaft engines to power the aircraft with conventional pusher propellers, like the image below.
Some concepts were also designed for later production and mission-capable applications of the technology.
Interestingly, the developers behind the X-Wing program never envisaged the X-Wing as a direct replacement for helicopters of fixed-wing aircraft. Rather, they envisaged them as a highly specialized form or aircraft that would need low-speed efficiency, and maneuverability of helicopters combined with the high cruise speed of fixed-wing aircraft.
These could include things like air-to-air and air-to-ground tactical operations, airborne early warning, electronic intelligence, antisubmarine warfare, and search and rescue. One example envisaged by Sikorsky was to produce a Navy variant capable of operating from the decks of DD963 class destroyers.
Part of the rationale was the fact that the X-Wing setup was thought to be an ideal choice for missions requiring high-speed dash capabilities, combined with short loiter and hover times. This aircraft could have been used for missions like OTH (Over the Horizon) Targeting, and Surveillance, and AFHSAR (Search and Rescue).
According to Sikorsky, "this aircraft [would have] used a 50 ft. diameter rotor/wing and two General Electric GE-CTSF-34 high by-pass fan engines. Gross weight was 30,000 lbs. Anti-torque was provided by a nozzle at the tail vectoring the thrust of the engines."
Did the Sikorsky X-Wing inspire the X-wings in Star Wars?
And now, it is probably time to address the elephant in the room. We know you've probably been mulling over this very question while reading this article too.
Given the overlap between the development of this aircraft, and the production of the first Star Wars film, Star Wars: A New Hope, it would seem logical that the now-iconic Incom T-65B X-Wing Starfighters in the franchise may have, at least in part, been inspired by this program.
Thankfully, one of the key designers, Colin Cantwell, for Lucasfilm's Industrial Light and Magic VFX addressed just this subject in a Reddit AMA a few years back.
When asked the question by one Redditor, Cantwell explained that "It had to be ultracool and different from all the other associations with aircraft, etc. In other words, it had to be alien and fit in with the rest of the story. A dart being thrown at a target in a British pub gave me the original concept, and then it went forward from there."
When you think about it, you really can see the dart-like nature of the X-wing. It seems this is just another one of those funny coincidences.
After years of development and overcoming many technical challenges, the Sikorsky X-Wing had come very close to actually flying. We can never know for sure if it would have flown successfully, but it is a shame that the program failed for financial, rather than technical, reasons.
Despite this disappointment, Sikorsky gained very valuable experience and knowledge in developing bearingless composite rotors, and fly-by-wire, and higher harmonic control systems, all of which were used in future aircraft designed by the company.
The future of the X-Wing concept is anyone's guess, but many enthusiasts of this unique aircraft hope that a mission capable X-Wing may take the air one day,
We can but hope.