The X-Wing concept is based on the initial work of Professor Ian Cheeseman of the University of Southhampton, England, and his experiments with what some called a “Flying Stovepipe”. Dr. Cheesman employed the Coanda principal to create and then modulate the lift on a cylindrical rotor blade. The Coanda principle is that if air is blown tangentially over a surface it will adhere and follow the surface until the curvature gets too great and it detaches. Thus it can be used as a virtual flap on an airfoil. He believed that such a rotor could be stopped and started in flight if used on a winged vehicle, and employed only for takeoff and landing. The rotor would then be stopped and stowed for cruise efficiency.
Engineers at the U.S. Navy’s David W. Taylor Naval Ship Research and Development Center (DTNSRDC) extended Professor Cheesman’s idea of lift control on a cylinder using circulation control by blowing tangential to a rounded trailing edge of a streamlined shape similar to a conventional airfoil. This then evolved into the X-Wing concept where circulation control airfoils could produce lift in either direction and allow the rotor/wing to maintain lift and control while starting and stopping rotation. In 1976 DARPA (the Defense Advanced Research Projects Agency) joined the Navy in X-Wing development.
Quoting the winter 1982 AIAA Student Journal in an article on the X-Wing by Robert M. Williams, DARPA Program Manager:
From the success of these tests, DARPA and the Navy, with additional support from NASA, decided to progress to a larger-scale flight test of the X-Wing rotor/wing. Sikorsky was contracted in 1982 for this development, with the idea of testing the rotor/wing on the recently delivered NASA/Army/Sikorsky RSRA (Rotor Systems Research Aircraft). The S-72 RSRA was a unique aircraft designed solely for testing advanced helicopter rotors. It had a variable incidence wing which could offload or overload the rotor’s lifting requirements. It had auxiliary propulsion and drag brakes to offload or overload the rotor’s horizontal thrust requirements. It thus would permit initial rotor/wing stopping and starting tests to be done in an orderly build-up fashion, without requiring the rotor to support its full design lift during initial conversions.
The S-72 RSRA is described in another section of this Sikorsky Product History. See:
The X-wing was perhaps the most technically challenging concept Sikorsky had ever worked on. The rotor blades have to carry lift without the conventional helicopter’s use of centrifugal force for blade stiffening. The blades also have to be extremely stiff since two of them become forward-swept wings in the stopped rotor flight mode. The blades have to be “double-ended” since the blades on the retreating side of the rotor disc must fly “backwards” in stopped rotor flight. By using the Coanda effect to control blade lift, the helicopter’s conventional mechanical swashplate is replaced with a valving system in the pneumatic lines feeding air to the rotor blades. The starting and stopping sequence requires complex high-frequency manipulation of the circulation control valving system to provide controllable forces and moments during the conversion process. This requires the latest in the state-of-the-art in computation capability to control the process.
Other technical challenges included the design of large compressor to provide the circulation control air, and a high energy clutch and a brake to start and stop the rotor. Hub moment feedback was required to control the rotor in conversion. This was truly a “high-risk/high-payoff” program. Both the technical challenges and achievement of the full aircraft potential were major steps in the state-of-the-art.
Two design teams were established at Sikorsky. The first concentrated on developing the X-Wing rotor/wing systems as they would be applied to a demonstrator aircraft. The second worked on how to test these systems on the RSRA. Much detailed work was required on all the disciplines involved. A detailed 10 foot diameter wind tunnel model was developed to optimize the design. Testing with this model began in December, 1985 at Sikorsky. In June, 1986 it was tested in the United Technologies large-scale wind tunnel for forward flight tests prior to the actual RSRA flight test.
The entire X-Wing dynamic system was fabricated and tested on a Propulsion System Test Bed (PSTB) at Sikorsky’s West Palm Beach Florida test facility. Initial tests used dummy blades for development of the pnuemodynamic system from the compressor inlet to the blade root. Then the blades were installed for a full system test.
A Vehicle Management System Laboratory (VMSL) was created to develop the Vehicle Management System (expanded flight control system) prior to its use on the aircraft. The four computers were tied to a full set of aircraft sensors, hydraulics, electric power and actuation components, as well as a fixed-base cockpit. All the aircraft instrumentation was duplicated. An additional computer facility provided a simulation of the RSRA/X-Wing vehicle. Hundreds of operational hours were accumulated on the system prior to the start of the flight test program.
This VMSL became the prototype for all subsequent Sikorsky flight control system integration laboratories.
All of this came together on the RSRA test aircraft. The roll-out of the RSRA/X-wing occurred on August 19, 1986.
In September 1986 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.
The flight test program was envisioned as a gradual buildup of the envelope. After taxi tests, the aircraft was flown without the X-Wing blades installed to establish baseline date.
RSRA was designed to fly new rotors when they were not producing lift, and to return to base in an emergency after the rotor blades had been severed from the rotor head. Thus it could fly as a fixed wing airplane without a rotor. When this test occurred it was a special event for the Sikorsky team. Igor Sikorsky spent his early career producing fixed-wing aircraft, including the large flying boats in the 1930s. But a new Sikorsky-designed fixed-wing aircraft had not flown in almost a half of a century
The plan was to then fly it first with two rotor blades, followed by four blades, with the rotors stopped. By mid-1987 the RSRA was scheduled to fly with the rotor/wing turning at full speed, and by year’s end the rotor/wing was to be stopped and started in flight.
But, that was not to be. In 1987 the Government had funding issues and higher priorities, and decided to terminate the contract. The program was terminated due to this lack of funding rather than for any technical issues.
The design and fabrication of the rotor/wing was the most difficult structural challenge. The blades could not rely on centrifugal stiffening like a conventional helicopter since they had to operate when the rotor was stopped. In addition, they had to be unusually stiff since when operating as an “X” wing two of the blades were swept forward at 45º. Composites were used to provide this extreme stiffness at an acceptable weight.
Rotor hub flexbeam design
The blade manufacturing and tooling challenges were unprecedented in the rotary wing field and involved development of a unique curing process for the very thick composite components. New processes had to be developed to characterize the material, define the allowable void size and establish a quality control procedure.
The following illustration shows the need for both leading and trailing edge blowing. In rotary wing mode, air is blown out the trailing edges of the blades. In the fixed wing mode, two of the blades are now flying “backwards” with blowing now required on what was the leading edge. During conversion the aerodynamics get much more complicated, with circulation control blowing required out of both the leading and trailing edges at the same time.
To provide the air for the circulation control system a two-stage axial compressor was used driven off the main gearbox. This unique compressor was developed by Pratt & Whitney in Florida. It fed air to a plenum mounted below the rotor concentric with the rotor shaft. This was a pneumatic swashplate, with the stationary element collecting air from the compressor, valves to control the flow, and a rotating element which passed the air out to the blades.
Both a high energy clutch and brake were provided for the rotor starting and stopping operations. The clutch was developed by Allison. The clutch concept became the basis of the clutch design eventually used to manage the lift fan on the Joint Strike Fighter, the F-35.
A full authority, quad redundant, digital fly-by-wire vehicle management system was used to control the blade aerodynamics up to the fourth harmonic with rapid response characteristics. This was done by controlling the pneumatic control valves in the plenum as well as the mechanical collective pitch change mechanism and the compressor. During the conversion process it also controlled the clutch, the brake, and the positioning index system.
The revolutionary flight control computer was developed by The Hamilton Standard Division of UTC. At the time it was the most sophisticated system of its kind other than the space shuttle.
In a concurrent effort, NASA and DARPA funded General Electric to develop the GE CEST TF34 convertible engine. This was successfully designed and built and then tested at NASA Lewis from 1984 to 1986. It was a modified TF-34-400B engine and produced up to 7950 pounds of thrust with the inlet guide vanes open, and 4650 shp plus 1650 pounds of thrust with the inlet vanes fully closed. This photograph shows the engine being tested at the NASA Lewis (now Glenn) Research Center.
General Arrangement – RSRA X-Wing
Characteristics and Performance
Demonstrator Aircraft Conceptual Designs
Production Aircraft Conceptual Design
Over the five years of the contract, a tremendous amount of technical work had been accomplished and many difficult technical challenges had been resolved. The powered wind tunnel effort, plus compressor/clutch development, vehicle management system laboratory, system and software validation activities, composite material development, blade and rotor head fabrication, and appropriate risk mitigation tests were accomplished in a very few short years.
At the time of termination the X-Wing technology was still in the development stage. The true potential of the concept was not yet demonstrated in flight. Perhaps some day it will be demonstrated and a mission capable X-Wing will enter service.
With the X-Wing program Sikorsky gained much technical experience on bearingless composite rotors, and fly-by-wire and higher harmonic control systems, which greatly advanced these technologies for future aircraft.
Additional Information Resources
“X-Wing Testbed Flies in Rotorless Configuration” Aviation Week and Space Technology, January 14, 1985
Linden, A., Biggers, J., “X-Wing Potential for Navy Applications”, American Helicopter Society 41st Annual Forum, Fort Worth, Texas, May 1985.
LAST UPDATE MARCH 26, 2013
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