The American aircraft manufacturer Sikorsky is trying to maintain a leading position in the aircraft industry, which is directly related to the search and implementation of new solutions. In recent years, she has been actively involved in high-speed helicopters with coaxial rotor and pusher rotor. Such a scheme was first implemented in the experimental project X2 and has proven itself well. Now she has found application in the development of new machines.
Experimental X2
The X2 project was experimental from the very beginning. His goal was to create and build a flying laboratory for testing a new aircraft layout. In the future, the machine could become a source of new technologies for other projects, but its direct implementation was not planned.
The design of the new helicopter was completed in the middle of the 2000s, and in 2008 it was ready for testing. The main work was carried out by Sikorsky independently, while some of the systems and units were created by other enterprises or purchased from them. In particular, the fly-by-wire control system was designed at Honeywell, the main rotors were made by Eagle Aviation Technologies, and the tail was made by Aero Composites.
The streamlined fuselage of the X2 housed the cockpit, the necessary equipment, the power plant and gearboxes to distribute power between the three propellers. In the tail section, a tail unit with steering surfaces was provided.
The X2 received a 1800 hp LHTEC T800-LHT-801 turboshaft engine. Power was given to two coaxial main rotor and to the tail pusher, necessary for acceleration. Control systems provided control over the rotational speed of the rotor. At speeds up to 200 knots (370 km / h), the maximum permissible rotational speed exceeded 440 rpm. Above 200 knots, the propeller was decelerated to 360 rpm or less to maintain optimal blade speed.
The X2 carrier system includes two coaxial four-bladed propellers rotating in different directions. The design of the blades uses engineering solutions aimed at increasing their rigidity and reducing twisting under aerodynamic loads. It is reported that similar solutions have already been used in American helicopter projects.
Due to the need to increase the flight speed and reduce the resistance, the propeller hub was closed with several fairings. Two disc fairings cover the roots of the blades. Another aerodynamic part sits between them and reduces the resistance of the vertical part of the hub.
To achieve maximum speeds, the X2 helicopter used a tail propeller to create the necessary thrust. Due to this, at high-speed flight conditions, the rotors create only lift, but not translational thrust. Due to this, the rotor speed is reduced, and the speed of the blades remains within acceptable limits.
For acceleration in horizontal flight, the X2 was responsible for a six-bladed tail propeller. The shape of the blades has been optimized to operate in the specified modes. The calculated thrust of such a propeller met the general requirements of the project.
The X2 had an EDSU capable of receiving data from a variety of sensors and responding to incoming information. Automation was supposed to monitor the operation of the units and the behavior of the machine, as well as issue the necessary commands to the actuators. Due to this, it was proposed to ensure the confident behavior of the machine in all flight modes.
Test results
The first flight of the experienced Sikorsky X2 took place on August 27, 2008 and lasted about half an hour. The first stages of testing, which provided for flight only at the expense of the carrier system, lasted about a year. Their result was a horizontal flight speed of the order of 250-300 km / h - at the level of other modern helicopters.
In mid-2009, a new stage of testing began, in which all available propellers were involved. In May 2010, they managed to get a speed of 180 knots (335 km / h), and after a few weeks the flying laboratory accelerated to 225 knots (417 km / h). This flight could claim a world record, but the results were not recorded according to the FAI rules.
On September 15 of the same year, a new record took place - the X2 reached a speed of 250 knots (460 km / h). A little later, the speed was increased by another 20 km / h. Flight tests continued until July 2011, but new records were no longer set. The testers flew in different modes in order to collect data on the behavior of the equipment.
After completing the tests, the experienced X2 parked as unnecessary. It was donated to the National Aerospace Museum in 2016. Now everyone can see the car.
Implementation of developments
The Sikorsky X2 was a purely experimental vehicle intended only for testing new technical solutions. During its tests, the data was collected, necessary for the development of new projects of practically applicable technology. Similar work started even before the end of the X2 tests.
In 2010, the US Army launched the Armed Aerial Scout program, which aims to build a helicopter to replace the aging OH-58D. The new machine should carry comparable cargo and show better flight characteristics. To participate in the AAC, Sikorsky has developed a new high-speed helicopter S-97 Raider, which is based on all the main developments on the X2 theme. The first flight of such a machine took place in May 2015, and so far three prototypes have been used in testing.
From the point of view of the general scheme, the S-97 does not differ from the previous flying laboratory. It has coaxial main rotor and a tail propeller. The propellers are driven by a General Electric YT706 2600 hp engine. There is a developed horizontal stabilizer that relieves the rotor during acceleration. A helicopter with a takeoff weight of less than 5 tons will be able to carry up to six paratroopers or comparable cargo or weapons.
The Raider's design cruising speed is 220 knots (410 km / h). The maximum is 250 knots. However, so far the real results look more modest. To date, the speed of test flights does not exceed 190-200 knots (no more than 370 km / h). It is expected that the S-97 will show all the required flight characteristics for the foreseeable future, and this will ensure it will win the AAS competition.
Later, the Sikorsky Boeing SB> 1 Defiant helicopter project appeared. It is being created to participate in the US Army's Future Vertical Lift program and should fill the niche of a medium multi-purpose transport helicopter. Like the S-97, the SB> 1 is based on the experimental X-2 and has the same design.
The Defiant has two four-bladed rotors and a thrusting eight-bladed rotor. The power plant is based on two Lycoming T55 engines. In the future, it is planned to replace them with motors with higher characteristics.
The first flight of SB> 1 took place on March 21, 2019. As part of the tests, the horizontal flight speed is constantly increasing, but it is still far from record values. In the future, after replacing the engines, it is planned to bring the cruising speed to 250 knots. At the same time, it is proposed to combine high speed with good efficiency. In terms of flight range, the Defiant will also have to surpass existing vehicles.
Direction prospects
The experimental project Sikorsky X2 can be considered unequivocally successful. The prototype machine coped with the tasks. It provided validation of new solutions and technologies, and also allowed the accumulation of the necessary amount of data. All this experience has already been used in two projects, and new helicopters of this kind may appear in the future.
The prospects for the S-97 Raider and SB> 1 Defiant can only be partially assessed. Two machines are being tested and show good technical results. The tasks of increasing the flight speed are gradually being solved, and the characteristics go to the specified level. There is every reason to believe that the two promising helicopters will indeed show the expected capabilities.
However, the commercial prospects of the new Sikorsky machines are still in doubt. Several aircraft manufacturers with different projects participate in the AAS and FVL competitions. In both cases, Sikorsky owns the most interesting developments, but high performance and technical courage may not be the deciding factor. In the foreseeable future, the Pentagon must choose the winner of two competitions and thereby determine the path of development of army aviation.
The future of Sikorsky's projects has not yet been determined, but the interim results look interesting. An experimental project ten years ago successfully solved the assigned tasks and opened the way for the development of new samples. In the near future, they will have to go through the necessary checks and compete for a place in the troops. So far, the chances of two X2-based helicopters look good enough.
