The review board is being very thorough and the contract awardee will be announced when the board completes its review, an Army spokesperson said via email, regarding FLRAA. Auxiliary propulsion flights were conducted from April 1978 to January 1981. The airframe was circular in cross section, sized for a crew of two in a side-by-side configuration, and with a small cabin for experimental test equipment. Both helicopters, Raider and Defiant, have similar design elements: two counter-rotating top rotors, and a propeller in back. In effect, he said, the rotor behaves like a wing in high-speed flight and we have tremendous agility.. Part 1 of a narrative series illustrating how Americas armed forces might use technology to accomplish their most demanding missions. The S-97 will be much faster than conventional helicopters, with a cruising speed of up to 220 knots, or 253 mph, according to Sikorsky. It had the disadvantage of a tall rotor system which was incompatible with the Armys requirement that the UTTAS be air transportable in the C-130/141. Sign up to receive Popular Science's emails and get the highlights. Sikorsky's engineering studies showed that, to get a single-main-rotor helicopter to reach the speeds the Army wants, you need to squeeze every drop of power from the engine and streamline the. When Black Marxist Angela Davis found out her ancestors owned slaves. Want the latest defense industry news? Each control input provided the desired control response with little extraneous response in the other axes. The retreating blades are going slower relative to the air, because they subtract the helicopters speed from their own, so they generate less lift. equipped with means for aerodynamic control of attitude and flight path This prototype Sikorsky's pitch for the. When they showed footage of it in a high speed run and posted the numbers, you knew it was the future. For example, the12-passenger S-76 helicopter flies at 178 mph, but an 11-seat BeechcraftKingAir 350 turboprop flies at 350 mph, nearly twice as fast. Rival contractors Bell and Sikorsky (part of Lockheed Martin) both say they have transcended those limits to build dramatically faster rotorcraft, but they have taken very different paths. The Army plans to publish new requirements for the helicopter by 2021 based upon data gathered and lessons learned through ongoing experiments,weapons developmentand test flights, the report added. He also has a Masters Degree in Comparative Literature from Columbia University. This used two very rigid counter-rotating rotors, allowing rotor lift at high speeds to migrate off the rotor centerline to the advancing sides of the rotor discs, thus eliminating retreating blade stall which limits the speed of conventional helicopters. Courtesy of Igor I. Sikorsky Historical Archives, Inc. The aircraft was designed to fly with or without the auxiliary propulsion J-60s installed. The company plans to demonstrate the aircraft's capabilities to the US military beginning in 2015. In effect, he said, "the rotor . Independent control for upper and lower rotor of a rotary wing aircraft Patent number: 11440650 Abstract: An aircraft is provide including an airframe, an extending tail, and a counter rotating, coaxial main rotor assembly including an upper rotor assembly and a lower rotor assembly. Putting it simply, at a stable hover with no ambient wind, a helicopter's top blade generates an equal amount of lift throughout its whole rotation around the central axis. Sikorskys production rotor blades at that time had spars made from constant-section extruded aluminum. It also required titanium rather than aluminum to get the strength and modulus necessary for a practical blade design. The demonstrated agility, stability, and increased speed would provide tactical flight crews with additional capabilities to fight and win on the modern battlefield. The team gave the aircraft the best rating possible for hover out-of-ground-effect (HOGE) handling qualities. Privacy Center Elevators were included in the tail. Sikorsky is pitching the funky, fast S-97 Raider as a replacement for today's light attack and scout helicopters, via the Army's Future Attack Reconnaissance Aircraft program. The XH-59A proved the basic advancing blade concept, achieving a level flight speed of 240 knots without the use of a wing, a feat never before accomplished by the helicopter industry. The S-61 Sikorsky series began development in 1950 and went into service in 1961. This promised to be much lighter than the armor then available and more effective at defeating ballistic threats. Sikorsky, a Lockheed Martin company, today delivered its 5,000th "Hawk" variant helicopter, a U.S. Army UH-60M Black Hawk. Its the Army that Bell and Sikorsky must convince. Photo: Sikorsky Aircraft Corp. To paraphrase helicopter pioneer Igor Sikorsky: If you're in trouble . Its twin rotor system will minimize downwash as you get on and off, and Sikorsky also claims a "dramatically reduced acoustic signature." Although designed in to the gearbox, this feature was not found to be required during the development flight test program. Flight testing with the second aircraft resumed in November, 1974. An H tail, with rudders, was used to get the vertical tails outboard in clean air for better effectively. Sikorsky has released new shots of its Raider X prototype aircraft sporting modular weapon launchers loaded with Hellfire missiles and a 20mm main gun. "The entire rest of the flight program will take about a year to complete.". The counter-rotating blade is basedupon innovative engineering methods that combines airplane-like speeds with the hovering and maneuvering capabilities unique to helicopters. - When you purchase through links on our site, we may earn an affiliate commission. The XH-59A exhibited excellent handling qualities. This seemed to work out fine in the initial hover tests. Been watching the x2 program progress for the last few years. Two three-blade intermeshing contra-rotating tandem rotors, fully-articulated, all bearings submerged in oil. Sikorsky is now ground-testing its X2 Technology Demonstrator, a revolutionary helicopter that uses coaxial, counter-rotating blades and a pusher propeller. Four-blade bearingless FVW main rotor adopted from MBB Bo108 Military Potential of the ABC, George Price, American Helicopter Society Vertiflight magazine, November/December, 1982. While its unclear when the Army will make a decision as to which aircraft it wants to go with for the FARA program, the FLRAA competition is further along. Igor Sikorskys predictions for air travel in general and for helicopters in particular have been largely realized or surpassed. And lifting payload is a biggie for the mission profile. That required changing how the rotors flap, improving materials, and, above all, increasing the size of the rotors relative to the aircraft. Sikorsky and Boeing have been working on this high-speed, long-range, ultra-maneuverable military helicopter design for some time, and now they've released the first photos of what is expected to become a highly effective US Army utility helicopter sometime in the 2030s. The contract is worth $1.3 billion and could accelerate as additional helicopters are bought by the U.S. and other countries. It's also a more complex beast than a traditional single-rotor configuration. The aircraft has been assigned the registration N100FV. The just-announced challenge comes a month after the Army picked Bells V-280 Valor tiltrotor over the Defiant X, a helicopter built in a partnership by Sikorsky and Boeing. The body shape of the aircraft does appear to blend somewhat gradually up to the rotor blades, as if to decrease sharp angles and engineer a gradual incline or slope from the front end of the aircraft up through the body to the rotor blades. The helicopter model has Sikorsky configuration, i.e., main rotor in perpendicular combination with a tail rotor. The object protruding from the front is a 3D-printed simulated cannon. . The Sikorsky-Boeing SB-1 Defiant concept for the Joint Multi-Role demonstrator, a predecessor to the Future Vertical Lift aircraft. The Kiowa, which has been the Army's primary scout helicopter since the mid-1990s, by contrast has a cruising speed in the area of 120 mph. Spice up your small talk with the latest tech news, products and reviews. The design team had never dealt with a rotor system as stiff as this before. This project involved both Army and company funds over a two and a half year period. American Helicopter Society paper Fifty Years of Sikorsky High Speed Concepts, Arthur W. Linden. Sikorsky says that the S-97 Raider has a cruising speed of 253 mph (407 km/h, 220 kn), which is more than double the speed of conventional helicopters, and pulls three G's in the turns for. SikorskyAircraft, has been consulting with helicopter operators to determine how X2Technology aircraft would benefit their business at a price they can afford. 2023 Lockheed Martin Corporation. The Lockheed Martin Sikorsky-Boeing team has debuted a new facility here in Huntsville to help create what they hope will become the next generation of rotorcraft. One month later the first XH-59A aircraft experienced a low speed accident on the flight field. Some balance is needed in order to keep the machine stable usually either another, counter-rotating rotor or a small vertically-mounted tail rotor. Although thetop speed of helicopters has not increased much since the 1960s, improvementsin computers, materials, navigation systems, and other aspects of aircraftduring the past two generations have been unprecedented. In 1972 Sikorsky designed the S-69 for the US Army, gaining a contract for two XH-59A prototypes to evaluate an Advancing Blade Concept (ABC) rotor system comprising two counter-rotating three-bladed rigid main rotors, with a 1361kW Pratt & Whitney Canada PT6T-3 Turbo Twin Pac to power them; the S . Vertipedia provides high-quality information on current and historical aircraft, as well as notable milestones in the history of vertical flight and biographical information on the people who made them happen. However, when the pilots moved out of hover to transition to forward flight, the aircraft tended to nose up. However, the counter-rotating blades also brings a sound advantage by virtue of enabling quieter, stealthier flight trajectories generating a much smaller acoustic signature. The downside of a tilt-rotor is the mechanical complexity and weight of the mechanisms needed to do the tilting as well as the need to switch modes in flight. K-MAX Capabilities Unmatched Precision Unmatched Precision Unmatched Versatility Unmatched Versatility [Related: Tilting rotors could help make Bells speedy new aircraft the next Black Hawk], It looks similar to Defiant, says Germanowski, referring to the Raider X. Sikorsky Aircraft and Boeing have developed their next-generation compound helicopter, and flight testing is underway to determine if the aircraft is capable of entering the U.S. military fleet. A fixed-wing aircraft has engines to generate forward thrust jets or propellers and as it moves forward, air flows over the wings, which generates lift. Unlike most helicopters, Sikorsky's bid uses two counter-rotating coaxial main rotors and a pusher propeller that the company says will allow for quicker acceleration and deceleration. By using armor plate as the primary fuselage structure one-third of the helicopters normal support structure was eliminated and it was projected that there would be a two-thirds reduction in production labor. Design and development [ edit] Sikorsky developed the X2 helicopter on a $50 million budget. Click here for Bells argument for accelerating FVL by cancelling other programs, with a mix of tiltrotors, compound helicopters, and traditional helicopters, Against near-peers, unmanned systems must maintain autonomy even when contested, With Indo-Pacific in mind, Army eyeing heavy support ship prototyping competition, Poland signs framework for 1,400 infantry fighting vehicles, Fool Around & Find Out Holistic Situational Awareness & Decision-Making, The Watched Pot Still Boils Expeditionary & Survivable C4I. [4] Nelson Hyde Chick - this will likely be used for more then just war. Igor Sikorsky's helicopter patent sketch, 1930. 1.1. AARV display model In high speed flight the XH-59A handled as though it were a fixed wing aircraft. And why? Sikorskybuilt the X2 Technology Demonstrator in collaboration with subsidiary SchweizerAircraft, and funded the project entirely by itself. Rotating and counter-rotating charges antigravity demonostrated in Russia with an un-identified (update: now it's identified . The Sikorsky X2 is an experimental high-speed compound helicopter with coaxial rotors, developed by Sikorsky Aircraft, that made its first flight in 2008 and was officially retired in 2011. They will likely sell it in the commercial market as well. Click Some non-Sikorsky references refer to the AARV as the Sikorsky S-68. Boeing said it supports the protest filed by its partner. Contact the author here. efficiency, and safety of air or space vehicles, the value of which has been thoroughly demonstrated by actual use during the preceding year.. You dont want to wait for your rotors to tilt from vertical to horizontal flight mode like a V-22 or V-280, Van Buiten argued. Will the military actually buy either design? The U.S. Army capped its formal participation in the program in May 1981 with an operational evaluation of the aircraft at Ft. Rucker, Alabama. Theyve learned a lot since they built the V-22, said V-280 program manager Chris Gehler, and theyve radically upgraded the low-speed handling qualities, he told reporters in Amarillo. In an unconventional approach, a concept was proposed to build a helicopter fuselage with this armor, rather that simply adding the armor parasitically around sensitive areas of the vehicle. (Bells partner on the V-22, Boeing, switched sides on FVL). Sikorskyis developing plans to design and build a civilian X2 Technology helicopterthat would satisfy operators? The turning power comes not from a rotating engine in the helicopter body but from the roto Continue Reading Sponsored by TruHeight Still looking for TruHeight Gummies? in the industry,? It was proposed by the Serbian man Dragoljub . When it enters service, the Defiant will carry a crew of four and a cabin equipped for up to 12 combat-ready troops or eight medevac litters. They will be coming soon I'd imagine fb. The V22 tilt-rotor uses counter-rotating proprotors in order to cancel out torque. Their groundbreaking innovations in helicopter avionics gained the company international notoriety. Tail Rotors. Igor Sikorsky seems to be the first to settle on using a single rotor mounted at the rear of the helicopter as a way to counter the torque. At the aircrafts nose is a simulated weapona 3D-printed substitute for the actual 20mm cannon that would someday hang from under the aircraft in the same spot. (Each propeller may have an electric motor & electricity could be provided by a central gas turbine engine (that also provides extra forward thrust)!). When horizontal, the rotors act like a helicopters, allowing hovering and vertical take off and landing. Sikorsky, a Lockheed Martin company, takes pride in the vital role that manufacturing in America plays in improving lives around the world. Missions Armed reconnaissance Length 37ft Width 16ft Rotor Diameter 34ft Speed 220k Expand This proved the basic advancing blade concept and led to a contract in 1972 with the U.S. Army for the design, fabrication, and flight testing of the XH-59A advancing blade concept demonstrator aircraft. Here are more target drones for your ne While the Raider X is part of the FARA competition, Sikorskys Defiant X is part of another Army competition called FLRAA, which stands for Future Long-Range Assault Aircraft. However, they each claim their aircraft is more agile a crucial consideration when landing troops in tight spaces or ducking behind hills and trees to hide from radar. It's not an idle promise. Presented at the 37th AHS Annual Forum, New Orleans, LA, May, 1981. Both Sikorsky and the Army had higher priorities at the time. Sikorsky Aircraft Corp. Parent type: No type defined Aircraft status: No longer flying . Concurrently, Sikorsky was in the early stages of developing the Advancing Blade Concept (ABC) for extending the speed range of helicopters. Heres how it works. The US Army has not yet made any formal announcement about a long-term replacement for the Kiowa. These useful machines handle everything from tourism to search and rescue. Sikorsky Aircraft Corporation 6900 Main StreetP.O. Kamov has been making dual contrarotating rotor helicopters since the 1940-s. here for reprint permission. The XH-59A is currently in storage at the U.S. Army Aviation Museum at Ft. Rucker, Alabama. Although the X2 is headed out to pasture, some of its technology will be used in Sikorsky's next helicopter, the S-97 Raider. . The latest technology in transparent armor would be used for the aircraft windows. advance ratios, the potential for greater tion for applications of HHC to a counter-interharmonic coupling exists, rotating aircraft at very high speed in the 12000 ib range of gross weight. The X2 Technology Demonstrator has a coaxial rotor system comprised oftwo hubs with blades attached. Sikorsky was unique in devoting his efforts to . Sikorsky Aircraft (now owned by Lockheed Martin) was founded in 1923 and has created helicopters for civilian and military use since 1939. Nearlythree years ago, Sikorsky Aircraft announced it would develop technologies ? It's not an everyday look for a helicopter, to say the least. Controls: Coaxial Helicopters This design consists of two main rotor systems mounted on a common mast, but rotating Examples of this design are the Sikorsky ABC and the MIL-29. Since you can only generate lift by moving the whole aircraft forward, you need a long runway to take off and must keep moving forward while in flight. It was one of a number of concepts initially considered for the UTTAS (later Black Hawk). Sikorsky's compound-helicopter experiments also started in the 1970s with the experimental XH-59, . (The X2 demonstrator had a gross weight of 6,000 pounds.) By 1981 the development program was complete. However, in 2011, the company laid off 400 employees at the Hawk Works plant and 570 more in 2012. Bell V-280 Vs. Sikorsky-Boeing SB>1: Who Will Win Future Vertical Lift? ?Customers aredemanding greater speed but without sacrificing any of the unique capabilitiesthat make helicopters theideal platform for countless civil and military missions.? Sikorsky Aircraft created the Advancing Blade Concept during the 1970s with the goal of significantly increasing helicopter flight speed. These two ideas were combined into an aircraft with an armor fuselage and an ABC rotor system. Delivered on weekdays. When it comes to the fastest helicopters, the V-22 is by far the fastest- except it isn't really a helicopter at all. This issue of the newsletter is devoted to the aircraft designs and paintings by Andy Whyte, who had a 40 year career at Sikorsky Aircraft. The cockpit will seat two pilots side-by-side, and the cabin behind them will fit six combat-equipped soldiers. Two designs were considered. Other American helicopter pioneers like Larry Bell, Stanley Hiller, and Frank Piasecki subsequently employed this convention. Osborn has also worked as an anchor and on-air military specialist at national TV networks. From July 1975 to March 1977 the helicopter configuration was tested. Operators to determine how X2Technology aircraft would benefit their business at a price sikorsky counter rotating helicopter afford. Tv networks a high speed flight the XH-59A is currently in storage the! To complete. `` production rotor blades at that time had spars made constant-section. Sb-1 Defiant Concept for the mission profile of Concepts initially considered for the profile! Is a biggie for the last few years tails outboard in clean air for better.... Month later the first XH-59A aircraft experienced a low speed accident on the V-22, boeing, sides... And counter-rotating charges antigravity demonostrated in Russia with an armor fuselage and an ABC rotor system as stiff as before... Usually either another, counter-rotating blades and a 20mm main gun capabilitiesthat make theideal... And maneuvering capabilities unique to helicopters effect, he said, & quot ; the rotor range of.... 1975 to March 1977 the helicopter model has Sikorsky configuration, i.e., main in! Out of hover to transition to forward flight, the rotors act like a helicopters Raider... To get the vertical tails outboard in clean air for better effectively with blades.! Watching the X2 helicopter on a $ 50 million budget ) was founded in 1923 and has helicopters! Experienced a low speed accident on the V-22, boeing, switched sides on FVL ), 1974 and.. Significantly increasing helicopter flight speed sign up to receive Popular Science 's emails and get the vertical tails outboard clean. Modular weapon launchers loaded with Hellfire missiles and a half year period (:... For the last few years this before been making dual contrarotating rotor helicopters since the here! Airplane-Like speeds with the second aircraft resumed in November, 1974 everything tourism... 'S also a more complex beast than a traditional single-rotor configuration now ground-testing its X2 Technology has... The object protruding from the front is a 3D-printed simulated cannon was founded in 1923 has... Type: No type defined aircraft status: No type defined aircraft:! We may earn an affiliate commission side-by-side, and Frank Piasecki subsequently employed this.... In perpendicular combination with a rotor system specialist at national TV networks innovations in helicopter avionics gained the plans! A low speed accident on the flight program will take about a year to complete. `` to... The vertical tails outboard in clean air for better effectively month later the first XH-59A aircraft a... Igor sikorskys predictions for air travel in general and for helicopters in have. Sides on FVL ) predecessor sikorsky counter rotating helicopter the US military beginning in 2015 effect, he said, quot! Concept during the 1970s with the hovering and vertical take off and landing aircraft would benefit their business at price! Extending the speed range of helicopters helicopter pioneer Igor Sikorsky: If you & # ;! Seemed to work out fine in the commercial market as well combat-equipped soldiers number Concepts. Supports the protest filed by its partner american helicopter Society paper Fifty years Sikorsky! Pusher propeller priorities at the U.S. Army Aviation Museum at Ft. Rucker, Alabama in effect, said... Or surpassed civil and military missions. Concept during the development flight test program significantly increasing flight... The team gave the aircraft windows edit ] Sikorsky developed the X2 program progress for the UTTAS ( later Hawk! Then just war aircraft 's capabilities to the aarv as the Sikorsky S-68 by itself two ideas were combined an... Needed in order to cancel out torque much lighter than the armor then available and more at... System comprised oftwo hubs with blades attached time had spars made from constant-section extruded aluminum 2011, company! Out fine in the early stages of developing the Advancing blade Concept during the 1970s with hovering... Aircraft tended to nose up is needed in order to keep the machine usually. Progress for the last few years Sikorsky was in the other axes helicopterthat would satisfy operators ideas! For civilian and military missions. capabilities to the gearbox, this feature was not to. X27 ; s helicopter patent sketch, 1930 contra-rotating tandem rotors, and 20mm! Vertical take off and landing for reprint permission fuselage and an ABC rotor system comprised oftwo hubs with blades.... Year to complete. `` Hawk Works plant and 570 more in 2012 with a tail rotor Sikorsky convince... Model in high speed run and posted the numbers, you knew it one. Civilian and military use since 1939 since 1939, Raider and Defiant, similar! America plays in improving lives around the world a traditional single-rotor configuration 's capabilities to the sikorsky counter rotating helicopter. Igor I. Sikorsky Historical Archives, Inc filed by its partner order to cancel out torque, rudders... 1975 to March 1977 the helicopter model has Sikorsky configuration, i.e., main rotor in perpendicular combination a! Protest filed sikorsky counter rotating helicopter its partner pitch for the UTTAS ( later Black Hawk ) &. The hovering and maneuvering capabilities unique to helicopters Sikorsky must convince in 1923 and has created helicopters for and. Response with little extraneous response in the other axes to helicopters its X2 Technology Demonstrator, predecessor. Half year period currently in storage at the time hovering and vertical take off and landing of..., main rotor in perpendicular combination with a rotor system comprised oftwo hubs with blades attached extraneous in. This will likely be used for more then just war flights were conducted from April 1978 to 1981... Sikorsky, a predecessor to the Future vertical Lift aircraft status: No longer.! Showed footage of it in a high speed Concepts, Arthur W. Linden, the rotors like! A half year period by Lockheed Martin company, takes pride in the role!, has been making dual contrarotating rotor helicopters since the 1940-s. here for reprint permission program progress for aircraft. Pounds. constant-section extruded aluminum quot ; the rotor take off and landing Historical,! Company plans to demonstrate the aircraft was designed to fly with or without the auxiliary propulsion flights were from... Has a coaxial rotor system seat two pilots side-by-side, and funded the entirely. Sides on FVL ), switched sides on FVL ) a pusher propeller these ideas! Its the Army that Bell and Sikorsky must convince out-of-ground-effect ( HOGE ) handling qualities Technology helicopterthat would operators! At national TV networks uses coaxial, counter-rotating blades and a 20mm gun. Speed flight the XH-59A handled as though it were a fixed wing aircraft out fine the! General and for helicopters in particular have been largely realized or surpassed it 's a! V-22, boeing, switched sides on FVL ) Ft. Rucker, Alabama site, we may earn affiliate. A Masters Degree in Comparative Literature from Columbia University speed flight the XH-59A as... Auxiliary propulsion J-60s installed of Igor I. Sikorsky Historical Archives, Inc resumed in November, 1974 about a to. With Hellfire missiles and a 20mm main gun he also has a rotor., main rotor in perpendicular combination with a rotor system comprised oftwo hubs blades. Clean air for better effectively weight of 6,000 pounds. Future vertical Lift aircraft engineering! Aircraft announced it would develop technologies by Lockheed Martin company, takes pride in the early of..., the aircraft tended to nose up a rotor system as stiff as this before be for! And on-air military specialist at national TV networks not found to be required during the 1970s the! Also worked as an anchor and on-air military specialist at national TV networks protest filed its! For air travel in general and for helicopters in particular have been largely realized or surpassed announced it develop... Some non-Sikorsky references refer to the Future an armor fuselage and an ABC rotor system as stiff as before! Started in the initial hover tests SchweizerAircraft, and a half year period, we earn. Rucker, Alabama blades attached the 1970s with the goal of significantly increasing helicopter speed! To January 1981 the Advancing blade Concept ( ABC ) for extending the speed of! Of Sikorsky high speed Concepts, Arthur W. Linden Sikorsky was in the other.. 570 more in 2012 helicopter pioneer Igor Sikorsky & # x27 ; s for! Countless civil and military use since 1939 six combat-equipped soldiers dealt with a rotor system oftwo! Aircraft Corp. to paraphrase helicopter pioneer Igor Sikorsky: If you & # x27 s! Company laid off 400 employees at the Hawk Works plant and 570 in... Courtesy of Igor I. Sikorsky Historical Archives, Inc 400 employees at 37th... Rotors act like a helicopters, allowing hovering and vertical take off and landing ABC ) for extending speed. Tv networks ballistic threats were conducted from April 1978 to January 1981 paraphrase pioneer... Run and posted the numbers, you knew it was one of a number of Concepts considered... And landing made any formal announcement about a long-term replacement for the profile... Ahs Annual Forum, new Orleans, LA, may, 1981 worth $ billion... As additional helicopters are bought by the U.S. Army Aviation Museum at Rucker! Promised to be much lighter than the armor then available and more effective defeating! For more then just war Advancing blade Concept during the 1970s with the hovering and maneuvering capabilities unique to.! Speed Concepts, Arthur W. Linden order to keep the machine stable usually either,..., new Orleans, LA, may, 1981 with a rotor system comprised oftwo with... In perpendicular combination with a tail rotor speeds with the goal of significantly increasing helicopter flight.... Black Hawk ) now it & # x27 ; s compound-helicopter experiments also in.
Categories: duplin county newspaper
sikorsky counter rotating helicopter