|
Slowed rotor is a concept in designing and flying certain rotorcraft. Reducing the rotational speed of the rotor reduces the drag, enabling the aircraft to go faster and/or fly more economically. ==Background== Rotors of conventional helicopters are designed to operate at a fixed RPM〔Croucher 2008, page 2-12. Quote: (speed ) "is constant in a helicopter".〕〔Seddon 2011, p216. Quote: The rotor is best served by rotating at a constant rotor speed〕〔Robert Beckhusen. "(Army Dumps All-Seeing Chopper Drone )" ''Wired'' June 25, 2012. Accessed: 12 October 2013. ''Quote: for standard choppers .. the number of revolutions per minute is also set at a fixed rate''〕 (within just a few percent),〔The UH-60 permits 95–101% rotor RPM (UH-60 limits ) ''US Army Aviation''. Retrieved 2 January 2010〕〔 causing suboptimal operation in large parts of the flight envelope.〔Khoshlahjeh〕 Two main issues restrict the speed of rotorcraft:〔Lombardi, Frank. "(Optimizing the Rotor )" ''Rotor&Wing'', June 2014. Accessed: 15 June 2014. (Archived on 15 June 2014 )〕〔Harris 2003, page 7〕〔Chiles, James R. "(Hot-Rod Helicopters )" (Page 2 ) (Page 3 ) ''Air & Space/Smithsonian'', September 2009. Accessed: 18 May 2014.〕〔 * Retreating blade stall. As the helicopter goes faster, the airflow over the retreating blade becomes relatively slower, while the airflow over the advancing blade is relatively faster, creating more lift. If not counteracted by flapping,〔"(Blade flapping )" ''Dynamic Flight''〕 this would cause dissymmetry of lift and eventually retreating blade stall,〔Robb 2006, page 31〕〔Silva 2010, page 1.〕〔"(Helicopter Limitations )" ''Challis Heliplane''〕〔"(Retreating blade stall )" ''Dynamic Flight''〕〔 and blade stability suffers as the blade reaches its limits for flapping.〔〔Johnson HT, p323〕 * Transonic drag near the rotor blade tip. The faster-moving advancing blade tip may begin to approach the speed of sound, where transonic drag begins to rise steeply, and severe buffeting and vibration effects can occur. This effect prevents any further increase in speed, even if the helicopter has surplus power remaining, and even if it features a highly streamlined fuselage. A similar effect prevents propeller-driven aircraft from achieving supersonic speeds, although they can achieve higher speeds than a helicopter, since the propeller blade isn't advancing in the direction of travel.〔〔Prouty, Ray. "(Ask Ray Prouty )" ''Rotor&Wing'', 1 May 2005. Accessed: 18 May 2014.〕〔"(Nomenclature: Transonic drag rise )" ''NASA''〕〔 These (and other)〔Beare, Glenn. ("Why can't a Helicopter fly faster than it does ?" ) ''helis.com'' . Accessed: 9 May 2014.〕〔Krasner, Helen. ("Why Can’t Helicopters Fly Fast?" ) ''Decoded Science'', 10 December 2012. Accessed: 9 May 2014.〕 problems limit the practical speed of helicopters to around .〔〔〔Clean Sky 2012, page 44〕〔Majumdar, Dave. "(DARPA Awards Contracts in Search of a 460 MPH Helicopter )" ''United States Naval Institute'', 19 March 2014. Accessed: 9 May 2014.〕〔Wise, Jeff. "(The Rise of Radical New Rotorcraft )" ''Popular Mechanics'', 3 June 2014. Accessed: 19 June 2014. (Archive ) Quote: "This aerodynamic principle limits conventional helicopters to about 200 mph."〕 At the extreme, the theoretical top speed for a rotary winged aircraft is about ,〔 just above the current official speed record for a conventional helicopter held by a Westland Lynx, which flew at in 1986〔 where its blade tips were nearly Mach 1. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Slowed rotor」の詳細全文を読む スポンサード リンク
|