The United States Armed Forces[N 1] are the military forces of the United States of America. They consist of the Army, Navy, Marine Corps, Air Force, and Coast Guard.[6] The U.S. has a strong tradition of civilian control of the military. The President of the United States is the military's overall head, and helps form military policy with the U.S. Department of Defense (DoD), a federal executive department, acting as the principal organ by which military policy is carried out. The DoD is headed by the Secretary of Defense, who is a civilian and Cabinet member. The Defense Secretary is second in the military's chain of command, just below the President, and serves as the principal assistant to the President in all DoD-related matters.[7] To coordinate military action with diplomacy, the President has an advisory National Security Council headed by a National Security Advisor. Both the President and Secretary of Defense are advised by a seven-member Joint Chiefs of Staff, which includes the head of each of the Defense Department's service branches as well as the chief of the National Guard Bureau. Leadership is provided by the Chairman of the Joint Chiefs of Staff and the Vice Chairman of the Joint Chiefs of Staff.[8] The Commandant of the Coast Guard is not a member of the Joint Chiefs of Staff. The Bell Boeing V-22 Osprey is an American multi-mission, military, tiltrotor aircraft with both a vertical takeoff and landing (VTOL), and short takeoff and landing (STOL) capability. It is designed to combine the functionality of a conventional helicopter with the long-range, high-speed cruise performance of a turboprop aircraft. The V-22 originated from the United States Department of Defense Joint-service Vertical take-off/landing Experimental (JVX) aircraft program started in 1981. The team of Bell Helicopter and Boeing Helicopters was awarded a development contract in 1983 for the tiltrotor aircraft. The Bell Boeing team jointly produce the aircraft.[4] The V-22 first flew in 1989, and began flight testing and design alterations; the complexity and difficulties of being the first tiltrotor intended for military service in the world led to many years of development. The United States Marine Corps began crew training for the Osprey in 2000, and fielded it in 2007; it is supplementing and will eventually replace their CH-46 Sea Knights. The Osprey's other operator, the U.S. Air Force, fielded their version of the tiltrotor in 2009. Since entering service with the U.S. Marine Corps and Air Force, the Osprey has been deployed in both combat and rescue operations over Iraq, Afghanistan and Libya. The Osprey is the world's first production tiltrotor aircraft, with one three-bladed proprotor, turboprop engine, and transmission nacelle mounted on each wingtip. It is classified as a powered lift aircraft by the Federal Aviation Administration.[88] For takeoff and landing, it typically operates as a helicopter with the nacelles vertical and rotors horizontal. Once airborne, the nacelles rotate forward 90° in as little as 12 seconds for horizontal flight, converting the V-22 to a more fuel efficient, higher speed turboprop aircraft. STOL rolling-takeoff and landing capability is achieved by having the nacelles tilted forward up to 45°.[68] Other orientations are possible, such as the "80 Jump" takeoff which uses nacelles at 80° to quickly achieve high altitude and speed.[89] Composite materials make up 43% of the V-22's airframe. The proprotors blades also use composites.[68] For compact storage and transport, partly on Marine launch ships, the V-22's rotors fold in 90 seconds and its wing rotates to align, front-to-back, with the fuselage.[90] Due to the requirement of folding the rotors their 38 feet diameter is 5 feet less than optimal for vertical takeoff, resulting in high disk loading.[89] Most Osprey missions use fixed wing flight 75 percent or more of the time, reducing wear and tear on the aircraft and reducing operational costs. This fixed wing flight is higher than typical helicopter missions allowing longer range line-of-sight communications for improved command and control.[24] The V-22's two Rolls-Royce AE 1107C engines are connected by drive shafts to a common central gearbox so that one engine can power both proprotors if an engine failure occurs.[53] However, if a proprotor gearbox fails that proprotor cannot be feathered, and both engines must be stopped before an emergency landing.[52] The aircraft's autorotation characteristics are poor partly because the rotors have low inertia.[52] Boeing has stated the V-22 design loses 10% of its vertical lift over a Tiltwing design when operating in helicopter mode because of airflow resistance due to the wings, but that the Tiltrotor design has better short takeoff and landing performance.[91] The rotorwash usually prevents usage of the starboard door in hover, and the rear ramp is used for rappelling and hoisting.[52]

• published: 19 Mar 2014
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