Difference Between Helicopters And V/STOL Aircraft
Helicopters are craft that fly through the air using one or more rotating blades mounted on top to provide lift, thrust, and control. Since the blade rotates in one direction, the helicopter body tends to rotate in the other direction according to Newton’s third law of motion. Often, two blades are used that rotate in opposite directions or a smaller vertical blade mounted on the tail is used to counter this rotation. The helicopter has the ability to take off vertically, hover—stay stationary in the air—and travel in any direction. It has many applications as a result of its maneuverability. It can be used for transportation between airports and suburbs and in power-line construction, mining and exploration, personnel transportation in hard-to-get-at sites, police work, and traffic control.
The rotor or rotors that enable a helicopter to fly are large, usually from 30 to 100 feet (9 to 30 meters) in diameter, and airfoil-shaped. A helicopter has two control sticks. The first is the cyclic pitch control that changes the rotor blade’s local angle of attack in a particular part of the circle in which it travels. The second control stick is the collective pitch control that changes the overall angle of attack of the blades as a whole. In moving the cyclic pitch control, there is a change in the local angle of attack of the blade that causes an asymmetry of the lift over the rotor disk. This in turn causes a tilt in the disk and a change of the rotor’s thrust in the desired direction. This collective change in the angle of attack causes the helicopter to either rise or fall.
The U.S. military has relied on helicopters to both attack enemy targets and to carry personnel into combat. These aircraft were used during the Korean War and even more extensively in Vietnam. In the early 21st century, such models as the Apache and the Black Hawk proved vital to the U.S.-led operations in Afghanistan and Iraq. Nevertheless, the helicopter has its drawbacks: it tends to handle poorly in bad weather and, because it often flies close to the ground, it is an attractive target for enemy fire.
A V/STOL—vertical/short takeoff and landing—aircraft takes off vertically like a helicopter but flies like a conventional plane. In the United States, NASA and the military developed the Tiltrotor XV-15—both a true helicopter and a turboprop fixed-wing airplane. The XV-15 has two large, three-bladed propellers mounted at the wing tips, which can be rotated from the vertical to the horizontal. On takeoff, the engines and propellers are turned straight up so that the XV-15 takes off like a helicopter, lifting its design weight of 13,220 pounds (6,000 kilograms). It can hover for more than an hour; once the craft is in the air, the engines and propellers are rotated forward. The vehicle picks up speed as lift is transferred from the propellers to the wings. The XV-15 inspired U.S. aerospace manufacturer Boeing to develop the V-22 Osprey, another military V/STOL.
The British-built Harrier military jet is a V/STOL that uses thrust vectoring—directing the thrust of its jet engines downward—to take off vertically. Once the jet is in the air, the engines are rotated so that the thrust is directed rearward, and the craft flies like a regular jet. The U.S. Marines acquired the Harrier in the early 1970s. Mechanical problems and safety concerns prompted a major redesign of the Harrier’s engines. Following this overhaul, accidents involving the V/STOL dropped significantly. The aircraft are now vital assets for the U.S. military in Iraq: the Harrier’s hovering ability makes it an efficient surveillance tool and an accurate attack plane.