Changes

</ref>

</ref>

Each blade in a helicopter rotor is an [[airfoil]], a [[wing]] with a curved top and a straight bottom. As the blade spins around, air travels faster over the top surface than under the bottom. This reduces air pressure above the blade and produces an upward force called lift. The pitch of the blades (the angle they make to the incoming airflow) controls the amount of lift. During takeoff, the pilot increases the pitch with a control called the collective pitch stick. The lift produced is greater than the helicopter's weight and this makes the helicopter rise upward. If the lift exactly equals the weight, the helicopter hovers. If the weight is greater than the lift, the helicopter descends to Earth.<ref>[http://www.explainthatstuff.com/helicopter.html How helicopters work]</ref>

Each blade in a helicopter rotor is an airfoil, a [[wing]] with a curved top and a straight bottom. As the blade spins around, air travels faster over the top surface than under the bottom. This reduces air pressure above the blade and produces an upward force called lift. The pitch of the blades (the angle they make to the incoming airflow) controls the amount of lift. During takeoff, the pilot increases the pitch with a control called the collective pitch stick. The lift produced is greater than the helicopter's weight and this makes the helicopter rise upward. If the lift exactly equals the weight, the helicopter hovers. If the weight is greater than the lift, the helicopter descends to Earth.<ref>[http://www.explainthatstuff.com/helicopter.html How helicopters work]</ref>

Bell Helicopter Textron, is a producer of commercial and military helicopters, and the pioneer of the revolutionary tiltrotor aircraft.  

Bell Helicopter Textron, is a producer of commercial and military helicopters, and the pioneer of the revolutionary tiltrotor aircraft.