direction or heading. It is the interaction of these controls that can make learning to hover difficult, since often an adjustment in any one control requires adjustment of the other two, necessitating pilot familiarity with the coupling of control inputs needed to produce smooth flight. Forward flight In forward flight, a helicopter's flight controls behave more like those in a fixed-wing aircraft. Moving the cyclic forward makes the nose pitch down, thus losing altitude and increasing airspeed. Moving the cyclic back makes the nose pitch up, slowing the helicopter and making it climb. Increasing collective (power) while maintaining a constant airspeed induces a climb, while decreasing collective (power) makes the helicopter descend. Coordinating these two inputs, down collective plus aft (back) cyclic or up collective plus forward cyclic causes airspeed changes while maintaining a constant altitude. The pedals serve the same function in both a helicopter and an airplane, to maintain balanced flight. This is done by applying a pedal input in the direction necessary to center the ball in the turn and bank indicator. It is important to consider the limitations of forward flight however, and the differences between fixed wing and rotary with aircraft. Unlike a fixed wing aircraft where the maximum airspeed is a function of the strength and stresses of the airframe, the maximum forward flight speed in a helicopter is limited by the RPM of the rotor and the effective airspeed over each blade. In a stationary hover, each rotor blade will experience the same airspeed at a constant RPM. In forward flight conditions, one rotor blade will be moving into the oncoming air stream, whilst the other moves away from it. At certain airspeeds, this can create a dangerous condition in which the receding rotor blade can stall, resulting in unstable flight characteristics. Autorotation Differential pitch control For helicopters with two horizontally-mounted rotors, changes in attitude often require having each rotor behave inversely in response to the standard control inputs from the pilot. Those with coaxial rotors (like a Kamov Ka-50) have both rotors mounted on the same mast, one above the other on concentric drive shafts contra-rotating—spinning in opposite directions on a shared axis—and make yaw changes by increasing the collective pitch of the rotor spinning in the direction of the desired turn while simultaneously reducing the collective pitch of the other, creating dissymmetry of torque. Tandem-rotor craft (like a Boeing CH-47 Chinook)