Coordinated flight
In aviation, coordinated flight of an aircraft is flight without sideslip.
When an aircraft is flying with zero sideslip a turn and bank indicator installed on the aircraft’s instrument panel usually shows the ball in the center of the spirit level. The occupants perceive no lateral acceleration of the aircraft and their weight to be acting straight downward into their seats.
Particular care to maintain coordinated flight is required by the pilot when entering and leaving turns.
Coordinated flight is usually preferred over uncoordinated flight for the following reasons:
Airplanes and helicopters are usually equipped with a turn and bank indicator to provide their pilots with a continuous display of the lateral balance of their aircraft so the pilots can ensure coordinated flight.
Glider pilots attach a piece of coloured string to the outside of the canopy to sense the sideslip angle and assist in maintaining coordinated flight.
An airplane has three axes of rotation:
Coordinated flight requires the pilot to use pitch, roll and yaw control simultaneously. See also flight dynamics.
If the pilot were to use only the rudder to initiate a turn in the air, the airplane would tend to "skid" to the outside of the turn.
If the pilot were to use only the ailerons to initiate a turn in the air, the airplane would tend to "slip" toward the lower wing.
If the pilot were to fail to use the elevator to increase the angle of attack throughout the turn, the airplane would also tend to "slip" toward the lower wing.
However, if the pilot makes appropriate use of the rudder, ailerons and elevator to enter and leave the turn such that sideslip and lateral acceleration are zero the airplane will be in coordinated flight.
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Wikipedia