Also known as the coordination ball and it is one of the simplest and useful devices in the cockpit of an aircraft. It shows the direction of the g-forces in a turn. This ball is not powered (gravity forces only) and you will find it in the turn coordinator / indicator. With an EFIS type cockpit it is displayed electronically on one of the screens.
Construction wise the ball sits in a pendulum bob filled with fluid and this dampens out erratic movement due to turbulence. During flight training the instructor will point it out regularly until the student is able to use the rudder properly. The basic rule is to 'step on the ball' to keep it centered!
Naturally this instrument is not gyro based but as it is usually included in a turn indicator/ coordinator, and part of correctly flying an aircraft, we felt the need to include it here.
When flying straight and level the ball will (should) be centered in the lower part. Should there be any unbalanced forces in flight (as in turbulence and using only ailerons and no rudder to keep the airplane level) the ball will move toward the lower wing inline with the aircraft weight.
During any turn (to the left or to the right) the ball will sit inline with the resultant of the weight and centrifugal force. In a balanced turn this resultant force must be inline with the resultant lift force. With more experience and flight hours on type, pilots can really feel this and truly fly by the seat of their pants and keep the ball centered without looking at it.
With an unbalanced turn the ball can sit either to the high or to the low wing. Towards the high wing is called skidding and towards the low wing slipping. Pilots not using rudder correctly will cause these indications on the ball and the nose of the aircraft will also move from left to right during aileron input.
To accomplish a coordinated turn to the right (or left) you need to move the stick or aileron to the right (or left) to obtain the correct bank angle and apply right (or left) rudder to keep the ball centered. Thats all there is to it and if you do it wrong the result will be a slipping or skidding out of balance turn, with an instructor yelling at you....
In short: you have to move the aileron and rudder controls in the same direction, the amount needed depends on what is required of the aircraft (bank angle and rate of turn) and the actual airspeed at the moment.
With a skidding turn the ball sits towards the outside of the turn or toward the higher wing. An good indication that too much rudder or not enough aileron is used. Application of either one will center the ball: thus step on the ball or apply more aileron in the direction of the turn.
Here, when the ball sits at the same side as the turn it is called slipping. This is an indication that not enough rudder or too much aileron in the direction of the turn is used. Instructors will see this usually more often with fresh students as they seem to forget to use their feet. And even more experienced pilots occasionally do not use their rudder enough.
Anytime an aircraft is slipping or skidding, airframe drag is higher than it should be. As a result height, airspeed and thus performance will be lost. When flying in turbulence it is a whole lot more comfortable for pilot and passengers (think about airsickness) to use the rudder together with the ailerons in the same direction to keep the aircraft balanced as much as possible. Aircraft equipped with a yaw damper may turn on that device for a much more comfortable flight.
Most low wing aircraft have the fuel tanks in the wings, during unbalanced flight the fuel will do the same thing as the coordination ball: seek the resultant of the centrifugal force and weight and thus fuel sloshes around in the tank. Sometimes this can lead to fuel spills through the tank vents (the DynAero MCR-01 suffers from this) or even air entering the fuel lines when fuel levels are getting really low in the tanks.
When performing the preflight the pilot should check for gyro rotation and possible failure flags if electrical or look for vacuum indications. When taxiing, check for correct indications of turn indicator / coordinator and ball: turning left/ skid right and turning right/ skid left.
During flight the pilot can fly a rate one turn (3° /sec or 180° / minute) and time this, it could be part of an air exercise or lookout turn before stalls are exercised.
At your parking location, check for a level ball (for example at the fuel station, this is handy when refueling the tanks to the brim) and no turn indication.