by Jonathan M. Stern
The turn coordinator is actually two instruments in one. The airplane replica in the middle of the instrument rolls proportionally to the roll rate of the airplane. When the bank angle is maintained, the replica indicates the rate of turn. If bank angle is maintained, when the right or left wing of the replica is aligned with the lower mark, the airplane is turning at a rate of 3° per second (a full 360° turn in two minutes). This rate of turn is known as standard rate.
The bank angle necessary to fly a standard rate turn varies with airspeed. It can be estimated by dividing true airspeed in knots by 10 and adding seven. For example, at a true airspeed of 130 knots, a bank angle of approximately 20° yields a standard rate turn. Give it a try. Take off in the Flight Simulator Cessna and climb to 2,000 feet. Try making turns at 80 knots indicated airspeed and then again at 130 knots indicated (see Figure 2.28).
Figure 2.28. At 130 knots, a bank angle of approximately 20° yields a standard rate turn.
Note the difference in bank angle required to maintain standard rate at the different speeds. Time a 360° turn to confirm that it takes two minutes at standard rate.
The second instrument in the turn coordinator is called an inclinometer. The inclinometer shows whether use of rudder and aileron is coordinated. If the ball in the liquid-filled glass tube moves outside of the center of the tube, the rudder and ailerons are not coordinated. If the ball moves to the outside of the turn, the airplane is skidding. If the ball moves to the inside of the turn, the airplane is slipping.
Uncoordinated flight can be corrected by applying more rudder pressure on the side that the ball is on. Student pilots are instructed to step on the ball, because the rudder is controlled by pedals, and pressure on the left pedal coordinates the turn if the ball is to the left of center.
Flight Simulator gives the option of flying with auto-coordination or with rudder controls. Rudder pedals are available from a variety of manufacturers. Auto-coordination permits you to control the ailerons and have the proper amount of rudder automatically applied. Independent rudder control is necessary to perform certain maneuvers in the airplane, such as slips, crosswind takeoffs and landings, and spins.