rudder pressure to level the miniature aircraft. Include the  miniature aircraft in the cross-check and correct for even  the smallest deviations from the desired position. When  this instrument is used to maintain straight flight, control  pressures must be applied very lightly and smoothly. The ball of the turn coordinator is actually a separate  instrument, conveniently located under the miniature  aircraft because the two instruments are used together.  The ball instrument indicates the quality of the turn. If the  ball is off-center, the airplane is slipping or skidding. That  is, if the coordinator’s miniature airplane is tilted left and  the ball is displaced to the right, the aircraft is in a skid.  [Figure 7-18]  If however, the miniature airplane is tilted to  the right with the ball off-center to the right, the aircraft is in  a slip.  [Figure 7-19]  If the wings are level and the airplane  is properly trimmed, the ball remains in the center, and the  airplane is in straight flight. If the ball is not centered, the  airplane is improperly trimmed. left rudder, right ball/right rudder), use aileron as necessary  for bank control and retrim.  To trim the airplane using only the turn coordinator, use  aileron pressure to level the miniature aircraft and rudder  pressure to center the ball. Hold these indications with control  pressures, gradually releasing them while applying rudder  trim sufficient to relieve all rudder pressure. Apply aileron  trim, if available, to relieve aileron pressure. With a full  instrument panel, maintain a wings-level attitude by reference  to all available instruments while trimming the airplane.  Unlike the turn coordinator that provides three indications  (roll, turn, and trim), the turn-and-slip indicator provides  two: turn-rate and trim. Although the turn-and-slip indicator  needle provides an indication of turn only, it provides an  indirect indication of aircraft attitude when used with roll  indicators, such as a heading indicator or magnetic compass.  As with the turn coordinator (after stabilizing from a roll),  when the turn-and-slip indicator’s needle is aligned with the  alignment marks, the aircraft is in a standard turn of 3 degrees  per second or 360° in 2 minutes. The ball of the turn-and-bank indicator provides important  trim in the same manner that the ball in the turn coordinator  does.  Figures 7-18  and  7-19  provide a comparison of the  two instruments. Power produces thrust which, with the appropriate angle of  attack of the wing, overcomes the forces of gravity, drag,  and inertia to determine airplane performance.  Power control must be related to its effect on altitude and  airspeed, since any change in power setting results in a change  in the airspeed or the altitude of the airplane. At any given  airspeed, the power setting determines whether the airplane  is in level flight, in a climb, or in a descent. If the power is  increased in straight-and-level flight and the airspeed held  constant, the airplane climbs. If power is decreased while  the airspeed is held constant, the airplane descends. On the  other hand, if altitude is held constant, the power applied  determines the airspeed.  The relationship between altitude and airspeed determines the  need for a change in pitch or power. If the airspeed is not the  desired value, always check the altimeter before deciding that  a power change is necessary. Think of altitude and airspeed  as interchangeable; altitude can be traded for airspeed by  lowering the nose or convert airspeed to altitude by raising  the nose. If altitude is higher than desired and airspeed is  To maintain  straight-and-level flight with proper trim, note  the direction  of ball displacement. If the ball is to the left of  center and the left wing is low, apply left rudder pressure  to center the ball and correct the slip. At the same time,  apply right  aileron pressure as necessary to level the wings,  cross-checking the heading indicator and attitude indicator  while  centering the ball. If the wings are level and the ball is  displaced  from the center, the airplane is skidding. Note the  direction of  ball displacement and use the same corrective  technique as  for an indicated slip. Center the ball (left ball/ 7-8 Turn-and-Slip Indicator (Needle and Ball) Power Control OFF  2 MIN TURN  DC ELEC  L R  Figure 7-18. Skid indication. Figure 5-18. Slip indication.  Figure 7-19. Slip indication. Figure 5-19. Skid indication.  OFF  2 MIN TURN  DC ELEC  L R 