If the altitude is held constant, power determines the airspeed.  For example, at a constant altitude, cruising power results  in cruising airspeed. Any deviation from the cruising power  setting results in a change of airspeed. When power is added  to increase airspeed, the nose of the helicopter pitches up and  yaws to the right in a helicopter with a counterclockwise main  rotor blade rotation.  [Figure 8-7]  When power is reduced  to decrease airspeed, the nose pitches down and yaws to the  left.  [Figure 8-8] The yawing effect is most pronounced  in single-rotor helicopters and is absent in helicopters with  counter-rotating rotors. To counteract the yawing tendency  of the helicopter, apply pedal trim during power changes.  To maintain a constant altitude and airspeed in level flight,  coordinate pitch attitude and power control. The relationship  between altitude and airspeed determines the need for a  change in power and/or pitch attitude. If the altitude is  constant and the airspeed is high or low, change the power to  obtain the desired airspeed. During the change in power, make  an accurate interpretation of the altimeter, then counteract  any deviation from the desired altitude by an appropriate  change of pitch attitude. If the altitude is low and the airspeed  is high, or vice versa, a change in pitch attitude alone may  return the helicopter to the proper altitude and airspeed. If  both airspeed and altitude are low, or if both are high, changes  in both power and pitch attitude are necessary. To make power control easy when changing airspeed, it is  necessary to know the approximate power settings for the  various airspeeds at which the helicopter is flown. When the  airspeed is to be changed by any appreciable amount, adjust  the power so that it is over or under that setting necessary  to maintain the new airspeed. As the power approaches the  desired setting, include the manifold pressure in the cross- check to determine when the proper adjustment has been  accomplished. As the airspeed is changing, adjust the pitch  attitude to maintain a constant altitude. A constant heading  should be maintained throughout the change. As the desired  airspeed is approached, adjust power to the new cruising  power setting and further adjust pitch attitude to maintain  altitude. The instrument indications for straight-and-level  flight at normal cruise and during the transition from normal  cruise to slow cruise are illustrated in  Figures 8-9  and  8-10 .  After the airspeed stabilizes at slow cruise, the attitude  indicator shows an approximate level pitch attitude.  The altimeter is the primary pitch instrument during level  flight, whether flying at a constant airspeed or during a  change in airspeed. Altitude should not change during  airspeed transitions, and the heading indicator remains the  primary bank instrument. Whenever the airspeed is changed  by an appreciable amount, the manifold pressure gauge is  momentarily the primary instrument for power control.  When the airspeed approaches the desired reading, the  airspeed indicator again becomes the primary instrument  for power control.  Figure 8-7.  Flight instrument indications in straight-and-level flight with power increasing. 3  4  I  0  2  2  I  3  VERTICAL SPEED  THOUSAND FT PER MIN  UP  DOWN  .5  .5  33  30  24  2I  I5  I2  6  3  2 MIN TURN  DC ELEC  L R  30.0  29.9  29.8  I 0 0 F E T I  4  5  6  7  9  2  0  8  3  CALIBRATED  TO 20,000 FEET  ALT  20 20 I0 I0 I0 I0 20 20 TEST  STBY PWR  IN Hg ALg. MANIFOLD  PRESS  25 5 15 30 20 10 35 110  100  90  80  70  60  50  110  100  90  80  70  60  50  E R  %RPM  8-8