lift  is developed.  This  feature provides the pilot  with complete  control   of  the  lift   developed  by  the  rotor blades. ROTOR   AREA One  assumption   made  is  that  the  lift    depends upon the entire  area of  the rotor  disc.   The rotor  disc area is  the  area of  the  circle,  the radius  of  which  is equal  to  the  length   of   the  rotor   blade.     Engineers determined that the  lift   of  a rotor  is in  proportion   to the  square of   the  length   of   the  rotor   blades.    The desirability of   large    rotor    disc   areas  is   readily apparent. However, the greater the rotor disc area, the greater the drag, which  results in  the need for  greater power requirements. ^.-. / PITCH    OF  ROTOR    BLADES If  the rotor  is operated at zero pitch  (flat pitch),  no lift  will  develop.  When the pitch  increases, the lifting force  increases until   the  angle  of  attack reaches the stalling  angle.   To even out the lift   distribution   along the length of the rotor  blade, it  is common practice to twist   the  blade.    With  the  twist,   a  smaller  angle  of attack results at the tip  than at the hub. SMOOTHNESS    OF  ROTOR    BLADES Tests have  shown   that   the  lift    of   a  helicopter increases by polishing  the rotor  blades to a mirrorlike surface.    By   making  the  rotor   blades  as smooth  as possible, the  parasite  drag  reduces.   Dirt,   grease, or abrasions on  the  rotor   blades  cause increased drag, which  decreases the lifting   power  of the helicopter. DENSITY    ALTITUDE In  formulas for  lift   and drag, the density of the air is an important  factor.   The mass or density of  the air reacting  in  a downward   direction   causes the lift   that supports the helicopter. ‘Density   is dependent on  two  factors.   One factor is  altitude,  since density  varies  from  a maximum  at sea level  to  a minimum   at  high   altitude.    The  other factor    is   atmospheric changes. Because   of   the atmospheric   changes  in   temperature,   pressure,   or humidity,  density  of  the air  may be different,  even at the same altitude. TORQUE Although   torque  is  not  unique  to  helicopters,  it does  present  some  special  problems. As  the  rotor turns   in   one   direction,    the  fuselage  rotates  in   the opposite   direction. Newton’s   third   law   of   motion (every   action   has  an  equal   and  opposite   reaction) applies.    This  tendency  for  the  fuselage to  rotate  is known  as the torque effect.   Since the torque effect on the  fuselage  is  a  direct  result  of  engine  power,  any change in  power  changes the torque.   The greater the engine  power,   the  greater  the  torque. There  is  no torque  when  the rotary-wing    head is  not  engaged or when the engine is not operating. The  usual  method  of   counteracting   torque  in   a single  main rotor  is by  a tail  (antitorque)  rotor.   This auxiliary   rotor  mounts vertically,  or  near vertical,  on the outer portion  of the tail  boom.   The tail  rotor  and its controls  serve as a means to counteract torque, and it   provides   a  means to   control   directional   heading. See figure  10-2. DISSYMMETRY OF  LIFT Dissymmetry of   lift     is   the   difference     in   lift existing  between the advancing blade half  of the disc and the retreating blade half.   The disc area is the area swept by the rotating  blades.  Dissymmetry  is created . . . . . . *.**..*:.., *.*,......*, . *.*.*.*.*.*. ..*...m.*.*. ..:.:.:.:.:. *.-**.'.*,- S.'.... ..,. DiRECTiON :.‘A’.‘.’ . v.*.*.*. .‘.‘.‘.‘. OF   TORQUE ot . . ...‘. :.>:.:. TAIL    ROTOR  THRUST -.*.*2.. ‘A*,‘, ‘.‘A*,~ ‘........ TO  COMPENSATE .:.:.:.: FOR   TORQUE Figure  lo-2.-Torque reaction. 10-2