NAVWEPS OD-80140  APPLICATION OF AERODYNAMICS  TO SPECIFIC PROBLEMS OF FLYING  following items depending on severity of the  stall:  blade must operate at a greater angle of attack.  If the blade pitch is increased or the forward  speed increased  the stalled portion of the rotor  disc becomes  larger with the stall progressing  in toward the hub from the tip of the retreating  blade. When approximately 15 percent of  the rotor disc is stalled, control of the heli-  copter will be impossible. Flight tests have  determined that control becomes  marginal and  the stall is considered severe when the outer  one-quarter of the retreating blade is stalled.  Retreating blade stall can be recognized by  rotor toughness, erratic stick forces, a vibration  and stick shake with a frequency determined  by the number of blades and the rotor speed.  Each of the blades of a three-bladed rotor will  stall as it passes  through the stall region and  create a vibration with three beats pet rotor  revolution.  Other evidence of retreating blade  stall is partial or complete loss of control or a  pitch-up tendency which can be uncontrollable  if the stall is severe.  1. Decrease  collective pitch  2. Decrease  airspeed  3. Increase  rotor RPM  4. Decrease severity of accelerated ma-  newer or control deflection  If the stall is severe  enough to result in pitch-up,  forward cyclic to attempt to control pitch-up  is ineffective and may aggravate the stall since  forward cyclic results in an increase in blade  angle of attack on the retreating blade.  The  helicopter will  automatically recover from  a severe  stall since the airspeed is decreased  in  the nose high attitude but recovery can be  assisted by gradual reduction in collective  pitch, increasing RPM, and leveling the heli-  copter with pedal and cyclic stick.  From the previous discussion, it is apparent  that there is some degree of retreating blade  stall even at moderate airspeeds.  However,  the helicopter is able to perform satisfactorily  until a sufficiently large area of the rotor disc  is stalled. Adequate warning of the impend-  ing stall is present when the stall condition  is approached slowly.  There is inadequate  warni,ng of the stall only when the blade pitch  or blade angle of attack is increased rapidly.  Therefore, unintentional severe stall is most  likely to occur during abrupt control motions  or rapid accelerated  maneuvers.  COMPRESSIBILITY EFFECTS. The highest  relative velocities occur at the tip of the ad-  vancing blade since the speed of the helicopter  is added to the speed due to rotation at this  point.  When the Mach number of the tip  section of the advancing blade exceeds the  critical Mach number for the rotor blade sec-  tion, compressibility effects result. The criti-  cal Mach number is reduced by thick, highly  cambered airfoils and critical Mach number  decreases  with increased lift coefficient. Most  helicopter blades have symmetrical sections  and therefore have relatively high critical  Mach numbers at low lift coefhcients. Since  the principal effects of compressibility are the  Conditions favorable for the occurrence of  retreating blade stall are those conditions that  result in high retreating blade angles of attack.  Each of the following conditions results in a  higher angle of attack on the retreating blade  and may contribute to retreating blade stall:  Recovery from a stalled condition can be  effected only by decreasing the blade angle of  attack below the stall angle.  This can be  accomplished by one or a combination of the  1. High airspeed  2. Low totor RPM-operation  at low  rotor RPM necessitates the use of  higher blade pitch to get a given  thrust from the rotor, thus a higher  angle of attack  3. High gross weight  4. High density altitude  5. Accelerated  flight, high load factor  6. Flight through turbulent air or gusts-  sharp updrafts result in temporary  increase in blade angle of attack  7. Excessive ot abrupt control deflections  during maneuvers 