NAVWEPS CID-ROT-R0  APP,LlCATlON OF AERODYNAMICS  TO SPECIFIC PROBLEMS OF FLYmING  large increase  in drag and rearward shift of the  airfoil aerodynamic center, compressibility ef-  fects on the helicopter increase the power te-  quired to maintain rotor RPM and cause rotor  roughness, vibration, stick shake, and an un-  desirable structural twisting of the blade.  Since compressibility effects become more  severe  at higher lift coefficients  (higher blade  angles of attack) and higher Mach numbers,  the following operating conditions represent  the most adverse conditions from the stand-  point of compressibility:  Compressibility effects will  vanish by de-  creasing the blade pitch.  The similarities in  the critical conditions for retreating blade  stall and compressibility should be noticed  but one basic difference  must be appreciated-  compressibility occurs at HIGH RPM while  retreating blade stall occurs at LOW RPM.  Recovery technique is identical for both with  the exception of RPM control.  AUTOROTATION  CHARACTERISTICS.  One of the unique characteristics  of helicopters  is their ability to take part of the energy of  the airstream to keep the rotor turning and  glide down to a landing with no power.  Consideration of the rotor during a vertical  autorotation will provide an understanding of  why the rotor continues to rotate without  power. During autorotation, the flow of air  is upward through the rotor disc and there is  a vertical velocity component equal to the  rate of descent of the helicopter. In addition,  there is a velocity component due to rotation  of the rotor.  The vector sum of these two  velocities is the relative wind for the blade  element. The forces  resulting from the relative  wind on each particular blade section will  provide the reason why the rotor will continue  to operate without power. First, consider, a  blade element near the tip of the blade as illus-  trated in figure 6.17.  At this point there is  a lift force acting perpendicular to the relative  wind and a drag force acting parallel to the  relative wind through the aerodynamic center.  Since the rotation of the rotor is affected only  by forces acting in the plane of rotation, the  important forces are components of the lift  and drag force in the plane of rotation.  In  this low angle of attack high speed tip section,  the net in-plane force is a drag force which  would tend to retard the rotor.  Next, con-  sider a blade section at about the half-span  position as illustrated in figure 6.17. In this  case, the same forces are present, but the iti-  plane component of lift force is greater than  the drag force and this results in a net thrust  or forward force in the plane of rotation which  tends to drive the rotor.  During a steady autorotation, there is a  balance of torque from the forces along the  blade so that the RPM is maintained in equi-  librium at some particular value. The region  of the rotor disc where there is a net drag force  on the blade is called the “propeller region”  and the region of the rotor disc where there  is a net in-plane thrust force is called the  “autorotation  region.”  These regions are  shown for vertical autorotation and forward  speed (or normal) autorotation in figure 6.17.  Forces acting on the rotor blades in forward  flight autorotation are similar to those in  vertical autorotation but the difference will  consist mainly of shifts of the autorotation  region to the left and the addition of reverse  405  1. High airspeed  2. High rotor RPM  3. High gross weight  4. High density altitude  5. Low temperature-the speed of sound  is proportional to the square root of  the absolute temperature. Therefore,  sonic velocity will  be more easily  obtained at low temperatures when  the sonic speed is lower.  6. Turbulent air-sharp  gusts momen-  tarily increase the blade angle of  attack and thus lower the critical  Mach number to the point where  compressibility effects may be en-  countered on the blade. 