Tác dụng nén sẽ biến mất nếu sân lưỡi cắt giảm. Có những điểm tương đồng trong những điều kiện quan trọng để rút lui gian hàng lưỡi và tính nén, nhưng một trong sự khác biệt phải được đánh giá cao. Nén xảy ra tại rpm rotor cao, trong khi lưỡi gian hàng xảy ra tại rpm rotor thấp. | HELICOPTER AERODYNAMICS WORKBOOK CHAPTER 5 4. High density altitude 5. Low temperature 6. Turbulent air Compressibility effects will vanish if blade pitch is decreased. There are similarities in critical conditions for retreating blade stall and compressibility but one difference must be appreciated. Compressibility occurs at high rotor rpm while blade stall occurs at low rotor rpm. Recovery technique is similar for both with the exception of rpm control. VORTEX RING STATE Vortex ring state power settling is an uncontrolled rate of descent caused by the helicopter rotor encountering disturbed air as it settles into its own downwash. This condition is based on the pilot s observation that even though the aircraft may have plenty of engine power the aircraft continues to sink rapidly. This condition may occur in powered descending flight at low airspeeds while out of ground effect. The vortex ring state is encountered when the rate of descent approaches or equals the induced flow rate figure 5-4 . Based on wind tunnel and flight tests flight in the vortex ring state begins at induced velocity peaks at induced velocity and disappears at 1 times the induced velocity. Depending on their disk loading various helicopters enter this phenomenon at a descent rate of 300 to 600 feet per minute and must exceed 1500 to 3000 feet per minute to get clear of it. Staying in this state for any length of time depends on maintaining a nearly vertical flight path. There is some evidence a glide slope of about 70 is worse than a true 90 descent. Approaches with glide slopes less than about 50 with forward speeds between 15 and 30 knots will introduce enough fresh air into the rotor system to blow the tip vortices away from the rotor and free it from the clutches of vortex ring state. Figure 5-4 FLIGHT PHENOMENA 5-5 CHAPTER 5 HELICOPTER AERODYNAMICS WORKBOOK The unsteadiness of the flow has been seen during wind-tunnel tests of model rotors using smoke for flow visualization. Figure 5-5
