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Rotary Wing AERODYNAMICS

Rotary Wing AERODYNAMICS. Airfoil Force Vectors. Aerodynamic Terms. Rotational Relative Wind Opposes Direction of Blade Rotation in Tip Path Plane Induced Flow Vertical Component of Airflow Drawn Through the Rotor System Resultant Relative Wind

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Rotary Wing AERODYNAMICS

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  1. Rotary WingAERODYNAMICS

  2. Airfoil Force Vectors

  3. Aerodynamic Terms • Rotational Relative Wind Opposes Direction of Blade Rotation in Tip Path Plane • Induced Flow Vertical Component of Airflow Drawn Through the Rotor System • Resultant Relative Wind Actual Wind that Acts on the Airfoil(Vector Sum of Rotational Relative Wind & Induced Flow) • Angle of Incidence Angle Between Chord Line & Rotational Relative Wind(Tip Path Plane)

  4. Aerodynamic Terms (Con’t) • Angle of Attack Angle Between Chord Line & Resultant Relative Wind • Lift Acts Perpendicular to Resultant Relative Wind • Drag Acts Parallel & Opposite to Resultant Relative Wind • Total Aerodynamic Force Vector Sum of Airfoil Lift & Drag

  5. Lift • Pressure Differential Between Upper & Lower Airfoil Surfaces Creates Lift • Lift Equation • Cambered Airfoil in Positive Lift

  6. Drag • Types of Drag • Induced: Caused by the Production of Lift • Parasite: All Drag Not Caused by Lift • Profile: Parasitic Drag of Rotor Blades Passing Through the Air • Drag Equation • Largest Contributor to Total Drag • Low Speed: Induced Drag • High Speed: Parasite/Profile Drag

  7. Helicopter Drag vs. Airspeed

  8. Airflow At A Hover - OGE

  9. Airflow At A Hover - IGE

  10. Translating Tendency • Tendency of Aircraft to Drift In the Direction of Tail Rotor Thrust at a Hover • Compensated for by Mixing Unit & Pilot Input

  11. Dissymmetry of Lift • Difference in Lift Associated with the Advancing & Retreating Sides of the Rotor System • Compensated for by Blade Flapping & Cyclic Feathering

  12. Blade Flapping • Up/Down Movement of the Rotor Blade About A Flapping Hinge • Causes Blowback (Rearward Tilt of Rotor Disk)

  13. Blade Lead & Lag (Hunting) • Fore & Aft Movement of the Blade in Tip Path Plane Due to Changes in Blade Speed • Coriolis Effect Angular Velocity Changes with Blade CG

  14. Retreating Blade Stall • Outboard Section of Retreating Blade Stalls at High Forward Airspeed • Causes Blade Flapping & Cyclic Feathering that Exceed Critical Angle • Aircraft Pitches Up & Rolls Left • Conditions Conducive to Retreating Blade Stall - High GWT - Low Rotor RPM - High DA - High “G” Maneuvers - Turbulent Air

  15. Retreating Blade Stall

  16. Compressibility • Outboard Section of Advancing Blade Exceeds the Speed of Sound at High Airspeed • Aerodynamic Center Moves Aft Large Down Pitching Moment at Outboard Tip Will Cause Structural Failure of Blade • Aircraft Pitches Down • Conditions Conducive to Compressibility - High Airspeed - High Rotor RPM - High GWT - High DA - Low Temperature - Turbulent Air

  17. Settling with Power(Vortex Ring State) • Formation of an Inner Vortex on the Blade Causes Substantial Loss of Lift • Increased Collective Results in Larger Vortex Rings & Higher Rates of Descent • Conditions Conducive to Settling with Power • Very Low Forward Airspeed • 20-100% of Available Power Applied • 300 ft/min Rate of Descent or Greater • Recover by Establishing Directional Flight

  18. Vortex Ring State • Induced Flow Before Vortex Ring State • Vortex Ring State

  19. Offset Hinges • Tends to Align the Helicopter with the RotorTip Path Plane • Offset Creates a Hub Moment Larger the Offset, Higher the Hub Moment • Results in Greater Maneuverability & Faster Aircraft Response

  20. Dynamic Rollover • Aircraft Exceeds Critical Rollover Angle with a Rolling Moment • Dynamic Rollover Criteria • Pivot Point • Rolling Moment • Lift Component and/or Hub Moment • Tail Rotor Contribution • Collective is Most Effective Control Cyclic is also Effective Due to Offset Hinges

  21. Fuselage Hovering Attitude • Nose High • Forward Tilt of Main Transmission • Aircraft CG Aft of Main Rotor Mast • Left Side Low • Left Cyclic Compensating for Translating Tendency

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