Aerodynamics 1
Rotary WingAERODYNAMICS
Aerodynamics 2
Airfoil Force Vectors
Aerodynamics 3
Aerodynamic Terms
• Rotational Relative WindOpposes Direction of Blade Rotation in Tip Path Plane
• Induced FlowVertical Component of Airflow Drawn Through the
Rotor System
• Resultant Relative WindActual Wind that Acts on the Airfoil
(Vector Sum of Rotational Relative Wind & Induced Flow)
• Angle of IncidenceAngle Between Chord Line & Rotational Relative Wind
(Tip Path Plane)
Aerodynamics 4
Aerodynamic Terms (Con’t)
• Angle of AttackAngle Between Chord Line & Resultant Relative Wind
• LiftActs Perpendicular to Resultant Relative Wind
• DragActs Parallel & Opposite to Resultant Relative Wind
• Total Aerodynamic ForceVector Sum of Airfoil Lift & Drag
Aerodynamics 5
Lift
• Pressure DifferentialBetween Upper & Lower Airfoil Surfaces Creates Lift
• Lift Equation
• Cambered Airfoil in Positive Lift
L V SCL
1
22
Aerodynamics 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
D V SCD
1
22
Aerodynamics 7
Helicopter Drag vs. Airspeed
Aerodynamics 8
Airflow At A Hover - OGE
Aerodynamics 9
Airflow At A Hover - IGE
Aerodynamics 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
Aerodynamics 11
Dissymmetry of Lift
• Difference in Lift Associated with the Advancing & Retreating Sides of the Rotor System
• Compensated for by Blade Flapping & Cyclic Feathering
Aerodynamics 12
Blade Flapping
• Up/Down Movement of the Rotor Blade About A Flapping Hinge
• Causes Blowback (Rearward Tilt of Rotor Disk)
Aerodynamics 13
Blade Lead & Lag (Hunting)
• Fore & Aft Movement of the Blade in Tip Path Plane Due to Changes in Blade Speed
• Coriolis EffectAngular Velocity Changes with Blade CG
Aerodynamics 14
Retreating Blade Stall
• Outboard Section of Retreating Blade Stalls at High Forward Airspeed
• CausesBlade 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
Aerodynamics 15
Retreating Blade Stall
Aerodynamics 16
Compressibility
• Outboard Section of Advancing Blade Exceeds the Speed of Sound at High Airspeed
• Aerodynamic Center Moves AftLarge 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
Aerodynamics 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
Aerodynamics 18
Vortex Ring State
• Induced Flow Before Vortex Ring State
• Vortex Ring State
Aerodynamics 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
Aerodynamics 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 ControlCyclic is also Effective Due to Offset Hinges
Aerodynamics 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