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