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Aerodynamics

Aerodynamics. FF-1/01. Aero Foundations. Translating Tendency Ground Effect Translational Lift Transverse Flow Dissymmetry of Lift Drag Total Aerodynamic Force Aero in Autorotations. Aerodynamic Difficulties. Engine Failure in a Left Turn Negative/Low G Vortex Ring State

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Aerodynamics

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  1. Aerodynamics FF-1/01

  2. Aero Foundations • Translating Tendency • Ground Effect • Translational Lift • Transverse Flow • Dissymmetry of Lift • Drag • Total Aerodynamic Force • Aero in Autorotations

  3. Aerodynamic Difficulties • Engine Failure in a Left Turn • Negative/Low G • Vortex Ring State • Loss of Tailrotor Effectiveness • Marginal Power Ops • Retreating Blade Stall

  4. Huey Hovering • Nose High • 5º Mast tilt • CG Forward / Aft • Left Skid Low • Translating Tendency • Antitorque (Tail Rotor) • Pushes Tail to the Right (Nose - Left) • Thrust also pushes fuselage right, below the CG • Compensated for by left cyclic • Thus, left skid lower than right skid

  5. Translating Tendency

  6. Ground Effect • Practically speaking, experienced up to one rotor disk above the ground. • In ground effect hover power up to 30% less than out of ground effect. Why? • Primary: Reduction in downward velocity of induced flow caused by proximity of the ground • Secondary: Disruption in size and number of wing tip vortices (downward and outward flow of air discourages vortex formation)

  7. The Transition to Directional Flight Because hovering is fun, but it isn’t getting us anywhere.

  8. Translational Lift • Additional lift because of horizontal movement with respect to the air • Airflow transitions from vertical at a hover to more horizontal in forward flight • Modifies induced flow changing the resultant RW and increasing the AOA

  9. Translational Lift Airflow at a Hover Airflow in Forward Flight

  10. Effective Translational Lift(ETL) • Definition • The point at which the aircraft outruns its own downwash • Results: • Lift increases with no collective change • Roll - right; Nose - up • Pitch up and Roll Caused by: • Transverse Flow • Dissymmetry of Lift

  11. Transverse Flow Effect • Definition • A condition of increased drag and decreased lift in the aft portion of the rotor disk caused by the air having a greater induced velocity and reduced angle of attack in the aft portion of the disk.

  12. Transverse Flow Effect Rotor Disk -Front Rotor Disk -Rear

  13. TransverseFlow Effect • Phase lag effect • felt 90º later • Result: • Right Roll • More lift in front half of rotor disk • Less lift in rear half

  14. Dissymmetry of Lift • The difference in lift between the advancing half and the retreating half of the rotor disk

  15. Dissymmetry of Lift L = 1/2  V2 S CL

  16. Dissymmetry of Lift • Compensate by • Flapping • Cyclic Feathering • Result • Nose pitches up • Referred to as “Blowback”

  17. Types of Drag • Induced • Profile • Skin Friction • Form • Parasite • Skin Friction • Form • Interference • Wave

  18. Drag

  19. PPC

  20. Total Aerodynamic Force • Lift is perpendicular to Resultant Relative Wind • Drag is parallel to Resultant Relative Wind • Lift is modified by Total Drag

  21. Autorotations • Blade Regions • Descent Rates/Rotor RPM • -10 Pg 9-11 • HV Diagram • -10 Pg 9-13 • FM 1-203 Pg 6-50

  22. Flight Characteristics • Engine Failure during left turn • -10 Pg 9-2 • Negative Gs / Low Gs • -10 Pg 8-10 • Right Roll • Mast Bumping Video

  23. AERODYNAMIC PROBLEMS The main reason why pilots need to know aero

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