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 Loss of Tailrotor Effectiveness Marginal Power Ops Retreating Blade Stall

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

Translating Tendency

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)

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

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

Translational Lift Airflow at a Hover Airflow in Forward Flight

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

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.

Transverse Flow Effect Rotor Disk -Rear Rotor Disk -Front

Transverse Flow Effect More lift in front half of rotor disk Less lift in rear half Phase lag effect felt 90º later Result: -Right Roll

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

Dissymmetry of Lift L = 1/2  V 2 S C L

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

Types of Drag Induced Profile –Skin Friction –Form Parasite –Skin Friction –Form –Interference –Wave

Drag

PPC

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

Autorotations Blade Regions Descent Rates/Rotor RPM –-10 Pg 9-11 HV Diagram –-10 Pg 9-13 –FM Pg 6-50

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

AERODYNAMIC PROBLEMS The main reason why pilots need to know aero