Aerodynamics

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 isnt 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
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

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

• Result
• Nose pitches up
• Referred to as Blowback

• Compensate by
• Flapping
• Cyclic Feathering

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