Transcript Aerodynamics - Baseops.net

Aerodynamics
FF-1/01
Aero Foundations
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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 -Front
Rotor Disk -Rear
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  V2 S CL
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
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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 1-203 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