AERODYNAMICS

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AERODYNAMICS Arizona Army National Guard Aviation Support Facility #1 REFERENCES FM 1-203, Fundamentals of flight TC 1-212, Aircrew Training Manual Learning Objectives Applied and simplified understanding of helicopter aerodynamic characteristics

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### AERODYNAMICS

Arizona Army National Guard

Aviation Support Facility #1

### REFERENCES

FM 1-203, Fundamentals of flight

TC 1-212, Aircrew Training Manual

Learning Objectives
• Applied and simplified understanding of helicopter aerodynamic characteristics
• Correlate relationships between these characteristics
Rotary Wing Aerodynamic Subject Areas
• Aerodynamic Factors
• Relative Wind
• Induced Flow Production
• Resultant Relative Wind
• Angle of Attack / Angle of Incidence
• Total Aerodynamic Force
• Lift
• Drag
• Airflow During a Hover
Rotary Wing Aerodynamics Subject Areas (Cont)
• Translating Tendency
• Mechanical and Pilot Inputs
• Dissymmetry of Lift
• Cyclic Feathering
Rotary Wing Aerodynamic Subject Areas (Cont)
• Compressibility
• Settling with Power
• Off Set Hinges
• Dynamic Rollover
Relative Wind
• Relative wind is defined as the airflow relative to an airfoil
• Relative wind is created by movement of an airfoil through the air
Induced Flow Production
• This figure illustrates how still air is changed to a column of descending air by rotor blade action
Resultant Relative Wind
• Airflow from rotation, modified by induced flow, produces the Resultant Relative Wind
• Angle of attack is reduced by induced flow, causing the airfoil to produce less lift
Angle of Attack
• Angle of Attack (AOA) (4) is the angle between the airfoil chord line and its direction of motion relative to the air (the Resultant Relative Wind)
Angle of Incidence
• Angle of Incidence (or AOI) is the angle between the blade chord line and the plane of rotation of the rotor system.
Total Aerodynamic Force
• A Total Aerodynamic Force (3) is generated when a stream of air flows over and under an airfoil that is moving through the air
Total Aerodynamic Force
• Total aerodynamic force may be divided into two components called lift and drag
• Lift acts on the airfoil in a direction perpendicular to the relative wind
• Drag acts on the airfoil in a direction parallel to the relative wind and is the force that opposes the motion of the airfoil through the air
Airflow at a Hover (IGE)
• Lift needed to sustain an IGEHover can be produced with a reduced angle of attack and less power because of the more vertical lift vector
• This is due to the ground interrupting the airflow under the helicopter thereby reducing downward velocity of the induced flow
Airflow at a Hover (OGE)
• Downward airflow alters the relative wind and changes the angle of attack so less aerodynamic force is produced
• Increase collective pitch is required to produce enough aerodynamic force to sustain an OGEHover
Rotor Tip Vortexes Effects
• At a hover, the Rotor Tip Vortex reduces the effectiveness of the outer blade portions
• When operating at an IGE Hover, the downward and outward airflow pattern tends to restrict vortex generation
• Rotor efficiency is increased by ground effect up to a height of about one rotor diameter for most helicopters
Translating Tendency
• The tendency for a single rotor helicopter to drift laterally, due to tail rotor thrust
Dissymmetry of Lift
• Definition
• Compensation
• Cyclic Feathering
Dissymmetry of Lift Definition

Dissymmetry of Lift is the difference in lift that exists between the advancing half of the rotor disk and the retreating half

• Blade Flapping is the up and down movement of a rotor blade, which, in conjunction with cyclic feathering, causes Dissymmetry of Lift to be eliminated.
Cyclic Feathering
• These changes in blade pitch are introduced either through the blade feathering mechanism or blade flapping.
• When made with the blade feathering mechanism, the changes are called Cyclic Feathering.
• Blade Lead / Lag Each rotor blade is attached to the hub by a vertical hinge (3) that permits each blade, independently of the others, to move back and forth in the rotational plane of the rotor disk thereby introducing cyclic feathering.
• A tendency for the retreating blade to stall in forward flight is inherent in all present day helicopters and is a major factor in limiting their forward speed
• When operating at high forward airspeeds, the following conditions are most likely to produce blade stall:
• Low Rotor RPM
• High Density Altitude
• Steep or Abrupt Turns
• Turbulent Air
• The major warnings of approaching retreating blade stall conditions are:
• Abnormal Vibration
• Nose Pitch-up
• The Helicopter Will Roll Into The Stalled Side
• When the pilot suspects blade stall, he can possibly prevent it from occurring by sequentially:
• Reducing Power (collective pitch)
• Reducing Airspeed
• Reducing "G" Loads During Maneuvering
• Increasing Rotor RPM to Max Allowable Limit
• Checking Pedal Trim
CompressibilityWhat Happens?
• Rotor blades moving through the air below approximately Mach 0.7 cause the air in front of the blade to move away before compression can take place.
• Above speeds of approximately Mach 0.7 the air flowing over the blade accelerates above the speed of sound, causing a shock wave (also known as a sonic boom) as the blade compresses air molecules faster than they can move away from the blade.
• The danger of this shock wave (Compressibility) is its effect on aircraft control and fragile rotor blade membranes.
CompressibilityCauses
• Conditions conducive to Compressibility
• High Airspeed
• High Rotor RPM
• High Gross Weight
• High Density Altitude
• Low Temperature
• Turbulent Air
CompressibilityIndications
• As Compressibility approaches:
• Power Required Increase as Lift Decreases and Drag Increases
• Vibrations Become More Severe
• Shock Wave Forms (Sonic Boom)
• Nose Pitches Down
Compressibility Corrective Actions
• When the pilot suspects Compressibility, he can possibly prevent it from occurring by:
• Slowing Down the Aircraft
• Decreasing Pitch Angle (Reduce Collective)
• Decreasing Rotor RPM
Settling with Power
• Settling With Power is a condition of powered flight where the helicopter settles into its own downwash.
• It is also known as Vortex Ring State
Settling with PowerCause
• Increase in induced flow results in reduction of angle of attack and increase in drag
• This creates a demand for excessive power and creates greater sink rate
• Where the demand for power meets power available the aircraft will no longer sustain flight and will descend
Settling With PowerConditions
• Conditions required for Settling with power are:
• 300-1000 FPM Rate of Descent
• Power Applied (> than 20% Available Power)
• Near Zero Airspeed (Loss of ETL)
• Can occur during:
• Downwind Approaches.
• Formation Approaches and Takeoffs.
• Steep Approaches.
• NOE Flight.
• Hover OGE.
Settling With PowerIndications
• Symptoms of Settling with Power:
• A high rate of descent
• High power consumption
• Loss of collective pitch effectiveness
• Vibrations
Settling With PowerCorrective Actions
• When Settling with Power is suspected:
• Establish directional flight.
• Lower collective pitch.
• Increase RPM if decayed.
• Apply right pedal.
Off Set Hinges
• The Offset Hinge is located outboard from the hub and uses centrifugal force to produce substantial forces that act on the hub itself.
• One important advantage of offset hinges is the presence of control regardless of lift condition, since centrifugal force is independent of lift.
Dynamic Rollover
• With a rolling moment and a pivot point if the helicopter exceeds a critical angle it will roll over.
Dynamic Rollover
• The critical rollover angle is further reduced under the following conditions:
• Right Side Skid Down Condition
• Crosswinds
• Lateral Center Of Gravity (CG) Offset
• Main Rotor Thrust Almost Equal to Weight
• Left Yaw Inputs
Dynamic Rollover
• Pilot Technique

When landing or taking off, with thrust (lift) approximately equal to the weight (light on the skids or wheels), the pilot should keep the helicopter cyclic trimmed (force trim/gradient) and prevent excessive helicopter pitch and roll movement rates. The pilot should fly the helicopter smoothly off (or onto) the ground, vertically, carefully maintaining proper cyclic trim.

Summary
• Websites containing additional and more detailed information on Helicopter Aerodynamics:
• http://www.dynamicflight.com/aerodynamics/
• http://www.copters.com/helo_aero.html
• http://www.helicopterpage.com/html/forces.html

Websites checked

as of 9 JUN 05

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