Arizona Army National GuardArizona Army National Guard

Aviation Support Facility #1Aviation Support Facility #1

AERODYNAMICSAERODYNAMICS

FM 1-203, Fundamentals of flightFM 1-203, Fundamentals of flight TC 1-212, Aircrew Training ManualTC 1-212, Aircrew Training Manual

REFERENCESREFERENCES

Learning ObjectivesLearning Objectives

Applied and simplified Applied and simplified understanding of understanding of helicopter helicopter aerodynamic aerodynamic characteristicscharacteristics

Correlate relationships Correlate relationships between these between these characteristics characteristics

Rotary Wing Aerodynamic Rotary Wing Aerodynamic Subject AreasSubject Areas

Aerodynamic FactorsAerodynamic Factors– Relative WindRelative Wind– Induced Flow ProductionInduced Flow Production– Resultant Relative WindResultant Relative Wind– Angle of Attack / Angle of IncidenceAngle of Attack / Angle of Incidence– Total Aerodynamic ForceTotal Aerodynamic Force

LiftLift DragDrag

Airflow During a HoverAirflow During a Hover

Rotary Wing Aerodynamics Rotary Wing Aerodynamics Subject Areas (Cont) Subject Areas (Cont)

Translating TendencyTranslating Tendency

– Mechanical and Pilot Mechanical and Pilot InputsInputs

Dissymmetry of LiftDissymmetry of Lift – Blade FlappingBlade Flapping– Blade Lead and LagBlade Lead and Lag– Cyclic FeatheringCyclic Feathering

Rotary Wing Aerodynamic Rotary Wing Aerodynamic Subject Areas (Cont)Subject Areas (Cont)

Retreating Blade Stall Retreating Blade Stall CompressibilityCompressibility Settling with Power Settling with Power Off Set HingesOff Set Hinges Dynamic RolloverDynamic Rollover

Relative WindRelative 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 ProductionInduced Flow Production

• This figure illustrates how still air is changed to a column of descending air by rotor blade action

Resultant Relative WindResultant Relative Wind

• Angle of attack is reduced by induced flow, causing the airfoil to produce less lift

• Airflow from rotation, modified by induced flow, produces the Resultant Relative Wind

Angle of AttackAngle 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 IncidenceAngle 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 ForceTotal 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 ForceTotal Aerodynamic Force

Total aerodynamic force may be divided Total aerodynamic force may be divided into two components called lift and draginto two components called lift and drag

LiftLift acts on the airfoil in a direction acts on the airfoil in a direction perpendicular to the relative wind perpendicular to the relative wind

DragDrag acts on the airfoil in a direction acts on the airfoil in a direction parallel to the relative wind and is the parallel to the relative wind and is the force that opposes the motion of the force that opposes the motion of the airfoil through the airairfoil through the air

Airflow at a Hover (IGE)Airflow at a Hover (IGE)• Lift needed to sustain an IGE Hover 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)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 OGE Hover

Rotor Tip Vortexes (IGE/OGE)Rotor Tip Vortexes (IGE/OGE)

Rotor Tip Vortexes EffectsRotor 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 TendencyTranslating Tendency

The tendency for a The tendency for a single rotor helicopter to single rotor helicopter to drift laterally, due to tail drift laterally, due to tail rotor thrustrotor thrust

Dissymmetry of LiftDissymmetry of Lift

DefinitionDefinition CompensationCompensation

– Blade FlappingBlade Flapping– Cyclic FeatheringCyclic Feathering– Blade Lead and LagBlade Lead and Lag

Dissymmetry of Lift DefinitionDissymmetry 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 Blade FlappingFlapping• 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.

Blade Blade FlappingFlapping

Cyclic FeatheringCyclic 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 and LagBlade Lead and Lag

• 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.

Retreating Blade StallRetreating Blade Stall

A tendency for the A tendency for the retreating blade to stall retreating blade to stall in forward flight is in forward flight is inherent in all present inherent in all present day helicopters and is day helicopters and is a major factor in a major factor in limiting their forward limiting their forward speedspeed

Retreating Blade StallRetreating Blade StallLift at a HoverLift at a Hover

Retreating Blade Stall Retreating Blade Stall Lift at CruiseLift at Cruise

Retreating Blade Stall Retreating Blade Stall Lift at Stall AirspeedLift at Stall Airspeed

Retreating Blade StallRetreating Blade StallCausesCauses

When operating at high forward airspeeds, When operating at high forward airspeeds, the following conditions are most likely to the following conditions are most likely to produce blade stall:produce blade stall: – High Blade Loading (high gross weight) High Blade Loading (high gross weight) – Low Rotor RPMLow Rotor RPM– High Density Altitude High Density Altitude – Steep or Abrupt Turns Steep or Abrupt Turns – Turbulent AirTurbulent Air

Retreating Blade StallRetreating Blade StallIndicationsIndications

The major warnings of approaching The major warnings of approaching retreating blade stall conditions are:retreating blade stall conditions are: – Abnormal Vibration Abnormal Vibration – Nose Pitch-up Nose Pitch-up – The Helicopter Will Roll Into The Stalled SideThe Helicopter Will Roll Into The Stalled Side

Retreating Blade StallRetreating Blade StallCorrective ActionsCorrective Actions

When the pilot suspects blade stall, he can When the pilot suspects blade stall, he can possibly prevent it from occurring by possibly prevent it from occurring by sequentially:sequentially: – Reducing Power (collective pitch)Reducing Power (collective pitch)– Reducing Airspeed Reducing Airspeed – Reducing "G" Loads During Maneuvering Reducing "G" Loads During Maneuvering – Increasing Rotor RPM to Max Allowable Limit Increasing Rotor RPM to Max Allowable Limit

– Checking Pedal TrimChecking Pedal Trim

CompressibilityCompressibility

CompressibilityCompressibilityWhat Happens?What Happens?

Rotor blades moving through the air below Rotor blades moving through the air below approximately Mach 0.7 cause the air in front of the approximately Mach 0.7 cause the air in front of the blade to move away before compression can take blade to move away before compression can take place.place.

Above speeds of approximately Mach 0.7 the air Above speeds of approximately Mach 0.7 the air flowing over the blade accelerates above the speed of flowing over the blade accelerates above the speed of sound, causing a shock wave (also known as a sound, causing a shock wave (also known as a sonic sonic boomboom) as the blade compresses air molecules faster ) as the blade compresses air molecules faster than they can move away from the blade. than they can move away from the blade.

The danger of this shock wave (The danger of this shock wave (CompressibilityCompressibility) is its ) is its effect on aircraft control and fragile rotor blade effect on aircraft control and fragile rotor blade membranes.membranes.

CompressibilityCompressibilityCausesCauses

Conditions conducive to CompressibilityConditions conducive to Compressibility– High AirspeedHigh Airspeed– High Rotor RPMHigh Rotor RPM– High Gross WeightHigh Gross Weight– High Density AltitudeHigh Density Altitude– Low TemperatureLow Temperature– Turbulent AirTurbulent Air

CompressibilityCompressibilityIndicationsIndications

As Compressibility As Compressibility approaches:approaches:– Power Required Power Required

Increase as Lift Increase as Lift Decreases and Drag Decreases and Drag IncreasesIncreases

– Vibrations Become Vibrations Become More SevereMore Severe

– Shock Wave Forms Shock Wave Forms (Sonic Boom)(Sonic Boom)

– Nose Pitches DownNose Pitches Down

CompressibilityCompressibility Corrective Actions Corrective Actions

When the pilot suspects Compressibility, he When the pilot suspects Compressibility, he can possibly prevent it from occurring by:can possibly prevent it from occurring by:– Slowing Down the AircraftSlowing Down the Aircraft– Decreasing Pitch Angle (Reduce Collective)Decreasing Pitch Angle (Reduce Collective)– Minimizing G LoadingMinimizing G Loading– Decreasing Rotor RPMDecreasing Rotor RPM

Settling with PowerSettling with Power

Settling With PowerSettling With Power is a condition of powered is a condition of powered flight where the helicopter settles into its own flight where the helicopter settles into its own downwash.downwash.

It is also known as It is also known as Vortex Ring StateVortex Ring State

Settling with PowerSettling with PowerCauseCause

Increase in induced flow results in reduction of angle of Increase in induced flow results in reduction of angle of attack and increase in dragattack and increase in drag

This creates a demand for excessive power and creates This creates a demand for excessive power and creates greater sink rategreater sink rate

Where the demand for power meets power available the Where the demand for power meets power available the aircraft will no longer sustain flight and will descendaircraft will no longer sustain flight and will descend

Settling With PowerSettling With PowerConditionsConditions

Conditions required for Settling with power Conditions required for Settling with power are:are:– 300-1000 FPM Rate of Descent 300-1000 FPM Rate of Descent – Power Applied (> than 20% Available Power)Power Applied (> than 20% Available Power)– Near Zero Airspeed (Loss of ETL)Near Zero Airspeed (Loss of ETL)

Can occur during:Can occur during:– Downwind Approaches.Downwind Approaches.– Formation Approaches and Takeoffs.Formation Approaches and Takeoffs.– Steep Approaches.Steep Approaches.– NOE Flight.NOE Flight.– Mask/Unmask Operations.Mask/Unmask Operations.– Hover OGE.Hover OGE.

Settling With PowerSettling With PowerIndicationsIndications

Symptoms of Settling with Power:Symptoms of Settling with Power:– A high rate of descentA high rate of descent– High power consumptionHigh power consumption– Loss of collective pitch effectivenessLoss of collective pitch effectiveness– VibrationsVibrations

Settling With PowerSettling With PowerCorrective ActionsCorrective Actions

When Settling with When Settling with Power is suspected:Power is suspected:– Establish directional Establish directional

flight.flight.– Lower collective Lower collective

pitch.pitch.– Increase RPM if Increase RPM if

decayed.decayed.– Apply right pedal.Apply right pedal.

Off Set HingesOff 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 RolloverDynamic Rollover

With a rolling moment and a pivot point if the helicopter exceeds a critical angle it will roll over.

Dynamic RolloverDynamic 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 Main Rotor Thrust

Almost Equal to WeightAlmost Equal to Weight– Left Yaw InputsLeft Yaw Inputs

Dynamic RolloverDynamic Rollover

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

SummarySummary

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

Websites checkedas of 9 JUN 05

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