LOS HALCONESCAMILO ISIDRO GUAYAZAN MORENO

INSTRUCTOR ESPECIALIDADES TERRESTRES

HELICOPTER AERODYNAMICS

Learning Objectives

Applied and simplified understanding of helicopter aerodynamic characteristics

Correlate relationships between these characteristics

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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 HoverCIGM

Rotary Wing Aerodynamics Subject Areas (Cont)

Translating Tendency – Mechanical and Pilot

Inputs Dissymmetry of Lift

– Blade Flapping– Blade Lead and Lag– Cyclic Feathering

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Rotary Wing Aerodynamic Subject Areas (Cont)

Retreating Blade Stall Compressibility Settling with Power Off Set Hinges Dynamic Rollover

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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

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Induced Flow Production

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

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Resultant 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

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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

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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

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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

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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

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Rotor Tip Vortexes (IGE/OGE)CIGM

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 CIGM

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Translating Tendency

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

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Dissymmetry of Lift

Definition Compensation

– Blade Flapping– Cyclic Feathering– Blade Lead and Lag

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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

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Blade Flapping• 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.

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Blade Flapping CIGM

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.

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Blade 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.

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Retreating Blade Stall

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

Retreating Blade StallLift at a Hover

Retreating Blade Stall Lift at Cruise

Retreating Blade Stall Lift at Stall Airspeed

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Retreating Blade StallCauses

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

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Retreating Blade StallIndications

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

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Retreating Blade StallCorrective Actions

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

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Compressibility

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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. CIGM

CompressibilityCauses

Conditions conducive to Compressibility– High Airspeed– High Rotor RPM– High Gross Weight– High Density Altitude– Low Temperature– Turbulent Air

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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

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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)– Minimizing G Loading– Decreasing Rotor RPM

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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

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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

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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.– Mask/Unmask Operations.– Hover OGE.

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Settling With PowerIndications

Symptoms of Settling with Power:– A high rate of descent– High power consumption– Loss of collective pitch effectiveness– Vibrations

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Settling With PowerCorrective Actions

When Settling with Power is suspected:– Establish directional

flight.– Lower collective

pitch.– Increase RPM if

decayed.– Apply right pedal.

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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. CIGM

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Dynamic Rollover

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

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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

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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.

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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

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QUIZClick on the link below to access the

Aerodynamics Quiz

http://ang.quizstarpro.com

Log-in and Click “Search” TabClass Name = Aerodynamics

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