Download ppt - Rotational Airflow (no forward movement) Rotational Airflow (no forward movement) Tip Speed 700 FPS Tip Speed 700 FPS Tip Speed 700 FPS Tip Speed 700 FPS

Rotational AirflowRotational Airflow(no forward movement)(no forward movement)

Rotational AirflowRotational Airflow(no forward movement)(no forward movement)

Tip SpeedTip Speed700 FPS700 FPS




Circular movement of the rotor blades….Circular movement of the rotor blades….Circular movement of the rotor blades….Circular movement of the rotor blades….

……produces basic rotational relative wind.produces basic rotational relative wind.Maximum speed is at the tip of the bladeMaximum speed is at the tip of the bladeand decreases uniformly to zero at the huband decreases uniformly to zero at the hub

……produces basic rotational relative wind.produces basic rotational relative wind.Maximum speed is at the tip of the bladeMaximum speed is at the tip of the bladeand decreases uniformly to zero at the huband decreases uniformly to zero at the hub

Typical Blade Tip SpeedsTypical Blade Tip SpeedsTypical Blade Tip SpeedsTypical Blade Tip Speeds

Acft RPM TIP SpeedAcft RPM TIP SpeedAcft RPM TIP SpeedAcft RPM TIP Speed

AH-1F(Cobra)AH-1F(Cobra) 324 324 746 (fps) 746 (fps)

OH-58(Kiowa)OH-58(Kiowa) 354 354 656 (fps) 656 (fps)

AH-64(Apache) 289AH-64(Apache) 289 726 (fps) 726 (fps)

UH-60(Black Hawk) 258UH-60(Black Hawk) 258 725 (fps) 725 (fps)

R-22(Snail)R-22(Snail) 530 530 672 (fps) 672 (fps)

EC-135(Bug)EC-135(Bug) 395 395 695 (fps) 695 (fps)

AH-1F(Cobra)AH-1F(Cobra) 324 324 746 (fps) 746 (fps)

OH-58(Kiowa)OH-58(Kiowa) 354 354 656 (fps) 656 (fps)

AH-64(Apache) 289AH-64(Apache) 289 726 (fps) 726 (fps)

UH-60(Black Hawk) 258UH-60(Black Hawk) 258 725 (fps) 725 (fps)

R-22(Snail)R-22(Snail) 530 530 672 (fps) 672 (fps)

EC-135(Bug)EC-135(Bug) 395 395 695 (fps) 695 (fps)

At flat pitch air leaves the trailing edge of the rotor in the At flat pitch air leaves the trailing edge of the rotor in the same direction that it moved along the on to the leading same direction that it moved along the on to the leading edge. No lift is being produced On a asymmetrical airfoil, air edge. No lift is being produced On a asymmetrical airfoil, air following the curved upper camber will leave the trailing following the curved upper camber will leave the trailing edge with a downward flow imparted on it.edge with a downward flow imparted on it.

At flat pitch air leaves the trailing edge of the rotor in the At flat pitch air leaves the trailing edge of the rotor in the same direction that it moved along the on to the leading same direction that it moved along the on to the leading edge. No lift is being produced On a asymmetrical airfoil, air edge. No lift is being produced On a asymmetrical airfoil, air following the curved upper camber will leave the trailing following the curved upper camber will leave the trailing edge with a downward flow imparted on it.edge with a downward flow imparted on it.

Each blade creates a greater downward column of air that Each blade creates a greater downward column of air that is ingested by the next blade which produces more is ingested by the next blade which produces more downflow and so on. downflow and so on.

The next slide will attempt to illustrate...The next slide will attempt to illustrate...

Each blade creates a greater downward column of air that Each blade creates a greater downward column of air that is ingested by the next blade which produces more is ingested by the next blade which produces more downflow and so on. downflow and so on.

The next slide will attempt to illustrate...The next slide will attempt to illustrate...

Development of Induced FlowDevelopment of Induced FlowDevelopment of Induced FlowDevelopment of Induced Flow

Development of Induced FlowDevelopment of Induced FlowDevelopment of Induced FlowDevelopment of Induced Flow

Still airStill airStill airStill air

Downward Downward column of aircolumn of airDownward Downward

column of aircolumn of air

Blade 1Blade 1Point APoint ABlade 1Blade 1Point APoint A




Since the air is disturbed each time that a blade passes a point in space,Since the air is disturbed each time that a blade passes a point in space,the air is accelerated downward until it either slows due to ground effectthe air is accelerated downward until it either slows due to ground effector is slowed by a high volume of undisturbed air well below the rotor (OGE)or is slowed by a high volume of undisturbed air well below the rotor (OGE)

Since the air is disturbed each time that a blade passes a point in space,Since the air is disturbed each time that a blade passes a point in space,the air is accelerated downward until it either slows due to ground effectthe air is accelerated downward until it either slows due to ground effector is slowed by a high volume of undisturbed air well below the rotor (OGE)or is slowed by a high volume of undisturbed air well below the rotor (OGE)

Initial Velocity = 0Initial Velocity = 0Initial Velocity = 0Initial Velocity = 0

Velocity InducedVelocity InducedVelocity InducedVelocity Induced

Velocity Final = 2X Velocity InducedVelocity Final = 2X Velocity InducedVelocity Final = 2X Velocity InducedVelocity Final = 2X Velocity Induced

At a hover, the rotor tip vortex (air swirl at the tip of the rotor blades) slightly reduces the effectiveness of the outer blade portions. Also, the vortexes of the preceding blade affect the lift of the following blades. If the vortex made by one passing blade remains a vicious swirl for some number of seconds, then two blades operating at 350 RPM create 700 long lasting vortex patterns per minute.

At a hover, the rotor tip vortex (air swirl at the tip of the rotor blades) slightly reduces the effectiveness of the outer blade portions. Also, the vortexes of the preceding blade affect the lift of the following blades. If the vortex made by one passing blade remains a vicious swirl for some number of seconds, then two blades operating at 350 RPM create 700 long lasting vortex patterns per minute.

Rotor Tip VortexRotor Tip Vortex

This continuous creation of new vortices and ingestion of existing vortices is a primary cause of high power requirements for hovering.

These vortices are little more than the blade out running the high pressure below the blade seeking the lower pressure above the blade

This continuous creation of new vortices and ingestion of existing vortices is a primary cause of high power requirements for hovering.

These vortices are little more than the blade out running the high pressure below the blade seeking the lower pressure above the blade

Rotor Tip Vorticies Rotor Tip Vorticies cont.cont.Rotor Tip Vorticies Rotor Tip Vorticies cont.cont.

Rotor Tip Vorticies Rotor Tip Vorticies cont.cont.Rotor Tip Vorticies Rotor Tip Vorticies cont.cont.

Affects of AirspeedAffects of AirspeedAffects of AirspeedAffects of Airspeed

Advancing BladeAdvancing Blade•Airspeed is added to the rotational relative wind speedAirspeed is added to the rotational relative wind speed•The greatest value will occur when the blade is at the 3 o’clock positionThe greatest value will occur when the blade is at the 3 o’clock position•Increases the velocity along the span of the advancing blade by a Increases the velocity along the span of the advancing blade by a

velocity equal to the forward airspeed.velocity equal to the forward airspeed.

Advancing BladeAdvancing Blade•Airspeed is added to the rotational relative wind speedAirspeed is added to the rotational relative wind speed•The greatest value will occur when the blade is at the 3 o’clock positionThe greatest value will occur when the blade is at the 3 o’clock position•Increases the velocity along the span of the advancing blade by a Increases the velocity along the span of the advancing blade by a

velocity equal to the forward airspeed.velocity equal to the forward airspeed.

Retreating bladeRetreating blade•Airspeed is subtracted from the rotational velocityAirspeed is subtracted from the rotational velocity•The minimum value will occur when the blade is at the 9 o’clock positionThe minimum value will occur when the blade is at the 9 o’clock position•Decreases velocity across the span of the retreating bladeDecreases velocity across the span of the retreating blade•Produces three “Produces three “NO LIFTNO LIFT” areas along the retreating blade” areas along the retreating blade

Retreating bladeRetreating blade•Airspeed is subtracted from the rotational velocityAirspeed is subtracted from the rotational velocity•The minimum value will occur when the blade is at the 9 o’clock positionThe minimum value will occur when the blade is at the 9 o’clock position•Decreases velocity across the span of the retreating bladeDecreases velocity across the span of the retreating blade•Produces three “Produces three “NO LIFTNO LIFT” areas along the retreating blade” areas along the retreating blade

Affects of Airspeed Affects of Airspeed cont.cont.Affects of Airspeed Affects of Airspeed cont.cont.

Blades over the nose and tail are affected minimally by forward airspeedBlades over the nose and tail are affected minimally by forward airspeedBlades over the nose and tail are affected minimally by forward airspeedBlades over the nose and tail are affected minimally by forward airspeed

Development of lift areas around the rotor system in forward flightDevelopment of lift areas around the rotor system in forward flightDevelopment of lift areas around the rotor system in forward flightDevelopment of lift areas around the rotor system in forward flight

The entire advancing blade is producing lift, However, the retreating The entire advancing blade is producing lift, However, the retreating blade produces five distinct lift areas:blade produces five distinct lift areas:The entire advancing blade is producing lift, However, the retreating The entire advancing blade is producing lift, However, the retreating blade produces five distinct lift areas:blade produces five distinct lift areas:

•Reverse Flow: Reverse Flow: Airflow is from trailing edge to leading edgeAirflow is from trailing edge to leading edge

•Negative Stall: Negative Stall: Airflow strikes blade from well above chord line Airflow strikes blade from well above chord line

•Negative Lift: Negative Lift: Airflow also above chord line but lift produced under bladeAirflow also above chord line but lift produced under blade

•Positive Lift: Positive Lift: Airflow is below chord line. Airflow is below chord line. This is desirableThis is desirable

•Positive Stall: Positive Stall: Airflow well below chord line. More drag than liftAirflow well below chord line. More drag than lift

•Reverse Flow: Reverse Flow: Airflow is from trailing edge to leading edgeAirflow is from trailing edge to leading edge

•Negative Stall: Negative Stall: Airflow strikes blade from well above chord line Airflow strikes blade from well above chord line

•Negative Lift: Negative Lift: Airflow also above chord line but lift produced under bladeAirflow also above chord line but lift produced under blade

•Positive Lift: Positive Lift: Airflow is below chord line. Airflow is below chord line. This is desirableThis is desirable

•Positive Stall: Positive Stall: Airflow well below chord line. More drag than liftAirflow well below chord line. More drag than lift

Reverse Flow, Negative Stall and Negative Lift are the three “NO LIFT” Reverse Flow, Negative Stall and Negative Lift are the three “NO LIFT” areas discussed on the previous slideareas discussed on the previous slideReverse Flow, Negative Stall and Negative Lift are the three “NO LIFT” Reverse Flow, Negative Stall and Negative Lift are the three “NO LIFT” areas discussed on the previous slideareas discussed on the previous slide

Reverse FlowReverse FlowReverse FlowReverse Flow

Negative StallNegative StallNegative StallNegative Stall

Negative LiftNegative LiftNegative LiftNegative Lift

Positive StallPositive StallPositive StallPositive Stall

Resultant AirflowResultant Airflow(120 KTS)(120 KTS)

Resultant AirflowResultant Airflow(120 KTS)(120 KTS)

The forward velocity is The forward velocity is addedaddedto the advancing blade….to the advancing blade….The forward velocity is The forward velocity is addedaddedto the advancing blade….to the advancing blade….

……while it is while it is subtractedsubtracted from fromthe retreating bladethe retreating blade……while it is while it is subtractedsubtracted from fromthe retreating bladethe retreating blade

1000 FPS Tip Speed1000 FPS Tip Speed1000 FPS Tip Speed1000 FPS Tip Speed

800 FPS = Rotation800 FPS = Rotation+200 FPS = Fwd Airspeed+200 FPS = Fwd Airspeed 800 FPS = Rotation800 FPS = Rotation+200 FPS = Fwd Airspeed+200 FPS = Fwd Airspeed

600 FPS Tip Speed600 FPS Tip Speed600 FPS Tip Speed600 FPS Tip Speed

800 FPS = Rotation800 FPS = Rotation- 200 FPS = Fwd Airspeed- 200 FPS = Fwd Airspeed 800 FPS = Rotation800 FPS = Rotation- 200 FPS = Fwd Airspeed- 200 FPS = Fwd Airspeed

Dissymmetry of LiftDissymmetry of LiftDissymmetry of LiftDissymmetry of Lift

The The potentialpotential for unequal lift to develop between the for unequal lift to develop between the advancing and retreating halves of rotor disk due to the advancing and retreating halves of rotor disk due to the differential velocity of wind flow across the advancing and differential velocity of wind flow across the advancing and retreating halves of the rotor system.retreating halves of the rotor system.

The The potentialpotential for unequal lift to develop between the for unequal lift to develop between the advancing and retreating halves of rotor disk due to the advancing and retreating halves of rotor disk due to the differential velocity of wind flow across the advancing and differential velocity of wind flow across the advancing and retreating halves of the rotor system.retreating halves of the rotor system.

The helicopter would become uncontrollable if dyssemmetry The helicopter would become uncontrollable if dyssemmetry of lift were permitted to manifest itself in the rotor system. of lift were permitted to manifest itself in the rotor system. A means to compensate for, overcome or eliminate its A means to compensate for, overcome or eliminate its effects must be available.effects must be available.

The helicopter would become uncontrollable if dyssemmetry The helicopter would become uncontrollable if dyssemmetry of lift were permitted to manifest itself in the rotor system. of lift were permitted to manifest itself in the rotor system. A means to compensate for, overcome or eliminate its A means to compensate for, overcome or eliminate its effects must be available.effects must be available.

Those means are:Those means are:Those means are:Those means are:

Blade FlappingBlade FlappingBlade FlappingBlade Flapping

The rotor system will compensate for dissymmetry of liftThe rotor system will compensate for dissymmetry of liftautomatically, without pilot input, through blade flapping automatically, without pilot input, through blade flapping The rotor system will compensate for dissymmetry of liftThe rotor system will compensate for dissymmetry of liftautomatically, without pilot input, through blade flapping automatically, without pilot input, through blade flapping

As the relative wind speed of the advancing blade increases,As the relative wind speed of the advancing blade increases,it gains lift and starts flapping up. It reaches its maximumit gains lift and starts flapping up. It reaches its maximumupflap velocity at the 3 o'clock position, where the wind upflap velocity at the 3 o'clock position, where the wind velocity is at its highest. The upflapping velocity creates a velocity is at its highest. The upflapping velocity creates a downward flow of air across the blade. This has the samedownward flow of air across the blade. This has the sameeffect as increasing the induced flow velocity and reducing effect as increasing the induced flow velocity and reducing angle of attack, decreasing lift across the advancing blade.angle of attack, decreasing lift across the advancing blade.

As the relative wind speed of the advancing blade increases,As the relative wind speed of the advancing blade increases,it gains lift and starts flapping up. It reaches its maximumit gains lift and starts flapping up. It reaches its maximumupflap velocity at the 3 o'clock position, where the wind upflap velocity at the 3 o'clock position, where the wind velocity is at its highest. The upflapping velocity creates a velocity is at its highest. The upflapping velocity creates a downward flow of air across the blade. This has the samedownward flow of air across the blade. This has the sameeffect as increasing the induced flow velocity and reducing effect as increasing the induced flow velocity and reducing angle of attack, decreasing lift across the advancing blade.angle of attack, decreasing lift across the advancing blade.

UpflappingUpflappingUpflappingUpflapping

As the relative wind speed of the retreating blade decreases,As the relative wind speed of the retreating blade decreases,the blade loses lift and starts flapping down. It reaches itsthe blade loses lift and starts flapping down. It reaches itsmaximum downflap at the 9 o’clock position, where the windmaximum downflap at the 9 o’clock position, where the windvelocity is the lowest. The downflapping velocity creates anvelocity is the lowest. The downflapping velocity creates anupward flow of air across the blade. The upflow reduces theupward flow of air across the blade. The upflow reduces theinduced flow velocity and increases the angle of attack, induced flow velocity and increases the angle of attack, increasing lift. increasing lift.

As the relative wind speed of the retreating blade decreases,As the relative wind speed of the retreating blade decreases,the blade loses lift and starts flapping down. It reaches itsthe blade loses lift and starts flapping down. It reaches itsmaximum downflap at the 9 o’clock position, where the windmaximum downflap at the 9 o’clock position, where the windvelocity is the lowest. The downflapping velocity creates anvelocity is the lowest. The downflapping velocity creates anupward flow of air across the blade. The upflow reduces theupward flow of air across the blade. The upflow reduces theinduced flow velocity and increases the angle of attack, induced flow velocity and increases the angle of attack, increasing lift. increasing lift.

DownflappingDownflappingDownflappingDownflapping

Increased velocity Increased velocity Increased velocity Increased velocity

Decreased velocityDecreased velocityDecreased velocityDecreased velocity

Advancing halfAdvancing halfAdvancing halfAdvancing half

Retreating halfRetreating halfRetreating halfRetreating half

Due to gyroscopic effect the maximum upflap takes place 90° Due to gyroscopic effect the maximum upflap takes place 90° after its maximum upflap velocity. Since the maximum upflap after its maximum upflap velocity. Since the maximum upflap velocity is at the 3 o’clock position, the maximum upflapvelocity is at the 3 o’clock position, the maximum upflapdisplacement is at the 12 o’clock position.displacement is at the 12 o’clock position.

Due to gyroscopic effect the maximum upflap takes place 90° Due to gyroscopic effect the maximum upflap takes place 90° after its maximum upflap velocity. Since the maximum upflap after its maximum upflap velocity. Since the maximum upflap velocity is at the 3 o’clock position, the maximum upflapvelocity is at the 3 o’clock position, the maximum upflapdisplacement is at the 12 o’clock position.displacement is at the 12 o’clock position.

Likewise, because the maximum downflap velocity is at the Likewise, because the maximum downflap velocity is at the 9 o’clock position, the maximum downflap displacement is 9 o’clock position, the maximum downflap displacement is at the 6 o’clock position.at the 6 o’clock position.

Likewise, because the maximum downflap velocity is at the Likewise, because the maximum downflap velocity is at the 9 o’clock position, the maximum downflap displacement is 9 o’clock position, the maximum downflap displacement is at the 6 o’clock position.at the 6 o’clock position.

Upflapping and downflapping do not change the amount of liftUpflapping and downflapping do not change the amount of liftproduced by the rotor system. The blades flap to equilibrium.produced by the rotor system. The blades flap to equilibrium.However, flapping changes the attitude of the rotor systemHowever, flapping changes the attitude of the rotor system(blowback) and therefore, the direction of the total lift vector. (blowback) and therefore, the direction of the total lift vector. This reduces helicopter speed.This reduces helicopter speed.

Upflapping and downflapping do not change the amount of liftUpflapping and downflapping do not change the amount of liftproduced by the rotor system. The blades flap to equilibrium.produced by the rotor system. The blades flap to equilibrium.However, flapping changes the attitude of the rotor systemHowever, flapping changes the attitude of the rotor system(blowback) and therefore, the direction of the total lift vector. (blowback) and therefore, the direction of the total lift vector. This reduces helicopter speed.This reduces helicopter speed.

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Lift differences beforeLift differences beforeconsidering phase lagconsidering phase lag

(speed < ETL)(speed < ETL)

Lift differences beforeLift differences beforeconsidering phase lagconsidering phase lag

(speed < ETL)(speed < ETL)

Lift differences afterLift differences afterthe effects of phasethe effects of phaselag are appliedlag are applied

Lift differences afterLift differences afterthe effects of phasethe effects of phaselag are appliedlag are applied

The flight profile of an aircraft experiencing The flight profile of an aircraft experiencing BLOW BACKBLOW BACK

Initial cyclic input made to start forward momentum, then Initial cyclic input made to start forward momentum, then cyclic is held in place with no further corrective actions cyclic is held in place with no further corrective actions takentaken

Since blade flapping alone would limit directional velocities to Since blade flapping alone would limit directional velocities to around ETL, another means of compensating for around ETL, another means of compensating for dissymmetry of lift must be available. dissymmetry of lift must be available. The pilot must be able to control the attitude of the rotorThe pilot must be able to control the attitude of the rotorto attain the desired direction and velocity.to attain the desired direction and velocity.

Since blade flapping alone would limit directional velocities to Since blade flapping alone would limit directional velocities to around ETL, another means of compensating for around ETL, another means of compensating for dissymmetry of lift must be available. dissymmetry of lift must be available. The pilot must be able to control the attitude of the rotorThe pilot must be able to control the attitude of the rotorto attain the desired direction and velocity.to attain the desired direction and velocity.

Cyclic FeatheringCyclic FeatheringCyclic FeatheringCyclic Feathering

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While both cyclic feathering and blade flapping are While both cyclic feathering and blade flapping are used to compensate for dissymmetry of lift, cyclic used to compensate for dissymmetry of lift, cyclic feathering is the primary means of compensating feathering is the primary means of compensating for dissymmetry of lift in normal cruise flight.for dissymmetry of lift in normal cruise flight.

While both cyclic feathering and blade flapping are While both cyclic feathering and blade flapping are used to compensate for dissymmetry of lift, cyclic used to compensate for dissymmetry of lift, cyclic feathering is the primary means of compensating feathering is the primary means of compensating for dissymmetry of lift in normal cruise flight.for dissymmetry of lift in normal cruise flight.

Other design features to reduce flapping:Other design features to reduce flapping:Other design features to reduce flapping:Other design features to reduce flapping:

Forward tilt to the rotor reduces flapping to a Forward tilt to the rotor reduces flapping to a minimum during normal cruise flightminimum during normal cruise flightForward tilt to the rotor reduces flapping to a Forward tilt to the rotor reduces flapping to a minimum during normal cruise flightminimum during normal cruise flight

Synchronized elevator/stabilator help maintain the Synchronized elevator/stabilator help maintain the desired fuselage attitude to reduce flapping desired fuselage attitude to reduce flapping Synchronized elevator/stabilator help maintain the Synchronized elevator/stabilator help maintain the desired fuselage attitude to reduce flapping desired fuselage attitude to reduce flapping