Lesson 2 basic aerodynamics

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  • 1. Lesson 2Lesson 2 Basic Aerodynamics andBasic Aerodynamics and Principles of Helicopter FlightPrinciples of Helicopter Flight
  • 2. OverviewOverview  This lesson will introduce basicThis lesson will introduce basic aerodynamics and principles ofaerodynamics and principles of helicopter flight.helicopter flight. ObjectivesObjectives  After completion of this lesson you willAfter completion of this lesson you will know the specifics of:know the specifics of:  The Four ForcesThe Four Forces  TorqueTorque  AirfoilsAirfoils  Rotor SystemsRotor Systems  Lift and DragLift and Drag  VibrationsVibrations  The Three AxesThe Three Axes
  • 3. The Four ForcesThe Four Forces DefinitionsDefinitions LIFTLIFT Upward force created by theUpward force created by the Airfoils.Airfoils. WEIGHTWEIGHT Downward force due to the pullDownward force due to the pull of gravity.of gravity. THRUSTTHRUST The force that propels theThe force that propels the helicopter through the air.helicopter through the air. DRAGDRAG The force due to the resistanceThe force due to the resistance of movement though the air.of movement though the air. Lift Thrust Weight Drag The four forces acting on a helicopter in forward flight
  • 4.  Equal upper and lower CamberEqual upper and lower Camber  AdvantagesAdvantages  More stableMore stable  The result is less flapping and lead/lagThe result is less flapping and lead/lag  DisadvantagesDisadvantages  Less lift for a given chord sizeLess lift for a given chord size  Main Rotor of:Main Rotor of:  R22, R44, Schweitzer 300R22, R44, Schweitzer 300  Tail Rotor of:Tail Rotor of:  Schweitzer 300Schweitzer 300 Symmetrical Asymmetrical Leading Edge Upper Camber Chord Line Lower Camber Trailing Edge AirfoilsAirfoils Asymmetrical SymmetricalSymmetrical
  • 5.  Different upper and lowerDifferent upper and lower CamberCamber  AdvantagesAdvantages  More lift for a given chord sizeMore lift for a given chord size (More Tail Rotor authority)(More Tail Rotor authority)  DisadvantagesDisadvantages  Less stableLess stable  More flapping and lead/lagMore flapping and lead/lag  Main Rotor of:Main Rotor of:  Chinook CH-47Chinook CH-47  Tail Rotor of:Tail Rotor of:  R22, R44R22, R44 Symmetrical Asymmetrical Leading Edge Upper Camber Chord Line Lower Camber Trailing Edge AirfoilsAirfoils AsymmetricalAsymmetrical
  • 6. AirfoilsAirfoils  Relative WindRelative Wind Velocity of air due to blade rotation Velocity of air due to induced flow Chord Line INDUCED FLOW Blade Rotation Relative Wind  Angle of AttackAngle of Attack  The angle between the Chord LineThe angle between the Chord Line and the Relative Wind.and the Relative Wind. Angle of Attack  This is an aerodynamic angle.This is an aerodynamic angle. That is, the angle changes due to theThat is, the angle changes due to the result of aerodynamic influences.result of aerodynamic influences.  The direction of airflowThe direction of airflow encountered by the airfoil.encountered by the airfoil.
  • 7. AirfoilsAirfoils  Tip Path PlaneTip Path Plane Tip Path Plane  Pitch AnglePitch Angle  The angle between the Chord LineThe angle between the Chord Line and the Tip Path Plane.and the Tip Path Plane. Tip Path Plane Tip Path Plane Direction of Airfoil Travel along the Tip Path Plane Pitch Angle Relative Wind Angle of Attack Chord Line  This is a mechanical angle.This is a mechanical angle. The angle varies according toThe angle varies according to mechanical changes.mechanical changes. (The collective.)(The collective.)  A circular plane described by theA circular plane described by the rotational path of the blade tips.rotational path of the blade tips.
  • 8. Venturi effectVenturi effect andand Bernoulli’s PrincipleBernoulli’s Principle  As the area of a passageAs the area of a passage decreasesdecreases, the velocity going, the velocity going through that passagethrough that passage increasesincreases.. LOW HIGH PRESSURE PRESSURE LOW HIGH  As the velocityAs the velocity increasesincreases, the, the pressurepressure decreasesdecreases.. Lift & DragLift & Drag  Venturi EffectVenturi Effect  Bernoulli’s PrincipleBernoulli’s Principle  It is not necessary for air to pass throughIt is not necessary for air to pass through an enclosed tube for Bernoulli’s principlean enclosed tube for Bernoulli’s principle to apply.to apply.  Any surface that alters airflowAny surface that alters airflow (i.e. an airfoil) causes a venturi effect.(i.e. an airfoil) causes a venturi effect.  Air traveling over the top surface mustAir traveling over the top surface must travel farther, therefore it must go fastertravel farther, therefore it must go faster in order to rejoin the lower layersin order to rejoin the lower layers beyond the trailing edge.beyond the trailing edge. LOW HIGH PRESSURE
  • 9.  This difference in air speed createsThis difference in air speed creates a pressure differential…a pressure differential…  This pressure differential is theThis pressure differential is the major sourcemajor source (70%)(70%) of lift for anof lift for an aircraft.aircraft.  30% of our lift comes from30% of our lift comes from deflection; any moving shape will bedeflection; any moving shape will be deflected due to the force of airdeflected due to the force of air hitting against it.hitting against it. Shape Velocity Deflection Airfoil Velocity LIFT (Think about sticking your hand out(Think about sticking your hand out the window of a moving car.)the window of a moving car.)  Newton’s third law says that for everyNewton’s third law says that for every action, there is an equal and oppositeaction, there is an equal and opposite reaction. So the downward deflection ofreaction. So the downward deflection of air mass causes a corresponding lift onair mass causes a corresponding lift on the mass of the airship.the mass of the airship. Venturi effectVenturi effect andand Bernoulli’s PrincipleBernoulli’s Principle Lift & DragLift & Drag ……which results in a lifting force.which results in a lifting force. Lift
  • 10. Lift & DragLift & Drag  The Equation of LiftThe Equation of Lift Lift = CLift = CLL ½½ pp VV22 SS Coefficient of LiftCoefficient of Lift Air DensityAir Density Airfoil VelocityAirfoil Velocity Surface Area of AirfoilSurface Area of Airfoil
  • 11. Lift & DragLift & Drag  The Equation of LiftThe Equation of Lift Lift = CLift = CLL ½½ pp VV22 SS Coefficient of LiftCoefficient of Lift Small A.O.A. Bigger A.O.A. No A.O.A. No Downwash No Lift Maximum A.O.A. STALL Small Downwash More Downwash Maximum Downwash Small Lift More Lift Maximum Lift Definition:Definition:  The Coefficient of Lift is essentially theThe Coefficient of Lift is essentially the Angle of AttackAngle of Attack..  As the Angle of Attack increases, theAs the Angle of Attack increases, the amount of generated lift increases.amount of generated lift increases. (up to the critical angle)(up to the critical angle)  When a symmetrical airfoil has zeroWhen a symmetrical airfoil has zero Angle of Attack, there is no Lift.Angle of Attack, there is no Lift.  Experimental results show that there is aExperimental results show that there is a maximum Angle of Attack, resulting inmaximum Angle of Attack, resulting in maximum lift …maximum lift …  …… beyond which the Lift drops offbeyond which the Lift drops off dramatically, creating a Stall condition.dramatically, creating a Stall condition.
  • 12. Lift & DragLift & Drag Next, let’s look at Air Density.Next, let’s look at Air Density.  In the context of the lift equation, Air Density representsIn the context of the lift equation, Air Density represents the mass of the air being encountered by the airfoil.the mass of the air being encountered by the airfoil.  Because of Newton’s third law, the more air mass we canBecause of Newton’s third law, the more air mass we can deflect downward with the airfoil, the more airship massdeflect downward with the airfoil, the more airship mass we can lift into the air.we can lift into the air.  The Equation of LiftThe Equation of Lift Lift = CLift = CLL ½½ pp VV22 SS Air DensityAir Density
  • 13. Lift & DragLift & Drag  Air DensityAir Density  As altitude increases, the air densityAs altitude increases, the air density decreases.decreases.  Plugging this into the lift equation tells us thatPlugging this into the lift equation tells us that the lift gets less as the helicopter get higher.the lift gets less as the helicopter get higher. Greater air density. More lift. Decreased air density. Less lift.
  • 14. Lift & DragLift & Drag VelocityVelocity Now let’s look at Velocity.Now let’s look at Velocity.  ““V” represents true airspeed of the airfoil moving through the air.V” represents true airspeed of the airfoil moving through the air.  ““V” is theV” is the rotor rpmrotor rpm and can be changed using the throttleand can be changed using the throttle  ““VV22 ” is a very significant element of the lift equation because it is “squared”. A small” is a very significant element of the lift equation because it is “squared”. A small change in velocity results in a large change in lift.change in velocity results in a large change in lift.  ““V” isV” is NOTNOT the velocity of the helicopter itself moving through the air.the velocity of the helicopter itself moving through the air.  ““V” is affected by helicopter movement, but the effect is small, except at higher speeds.V” is affected by helicopter movement, but the effect is small, except at higher speeds.  The Equation of LiftThe Equation of Lift Lift = CLift = CLL ½½ pp VV22 SS
  • 15. Lift & DragLift & Drag Surface AreaSurface Area Finally, Surface Area.Finally, Surface Area.  ““S” represents the surface area of the airfoil.S” represents the surface area of the airfoil.  This implies that the larger the surface area theThis implies that the larger the surface area the more lift that gets produced.more lift that gets produced.  Surface area of an airfoil on a Helicopter is fixed.Surface area of an airfoil on a Helicopter is fixed.  In the R22, it is our chord line (7.2 inches)In the R22, it is our chord line (7.2 inches)  The Equation of LiftThe Equation of Lift Lift = CLift = CLL ½½ pp VV22 SS
  • 16. Lift & DragLift & Drag What two factors of the lift equation can the Pilot control?What two factors of the lift equation can the Pilot control?  Coefficient of Lift - (Angle of Attack)Coefficient of Lift - (Angle of Attack)  Pitch Angle (via Cyclic)Pitch Angle (via Cyclic)  Angle of Attack (via Collective and Cyclic)Angle of Attack (via Collective and Cyclic)  The Equation of LiftThe Equation of Lift Lift = CLift = CLL ½½ pp VV22 SS VelocityVelocityCoefficient of LiftCoefficient of Lift  Velocity - (blade speed)Velocity - (blade speed)  Rotor RPM (via Throttle)Rotor RPM (via Throttle)
  • 17. Lift & DragLift & Drag  The Drag FormulaThe Drag Formula Drag = CDrag = CDD ½½ pp VV22 SS Coefficient of DragCoefficient of Drag -4 0 4 8 12 16 Angle of Attack (degrees) CD  As the Angle of Attack increases, theAs the Angle of Attack increases, the amount of Drag increases.amount of Drag increases.  The amount of Drag will increaseThe amount of Drag will increase dramatically after the stall point.dramatically after the stall point. B C D E F A Definition:Definition:  The Coefficient of Drag represents theThe Coefficient of Drag represents the potential of a body to interfere with thepotential of a body to interfere with the smooth flow of air.smooth flow of air. The airfoil will disrupt the flow therebyThe airfoil will disrupt the flow thereby creating dragcreating drag (and perhaps some lift)(and perhaps some lift). The. The shape of the object and it’s angle ofshape of the object and it’s angle of attack will determine how much drag willattack will determine how much drag will be produced.be produced.
  • 18. Lift & DragLift & Drag  3 Types of Drag3 Types of Drag  Any body not producing liftAny body not producing lift generates Parasite Drag.generates Parasite Drag. Definition:Definition: Drag = CDrag = CDD ½½ pp VV22 SS 0 25 50 75 100 Indicated Airspeed (KIAS) Drag  The Graph of Parasite DragThe Graph of Parasite Drag ParasiteDrag  Parasite DragParasite Drag Induced Drag Profile Drag Form Drag Skin Friction  All non-lifting forms must be stream-All non-lifting forms must be stream- lined to reduce Parasite drag.lined to reduce Parasite drag.  As the airspeed of the helicopter increases,As the airspeed of the helicopter increases, parasite drag increases.parasite drag increases.
  • 19. Parasite Drag  Induced DragInduced Drag Profile Drag Form Drag Skin Friction Lift & DragLift & Drag  3 Types of Drag3 Types of Drag  Induced DragInduced Drag is a by-product of theis a by-product of the creation of lift.creation of lift. Definition:Definition: The higher air pressure below the blade…The higher air pressure below the blade… ……escapes around the end of the blade tipescapes around the end of the blade tip And because the blade is moving…And because the blade is moving… ……the result is a vortex spiral.the result is a vortex spiral.  The pressure differential betweenThe pressure differential between the upper and lower surfacesthe upper and lower surfaces causescauses vorticesvortices at the wing tips.at the wing tips.
  • 20. Parasite Drag • Induced DragInduced Drag Profile Drag Form Drag Skin Friction Lift & DragLift & Drag  3 Types of Drag3 Types of Drag 0 25 50 75 100 Indicated Airspeed (KIAS) Drag ParasiteDrag Profile D rag InducedDrag  As the aircraft speed increases, theAs the aircraft speed increases, the Tip Vortices decrease.Tip Vortices decrease. (They get “washed out”)(They get “washed out”)  The graph of Induced Drag looks like this:The graph of Induced Drag looks like this:  Thus, Induced Drag is greatest at lowThus, Induced Drag is greatest at low airspeeds and decreases at higherairspeeds and decreases at higher speeds.speeds.
  • 21. Lift & DragLift & Drag  Profile DragProfile Drag is the sum of Skinis the sum of Skin Friction and Form DragFriction and Form Drag Definition:Definition: Profile DragProfile Drag  Skin FrictionSkin Friction  Form DragForm Drag  Form DragForm Drag is created by theis created by the shape of the airfoil.shape of the airfoil.  Skin FrictionSkin Friction is the result of surfaceis the result of surface roughness on the blades (dirt, ice, etc)roughness on the blades (dirt, ice, etc) A rough surface causes lots ofA rough surface causes lots of disruption to the flow next to thedisruption to the flow next to the surface and thus, more drag.surface and thus, more drag. A smoother surface causes farA smoother surface causes far less disruption to the flow andless disruption to the flow and therefore, much less drag.therefore, much less drag.
  • 22. Lift & DragLift & Drag  3 Types of Drag3 Types of Drag  Profile DragProfile Drag is the sum ofis the sum of Form Drag and Skin FrictionForm Drag and Skin Friction Definition:Definition: Parasite Drag Induced Drag  Profile DragProfile Drag Form DragForm Drag Skin FrictionSkin Friction 0 25 50 75 100 Indicated Airspeed (KIAS) Drag ParasiteDrag  The Graph of Profile DragThe Graph of Profile Drag  At a hover, or low speeds, the effects ofAt a hover, or low speeds, the effects of Profile Drag on the blades tend to cancelProfile Drag on the blades tend to cancel each other out.each other out. Profile D rag  At higher airspeeds there starts to be anAt higher airspeeds there starts to be an accumulating effect of profile drag.accumulating effect of profile drag.
  • 23. Total Drag is the sum of the three types of drag.Total Drag is the sum of the three types of drag. Lift & DragLift & Drag  Total DragTotal Drag 0 25 50 75 100 Indicated Airspeed (KIAS) Drag ParasiteDrag Profile D rag InducedDrag  Parasite DragParasite Drag  Induced DragInduced Drag  Profile DragProfile Drag TOTALDrag  Note that there is a largeNote that there is a large amount of drag at low speeds,amount of drag at low speeds, andand at high speeds.at high speeds.  The minimum drag occurs atThe minimum drag occurs at an intermediate speed.an intermediate speed. (53 KIAS on the R-22).(53 KIAS on the R-22).
  • 24. The Three AxesThe Three Axes PITCHPITCH ROLLROLL YAWYAW Lateral axisLateral axis Longitudinal axisLongitudinal axis Vertical axisVertical axis Controlled by CyclicControlled by Cyclic Controlled by CyclicControlled by Cyclic Controlled by PedalsControlled by Pedals
  • 25. TorqueTorque Newton’s Third Law of MotionNewton’s Third Law of Motion Blade Rotation Blade Rotation Tail Rotor Thrust Torque-induced turning  The anti-torque rotor, or tail rotor, actsThe anti-torque rotor, or tail rotor, acts to prevent the fuselage from turning.to prevent the fuselage from turning.  This causes the helicopter fuselage toThis causes the helicopter fuselage to turn in the opposite, or clockwiseturn in the opposite, or clockwise direction.direction.  The engine turns the Rotor Blades in aThe engine turns the Rotor Blades in a counter-clockwise directioncounter-clockwise direction (on an R-22)(on an R-22).. For every action there is an equal and opposite reaction.For every action there is an equal and opposite reaction.  As the engine generates more or lessAs the engine generates more or less power, the tail rotor must produce apower, the tail rotor must produce a corresponding amount of thrust.corresponding amount of thrust.  The anti-torque pedals vary the pitchThe anti-torque pedals vary the pitch of the tail rotor, thereby controlling theof the tail rotor, thereby controlling the amount of thrust produced.amount of thrust produced.
  • 26.  There are three types of Rotor Systems:There are three types of Rotor Systems: Rotor SystemsRotor Systems  Each system is configured to allow one orEach system is configured to allow one or more of the following types of rotor behaviors:more of the following types of rotor behaviors:  Fully ArticulatedFully Articulated  Semi-RigidSemi-Rigid  RigidRigid  FlappingFlapping  FeatheringFeathering  Lead / LagLead / Lag
  • 27. Rotor SystemsRotor Systems FlappingFlapping FeatheringFeathering Lead / LagLead / Lag
  • 28. Rotor SystemsRotor Systems  Fully ArticulatedFully Articulated A generic representation of aA generic representation of a fully-articulated rotor lets usfully-articulated rotor lets us see the three movements…see the three movements… FeatheringFeathering Drag Hinge Feather Bearing Flapping Hinge  Able to independently:Able to independently:  FeatherFeather  FlapFlap  Lead / LagLead / Lag  Three separate hingesThree separate hinges
  • 29. Rotor SystemsRotor Systems FlappingFlapping Drag Hinge Feather Bearing Flapping Hinge  Fully ArticulatedFully Articulated  Able to independently:Able to independently:  FeatherFeather  FlapFlap  Lead / LagLead / Lag  Three separate hingesThree separate hinges A generic representation of aA generic representation of a fully-articulated rotor lets usfully-articulated rotor lets us see the three movements…see the three movements…
  • 30. Rotor SystemsRotor Systems Lead / LagLead / Lag Drag Hinge Feather Bearing Flapping Hinge  Fully ArticulatedFully Articulated  Able to independently:Able to independently:  FeatherFeather  FlapFlap  Lead / LagLead / Lag  Three separate hingesThree separate hinges
  • 31. Rotor SystemsRotor Systems  Semi-RigidSemi-Rigid  The Rotors are rigidlyThe Rotors are rigidly attached to the rotor hub.attached to the rotor hub.  Feathering occurs throughFeathering occurs through the feather bearings.the feather bearings. Feather Bearings FeatheringFeathering  Able to:Able to:  FeatherFeather  FlapFlap Lead / LagLead / Lag
  • 32. Rotor SystemsRotor Systems TeeteringTeetering (flapping)(flapping) Teetering Hinge  Semi-RigidSemi-Rigid  Able to:Able to:  FeatherFeather  FlapFlap Lead / LagLead / Lag  The entire rotor assembly isThe entire rotor assembly is able to “teeter” about this pivotable to “teeter” about this pivot point to allow the blades to flappoint to allow the blades to flap as a single unit.as a single unit. (Rigid in-plane)(Rigid in-plane)..  The hub is attached to the rotorThe hub is attached to the rotor mast by an elevated trunnionmast by an elevated trunnion bearing or teetering hinge.bearing or teetering hinge. This gives the rotor system anThis gives the rotor system an “under-slung” configuration.“under-slung” configuration.
  • 33. Rotor SystemsRotor Systems  RigidRigid  Mechanically simple but structurally complex.Mechanically simple but structurally complex.  Operating loads must be absorbed by the blades bendingOperating loads must be absorbed by the blades bending and flexing instead of pivoting around hinges.and flexing instead of pivoting around hinges.  Very expensive due to the use of specialized high-endVery expensive due to the use of specialized high-end composite materials and mathematically intensive designs.composite materials and mathematically intensive designs.  Able to:Able to:  FeatherFeather FlapFlap Lead / LagLead / Lag
  • 34.  Ground ResonanceGround Resonance VibrationsVibrations  While in flight, aWhile in flight, a (fully articulated)(fully articulated) three blade rotor system withthree blade rotor system with drag hinges will automaticallydrag hinges will automatically adjust the relative anglesadjust the relative angles between the blades so thatbetween the blades so that they are equal.they are equal.  This places the center ofThis places the center of mass directly in the center ofmass directly in the center of rotation.rotation. 120 120 120
  • 35.  Ground ResonanceGround Resonance VibrationsVibrations  If on landing, you create aIf on landing, you create a pivot point or land hard onpivot point or land hard on one skid…one skid…  ……the rotors can get jolted andthe rotors can get jolted and lose their symmetry and thelose their symmetry and the center of mass shifts awaycenter of mass shifts away from the center of rotation,from the center of rotation, causing the system tocausing the system to become unbalanced.become unbalanced.
  • 36.  Ground ResonanceGround Resonance VibrationsVibrations 116 122 122  If the skid corner remains inIf the skid corner remains in contact with the ground, thiscontact with the ground, this condition can cause acondition can cause a helicopter to self-destruct in ahelicopter to self-destruct in a matter of seconds.matter of seconds.
  • 37.  Ground ResonanceGround Resonance VibrationsVibrations  If the rpm is normal, theIf the rpm is normal, the corrective action is tocorrective action is to lift offlift off..  If you lift off but let the skidsIf you lift off but let the skids re-contact the ground againre-contact the ground again before the blades arebefore the blades are realigned, a second shockrealigned, a second shock could jolt the blades againcould jolt the blades again and make matters worse.and make matters worse. 116 122 122  This condition does not occurThis condition does not occur in semi-rigid rotor systemsin semi-rigid rotor systems because there are no lead/lagbecause there are no lead/lag hinges.hinges. https://www.youtube.com/watch?v=6vICf8l-KV0 https://www.youtube.com/watch?v=0GEj69NANc8
  • 38.  Sympathetic ResonanceSympathetic Resonance  In the range of 60% - 70% main rotor rpm, the frequency of the main rotorIn the range of 60% - 70% main rotor rpm, the frequency of the main rotor rotation can begin to interact with the frequency of the tail rotor rotation.rotation can begin to interact with the frequency of the tail rotor rotation. VibrationsVibrations Main Rotor Tail Rotor Resonance  The two frequencies can create a state of harmonic oscillationThe two frequencies can create a state of harmonic oscillation  The result can cause the tail boom to start to oscillating up and down inThe result can cause the tail boom to start to oscillating up and down in combined synchronization with both rotors.combined synchronization with both rotors.  This condition can accelerate such that the tail rotor drive shaft suffersThis condition can accelerate such that the tail rotor drive shaft suffers excessive stress leading to tail rotor drive shaft failure.excessive stress leading to tail rotor drive shaft failure.  The preventative action is to not allow the main rotor rpm to stay in the rangeThe preventative action is to not allow the main rotor rpm to stay in the range of 60% - 70%, hence the corresponding yellow band on the tachometer.of 60% - 70%, hence the corresponding yellow band on the tachometer.
  • 39.  Aircraft VibrationsAircraft Vibrations  The relative frequency can indicate the source of the vibration.The relative frequency can indicate the source of the vibration. (The following is for the R-22)(The following is for the R-22)  Low FrequencyLow Frequency  Main Rotor (530 RPM)Main Rotor (530 RPM)  Medium FrequencyMedium Frequency  Engine (2652 RPM)Engine (2652 RPM)  High FrequencyHigh Frequency  Tail Rotor (3396 RPM)Tail Rotor (3396 RPM) VibrationsVibrations
  • 40. ConclusionConclusion  This ends this lesson in which you have been introducedThis ends this lesson in which you have been introduced to basic aerodynamics and principles of Helicopter flight.to basic aerodynamics and principles of Helicopter flight. ObjectivesObjectives  Having completed this lesson you should know theHaving completed this lesson you should know the specifics of:specifics of:  The Four ForcesThe Four Forces  TorqueTorque  AirfoilsAirfoils  Rotor SystemsRotor Systems  Lift and DragLift and Drag  VibrationsVibrations  The Three AxesThe Three Axes