Helicopter Aerodynamics and Flight Principles Explained

Lesson 2Lesson 2
Basic Aerodynamics andBasic Aerodynamics and
Principles of Helicopter FlightPrinciples of Helicopter Flight

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

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

 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

 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

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.

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.

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

 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
……which results in a lifting force.which results in a lifting force.
Lift

 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

 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.

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.
SS
Air DensityAir Density

 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.

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.
SS

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)
SS

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)
SS
VelocityVelocityCoefficient of LiftCoefficient of Lift
 Velocity - (blade speed)Velocity - (blade speed)
 Rotor RPM (via Throttle)Rotor RPM (via Throttle)

 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
 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.

 3 Types of Drag3 Types of Drag
 Any body not producing liftAny body not producing lift
generates Parasite Drag.generates Parasite Drag.
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.

Parasite Drag
 Induced DragInduced Drag
Profile Drag
Form Drag
Skin Friction
 Induced DragInduced Drag is a by-product of theis a by-product of the
creation of lift.creation of lift.
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.

Parasite Drag
• Induced DragInduced Drag
Profile Drag
Form Drag
Skin Friction
0 25 50 75 100
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.

 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.

 Profile DragProfile Drag is the sum ofis the sum of
Form Drag and Skin FrictionForm Drag and Skin Friction
Parasite Drag
Induced Drag
 Profile DragProfile Drag
Form DragForm Drag
Skin FrictionSkin Friction
0 25 50 75 100
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.

Total Drag is the sum of the three types of drag.Total Drag is the sum of the three types of drag.
 Total DragTotal Drag
0 25 50 75 100
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).

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

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.

 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

FlappingFlapping FeatheringFeathering Lead / LagLead / Lag

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

FlappingFlapping
Drag Hinge
Feather Bearing
Flapping Hinge
 FeatherFeather
 FlapFlap
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…

Lead / LagLead / Lag
Drag Hinge
Feather Bearing
Flapping Hinge
 FeatherFeather
 FlapFlap

 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

TeeteringTeetering (flapping)(flapping)
Teetering Hinge
 FeatherFeather
 FlapFlap
 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.

 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.
 FeatherFeather
FlapFlap

 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

 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.

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.

 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

 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.
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.

 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)

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

Helicopter Aerodynamics and Flight Principles Explained

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