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Helicopter Flying Handbook > Chapter 2: Aerodynamics of Flight > Flashcards

Chapter 2: Aerodynamics of Flight Flashcards

1
Q

What are the 4 forces acting on a helicopter?

A

Lift

Thrust

Weight

Drag

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2
Q

Thrust

A
  • Forward force produced by rotor.
  • Opposes/overcomes drag force
  • Generally acts parallel to longitudinal axis
  • generated by rotation of main rotor system
  • Types: forward, reward, sideward or vertical
  • Combined Lift and thrust determine direction of movement for helicopter
  • Tail rotor also produces variable thrust and is used to control the Yaw
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3
Q

Drag

A
  • Force resisting movement through air.
  • Produced when lift is developed
  • Overcome by engine powering transmission to turn the rotor
  • Always acts parallel to relative wind
    • Direction of movement fluid relative to airfoil/aircraft. Opposite direction of movement of airfoil/aircraft.
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4
Q

Weight

A
  • rotor system must generate enough lift to overcome
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5
Q

Fixed Weight Influences

A
  • fuel, weight of helicopter, occupants, cargo, etc
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6
Q

Variable Weight Influences

A
  • Aerodynamic loads
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7
Q

Aerodynamic Loads

A
  • banking while maintaining constant altitude causes load factor(G load) to increase
  • Aerodynamic forces effect every movement in helicopter=never push limits of your machine
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8
Q

Load Factor(G Load)

A
  • actual load on the rotor blades at any time divided by normal load(aka gross weight of helicopter)
  • To overcome increase in G load, more lift must be produced. If extra engine power isn’t available, helicopter either descends or must decelerate to maintain same altitude. Over 30 degree bank G load soars
  • Turbulent Air also causes large G load:
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9
Q

Netwon’s 1st Law

A
  • : Every object in uniform motion remains in that state of motion unless external force(lift) is applied
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10
Q

Lift

A
  • Generated when an object changes the direction of flow of a fluid
  • When the object and fluid move relative to each other and the object deflects the fluid flow in a direction perpendicular to that flow, the force required to do this work creates and equal and opposite force(lift)
  • The flow meeting the leading edge of the airfoil is forced to split over/under the object
    • Fluid flow is accelerated above airfoil: Creates area of low pressure to form behind leading edge of the upper surface of airfoil
    • Fluid flow is slowed/stagnated below airfoil: Creates area of low pressure

Bottom Line: as blade spins, it forces air over it’s curved surface then throws it down behind it toward the ground, producing lift(majority of lift).

    * Both fluid flows leave trailing edge of airfoil with a downward momentum(lift)
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11
Q

Lift by Airfoil Depends on what?

A
  • Speed of airflow
  • Density of air
  • Total area of the segment or airfoil
  • Angle of attack(AoA) between air and airfoil
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12
Q

Angle of Attack

A
  • Angle at which airfoil(rotor blade) meets oncoming airflow(fluid) and vice versa.
  • Positive AOA: Symmetrical airfoil must have to generate positive lift
  • Zero AOA=no lift
  • Negative AOA=Negative Lift
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13
Q

Bernouli’s Principle

A
  • As the speed(velocity) of a moving fluid increases, the pressure within the fluid decreases
    • Fluid speeds up in direct proportion to reduction in area(venturi effect
  • Ventruri Effect: reduction in fluid pressure that results when a fluid flows through a constricted section of pipe
  • Conservation of Energy: energy cannot be created/destroyed and amount of energy entering system must also exit
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14
Q

Additional Lift provided by?

A
  • Air striking underside of rotor blade deflected downward(newton’s third law)
    • impact pressure and the deflection of air from the lower surface of the rotor blade provides a comparatively small percentage of the total lift
    • majority of lift is the result of decreased pressure above the blade, rather than the increased pressure below it.
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15
Q

What comprises Total Drag?

A
  • Profile Drag
  • Induced Drag
  • Parasitic Drag
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16
Q

Profile Drag

A
  • Frictional Resistance of blades passing through the air.
  • Moderate increase with airspeed increase
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17
Q

What comprises Profile Drag?

A
  • Form Drag
  • Skin Drag
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18
Q

Form Drag

A
  • resultant of turbulent wake causes by separation of airflow from surface of the structure
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19
Q

Skin Drag

A
  • surface roughness
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20
Q

Induced Drag

A
  • Component of lift that is acting in reward direction. airflow circulation around rotor blade as it creates lift
  • Vortex produced by H pres. Area beneath blade joining L pres. Area above at trailing edge And rotor tips
  • Vortices deflect airstream downward creating increase in downwash
  • Decreases as airspeed increases and increases as airspeed decreases
  • Main case of drag at low airspeeds
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21
Q

Downwash

A
  • Change in direction of air deflected by aerodynamic action of airfoil while producing lift.
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22
Q

Parasite Drag

A
  • Always present when helicopter moves through air
  • Cabin, rotor mast, tail, landing gear(i.e non-lifting components)
  • Rapid Increases with increasing airspeed
  • Main cause of drag at high airspeeds
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23
Q

Total Drag

A
  • Sum of Profile, Induced and Parasitic Drag
  • Airspeed increase=Parasite drag increase, induced drag decrease, profile drag remains mostly constant with some increase at higher speeds.
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24
Q

What are the Uses of an Airfoil

A
  • Lift: aerodynamic forces produced when air passes around airfoil
  • Stability: Fin
  • Control: Elevator
  • Thrust: propeller or Rotor
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25
Q

What can be an Airfoil on a Helicopter?

A
  • Main Rotor
  • Tail Rotor
  • Vertical/Horizontal Stabilizers
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26
Q

Blade Span

A
  • the length of the rotor blade from center of rotation to tip of the blade
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27
Q

Chord line

A
  • a straight line intersecting leading and trailing edges of the airfoi
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28
Q

Chord

A
  • the length of the chord line from leading edge to trailing edge; it is the characteristic longitudinal dimension of the airfoil section
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29
Q

Mean camber line

A
  • a line drawn halfway between the upper and lower surfaces of the airfoil
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30
Q

Camber

A
  • : curvature of the airfoil
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31
Q

Leading Edge

A
  • front edge of airfoil
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32
Q

Trailing Edge

A
  • rearmost edge of airfoil
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33
Q

Induced Flow

A
  • downward flow of air through rotor disc
  • Aka downwash
  • Each blade has a decreased AOA due to downwash b/c rotor blade action(pitch change) changes still air to column of descending air.
  • Most pronounced during Hover w/ no –wind conditions
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34
Q

Relative wind

A
  • Direction of movement fluid relative to airfoil/aircraft.
  • Opposite direction of movement of airfoil/aircraft(may not be exact opposite)
  • Moves in a parallel but opposite direction to movement of airfoil
  • Rotational for rotary wing.
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35
Q

Rotational Relative Wind

A
  • rotation of rotor blades as they turn about the mast produces rotational relative wind
  • Aka Tip-Path Plane
  • Flows opposite physical flightpath of airfoil.
  • Highest at blade tips decreasing to zero at axis of rotation(center of mast)
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36
Q

Resultant relative wind

A
  • : relative wind modified by induced flow
  • Opposite effective flightpath of airfoil rather than physical flightpath(rotational relative wind)
  • Serves as ref. plane for dev. Of lift, drag and TAF(total aerodynamic force vectors on airfoil)
37
Q

Angle of attack(AOA)

A
  • angle measured between resultant relative wind and chord line
  • Hard to measure b/c it’s a ‘aerodynamic angle’
  • Can change with no change in blade pitch angle(AOI)
  • Increase in AOA=air flowing over airfoil is diverted over greater distance, results in increase air velocity(more lift)
    • Further increase becomes more difficult for air to flow smoothly over top of airfoil(approaches a turbulent pattern)= large increase in Drag/Loss of Lift
  • Pilot has little direct control over AOA. Adjust AOA thru normal control manipulation of pitch angle of blades.
    • Pitch angle(AOI) increases, AOA increases
  • Indirectly changed by Colletive/Cyclic inputs on AOI
  • Majority of changes come from change in airspeed and rate of climb/descent
38
Q

Angle of Incidence(AOI)

A
  • angle between chord line of main/tail rotor blade and rotor hub
  • AKA Blade pitch angle
  • Collective input + cyclic feathering change AOI
  • Change in AOI changes AOA which therefore changes lift produced by airfoil
  • Zero induced flow means AOA=AOI
  • When induced flow, upflow, or airspeed changes relative wind, AOA different AOI
  • Easy to measure b/c it’s a ‘mechanical angle’
39
Q

Center of Pressure

A
  • : point along chord line through which all aerodynamic forces are considered to act.
  • Pressure variances caused by AOA changes cause center to move along chord line.
40
Q

FlightPath Velocity

A
  • speed + direction of airfoil passing thru air.
  • Rotational velocity +/- a component of directional airspeed
41
Q

Hub

A
  • center point on mast and attaching point for root of blade
42
Q

Root

A
  • inner end of blade that attaches to hub
43
Q

Tip

A
  • farthest outboard section of blade
44
Q

Twist

A
  • change in blade incidence from root to outer blade
45
Q

In Ground Effect(IGE)

A
  • increased efficiency or rotor system by interference of airflow when near ground
  • Air pressure is increased acting to decrease downward velocity of air
  • Permits relative wind to be more horizontal, lift vector to be more vertical and reduced induced drag
  • Smooth hard surfaces=most efficiency produces
  • Overall reduces power required to hover IGE
46
Q

Out of Ground effect(OGE)

A
  • efficiency is lost above IGE altitude
  • Induced flow velocity is increased=decrease in AOA and decrease in lift
  • Downward flow can become so localized that helicopter and locally disturbed air will sink at high rate
  • High blade pitch angle required to maintain same AOA in IGE hover but this also creates more drag and therefore requires more power to hover.
47
Q

What are the two types of Airfoils?

A
  • Symmetical
  • Non Symmetrical
48
Q

Symmetrical Airfoil

A
  • : identical upper/lower surfaces
  • Produces no lift at zero AOA
49
Q

Non Symmetrical Airfoil

A
  • different upper/lower surfaces
  • AKA Cambered
  • Greater curvature above chord line than below
  • Produces lift at zero AoA
  • Advantage: more lift at given AOA then symmetrical, improved lift-to-drag ratio, better stall characteristics
  • Disadvantage: Center of Pressure can travel up to 20% of chord line=creates undesirable torque on airfoil structure, and greater production costs
50
Q

Blade Twist

A
  • Because of lift differential due to differing rotational relative wind values along the blade, the blade should be designed with a twist to alleviate internal blade stress and distribute the lifting force more evenly along the blade
  • . Blade twist provides higher pitch angles at the root where velocity is low and lower pitch angles nearer the tip where velocity is higher. This increases the induced air velocity and blade loading near the inboard section of the blade.
51
Q

Rotor Blade Angles

A
  • two angles that enable a rotor system to produce lift
52
Q

Critical Angle of Attack

A
  • increases in AOA under CAA will increase lift. Increases in AOA past CAA will produce a stall/rapid decrease in lift.
53
Q

Powered Flight Can Consist of what?

A
  • Hovering
  • Vertical
  • Forward
  • Sideward
  • Reward
54
Q

Powered Flight

A
  • Total Lift and thrust forces of a rotor are perpendicular to the tip-path plane(rotational relative wind) or plane of rotation of the rotor
55
Q

Hovering Flight

A
  • Constant control inputs + corrections required
  • Cyclic used to eliminate drift in horizontal plane
  • Throttle(if not governor controlled) used to control RPM’s
  • Collective used to maintain altitude
  • Pedals used to control nose direction(heading)
  • Lift must equal weight and thrust must equal any wind + tail rotor thrust to maintain position.
  • Power must also be sufficient to turn rotors and overcome various drags(mostly induced drag)/frictions involved
56
Q

Translating Tendency(Drift)

A
  • during hovering, single main rotor heli tends to move laterally/sideward in direction of tail rotor thrust
57
Q

How do you counter Translating Tendency(drift)?

A
  • Mount main tranny at slight angle to left so rotor mast has built-in tilt to oppose tail rotor thrust
  • Rig flight controls so rotor disc is tilted slightly to left when cyclic is centered
58
Q

Pendular Action

A
  • naturally occurring but can be exaggerated by over controlling. Therefore, Control movements should be smooth and not exaggerated.
  • Occurs b/c heli body has mass and is suspended from a single point(rotor mast head)
  • Horizontal Stabilizer: levels airframe in forward flight
59
Q

Horizontal Stabilizer

A
  • levels airframe in forward fight
60
Q

Tail Strike

A
  • in reward flight the horizontal stabilizer can press tail downward
61
Q

Centrifugal force

A
  • rotation of rotor system creates cent. Force(inertia) which tends to pull blades straight outward from main rotor hub
  • Give blades their rigidity and strength to support Heli
62
Q

Coning

A
  • : during takeoff two major forces are acting at same time
  • Centrifugal force acting outward
  • Lift acting upward.
  • Result is blades assume a slight conical path instead of remaining in plane perpendicular to mast.
  • Too low rotor RPM=center. Force becomes smaller and coning angle becomes much larger(i.e. should rpm decrease too much, some point rotor blades could fold up with no change of recovery)
63
Q

Coriolis Effect

A
  • rotating body continue to rotate with same rotational velocity until some external force is applied to change speed of rotation
  • Aka law of conservation of angular momentum
  • Changes in angular velocity occur as mass of rotating body is move closer(angular acceleration) or farther away(angular deceleration) from axis of rotation. Example: Figure skater on ice
  • When rotor blade flaps/cone upward, center of mass of that blade moves closer to axis of rotation and blade acceleration takes place in order to conserve angular momentum
64
Q

Gyroscopic precession

A
  • Gyroscopic precession is the resultant action or deflection of a spinning object when a force is applied to this object.
  • This action occurs approximately 90° in the direction of rotation from the point where the force is applied (or 90° later in the rotation cycle).
65
Q

Vertical Flight

A
  • Increasing the angle of incidence of the rotor blades (pitch) while keeping their rotation speed constant generates additional lift and the helicopter ascends.
  • Decreasing the pitch causes the helicopter to descend.
66
Q

Forward Flight

A
  • 4 forces(lift, drag, thrust, weight) must be in balance
  • Once the tip-path plane is tilted forward, the total lift-thrust force is also tilted forward. This resultant lift-thrust force can be resolved into two components—lift acting vertically upward and thrust acting horizontally in the direction of flight.
  • In order to maintain unaccelerated flight, the pilot must understand that with any changes in power or in cyclic movement, the helicopter begins either to climb or to descend
  • Once straight-and-level flight is obtained, the pilot should make note of the power (torque setting) required and not make major adjustments to the flight controls.
67
Q

Straight and Level Flighta

A
  • : flight w/ constant heading and altitude
68
Q

Forward Flight Airflow

A
  • Differs from airflow in hovered flight
  • Fluid flows opposite aircraft’s flightpath
  • Fluid velocity=Heli forward speed
  • The highest velocity of airflow occurs over the right side (3 o’clock position) of the helicopter (advancing blade in a rotor system that turns counterclockwise)
  • decreases to rotational velocity over the nose.
  • It continues to decrease until the lowest velocity of airflow occurs over the left side (9 o’clock position) of the helicopter (retreating blade).
69
Q

Advancing blade

A
  • rotor blade moving in same direction as aircraft
  • In forward flight, advancing blade has higher airspeed than retreating blade, creating unequal lift across rotor disc(produces more lift)
  • As the relative wind speed of the advancing blade increases, the blade gains lift and begins to flap up. It reaches its maximum upflap velocity at the 3 o’clock position, where the wind velocity is the greatest. This upflap creates a downward flow of air and has the same effect as increasing the induced flow velocity by imposing a downward vertical velocity vector to the relative wind which decreases the AOA.
70
Q

Retreating Blade

A
  • rotor blade moving in opposite direction as aircraft
  • As relative wind speed of the retreating blade decreases, the blade loses lift and begins to flap down. It reaches its maximum downflap velocity at the 9 o’clock position, where wind velocity is the least. This downflap creates an upward flow of air and has the same effect as decreasing the induced flow velocity by imposing an upward velocity vertical vector to the relative wind which increases the AOA.
71
Q

Dissymmetry of Lift

A
  • Unequal(differential) lift between advancing and retreating halves of rotor dick causes by different wind flow velocity across each half
  • Blades flap and feather automatically as a unit(when on flaps up the other flaps down) to equalize lift across rotor disc via teetering hinge
72
Q

Important Reading

A

as the rotor blade reaches the advancing side of the rotor disk (A), it reaches its maximum up flap velocity. When the blade flaps upward, the angle between the chord line and the resultant relative wind decreases. This decreases the AOA, which reduces the amount of lift produced by the blade. At position (C), the rotor blade is now at its maximum down flapping velocity. Due to down flapping, the angle between the chord line and the resultant relative wind increases. This increases the AOA and thus the amount of lift produced by the blade.

73
Q

Important Reading Cont

A
  • The combination of blade flapping and slow relative wind acting on the retreating blade normally limits the maximum forward speed of a helicopter. At a high forward speed, the retreating blade stalls because of a high AOA and slow relative wind speed. This situation is called retreating blade stall and is evidenced by a nose pitch up, vibration, and a rolling tendency—usually to the left in helicopters with counterclockwise blade rotation.
74
Q

Important Reading Cont

A
  • Pilots can avoid retreating blade stall by not exceeding the never-exceed speed.
  • Blade flapping compensates for dissymmetry of lift in the following way. At a hover, equal lift is produced around the rotor system with equal pitch (AOI) on all the blades and at all points in the rotor system (disregarding compensation for translating tendency). The rotor disk is parallel to the horizon. To develop a thrust force, the rotor system must be tilted in the desired direction of movement. Cyclic feathering changes the angle of incidence differentially around the rotor system. Forward cyclic movements decrease the angle of incidence at one part on the rotor system while increasing the angle in another part.
75
Q

Important Reading Cont

A
  • When transitioning to forward flight either from a hover or taking off from the ground, pilots must be aware that as the helicopter speed increases, translational lift becomes more effective and causes the nose to rise, or pitch up (sometimes referred to as blowback). This tendency is caused by the combined effects of dissymmetry of lift and transverse flow. Pilots must correct for this tendency to maintain a constant rotor disk attitude that will move the helicopter through the speed range in which blowback occurs. If the nose is permitted to pitch up while passing through this speed range, the aircraft may also tend to roll to the right. To correct for this tendency, the pilot must continuously move the cyclic forward as velocity of the helicopter increases until the takeoff is complete and the helicopter has transitioned into forward flight.
76
Q

Important Reading Cont

A
  • Figure 2-36 illustrates the changes in pitch angle as the cyclic is moved forward at increased airspeeds. At a hover, the cyclic is centered and the pitch angle on the advancing and retreating blades is the same. At low forward speeds, moving the cyclic forward reduces pitch angle on the advancing blade and increases pitch angle on the retreating blade. This causes a slight rotor tilt. At higher forward speeds, the pilot must continue to move the cyclic forward. This further reduces pitch angle on the advancing blade and further increases pitch angle on the retreating blade. As a result, there is even more tilt to the rotor than at lower speeds.
77
Q

Important Reading Cont

A
  • A horizontal lift component (thrust) generates higher helicopter airspeed. The higher airspeed induces blade flapping to maintain symmetry of lift. The combination of flapping and cyclic feathering maintains symmetry of lift and desired attitude on the rotor system and helicopter.
78
Q

Translational Lift

A
  • Improved rotor(main and tail) efficiency from directional flight
  • Efficiency greatly improved with each knot of incoming wind due to vortices left behind as incoming wind enters rotor system producing air flow that becomes more horizontal
79
Q

Effective Translational Lift(ETL)

A
  • During transition from hover to forward @ 16-24 knots
  • Rotor system completely outruns the recirculation of old vortices and begins to work in relatively undisturbed air
  • The flow of air through the rotor system is more horizontal; therefore, induced flow and induced drag are reduced.
  • The AOA is effectively increased, which makes the rotor system operate more efficiently.
  • This increased efficiency continues with increased airspeed until the best climb airspeed is reached, and total drag is at its lowest point.
  • Once the helicopter is transitioning through ETL, the pilot needs to apply forward and left lateral cyclic input to maintain a constant rotor-disk attitude.
80
Q

Translational Thrust

A
  • Occurs when tail rotor becomes more efficient due to ETL (16-24 knots)
  • As the tail rotor works in progressively less turbulent air, this improved efficiency produces more antitorque thrust, causing the nose of the aircraft to yaw left
    • Countered with right pedal(decreasing AOA in tail rotor blade)
  • as the helicopter achieves ETL, you must reduce tail rotor thrust by pedal input at about the same time that you need to make cyclic adjustments for lateral tracking, acceleration, and climb.
81
Q

Induced Flow

A
  • As the rotor blades rotate, they generate what is called rotational relative wind. This airflow is characterized as flowing parallel and opposite the rotor’s plane of rotation and striking perpendicular to the rotor blade’s leading edge. This rotational relative wind is used to generate lift
  • . As rotor blades produce lift, air is accelerated over the foil and projected downward. Anytime a helicopter is producing lift, it moves large masses of air vertically and down through the rotor system
  • This downwash or induced flow can significantly change the efficiency of the rotor system. Rotational relative wind combines with induced flow to form the resultant relative wind. As induced flow increases, resultant relative wind becomes less horizontal. Since AOA is determined by measuring the difference between the chord line and the resultant relative wind, as the resultant relative wind becomes less horizontal, AOA decreases.
82
Q

Transverse Flow Effect

A
  • Difference in lift between fore and aft portions of rotor disc caused when induced flow drops to near zero at forward disc area/increases at aft disc area as heli accelerates in forward flight.
  • Recognized by increased vibrations of Heli at airspeeds just below ETL on takeoff/landing.
  • Cyclic input to left usually counters this.
83
Q

Sideward Flight

A
  • Rotational Relative Wind(aka tip path plane) tilted in direction that flight is desired
  • Horizontal(thrust) component now acts sideward with drag acting to opposite side
  • Vertical(lift) still acts upward and weight downward
  • Very unstable due to Parasitic drag of fuselage + lack of horizontal stabilizer
84
Q

Reward Flight

A
  • Rotational Relative Wind(aka tip path plane) tilted in reward
  • Drag acts forward/Thrust reward
  • Lift straight up and weight straight down
  • Due to position of horizontal stabilizer, tail of heli tends to pitch downward
85
Q

Turning Flight

A
  • In forward flight, the rotor disk is tilted forward, which also tilts the total lift-thrust force of the rotor disk forward. When the helicopter is banked, the rotor disk is tilted sideward resulting in lift being separated into two components. Lift acting upward and opposing weight is called the vertical component of lift. Lift acting horizontally and opposing inertia (centrifugal force) is the horizontal component of lift (centripetal force). [Figure 2-44]
  • As the angle of bank increases, the total lift force is tilted more toward the horizontal, thus causing the rate of turn to increase because more lift is acting horizontally. Since the resultant lifting force acts more horizontally, the effect of lift acting vertically is decreased. To compensate for this decreased vertical lift, the AOA of the rotor blades must be increased in order to maintain altitude. The steeper the angle of bank is, the greater the AOA of the rotor blades required to maintain altitude.
  • Thus, with an increase in bank and a greater AOA, the resultant lifting force increases and the rate of turn is higher. Simply put, collective pitch must be increased in order to maintain altitude and airspeed while turning. Collective pitch controls the angle of incidence and along with other factors, determines the overall AOA in the rotor system.
86
Q

Autorotation

A
  • Main rotor system being turned by air moving through rotor instead of engine power
  • In normal, powered flight, air is drawn into the main rotor system from above and exhausted downward, but during autorotation, air moves up into the rotor system from below as the helicopter descends
87
Q

Freewheeling unit

A
  • separate clutch mechanical component that permits autorotations
  • If the engine fails, the freewheeling unit automatically disengages the engine from the main rotor allowing the main rotor to rotate freely
88
Q
A

Helicopter Flying Handbook (4 decks)

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