Arizona Army National Guard

1
AERODYNAMICS

• Arizona Army National Guard
• Aviation Support Facility 1

2
REFERENCES

• FM 1-203, Fundamentals of flight
• TC 1-212, Aircrew Training Manual

3
Learning Objectives

• Applied and simplified understanding of
  helicopter aerodynamic characteristics
• Correlate relationships between these
  characteristics

4
Rotary Wing Aerodynamic Subject Areas

• Aerodynamic Factors
• Relative Wind
• Induced Flow Production
• Resultant Relative Wind
• Angle of Attack / Angle of Incidence
• Total Aerodynamic Force
• Lift
• Drag
• Airflow During a Hover

5
Rotary Wing Aerodynamics Subject Areas (Cont)

• Translating Tendency
• Mechanical and Pilot Inputs
• Dissymmetry of Lift
• Blade Flapping
• Blade Lead and Lag
• Cyclic Feathering

6
Rotary Wing Aerodynamic Subject Areas (Cont)

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

7
Aerodynamic Factors
8
Relative Wind

• Relative wind is defined as the airflow
  relative to an airfoil
• Relative wind is created by movement of an
  airfoil through the air

9
Induced Flow Production

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

10
Resultant Relative Wind

• Airflow from rotation, modified by induced flow,
  produces the Resultant Relative Wind

• Angle of attack is reduced by induced flow,
  causing the airfoil to produce less lift

11
Angle of Attack

• Angle of Attack (AOA) (4) is the angle between
  the airfoil chord line and its direction of
  motion relative to the air (the Resultant
  Relative Wind)

12
Angle of Incidence

• Angle of Incidence (or AOI) is the angle
  between the blade chord line and the plane of
  rotation of the rotor system.

13
Total Aerodynamic Force

• A Total Aerodynamic Force (3) is generated when
  a stream of air flows over and under an airfoil
  that is moving through the air

14
Total Aerodynamic Force

• Total aerodynamic force may be divided into two
  components called lift and drag
• Lift acts on the airfoil in a direction
  perpendicular to the relative wind
• Drag acts on the airfoil in a direction parallel
  to the relative wind and is the force that
  opposes the motion of the airfoil through the air

15
Airflow During a Hover
16
Airflow at a Hover (IGE)

• Lift needed to sustain an IGE Hover can be
  produced with a reduced angle of attack and less
  power because of the more vertical lift vector
• This is due to the ground interrupting the
  airflow under the helicopter thereby reducing
  downward velocity of the induced flow

17
Airflow at a Hover (OGE)

• Downward airflow alters the relative wind and
  changes the angle of attack so less aerodynamic
  force is produced
• Increase collective pitch is required to
  produce enough aerodynamic force to sustain an
  OGE Hover

18
Rotor Tip Vortexes (IGE/OGE)
19
Rotor Tip Vortexes Effects

• At a hover, the Rotor Tip Vortex reduces the
  effectiveness of the outer blade portions
• When operating at an IGE Hover, the downward
  and outward airflow pattern tends to restrict
  vortex generation
• Rotor efficiency is increased by ground effect
  up to a height of about one rotor diameter for
  most helicopters

20
Translating Tendency
21
Translating Tendency

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

22
Dissymmetry of Lift
23
Dissymmetry of Lift

• Definition
• Compensation
• Blade Flapping
• Cyclic Feathering
• Blade Lead and Lag

24
Dissymmetry of Lift Definition
Dissymmetry of Lift is the difference in lift
that exists between the advancing half of the
rotor disk and the retreating half
25
Blade Flapping

• Blade Flapping is the up and down movement of a
  rotor blade, which, in conjunction with cyclic
  feathering, causes Dissymmetry of Lift to be
  eliminated.

26
Blade Flapping
27
Cyclic Feathering

• These changes in blade pitch are introduced
  either through the blade feathering mechanism or
  blade flapping.
• When made with the blade feathering mechanism,
  the changes are called Cyclic Feathering.

28
Blade Lead and Lag

• Blade Lead / Lag Each rotor blade is attached
  to the hub by a vertical hinge (3) that permits
  each blade, independently of the others, to move
  back and forth in the rotational plane of the
  rotor disk thereby introducing cyclic feathering.

29
Retreating Blade Stall
30
Retreating Blade Stall

• A tendency for the retreating blade to stall in
  forward flight is inherent in all present day
  helicopters and is a major factor in limiting
  their forward speed

31
Retreating Blade StallLift at a Hover
32
Retreating Blade Stall Lift at Cruise
33
Retreating Blade Stall Lift at Stall Airspeed
34
Retreating Blade StallCauses

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

35
Retreating Blade StallIndications

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

36
Retreating Blade StallCorrective Actions

• When the pilot suspects blade stall, he can
  possibly prevent it from occurring by
  sequentially
• Reducing Power (collective pitch)
• Reducing Airspeed
• Reducing "G" Loads During Maneuvering
• Increasing Rotor RPM to Max Allowable Limit
• Checking Pedal Trim

37
Compressibility
38
Compressibility
39
CompressibilityWhat Happens?

• Rotor blades moving through the air below
  approximately Mach 0.7 cause the air in front of
  the blade to move away before compression can
  take place.
• Above speeds of approximately Mach 0.7 the air
  flowing over the blade accelerates above the
  speed of sound, causing a shock wave (also known
  as a sonic boom) as the blade compresses air
  molecules faster than they can move away from the
  blade.
• The danger of this shock wave (Compressibility)
  is its effect on aircraft control and fragile
  rotor blade membranes.

40
CompressibilityCauses

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

41
CompressibilityIndications

• As Compressibility approaches
• Power Required Increase as Lift Decreases and
  Drag Increases
• Vibrations Become More Severe
• Shock Wave Forms (Sonic Boom)
• Nose Pitches Down

42
Compressibility Corrective Actions

• When the pilot suspects Compressibility, he can
  possibly prevent it from occurring by
• Slowing Down the Aircraft
• Decreasing Pitch Angle (Reduce Collective)
• Minimizing G Loading
• Decreasing Rotor RPM

43
Settling With Power
44
Settling with Power

• Settling With Power is a condition of powered
  flight where the helicopter settles into its own
  downwash.
• It is also known as Vortex Ring State

45
Settling with PowerCause

• Increase in induced flow results in reduction of
  angle of attack and increase in drag
• This creates a demand for excessive power and
  creates greater sink rate
• Where the demand for power meets power available
  the aircraft will no longer sustain flight and
  will descend

46
Settling With PowerConditions

• Conditions required for Settling with power are
• 300-1000 FPM Rate of Descent
• Power Applied (gt than 20 Available Power)
• Near Zero Airspeed (Loss of ETL)
• Can occur during
• Downwind Approaches.
• Formation Approaches and Takeoffs.
• Steep Approaches.
• NOE Flight.
• Mask/Unmask Operations.
• Hover OGE.

47
Settling With PowerIndications

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

48
Settling With PowerCorrective Actions

• When Settling with Power is suspected
• Establish directional flight.
• Lower collective pitch.
• Increase RPM if decayed.
• Apply right pedal.

49
Off Set Hinges
50
Off Set Hinges

• The Offset Hinge is located outboard from the
  hub and uses centrifugal force to produce
  substantial forces that act on the hub itself.
• One important advantage of offset hinges is the
  presence of control regardless of lift condition,
  since centrifugal force is independent of lift.

51
Dynamic Rollover
52
Dynamic Rollover

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

53
Dynamic Rollover

• The critical rollover angle is further reduced
  under the following conditions
• Right Side Skid Down Condition
• Crosswinds
• Lateral Center Of Gravity (CG) Offset
• Main Rotor Thrust Almost Equal to Weight
• Left Yaw Inputs

54
Dynamic Rollover

• Pilot Technique
• When landing or taking off, with thrust (lift)
  approximately equal to the weight (light on the
  skids or wheels), the pilot should keep the
  helicopter cyclic trimmed (force trim/gradient)
  and prevent excessive helicopter pitch and roll
  movement rates. The pilot should fly the
  helicopter smoothly off (or onto) the ground,
  vertically, carefully maintaining proper cyclic
  trim.

55
Summary

• Websites containing additional and more detailed
  information on Helicopter Aerodynamics
• http//www.dynamicflight.com/aerodynamics/
• http//www.copters.com/helo_aero.html
• http//www.helicopterpage.com/html/forces.html

Websites checked as of 9 JUN 05
56
QUIZ

• Click on the link below to access the
• Aerodynamics Quiz
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