Airfoil terminology

1. Airfoil TerminologyAirfoil Terminology
SpanSpan
Chord LineChord Line
Mean Chamber LineMean Chamber Line
Upper ChamberUpper Chamber
Lower ChamberLower Chamber
Leading EdgeLeading Edge
Trailing EdgeTrailing Edge
Center of PressureCenter of Pressure

2. Types of AirfoilsTypes of Airfoils
SymmetricalSymmetrical
•Equal chamber on each sideEqual chamber on each side
•Each half mirror image of otherEach half mirror image of other
•Mean chamber line and chord line are coincidentalMean chamber line and chord line are coincidental
•Produces zero lift at zero angle of attackProduces zero lift at zero angle of attack
•Constant center of pressure with varying angles of Constant center of pressure with varying angles of
attackattack
NonsymmetricalNonsymmetrical
•Greater curvature above the chord line then belowGreater curvature above the chord line then below
•Chord and chamber line are not coincidentalChord and chamber line are not coincidental
•Produces useful lift even at negative angles of attackProduces useful lift even at negative angles of attack
•Produces more lift at a given angle of attack than Produces more lift at a given angle of attack than
symmetricalsymmetrical
•Better stall characteristics than symmetricalBetter stall characteristics than symmetrical
•Good lift to drag ratioGood lift to drag ratio
•Limited to low relative wind velocity, <300 knotsLimited to low relative wind velocity, <300 knots
•Excessive center of pressure travel up to 20% of chord lineExcessive center of pressure travel up to 20% of chord line

3. Airfoil (Rotor Blade) AnglesAirfoil (Rotor Blade) Angles
Angle of IncidenceAngle of Incidence
(pitch angle)(pitch angle)
Chord Line
Chord Line
Tip Path Plane
Tip Path Plane
The mechanical angle between the The mechanical angle between the chord linechord line of the airfoil of the airfoil
and the and the plane of rotation of the rotorplane of rotation of the rotor ( (tip path planetip path plane).).
Changed by collective and cyclic feathering. Any change inChanged by collective and cyclic feathering. Any change in
the angle of incidence changes the angle of attack.the angle of incidence changes the angle of attack.

4. Airfoil (Rotor Blade) AnglesAirfoil (Rotor Blade) Angles
Chord Line
Chord Line
Tip Path Plane
Tip Path Plane
Induced FlowInduced Flow
Angle of AttackAngle of Attack
(aerodynamic angle)(aerodynamic angle)
Resultant RW
Resultant RW
The acute angle formed between the The acute angle formed between the chord linechord line of an airfoil of an airfoil
and the and the resultant relative windresultant relative wind. As an aerodynamic angle the . As an aerodynamic angle the
angle of attack can change with no apparent change inangle of attack can change with no apparent change in
angle of incidence.angle of incidence.

5. StallStall
6° Angle of Attack6° Angle of Attack 12° Angle of Attack12° Angle of Attack
18° Angle of Attack18° Angle of Attack 24° Angle of Attack24° Angle of Attack
CC
L L MaxMax

6. Questions?Questions?

7. Enabling Learning Objective #5Enabling Learning Objective #5
From memory, the student will identify, by writing or From memory, the student will identify, by writing or
selecting from a list, the principles of cyclic and selecting from a list, the principles of cyclic and
collective feathering and the importance of rotary-collective feathering and the importance of rotary-
wing flight, the significance of blade flapping and the wing flight, the significance of blade flapping and the
significance of blade hunting and the forces significance of blade hunting and the forces
involved with hunting IAW FM 1-203involved with hunting IAW FM 1-203

8. Rotational AirflowRotational Airflow
(no forward movement)(no forward movement)
Tip SpeedTip Speed
700 FPS700 FPS
Tip SpeedTip Speed
700 FPS700 FPS
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 blade
and decreases uniformly to the huband decreases uniformly to the hub

9. FeatheringFeathering
Feathering is the rotation of the blade about its Feathering is the rotation of the blade about its
span-wise axisspan-wise axis
•Feathering can be uniform throughout the rotor through Feathering can be uniform throughout the rotor through
collective inputs.collective inputs.
•Feathering can be adjusted differentially through cyclic Feathering can be adjusted differentially through cyclic
manipulationmanipulation
Lets look at some examples of Lets look at some examples of feathering...feathering...

10. Collective FeatheringCollective Feathering
• The changing of the angle of incidence equally and in the The changing of the angle of incidence equally and in the
same direction on all of the rotor blades simultaneouslysame direction on all of the rotor blades simultaneously
• Changes the angle of attack, which changes the Changes the angle of attack, which changes the
coeffiecient of lift, which changes the overall lift of the rotorcoeffiecient of lift, which changes the overall lift of the rotor
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11. Cyclic FeatheringCyclic Feathering
•Fore or aft cyclic movements result in changes in angle of Fore or aft cyclic movements result in changes in angle of
incidence at the 3 and 9 o’clock positions around the rotorincidence at the 3 and 9 o’clock positions around the rotor
•Lateral cyclic movements result in the angle of incidence Lateral cyclic movements result in the angle of incidence
changing at the 12 and 6 o’clock positions around the rotorchanging at the 12 and 6 o’clock positions around the rotor
Differential change in angle of incidence around the rotorDifferential change in angle of incidence around the rotor

12. Forward cyclic inputsForward cyclic inputs
A forward cyclic input increases pitch angle at the 9 o’clock A forward cyclic input increases pitch angle at the 9 o’clock
position, and decreases it at the 3 o’clock position. Due to position, and decreases it at the 3 o’clock position. Due to
phase lag,phase lag, the greatest upflap occurs at the 6 o’clock the greatest upflap occurs at the 6 o’clock
position. Total aerodynamic force inclines forward.position. Total aerodynamic force inclines forward.
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13. Aft cyclic inputsAft cyclic inputs
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An aft cyclic input increases in the pitch of the blade at the An aft cyclic input increases in the pitch of the blade at the
3 o’clock position while decreasing it at the 9 o’clock position. 3 o’clock position while decreasing it at the 9 o’clock position.
Due to phase lag, the highest upflap occurs at the 12 o’clock Due to phase lag, the highest upflap occurs at the 12 o’clock
position. Total aerodynamic force inclines to the rear.position. Total aerodynamic force inclines to the rear.

14. Lateral Cyclic InputsLateral Cyclic Inputs
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Lateral cyclic inputs change the pitch angle at the 12 o’clockLateral cyclic inputs change the pitch angle at the 12 o’clock
and 6 o’clock position. Due to phase lag those changes areand 6 o’clock position. Due to phase lag those changes are
manifested in the rotor system 90 degrees later. The resultingmanifested in the rotor system 90 degrees later. The resulting
rotor attitude change causes the helicopter to move in the rotor attitude change causes the helicopter to move in the
desired directiondesired direction

15. FlappingFlapping
Flapping is the up and down movement of the rotor blades Flapping is the up and down movement of the rotor blades
about a flapping hinge (or flexible hub)about a flapping hinge (or flexible hub)
•Blades flap in response to changes in lift caused by Blades flap in response to changes in lift caused by
changes in velocity of the relative wind across the airfoil, or changes in velocity of the relative wind across the airfoil, or
by cyclic feathering by cyclic feathering
•No flapping occurs when the tip path plane is perpendicular No flapping occurs when the tip path plane is perpendicular
to the mastto the mast
ContributionsContributions
•Helps prevent dyssemmetry of liftHelps prevent dyssemmetry of lift
•Allows the rotor system to tilt in the desired direction in Allows the rotor system to tilt in the desired direction in
response to cyclic inputsresponse to cyclic inputs

16. Lead and LagLead and Lag
Rotor blades in an articulated system lead ahead Rotor blades in an articulated system lead ahead
and lag behind their normal position in the rotor and lag behind their normal position in the rotor
systemsystem
CausesCauses
•Angle of attack changes and drag forcesAngle of attack changes and drag forces
•Coriolis force, or the change in the relative Coriolis force, or the change in the relative
center of gravity along the span of the bladecenter of gravity along the span of the blade

17. R R
11
R R
22
Sequence when blade flaps upSequence when blade flaps up
As the center of gravity moves inboard, a smaller radius of travel isAs the center of gravity moves inboard, a smaller radius of travel is
produced. This causes the advancing blade to speed up or hunt. A vertical produced. This causes the advancing blade to speed up or hunt. A vertical
hinge pin (articulated rotor) allows the blade to sweep forward and hinge pin (articulated rotor) allows the blade to sweep forward and
absorbs stress that would otherwise be transmitted to the blade.absorbs stress that would otherwise be transmitted to the blade.
Blade CGBlade CG