Airfoil TerminologySpan Center of Pressure Upper Chamber Leading EdgeMean Chamber Line Chord Line Lower ChamberTrailing Edge
Types of Airfoils •Equal chamber on each side Symmetrical •Each half mirror image of other •Mean chamber line and chord line are coincidental •Produces zero lift at zero angle of attack •Constant center of pressure with varying angles of attackNonsymmetrical •Greater curvature above the chord line then below •Chord and chamber line are not coincidental •Produces useful lift even at negative angles of attack •Produces more lift at a given angle of attack than symmetrical •Better stall characteristics than symmetrical •Good lift to drag ratio •Limited to low relative wind velocity, <300 knots •Excessive center of pressure travel up to 20% of chord line
Airfoil (Rotor Blade) Angles Angle of Incidence (pitch angle) rd Line C ho Tip Path PlaneThe mechanical angle between the chord line of the airfoiland the plane of rotation of the rotor (tip path plane).Changed by collective and cyclic feathering. Any change inthe angle of incidence changes the angle of attack.
Airfoil (Rotor Blade) Angles Angle of Attack (aerodynamic angle) ine rd L Cho Resultant R W Induced Flow Tip Path PlaneThe acute angle formed between the chord line of an airfoiland the resultant relative wind. As an aerodynamic angle theangle of attack can change with no apparent change inangle of incidence.
6° Angle of Attack 12° Angle of Attack18° Angle of Attack 24° Angle of Attack CL Max Stall
Enabling Learning Objective #5From memory, the student will identify, by writing orselecting from a list, the principles of cyclic andcollective feathering and the importance of rotary-wing flight, the significance of blade flapping and thesignificance of blade hunting and the forcesinvolved with hunting IAW FM 1-203
Rotational Airflow (no forward movement) Tip Speed 700 FPSCircular movement of the rotor blades... ...Produces basic rotational relative wind.Tip Speed Maximum speed is at the tip of the blade 700 FPS and decreases uniformly to the hub
FeatheringFeathering is the rotation of the blade about itsspan-wise axis •Feathering can be uniform throughout the rotor through collective inputs. •Feathering can be adjusted differentially through cyclic manipulation Lets look at some examples of feathering...
Collective Feathering• The changing of the angle of incidence equally and in thesame direction on all of the rotor blades simultaneously• Changes the angle of attack, which changes thecoeffiecient of lift, which changes the overall lift of the rotor + + + +
Cyclic Feathering Differential change in angle of incidence around the rotor•Fore or aft cyclic movements result in changes in angle ofincidence at the 3 and 9 o’clock positions around the rotor•Lateral cyclic movements result in the angle of incidencechanging at the 12 and 6 o’clock positions around the rotor
Forward cyclic inputs + -A forward cyclic input increases pitch angle at the 9 o’clockposition, and decreases it at the 3 o’clock position. Due tophase lag, the greatest upflap occurs at the 6 o’clockposition. Total aerodynamic force inclines forward.
Aft cyclic inputs - +An aft cyclic input increases in the pitch of the blade at the3 o’clock position while decreasing it at the 9 o’clock position.Due to phase lag, the highest upflap occurs at the 12 o’clockposition. Total aerodynamic force inclines to the rear.
Lateral Cyclic Inputs - +Lateral cyclic inputs change the pitch angle at the 12 o’clockand 6 o’clock position. Due to phase lag those changes aremanifested in the rotor system 90 degrees later. The resultingrotor attitude change causes the helicopter to move in thedesired direction
FlappingFlapping is the up and down movement of the rotor bladesabout a flapping hinge (or flexible hub) •Blades flap in response to changes in lift caused by changes in velocity of the relative wind across the airfoil, or by cyclic feathering •No flapping occurs when the tip path plane is perpendicular to the mast Contributions•Helps prevent dyssemmetry of lift•Allows the rotor system to tilt in the desired direction inresponse to cyclic inputs
Lead and LagRotor blades in an articulated system lead aheadand lag behind their normal position in the rotorsystem Causes •Angle of attack changes and drag forces •Coriolis force, or the change in the relative center of gravity along the span of the blade
Sequence when blade flaps up Blade CG R2 R1As the center of gravity moves inboard, a smaller radius of travel isproduced. This causes the advancing blade to speed up or hunt. A verticalhinge pin (articulated rotor) allows the blade to sweep forward andabsorbs stress that would otherwise be transmitted to the blade.