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Chapter 5
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Chapter 5

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  • Normal stress on a wing plusCentrifugal force coupled with motionInduces vibrationGoal – to build most reliable, maintenance free, best performing Advantages & disadvantages Final result is a compromiseMost are of two basic types Semi rigid fully articulated
  • Semi rigid2 bladed underslung flapping axis, with seesaw action
  • Fully articulatedCompensates for dissymmetry of liftIndependent flapping of the bladesIndependent lead-lagRequires damper to prevent geometric imbalancePitch change
  • Heads can be classified by lubrication system usedGreaseOilNoneUsually steel or aluminiumTime-life limitedFibreglass head that does not currently have a lifeBell 47 – Early designParts change from design to design – nomenclature remains the sameBell 47 – Steel yoke Counter weight eases load in feathering axis
  • Bell 206Steel yoke with 2.5 degree preconeHollow blade bolts for balancingNo drag bracesFlap restraint to limit flapping motion at low RPM
  • Strap PackIt’s the reason for the heads light weight and simplicityReacts to centrifugal load Replaces flapping and feathering hinge15 straps, .009” thick, .004” Teflon impregnated cloth between each strap.The Teflon eliminates fretting corrosion
  • Mechanical damper attached at the inboard trailing edge of each bladeDampens hunting action of the lead-lag hingeVibration damper2 part pendulum to detune the first and second harmonics of the main rotor blades3 per & 5 per vibrations
  • 412 head is referred to as a soft in plane rotor.
  • Laminated wood bladesVarious types in combinationBirchSprucePineBalsaSteel core for mass balanceExterior covered with fibreglass clothLast 2/3 rds of the leading edge covered with stainless for abrasion resistance.Flush with contourRiveted into placeMetal plates at root for grip attachmentTow pinsTacks with heads protrudingFor center of gravity & center pressureTrim tab riveted to trailing edgeTip pocket used for spanwise balanceMatched pairs
  • Metal bladesEasy quality control during manufactureCan change a single bladeMatched to a masterWeight added to obtain the best aerodynamic & sufficient inertiaLife limitedBonded constructionEven distribution of stressesContinuous contact between surfacesSmoother contoursFlexible jointsReduced weightDisadvantage is can’t rebond in fieldUsually aluminum Skin for aerodynamic shapeStainless steel leading edge
  • HoneycombSame basic techniques as early bladesHoneycomb core to support and shape bladeTwisted to help equalize lift along span
  • Crack detectionOperated by centrifugally operated mercury switch
  • Fibreglass blades has a fibreglass sparComposite blades have a metal sparBiggest advantage of fibreglass blade is the life
  • Blade alignment required on semi rigid rotor systems
  • Bell medium helicopters use a drag brace to align blades and adjust chordwise balanceWhen balancing blades may only be swept aft of the alignment points.
  • Both static and dynamic balance is required for semi rigid rotor systems
  • Unbalanced condition causes vibrationsLow frequency vibrations occur between 0 – 500 cycles per minute
  • What are the two factors of a vibration that we are concerned withAmplitude (strength)Frequency (how fast)Tail rotor vibrations will cause a tingling sensation in the pilots feet.
  • Power Cable - connects to the aircraft's 12 or 28 VDC supply Receptacles - for cable from transducer and magnetic pickup from the left prop Receptacles - for cable from transducer and magnetic pickup from the right prop Magnetic Pickup Switch - "independent" position creates two independent circuits enabling the user to record balance reading from two propellers in one "run." The "common" position is used for helicopter work where two transducers are referenced to one magnetic pickup. Function Switch - selects propeller track function, or either left or right balance circuits IPS Meter - shows vibration level RPM Tune Dial - tunes the band pass filter to the RPM of the vibration RPM Range Switch - multiplies RPM of the RPM tune dial reading by factors of 1, 10, and 100 Push For Scale #2 - when depressed, changes IPS meter sensitivity from 0 to 1.0 IPS to 1.0 to 10.0 IPS Verify Tune Button - used in conjunction with RPM tune dial to adjust balancers band pass filter to the exact propeller or vibration frequency. Interrupter Logic Switch - single position is used for propeller balance work and the double position for most helicopter smoothing Test Button - verifies magnetic pickup signal and proper phazor operation Phazor - reads phase angle (or clock angle) between transducer and magnetic pickup signals

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