Chapater Four

7/29/2019 Chapater Four

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Chapater Four

Main rotor and Swash plate

4.1 Main rotorThe main rotor is designed to create the lift and thrust required for the helicopter

translational flight. Besides, the main rotor serves to change the helicopter attitude relative to thelongitudinal and lateral axes through the employment of the swashplate assembly. The metalblades are attached to the hub by two bolts.

The main rotor comprises the hub assembly and five blades. Given below are thebasic data of the main rotors;

Diameter, m_____________________________________21.3

Weight of blade, kg_____________________________700

Direction of rotation________________________CCW, looking from below (MGB side)

Blade chord length, mm_______________________________520

Blade plateform________________________________ rectangular

1-blade2-blade root fairing3-blade-to-hub attachment bolt4-slip ring unit5-hub6-swash plate


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4.1.1 Main rotor blades

The basic load-carrying element of the blade (Fig.) is the spar whose flanges and rear webserve for attachment of tail sections incorporating honeycomb filler. The tail sections togetherwith the spar form the blade contour. The thicker root portion of the spar carries a blade-to-hub

attachment end piece. The blade leading edge accommodates counterweights and its tip carriesan assembly for installing balance weights. The blades are provided with a spar damagepneumatic indicating system which includes the following components:- plugs installed in the blade root and tip portions and designed for sealing the spar cavity ;- an air pressure indicator which operates as the air pressure in the spar decreases by

0.15 - 0.2 atm gaug;- a valve with a slide and a cap.Sections Nos 16 and 17 have tabs, 40 mm wide, serving mainly for changing the blademoment characteristics. The tabs are used for adjustment of the main rotor blade trackingand for elimination of the control stick creeping.The main rotor blades are providedwith a spar damage indicating system. The blades are protected against ice formation by

an electrothermal de-icing system.

Spar

The spar is a hollow beam with a uniform-section inner contour. The spar outer surfaceis machined to fit the theoretical contour of the blade. The spar is made of a pressedaluminium alloy section.

The inner surfaces of the spar upper and lower flanges are provided with ribs smoothlyincreasing in width. The ribs located closer to the blade leading edge serve as guides forcounterweights. The thickness of the spar flanges at the places of attachment of the tail sectionsvaries from 6.6 mm. (approx.) at the place of attachment of section No.2 to 5 mm (approx.) atthe places of attachment of sections Nos 4 - 21.The spar root portion is made thicker to fasten theblade-to-hub attachment end-piece.

Blade root

The spar in the blade root portion (Fig. ) mounts a steel end-piece secured by means of 21bolts. The nine middle bolts are through ones; they are lock-punched. The remaining bolts (sixon each side) are installed in sleeves and are wire-locked. Placed in the inner cavity of the spar inthe vicinity of the through bolts is a textolite spacer. The end-piece is secured to the spar bymeans of an adhesive film. The spar end face is protected with a cover which mounts a plugconnector and a valve for inflating the spar cavity. The cover also serves as a spar root plug.Mounted on the spar rear web near the root end face is the pressure indicator ofthe spar damage

indicating system.


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Fig Blade root

1 - spar; 2 - bolt securing end-piece to spar; 3 - pressure indicator; 4 - sparCover (plug); 5 - inflating valve with slide and cap; 6 - end-piece; 7 - plug connector

Blade trailing edge

The blade trailing edge is composed of 21 separate sections (Fig.).Each section consists of analuminium alloy skin, 0.3 mm thick, cemented to the honeycomb filler, two side ribs, and a tailstringer. The skin is not cut at the tail stringer but envelops it. The honeycomb filler comprises astack of aluminium foil bands, 0.04 mm thick, milled over the theoretical contour. The bands arecemented and stretched, thus forming honeycombs with hexahedron sides measuring 5 mm.

The ribs are made of aluminium alloy, 0.4 mm thick. In places where the ribs are cementedto the spar the rib webs are bent to form lugs for attachment to the spare.

The tail stringer is made of textolite.The sections are cemented to the spar with the aid of an adhesive film.The sections are interlaid with sealing inserts to prevent air flow between the sections.The tail portion of the end rib is provided with a drain hole to remove moisture from the

sections.


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The blade leading edge are protected by adhesive tapes(seeAdhesive tape

Counterweight

To obtain the required lateral balance of the blade, the spar nose portion mounts a counterweightconsisting of ten separate blocks, 400 mm long and weighing1 kg each. -

The blocks are covered with rubber, which makes it possible to fit them tightly along thefront stiffening ribs into the spar cavity (see Fig.).

The centrifugal forces created by the counterweight during rotation of the blade are takenup by a screw stop mounted inside the spar.

Fig Typical blade sections

1 - Inter-section insert; 2 - honeycomb filler; 3 - lug; 4 - de-icer; 5 - counterweight; 6 - spar;7 - rib; 8 - .kin; 9 - tail stringer


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Tracking light

The tracking light bulb is installed in the blade tip fairing. The wire running to the bulb isplaced in a groove provided in the spar front portion and is brought to a plug connectorinstalled in the blade root.

Blade tip

The blade tip carries a fairing with a detachable (screwed) front portion. The tip nose isprovided with a safety cover-plate made of material Xl8H9T to increase resistance to abrasion.

When the fairing is removed, access is provided to the balance weights attachment assembly and

to the tracking light. The balance weight is secured on two studs to the end plug of the indicating

system. The plug is screwed to the spar.

Blade paint coating

The outer surfaces of the blades are coated with A-3A primer and painted with XB-16vinyl perchloride enamel with admixture of 2% of aluminium greyish-blue powder. The blade tipis painted with XB-3l yellow enamel. Lines applied along the upper and 10Vier surfaces of theblade mark the boundary of the honeycomb filler. The lines are applied near the spar rear flangeat the honeycomb filler boundary.


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4.2 Main Hub

The hub is intended for transmitting rotation from the main gearbox to the blades, as wellas for taking up and transmitting aerodynamic forces acting on the main rotor to the fuselage.

The blades are jointed to the hub body through the connection of flapping, drag, and

feathering hinges. The flapping hinges allow the blades to perform flapping motions (oscillationsin the vertical plane) under the action 'of aerodynamic forces variable in azimuth. The draghinge allows the blades to perform oscillatory motions in the plane of rotation. The featheringhinges are designed for changing the blade pitch angles.

General

Number of blades pc----------------------------------------------5Type----------------------------------------------------------------------------------- with offset and turnedflapping hinges, with drag hinges, flapping compensator, and centrifugal droop limiters

Flapping hinge turning angle------------------------------504'19"

Damper type-------------------------------------------------hydraulic

Flapping angle--------------------------------------------250

Droop angle With blade resting on link------------------4 0+10'

-20'

With blade resting on centrifugal droop---------1040+20'

Angle of turn relative to drag hinge (from position normal to flapping hinge axis): -forward (in direction of rotation)........................130151

- rear ward (against direction of rotation)...............110101

Hub dry weight, kg ..............................................610.525

Hub overall dimensions:

diameter mm------------------1744

height, mm-------------------------- 321

Adjustment of centrifugal droop limiters:

Operation of mechanisms with main rotor accelerated, rpm.......................1083Operation of mechanisms with main rotor decelerated, rpm.....................95 3

Oscillations of the blades relative to the drag hinges are suppressed by hydraulic dampers. Thehub is so designed that with the blade flapping about the flapping hinge at angle "", the bladepitch true angle decreases by the value to be determined as follows:Proportionality factor "k" is called a flapping compensator factor. The main rotor hub flapping


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compensator factor equals 0.5.In order to decrease drooping of the blades and to ensure the required clearances between

the blades and the tail boom when the main rotor is immovable or revolves at low speeds, thehub is provided with centrifugal droop limiters. The basic components of the main rotor hub areas follows. (Fig): hub body, links, feathering hinge trunnions, feathering hinge bodies and blade

turning arms.Hub body is connected to the MGB shaft by means of involute splines and located by lowerbronze cone having one cut and upper steel cone made up of two halves. .The body is secured onthe shaft by nut locked with pins .The nut is tightened by a special hydraulic wrench. The bodyhas five wide lugs (for fiveblades) arranged in one plane at an angle of 720 to one another andforming (together with links) flapping hinges.Each lug of the hub body has a cavity "" to befil1ed with oil for lubricating the flapping hinge bearings. -

Outer rings of the needle bearings installed in the hub body ensure supply of oil to themost loaded needles (under the action of centrifugal forces) even when cavity "" is filled withoil partially. The lug cavities are filled with oil through holes closed with plugs. Oil is drainedthrough drain holes closed with plugs. The lugs have upper and lower stops which limit theflapping motion of the blades. Lower stops are detachable and can be replaced in service if theydisplay peening or other defects.

Provided in the lower portion of the hub body is a hole for locking the bracket of theswashplate spider shackle.

The needle bearings of the flapping hinge consist of outer and inner rings and a set ofneedles, 6.5x60 mm in size, each bearing incorporating 43 needles. Outer rings are secured in thebody by nuts. The nuts are locked with plates screwed to the body. Bearing inner rings, as wellas rings (two) are mounted on flapping hinge pin and are held between the lugs of the link bynut. The pin rests against the link with its split snap ring and is held against turning by key .Thehollow of pin is guarded against the ingress of moisture at the snap ring end by a rubber closure,the opposite end of the pin is fitted with a plug intended for mooring the blades.

Installed between the outer rings of the needle bearings and thrust washer are two bronzerings taking up axial forces which appear when the blade deflects from the direction normal tothe axis of the flapping hinge.

The flapping binge is sealed with the aid of fixed rubber rings (two) and reinforced cups.Cup has an additional sealing band to protect the main working edge against dust and prematurewear.

Link is a box-section part. Provided at the ends of the link are lugs for connecting the linkto hub body and to feathering hinge trunnion.Tbe lug axes are arranged at right angles to eachother. The link is connected to the trunnion, thus forming the drag hinge. The construction of thedrag hinge is similar to that of the flapping hinge. Two needle bearings are enclosed in thecylindrical cavity of the bead of trunnion.

The bearings comprise outer and inner rings with a set of needles. Outer rings are securedin the trunnion by nuts. The nuts are locked with plates screwed to the trunnion. Bearing innerrings, thrust washer, and rings are fitted onto drag hinge pin and are clamped between the linklugs by nut. To take up axial forces, the drag binge is provided with bronze washers locatedbetween the faces of outer rings and thrust washer.

The drag binge is sealed by means of reinforced cups and rubber rings (two) similar to therespective parts incorporated in the flapping binge. The upper portion of pin carries sleeve. Plugcloses the hole through which the sleeve is filled with oil. Oil flows to the needle bearings via


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drilled passages in the sleeve and in the pin. The sealing rings prevent oil leakage from sleeve.To completely expel air from the drag hinge, the lower portion of the sleeve is fitted with a

grease fitting through which oil is injected in the course of assembly. When oil is injectedthrough the grease fitting, it flows through the holes in lower.Openings for filling and draining oil closed by plugs are provided in the feathering hinge

housing. Mounted on the feathering hinge housing safety valve which consists of a body, a coverand a rubber diaphragm. The feathering hinge is sealed by stationary rubber sealing rings, andcups (Two). Cup protects main cup from dust and dirt to prevent its working edge fromexcessive wear. The cup working edges move over the surface of thrust ring pressed onto thetrunnion stem.Feathering hinge housing is made in the form of a sleeve whose bottom is provided with alugged fitting serving for attachment of the blade. The other end of the housing is threaded tomount nut and provided with a flange to which blade pitch lever is bolted by four bolts. Boltsare unloaded from the shearing loads by bushings.The blade flapping angles are limited by fixed upper and lower stops installed at the hub bodyand the cleats.The cleat mounts a centrifugal blade droop restrainer (Fig.) limiting the blade droop angles with

the main rotor not rotating and at low rotational speed. Counterweight is attached to the cleat bymeans of pin and connected to one end of pawl through control rod .The pawl rotates on pininserted into the cleat lugs. The second end of the pawl serves as a stop limiting the blade droopangle.With the main rotor rotating at speeds below (1455) r/min, spring retains the counterweight andthe pawl to limit the blade droop angle to 3 040' +20.Upon attainment of a speed equal to(1455) r/min the counterweight rotates, being acted upon by the centrifugal force, to expandspring (3) and turn pawl (6) until fork (8) sets against the cleat. In this case, a clearance iscreated between the housing stop and the pawl, and the blade droop angle is limited only

by the cleat fixed stops which allow the blade to droop to 60+10(-20). As soon as the main rotor

speed drops down to (1305) r/min the pawl returns to the position corresponding to a blade

droop angle of 3040' +20. Transition from one limit setting to another should be abrupt and freefrom any lag.


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Centrifugal droop restrainer


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Hydraulic damperThe blade oscillations relative to the drag hinges are damped by hydraulic dampers having morestable performance characteristics as compared with the friction dampers.

The hydraulic damper design consists; Two cylinder trunnions of hydraulic damper are insertedin needle bearings installed at brackets bolted by eight wedge bolts to the flanges of feathering

hinge trunnion . Nut with a lubricator for pressure filling is threaded into the bracket.One end of hydraulic damper cylinder is closed with cover fastened to it by eleven bolts. Press-fitted into the cylinder bottom and the cover are bronze bushings (two) inside which rod moves.The rod is made integral with a piston in whose-middle portion eight bypass valves are installed,four at either side. The piston is provided with three piston rings. Bypass valve consists ofhousing, cone and spring

The spring setting against nut presses cone to the seat of housing Stop housing is threadedonto the rod threaded end. Shock absorber comprising two steel plates and a rubber padvulcanization-bonded between them is secured by six bolts to stop housing. The shockabsorber is intended for softening the shock against the drag hinge rear lug stop takingplace upon acceleration of the main rotor.

Stop housing is connected by means of shackle to the flapping hinge pin. The shackle hinges arelocated at a right angle to each other to compensate possible misalignments upon installation ofthe hydraulic damper. Bronze bushing put onto pin serves as the bearing for the hinge connectingshackle with stop housing. The hinge connecting the shackle e to the flapping hinge pin employsneedle rollers mounted on pin .The shackle hinges are sealed with rubber circular rings.

Cover protecting the hydraulic damper rod from dirt is put onto stop housing and cylinder. Therod is sealed by rubber sealing rings (two) placed in grooves of cylinder and cover betweenfluoroplastic rings. Rubber sealing ring is placed between cover and cylinder. Attached to thecover is sleeve blanking the rod exposed end.Since the balls are floating both chambers of the cylinder communicate with groove with thehydraulic damper inoperative. Groove communicates through the elbow connection and hoses

with a compensation tank attached to the hub body top and intended for replenishment of thehydraulic damper to compensate lost hydraulic fluid, as well as for egress of air bubbles(emulsion) from the hydraulic damper cylinder. Due to this fact no air accumulates in thehydraulic damper and the excessive hydraulic fluid freely flows into the compensation tank whenthe ambient temperature rises.With the hydraulic damper operating one of the balls (M or H) is set by the hydraulic fluidpressure against the stop housing seat to cut off the compensation tank from the HP chamber andshifts the second ball (H or M) through ball K off the seat. As ittakes place, the LP chamberremains communicated with the compensation tank. Such a system ensures reliable and trouble-free operation of the hydraulic dampers. The compensation system is vented through a hole inthe compensation tank casing.

The hydraulic damper tank is cast of A9 aluminium,alloy. An organic glass cap adhesive-bonded to itensures good visibility of the tank hydraulic fluid level. The hydraulic damper functional diagram and

diagram depicting the rod force P versus blade angle of deflection relative to the drag hinge (at a

constant oscillation frequency of 242 oscillations per minute) is shown in Fig. Point M indicates the

valves opening point and corresponds to force P approximately equal to 1000 kgf.


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Functional Diagram and Characteristics of Hydraulic Damper at Constant

Oscillation Frequency

Chapater Four

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