United States Army Aviation Warfighting Center
Fort Rucker, Alabama
February 2008
UH-60A
STUDENT HANDOUT
UH-60A Powertrain/ Rotor System
4745-3
PROPONENT FOR THIS STUDENT HANDOUT IS:
110TH AVIATION BRIGADE
ATTN: ATZQ-ATB-AD-C
Fort Rucker, Alabama 36362-5000
FD5: This product/publication has been reviewed by the product developers in coordination with the
USAAWC, Foreign Disclosure Officer, Fort Rucker, AL foreign disclosure authority. This product is
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releasable to students from all requesting foreign countries without restrictions.
Terminal Learning Objective:
Action: Describe the operational procedures that pertain to the UH-60 Powertrain and Rotor systems.
Condition: Without reference, given the aircraft systems examination and an answer sheet.
Standard: Be able to describe the operational characteristics that pertain to the UH-60 Powertrain/ Rotor
system. This includes the operational characteristics of the Main Transmission System, Transmission
Lubrication System, Transmission Chip Detector System, the Gust Lock, and Main and Tail Rotor
Components.
Safety Requirements: Use care when operating training aids and/or devices.
Risk Assessment Level: Low.
Environmental Considerations: It is the responsibility of all soldiers and DA civilians to protect the
environment from damage.
Evaluation: You must answer (4) out of (6) questions correctly on in this scorable unit on the systems
examination to receive a "GO". You will have one hour to complete the system examination.
Learning Step/Activity 1. Describe the operational characteristics of the Main Transmission System.
a. Main Transmission System
(1) The components of the power train system are the inputs from the two engines, a main
transmission, intermediate gear box, tail gear box, and connecting drive shafting. The main Transmission
(XMSN) is mounted on top of the cabin between the two engines. The main XMSN mounts and powers
the main rotor head, changes the angle of drive from the engines, reduces the rpm from the engines,
powers the tail rotor drive shaft and drives the accessory modules.
(2) The XMSN system carries engine torque to the main rotor and tail rotor. It consists of a main
transmission with oil cooler, intermediate gear box, tail gear box, and drive shafts.
(3) The XMSN system has oil pressure and oil temperature indicating systems, hot oil and low oil
pressure warning systems, and a chip detector system The main XMSN drives the main rotor, tail rotor,
main XMSN oil cooler fan, No. 1 and No. 2 hydraulic pump modules, and No. 1 and No. 2 generators.
(4) The Transmission consists of five modules: a main module, two input modules, and two
accessory modules.
(a) Input modules. The two input modules are interchangeable; right to left and left to right.
(b) Accessory modules. The two accessory modules are interchangeable; right to left and left to
right.
(c) The transmission main module can operate with no oil pressure at cruise flight for 30 minutes.
(d) The XMSN is limited to rotor RPM or engine Torque (TRQ), whichever occurs first.
(5) Main Module Mounting. The main module is mounted to the top of the cabin fuselage with a builtin 3º forward tilt, and is rigidly attached to the fuselage by eight mounting bolts.
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b. Main Module
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(1) The main module is a single-stage planetary, gear-type XMSN. The components of the main
module are a main bevel gear and planetary gears. The upper bevel gear is driven by the two input
modules which drives the planetary gears and tail rotor. This assembly is referred to as the main bevel
gear.
(2) The planetary gears are driven by the sun gears on the lower end of the outer shaft. The XMSN
mast is directly connected to the planetary gears and passes back up through the outer shaft after the
final gear reduction occurs.
(3) The helicopter's power to the XMSN begins at the engines. The engine Np section, turning at
20900 RPM (100% Nr), provides the power to the input module, which then the input module drives the
main module and accessory modules. The input module reduces engine input rpm to 5750 rpm and also
allows the drive angle to be changed from the engine to the main module.
(4) The main module then provides reduction for the main rotor head down to 258 rpm and a
reduction for the tail drive shaft and oil cooler to 4116 rpm.
(5) The intermediate gear box provides a reduction to 3319 rpm, plus changes the angle of drive
about 58°.
(6) A double thrust bearing supports the mast and all radial and axial loads. The double thrust
bearing on the lower end of the mast is in the sump case of the XMSN. The mast is directly connected to
the planetary gear of the XMSN and is supported by the double thrust bearing. It extends through the top
of the XMSN and connects the mast extension by means of external splines and threads.
(7) The XMSN oil pumps supply lubrication and provide a means of scavenging XMSN oil. The
planetary gears of the XMSN drive the oil pumps.
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c. Input Modules
(1) The two input modules are mounted on the left and right front side of the main module. They
connect the main module to the engines by shafting and gears.
(2) Each input module is identical and directly interchangeable.
(3) The internal body of the input module has an input bevel pinion and gear, and free-wheeling unit.
The free-wheeling unit allows for engine disengagement during autorotation, or in case of a nonoperating
engine or engine malfunction.
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(4) The accessory module is driven by the bevel gear of the input module, and during autorotation,
the accessory module will continue to be driven by the main rotor, providing drive for the accessories
installed on the accessory module. The input drive module provides the first gear reduction between
engine and main module.
(5) Free Wheeling Unit
(a) Engaged: The free-wheeling unit is engaged by the input drive gear of the engine and a
compression spring. As the engine drive gear turns the spring, it assists in forcing the rollers out of their
cage, which forms a coupling between the engine input drive gear and transmission cam.
(b) Disengaged: When the engine input drive gear slows down below the speed of the
transmission cam, the rollers are forced back into their cage. This disengages the engine drive from the
transmission drive in the input module and allows the main rotor to continue to drive the accessory and
main modules during autorotation or single-engine operation.
d. Accessory Modules
(1) The input module provides the drive to the accessory module. The accessory module is mounted
on the front side of the input module and driven by a bevel gear of the input module. The accessory
module provides mounting flanges, lubrication, and drive for the accessories installed on the accessory
module.
(2) When performing an autorotation, the input and accessory modules are driven by the main rotor,
providing drive for the accessories installed on the accessory module.
(3) The left module has a low oil pressure switch and a chip detector, located at the farthest point
from the pump, which will cause the MAIN XMSN OIL PRESS caution light to come on when the pressure
drops to 14 ± 2 PSI.
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(4) The accessory modules drive the electrical generators and the hydraulic pump modules. Both
modules also contain a chip detector to monitor lubricating oil for possible metal contamination.
Learning Step/Activity 2: Describe the operational characteristics of the Main Transmission Lubrication
System.
a. Main Transmission Lubrication System
(1) The main XMSN is a wet sump lubrication system that cools and filters the oil to all the gears and
bearings.
(2) The No. 1 and No. 2 generators also receive oil for cooling by way of internal lubrication lines.
The oil is pumped through internally cored oil lines, except for the oil cooler inlet and outlet lines.
(3) The main XMSN has an oil capacity of about 7 gallons. A dipstick is used for checking oil quantity.
The dual scale dipstick is for checking cold or hot oil levels. Use the appropriate scale when checking the
oil level.
(4) Read the hot side of the dipstick when checking hot oil (immediately to 2 hours after shutdown), or
the cold side of the dipstick when checking cold oil (at least 2 hours after shutdown).
(5) The system includes two pressure and scavenge, vane-type lubricating pumps that have pressure
regulating and bypass valves, a two-stage oil filter, an oil cooler and blower, and warning and indicating
systems.
b. Oil Pumps
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(1) The main XMSN lubrication system has two oil pumps that are connected into a common manifold
(combining their output) for pressure lubrication and a common oil return manifold for oil scavenging.
(2) Each pump is located in the XMSN sump case, one on the left hand side and one on the right
hand side. The main XMSN lubrication pumps, driven internally by the main module, are combination
pressure and scavenge vane-type pumps, operating in parallel.
(3) Characteristics
(a) The pressure side of the pumps supply oil at 15 Gallons Per Minute (GPM) at 50-55 psi. The
scavenge side returns oil at a rate of 7 GPM, at a pressure between 50-55 psi to the sump.
(b) The main XMSN oil pressure may fluctuate when the aircraft is known to be in a nose-up
attitude (i.e., slope landings or hover with an extreme aft Center of Gravity (CG).
(c) With a loss of all oil, the main XMSN will continue to operate in flight for another 30 minutes.
(d) If the oil pressure exceeds 55 psi, the bypass valve will open and return the excess oil back to
the inlet side of the pump preventing damage to the lubricating system.
c. Oil Manifolds
(1) The lubrication system contains two manifolds, pressure and return.
(a) The pressure manifold connects the two oil pumps in parallel for lubrication, and routes the oil
under pressure, through the filter and oil cooler to the five modules.
(b) The return manifold connects the returning oil to the scavenge side of the two oil pumps in
parallel.
d. Oil Filter
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(1) The two-stage oil filter, located at the right rear section of the sump and accessible from inside the
cabin, protects the lubrication system by removing lubricant contaminants.
(2) The filter elements are paper, throw-away types. The UH-60A and EH-60A model aircraft have
two models of filters available for use.
(3) Both filters have these similar characteristics.
(a) When the primary filter begins to clog, and pressure drops between 9 - 15 psi, the indicator
button extends from the bottom of the filter bowl.
(b) The indicator cannot be reset unless the filter elements are replaced. A thermal lockout
prevents the red indicator button popping when oil is cold and thick.
(c) The first stage filter will protect the system up to a differential pressure of 16 - 24 psi.
(d) At this point, the flow is bypassed to the second stage filter element, which will protect the
system up to 30 - 40 psi differential pressure.
e. Oil Flow
(1) The two oil pumps pick up the oil and pressurize the system, while the manifold combines the
pressure output of the two pumps, and routes the oil to the filter.
(2) Oil is then forced through the first and second stage elements of the oil filter to the oil cooler.
(3) As oil enters the oil cooler, the oil is routed to the thermostatic control bypass valve.
(4) If the oil temperature is below 71 º ±1C or the oil cooler is clogged, the oil will bypass the oil
cooler. Should the temperature be above 71 º ±1C, the oil is routed from the oil cooler back into the main
module pressure manifold.
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(5) As oil enters the main pressure module manifold, the oil is divided between the lubrication jets of
the main module and oil passages to the input modules, accessory modules, and the AC generators.
(6) The high oil temperature switch controls the transmission oil temperature caution light, MAIN
XMSN OIL TEMP. This switch causes the caution light to illuminate if the oil temperature exceeds the
safe operating temperature (120 ºC).
(7)The main XMSN oil pressure switch controls the transmission oil pressure caution light, MAIN
XMSN OIL PRESS. This switch causes the caution light to illuminate if the oil pressure drops to a
minimum of 14±2 psi.
(8) The left and right hand accessory chip detectors control the caution lights CHIP ACCESS MDLLH and CHIP ACCESS MDL-RH on the caution/advisory panel.
(a) The left and right hand input module chip detectors control the caution lights CHIP INPUT
MDL-LH and CHIP INPUT MDL-RH on the caution/advisory panel.
(b) The magnetic plug on each chip detector attracts ferrous particles, sending a signal
illuminating the respective caution light. The main module chip detector controls the caution light CHIP
MAIN MDL-SUMP on the caution/advisory panel.
(c) The magnetic plug on this chip detector attracts ferrous particles, sending a signal illuminating
the respective caution light. This chip detector is also connected to a 30 second time delay relay, allowing
small chips and fuzz to burn off and /or wash away.
(9) The scavenge (oil return) system includes the oil pumps and return manifold. The oil pumps are
pressure, or scavenge (dual-element) pumps; therefore, they scavenge oil back into the oil sump. This
provides an ample supply of oil for lubrication at all times. The return manifold combines the scavenge
capability of the two oil pumps in the return of the oil to the sump.
f. Oil Cooler and Blower
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(1) The oil cooler and fan, in the rear of the main rotor pylon, consists of a radiator, duct, fan and
shafting. The fan, driven by the tail rotor drive shaft, forces air through the radiator.
(2) Also, if the radiator becomes clogged, the oil will bypass. Oil is routed from the oil cooler to the
main module manifold to be divided between the lubrication jets in the main module and the oil passages
to the input modules, accessory modules, and generators.
(3) When inspecting the oil cooler, debris (i.e. dirt and grass) may become trapped inside the pockets
between the fan blades. A build up of debris can affect the balance of the oil cooler fan.
g. Oil Pressure and Monitoring System
(1) The oil pressure monitoring system has a XMSN oil pressure switch, sensor, indicator, and
caution light.
(a) The sensor is a transmitter, located in the left rear of the main module that converts fluid
pressure to an electrical pressure reading to the cockpit indicator.
(b) The transmission oil pressure switch is activated at 14±2 psi, and sends a signal to the
Caution/Advisory panel activating the MAIN XMSN OIL PRESS caution light.
(c) The oil pressure switch, located on the No. 1 accessory module (farthest point from the
pumps), is a pressure-type switch (i.e., the switch requires pressure to keep the contacts open).
(d) The caution light, located on the caution/advisory panel, receives a signal from the pressure
switch when it closes, illuminating the caution light.
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(2) Located on the central display unit, the indicator is a Vertical Instrument Display System (VIDS)
indicator with markings that range from: Minimum (Red) 0-20 psi, Idle and Transient (Amber) 20-30 psi,
Continuous (Green) 30-65 psi, Precautionary (Amber) 65-130 psi, and Maximum (Red) 130 psi.
(a) Verify all limitations referenced with the most current TM 1-1520-237-10, Chapter 5.
h. Oil Temperature Monitoring System
(1) The oil temperature monitoring system has a XMSN oil temperature switch, sensor, indicator, and
caution light.
(a) The sensor is a wet bulb sensor, located in the bottom forward side of the main module oil
sump. As oil increases in heat, the oil creates gases in the temperature bulb to expand and increase
their electrical impulse output to the instrument.
(b) Without a fluid flow and heat, the sensor becomes inoperative.
(c) The oil temperature switch is located on the XMSN at the input of the lubrication cycle after
the oil has passed through the oil cooler. The XMSN oil temperature switch will cause the XMSN oil
temperature caution/advisory light to illuminate if the oil temperature exceeds 120 ºC.
(d) The caution light, located on the caution/advisory panel, receives a signal from the
temperature switch when the temperature exceeds 120 ºC, which illuminates the caution light.
(2) Located on the central display unit, the indicator is a VIDS indicator with markings that range from:
Normal (Green) -50 º to +105 ºC, Limited (Amber) +105 º to +120 ºC, and Maximum (Red) +120 º to
+170 ºC.
(a) Verify all limitations referenced with the most current TM 1-1520-237-10, Chapter 5
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Learning Step/Activity 3. Describe the operational characteristics that pertain to the UH-60 Main
Transmission Chip Detector System.
a. Chip Detector Switches
(1) The XMSN chip detector system consists of chip detectors on the left and right input modules, left
and right accessory modules, and the main gearbox module.
(a) These detectors provide warning of chips in any of the five areas of the main XMSN system.
(b) Each chip detector, with the exception of the main module chip detector, incorporates a selfsealing sleeve so that it can be removed for visual inspection without loss of oil.
(c) The magnetic plugs on each chip detector attract ferrous particles at any of the detector
locations.
(2) A fuzz burn-off feature prevents false warnings by burning off small chips and fuzz from the
detector. This fuzz burn-off feature is deactivated when oil temperature reaches 140 °C.
(a) Deactivation of the fuzz burn-off feature does not disable detection and illumination of caution
lights.
(b) The system is powered by the DC essential bus through a circuit breaker on the upper
console circuit breaker panel marked CHIP DET.
b. Main Module Chip Detector
(1) The main gear box has one chip detector, mounted on the sump assembly that constantly
monitors lubricating oil for possible metal contamination.
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(2) Metal chips that accumulate within the chip detector gaps, close an electrical circuit that
illuminates the CHIP MAIN MDL SUMP capsule on the caution/advisory panel.
(3) The main XMSN chip detector is connected to a 30 second time delay relay to allow small chips
and fuzz to burn off and/or wash away.
c. Input Module Chip Detector
(1) Two chip detectors on the sump assembly constantly monitor the lubricating oil for the input
modules, for possible metal contamination.
(2) Any metal chips that accumulate within the chip detector gaps, close an electrical circuit that lights
either the CHIP INPUT MDL - LH or CHIP INPUT MDL - RH capsule on the caution/advisory panel.
d. Accessory Module Chip Detector
(1) There are two chip detectors, one mounted on each accessory module, which constantly monitor
the lubricating oil for possible metal contamination in the accessory modules.
(2) Any metal chips that accumulate within the chip detector gaps, close an electrical circuit that lights
either the CHIP ACCESS MDL-LH or CHIP ACCESS MDL-RH capsule on the caution/advisory panel.
e. Built In Test
NOTE: The MASTER CAUTION PRESS TO RESET caution may or may not disappear after being
pressed to reset while the chip detectors BIT is in progress.
(1) The Built In Test (BIT) circuits for the chip detectors will automatically test for a continuous circuit
from the caution/advisory panel to each individual chip detector when power is first applied.
(2) The chip detector caution illuminates during the test and extinguishes after successful completion
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of test.
f. Caution Lights
(1) The transmission chip detector system consists of chip detectors on the left and right input
modules, left and right accessory modules, the main module, and cautions marked CHIP INPUT MDL-LH,
CHIP INPUT MDL-RH, CHIP ACCESS MDL-LH, CHIP ACCESS MDL-RH, and CHIP MAIN MDL SUMP.
(2) The caution light will illuminate when the gap is closed on the chip detector switch. The pilot or
maintenance personnel must check for caution/advisories before removing power to determine the
location of the chip.
Learning Step/Activity 4. Describe the operational characteristics that pertain to the UH-60 Gust Lock
System.
a. Gust Lock
(1) The locking portion of the gust lock assembly is located at the tail rotor takeoff flange on the main
XMSN.
(2) The gust lock control handle is located in the cabin ceiling, aft left hand side of the main XMSN,
with a release button as part of the handle.
(3) The gust lock prevents the blades from rotating when the helicopter is parked.
(4) The gust lock is designed to withstand torque from one engine at idle, thus allowing engine
maintenance checks independent of drive train rotation.
b. Gust Lock Warning Schematic
WARNING: Before engine operations can be performed with the gust lock engaged, all main rotor
tiedowns shall be removed.
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(1) A micro switch, located in the control handle linkage, controls the GUST LOCK caution light on the
caution/advisory panel.
(2) When the gust lock is engaged, the micro switch plunger is depressed, causing the GUST LOCK
caution light to illuminate.
(3) When the gust lock is disengaged, the micro switch plunger is released, extinguishing the light.
(4) Operation.
(a) To engage the gust lock system, press the release button, then move the handle up.
(b) To disengage the gust lock system, press the release button, then move the handle down.
c. Gust Lock Limitations
(1) Dual-engine operation with the gust lock engaged is prohibited.
(2) Single-engine operation with the gust lock engaged will be performed by authorized pilot(s) at
IDLE only.
(3) The gust lock shall not be disengaged with the engine running.
Learning Step/Activity 5. Describe the operational characteristics that pertain to the UH-60 Tail Rotor
System.
a. Tail Rotor Drive Shafts
(1) Tail Rotor Sections
(a) The tail rotor drive shaft runs from the tail takeoff flange on the rear of the main transmission,
then drives the oil cooler blower. From there, section II and III run to the intermediate gear box. Section IV
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runs up from the intermediate gear box to the tail rotor.
(b) The drive shafts are broken down into four sections containing six shafts, and transmit torque
from the engines to the tail rotor. The sections are joined together by flexible couplings which eliminate
universal joints. Each shaft is dynamically balanced tubular aluminum.
(c) The shafts are ballistically tolerant if hit by a projectile, and are suspended at four points in
viscous-damper bearings mounted in adjustable plates bolted to the fuselage support brackets.
(2) Section l Drive Shaft
(a) The Section I drive shaft of the tail rotor drive shaft system is a single section of drive shaft
located between the aft section of the transmission and the transmission oil cooler.
(b) This drive shaft is not interchangeable with any other drive shafting.
(3) Section ll Drive Shaft
(a) The Section II drive shaft of the tail rotor drive shaft system is made up of three section of
drive shafts located between the transmission oil cooler and the Section III drive shaft, which is forward of
the intermediate gear box.
(b) These three drive shafts are identical and interchangeable.
(4) Section lll Drive Shaft
(a) The Section III drive shaft of the tail rotor drive shaft system is a single section of drive shaft
that connects the Section II drive shafts to the intermediate gear box.
(b) This drive shaft is not interchangeable with any other drive shafting.
(5) Section lV Drive Shaft
(a) The Section IV drive shaft of the tail rotor drive shaft system is a single section of drive
shafting that connects the intermediate gear box to the tail rotor gear box.
(b) This drive shaft is not interchangeable with any other drive shafting.
b. Flexible Couplings
(1) At the end of each drive shaft flexible couplings are provided. These couplings maintain the
alignment of the drive shaft during aircraft operating.
(2) The flexible couplings used throughout the tail rotor drive shaft system provide the connection
point of each section to the other, and serves the same purpose.
(3) When inspecting the flexible couplings, check for buckling, cracks, and security.
c. Support Bearing
(1) At the aft end of each shaft of Section II, a viscous type support bearing is used. This bearing
provides support for each of the three shafts in Section II, and is filled with a dampening fluid.
(2) The support bearing should be inspected for fluid leakage by looking for a bubble at the forward
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upper portion of the bearing containing the fluid (0 to 1 inch bubble is serviceable, 1 to 2 inch bubble
requires servicing, and a 2 inch or greater bubble requires replacement of the bearing).
d. Intermediate Gear Box
(1) The intermediate gear box is mounted at the base of the tail pylon.
(2) The intermediate gear box carries main transmission torque to the tail rotor gear box and changes
the angle of drive about 58°.
(3) The intermediate gear box also reduces tail rotor drive shaft input speed of 4110 rpm to 3319 rpm
pylon shaft output speed.
(4) Intermediate Gear Box Lubrication
(a) Lubrication of the intermediate gear box is accomplished by a splash type lubrication system.
As the oil is forced through the tapered input bearing, it is forced uphill to the output bearing through oil
ports. The oil drains back down the housing and internal gears in the same manner. Venting of the gear
box is accomplished through the output end flange.
(b) Located on the same side of the housing, directly below the filler plug is a sight gauge/plug.
The sight gauge is used to check the level of oil inside the gear box.
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(c) The intermediate gear box may run at cruise flight for 30 minutes with loss of all oil. The
intermediate gear box is serviced with oil (MIL-L 23699 or MIL-L 7808) at the filler plug located on the left
hand side of the housing.
e. Intermediate Gear Box Chip Detector Operation
(1) The intermediate gearbox has a chip detector/temperature sensor located on the right hand side
of the gear box. The chip detector is self-sealing to permit removal for inspection without the loss of oil.
(2) The chip detectors incorporate a fuzz burn-off feature which eliminates false warning due to fuzz
and small particles. When a chip is detected and will not burn off, the CHIP INT XMSN caution will
appear.
(3) The oil temperature sensor is a bimetal strip that reacts to temperatures. When the oil
temperature reaches 140 ºC, a switch closes and activates the INT XMSN OIL TEMP caution.
f. Tail Rotor Gear Box
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(1) Located on top of the tail pylon is the tail rotor gear box. The tail gear box mounts the tail rotor
and servo, changes the angle of drive 105º, and gives a gear reduction from 3,319 RPM input to 1,190
RPM output.
(2) The tail rotor gear box transmits torque to the tail rotor head.
(3) It also enables pitch changes of the tail rotor blades through the flight control system.
(4) Lubrication of the gear box is accomplished by a splash-type lubrication system.
(5) The gear box housing is magnesium, has an oil sight gauge for a fluid level check, and a filler cap
for servicing and venting of the gear box.
(6) The tail gear box may run at cruise flight for 30 minutes, with the loss of all oil.
(7) An internal fuzz suppression metal chip detector/temperature sensor detects metal particles, and
gear box over temperature conditions, to illuminate caution lights marked CHIP TAIL XMSN and TAIL
XMSN OIL TEMP.
g. Tail Rotor Assembly
(1) The tail rotor assembly incorporates two cross beam tail rotor blades with flexible spars that
accommodate flapping and pitch change. This eliminates the use of bearings.
(2) A pitch change beam, on the pitch control shaft, changes the angle of the tail rotor blades through
the pitch change links.
(3) Tail Rotor System Function
(a) The tail rotor head and blades are installed on the right side of the tail pylon, canted 20°
upward.
(b) In addition to providing directional control and anti-torque reaction, the tail rotor provides 2.5%
of the total lifting force at a hover. (Approximately 400 pounds) This allows for a shorter nose, provides a
longer Center of Gravity (CG) travel, and provides a greater hover and low speed flight capability.
(c) Spring loaded feature. If both tail rotor control cables fail, a centering spring will position the
tail rotor servo linkage to provide 10.5 degrees of pitch. This will allow trimmed flight at about 25 KIAS
and 145 KIAS (these speeds will vary with gross weight).
(4) The tail rotor power train system is comprised of seven major components; tail rotor gear box,
retention plates, pitch change beam, pitch change links, counter weights, bonding cables, and the tail
rotor paddles.
h. Tail Rotor Components
(1) Pitch Change Beam. The pitch change beam is attached to the outboard end of the pitch control
shaft. The pitch change beam is secured to the pitch control shaft by a retaining nut and washer. It
provides the attachment for the pitch change links.
(2) Bonding Cables
(a) The bonding cables are attached at the outboard end of the pitch beam and pitch horn of the
tail rotor blade.
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(b) A bonding cable is mounted with the each pitch change link at the pitch beam and paddle
pitch horn for grounding purposes and static electricity discharge.
(3) Retention Plates
(a) The two retention plates, inboard and outboard, attach the tail rotor blades to the tail rotor
assembly.
(b) The outboard retention plate, and the inboard retention plate, provides installation and
security of the two tail rotor paddle spars.
(c) The outboard retention plate compresses the two tail rotor blade spars together.
(4) Inboard Retention Plate
(a) The inboard retention plate is centered on the tail rotor pitch change shaft by the two sets of
split cones and secured by a retaining nut.
(b) The two sets of split cones are similar to the split cones used on the main rotor; they are a
matched set and if not seated properly, will create an unbalanced condition at the tail rotor.
(5) Tail Rotor Blades
(a) The tail rotor blades compose a cross-beam tail rotor blade system providing anti-torque
action and directional control. The blades have an incorporated 18 degree negative twist.
(b) The tail rotor blade is built around two graphite composite spars running from tip-to-tip and
crossing each other at the center to form the four blades.
(c) The two spars are interchangeable and can be replaced individually.
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(d) The tail rotor blade spar is a graphite structure. The graphite structure of the tail rotor blade
eliminates the use of bearings, allowing for the pitch change motions and flapping of the tail rotor.
(e) The tail rotor blade pivots at the torque rib on the blade, transmitting the inputs from the pitch
change links to the blade spar through the pivot bearing.
(f) The blade cover is fiberglass, the inboard fiberglass section is not bonded to the spar,
however, the outboard section is bonded to the spar over the blade core.
(h) The tip cap of the tail rotor paddle is made of Kevlar and is replaceable in the field. The
outboard end of the paddle has a core of Nomex and aluminum honeycomb. The aluminum honeycomb
covers the top and bottom near the spar, while the Nomex is used aft of the spar.
(i) Filler and lead weights cover the leading edge of the spar. The filler provides shape, and the
weights balance the paddle with the master weight.
(j) For a de-ice system, the paddle has a de-ice heater mat and a de-ice electrical connector. The
de-ice heater mat is an electro thermal blanket that is bonded into the paddle leading edge. The electrical
connector provides the electrical continuity between the de-ice heater blanket and the de-ice system.
(6) Pivot Bearing. The pivot bearing maintains the blade centered during pitch changes. The pivot
bearing is bolted to the blade and held to the spar by means of a bonded retention plate.
(7) Tail Rotor Boot. The tail rotor boot prevents the entry of dirt and debris at the pitch change horn of
the tail rotor paddle and is held in place with a tie wrap on each end.
(8) Pitch Change Links. The pitch change links are attached to the outboard ends of the pitch beam
and the pitch horn of each the tail rotor blades.
(a) Four pitch change links are installed on the tail rotor assembly. Each link connects an arm of
the pitch change beam to a pitch control horn on the paddle.
(b) The links transmit movement necessary for paddle pitch changes from the pitch change
beam. Each link consists of two rod ends, locking devices, and a link. The rod end that is connected to
the pitch change beam is marked for proper installation.
(9) Counter Weights.
(a) The counter provides a centering function for the tail rotor blade in the event of a pitch control
rod failure.
(b) There is a counter weight on the tail rotor blade to keep the blade centered if the pitch control
rod should fail. It functions by countering the weight of the pitch horn and holding the tail rotor in a neutral
pitch of 7.5 degrees.
(c) When inspecting the area located near the counter weight, check the blade de-ice cable for
security and the mount for cracks.
Learning Step/Activity 6. Describe the operational characteristics that pertain to the Main Rotor system.
a. Main Rotor Head Assembly
(1) The main rotor head is the mounting platform for the main rotor hub, spindle modules, droop
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stops, anti-flap restrainers, dampers, and four fully articulating blades.
(2) The principal components of the main rotor head are the bifilar vibration absorber, main rotor hub,
spindle modules, droop stops, antiflap restrainers, dampers, and four fully articulating blades.
b. Bifilar Assembly
(1) The bifilar assembly is a vibration absorber located on top of the main rotor mast, and secured to
the mast extension.
(2) The bifilar assembly absorbs main rotor vibrations and stresses, which contributes to the life of all
components and provides a smoother ride for the crew and passengers.
(3) The bifilar is a cross shaped aluminum forging. A steel weight pivots on two points at the end of
each arm. Weights are secured to the arms of the bifilar by two pins.
(4) During operation, the weights are placed in a balanced position by centrifugal force to dynamically
absorb vibrations created by the main rotor.
c. Hub Assembly
(1) The hub assembly is the mounting platform for the spindles, dampers, pitch control rods, antiflapping assemblies and bifilar assembly.
(2) The hub assembly is a titanium, one-piece casting with four arms for installation of the four
spindles. Internal splines mate with the external splines of the mast extension.
(3) Each arm is prelagged and preconed 7 degrees to reduce the torque bending movement from the
head and also reducing vibrations.
d. Spindle Module
(1) The main rotor spindle module is the mounting platform for the blade, and has an elastomeric
bearing.
(2) The spindle and liner assembly contains the anti-flap assembly, droop stop support ring, balance
weights and bracket, damper bracket, spindle horn, spindle cuff lug, and the tie rod.
(3) The tie rod is attached through the elastomeric bearing and the bearing allows the blade to lead,
lag, flap, and permits movement of the blade about its axis for pitch changes.
(4) Elastomeric bearing.
(a) The elastomeric bearing is a series of laminated rubber and steel bonded to form the two
section module.
(b) One section of the bearing, the cylindrical bearing, provides the pitch change axis.
(c) The other section of the bearing, the conical bearing, allows for the hunting axis (lead, lag,
and flapping).
(5) Spindle Module Purpose
(a) The purpose of the spindle module is five part.
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(b) The first is providing the main rotor blades an attachment point. The main rotor blades are
attached to lugs on the outboard end of the spindle module.
(c) The second part is pitch change.
(d) The third and fourth are hunting, and flapping. Hunting, or lead and lag, is provided by the
module during the rotation of the main rotor head. Flapping is also a purpose provided by the spindle
module.
(e) Lastly, the spindle module absorbs centrifugal force loads with the aid of the expandable pins.
(6) Droop Stop Location/Purpose
(a) The droop stop is mounted to the elastomeric bearing ears, with the droop stop support ring
mounted to the spindle.
(b) The droop stops prevent excessive blade droop during low rotor rpm or when the rotor stops.
(c) During run up, as the rotor rpm accelerates above 58 percent, the centrifugal force starts to
override the spring tension, causing the droop stop to start moving outward.
(d) As the rotor rpm continues to increase (70 to 75 percent Nr), the centrifugal force increases to
sufficiently move the stops to the full "OUT" position, allowing full vertical movement to the main rotor
blade.
(e) During low rotor rpm at 50 percent Nr, the return spring tension overrides the centrifugal force,
forcing the droop stop to move into the "IN" position.
(f) This reduces blade droop, providing protection to the helicopter and ground crew. The ground
crew should check for proper operation, especially during run up and shutdown procedures.
NOTE: If one or more droop stops do not go in during rotor shutdown, shut down an engine to lower
rotor idling RPM in an attempt to seat the droop stops. If droop stops still do not go in, accelerate rotor to
above 75% RPM R. Repeat rotor shutdown procedures slightly displacing cyclic in an attempt to dislodge
jammed droop stop. If droop stops still do not go in, make certain that rotor disc area is clear of
personnel and proceed with normal shutdown procedures while keeping cyclic in neutral position.
(7) Anti-flap Restrainers Location/Purpose
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(a) The anti-flap restrainer is mounted to the upper portion of the spindle and liner assembly.
(b) These are spring-loaded locks that prevent the blade from flapping when the rotor is at a low
speed or stopped.
(c) Anti-flap Restrainers Operation. The anti-flap restrainers operate in the same way as the
droop stops, except the operating rpm is lower.
(d) Above 35 percent Nr, the anti-flap restrainer moves out of the "LOCK" position to permit
flapping and coning of the main rotor blades.
(e) Below 35 percent Nr, the anti-flap restrainer moves back to the "LOCK" position.
CAUTION: During engine start and run-up, ensure that cyclic is kept in neutral, collective no more than
one inch above full down and pedals centered until % RPM R reaches 50% minimum to prevent damage
to anti-flap bracket bushings. To prevent damage to anti-flap stops, do not increase collective pitch at
any time during rotor coast-down.
e. Damper
(1) There are four dampers mounted to the main rotor hub; one between each of the spindles and
hub assembly.
(2) The dampers provide stops to restrain hunting (lead and lag motions) of the blades during
rotation keeping the blades in the proper position in relation to each other.
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(3) They will also absorb rotor head engagement loads and absorb some of the shock to the main
rotor system when collective inputs are made.
(4) The stops of the dampers allow for a 17-degree lag and a 3-degree lead of the main rotor
blade.
(5) The damper is a hydraulic type, and is supplied with pressurized fluid from a reservoir
mounted on the side of the damper. The reservoir is serviced with MIL- H-5606 hydraulic fluid, and can
be checked for fluid level by an indicator attached to the reservoir.
(6) A visible gold band on the indicator piston indicates the reservoir does not require servicing.
f. Main Rotor Blade Description
(1) The main rotor blade is 24 feet 4 inches in length, weighs 214 lbs, the chord is 21.75 inches,
and the rotor disk is 53 feet 8 inches.
(2) Main Rotor Blade Expandable Pins
(a) The main rotor blade is attached to the spindle by two expandable pins.
(b) No tools are required for removal or installation of the expandable pins. When inspecting
the expandable pins, check the expandable pin arm (locking handle) for positive locking.
(c) As the arm of the pin is opened or closed, the sleeves around the pin expand or contract
to ensure a secure attaching point between the blade and the spindle.
(d) Adjustments to the expandable pin are accomplished by the adjustment nut, located at
the bottom of the pin.
(3) Main Rotor Blade Construction
(a) The main rotor blade is constructed of a one piece titanium alloy spar, nickel abrasive
strip, wire mesh for lightning protection, heater mat for blade de-ice, fiberglass skin covering aft of the
leading edge, and a Nomex honeycomb core.
(b) An 18 degree negative twist is built into the main rotor blade, providing equal lift across
the blade span during main rotor operation.
(c) An adjustable trim tab is located on the trailing edge of each blade to allow for rotor
smoothing, and minimizing vibrations to the aircraft.
(4) Main Rotor Blade Tip Cap
(a) The tip cap is swept aft 20 degrees.
(b) This reduces the vortices at the tip and permits forward air speed to be increased
approximately 6 knots. It also reduces the noise signature of the aircraft.
The blade is made and balanced in relation to a master blade.
(5) Nitrogen Charge
(a) A nitrogen charge of approximately 10 psi, is located in the cavity of the spar.
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(b) The nitrogen charge provides a means of monitoring the main rotor blade for an unforeseen
combination of events that may impair the structural integrity of the spar, or allow nitrogen to escape
through leaking seals.
(c) A spar (BIM) Blade Inspection Method is located on each main rotor blade at the root end of
the trailing edge.
(d) The BIM indicator provides a visual indication when the spar structural integrity is degraded.
If a spar cracks or a seal leaks, the nitrogen will escape from the spar. When pressure drops below the
minimum, the indicator will show red bands.
(e) The spar indicator BIM compares a reference pressure built into the indicator to the pressure
within the spar. This compensates for temperature changes.
(f) A daily check of the BIM is a visual, checking for yellow bands.
g. Pitch Control Rods
(1) The pitch control rods are adjustable flight control rods extending from the swashplate to the pitch
horn of the main rotor spindle.
(2) Control inputs made in the cockpit area are transmitted through the flight controls, swashplate,
pitch control rods, and to the blades.
(3) Pitch Control Rod Bearings
(a) There are two types of main rotor pitch control rod bearings, spherical and elastomeric.
(b) These rod end bearings are a non-lubricated bearing and cannot be mixed.
(c) Each pitch control rod is color coded per main rotor blade color code.
h. Swashplate Assembly
(1) The swashplate slides on the main rotor shaft and tilting in any direction following the motion of
the flight controls.
(2) The swashplate has stationary and rotating discs joined by a bearing.
(3) The swashplate transmits flight control movement to the main rotor head through the four pitch
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control rods.
(4) The swashplate is permitted to slide on the main rotor shaft and tilt in any direction following the
motion of the flight controls.
(5) Swashplate Assembly Components . The major components of the swashplate assembly can be
separated into three areas, uni-ball bearing, controls, and the swashplate bearing.
(6) Swashplate Bearing.
(a)The swashplate bearing (duplex bearing) separates the stationary and rotating swashplates,
and is covered by an overlapping plate (stationary bearing retaining plate). This bearing is a lubricated
type ball bearing that performs radial and axial load sharing.
(b) When inspecting the swashplate for signs of wear of the duplex bearings, visually inspect the
rotating and stationary disks at the stationary bearing retaining plate for metal particles in extruded
grease.
(c) Extruded grease is not a cause for replacement of the swashplate, however, if metal particles
are found in the grease, or grinding is experienced, replace the swashplate.
(7) Uni-Ball Bearing
(a) The uni-ball bearing is a spherical type bearing, contained in the stationary swashplate, and
provides universal movement for the swashplate.
(b) To maintain smoothness of motion, there is a teflon liner between the swashplate guide to
uni-ball, and uni-ball to swash plate.
(c) When inspecting the uni-ball look for peeling of the plating or pieces of teflon coming out of
the bearing. Signs of a reddish brown powder between the swashplate and uni-ball are normal.
(d) A collective chatter (ratcheting) in the flight controls is an indication of binding between the
uni-ball and the swashplate guide. A cyclic chatter (ratcheting) in the flight controls is an indication of
binding between the uni-ball and stationary swashplate.
(8) Controls
(a) There are three servo links (forward, aft and lateral) which attach to the inner stationary ring of
the swashplate.
(b) These links transmit the inputs from the cockpit to the swashplate.
(c) During inspection of the expandable pins, check the nut for the cotter pin installation.
i. Bridge Assembly
(1) The bridge assembly is a framed mounting point for the bell cranks and control rods. The bridge
assembly is a deck-mounted frame with bell cranks and control rods.
(2) The bridge assembly is located on the forward side of the main transmission, and provides a
mechanical connection from the flight control servos to the main rotor swashplate.
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(3) The three control inputs connecting the primary servos to the main rotor swashplate are the
forward, aft, and lateral control rods. The forward control rod is located on the right hand side of the
bridge. The aft control rod is located at the center of the bridge. While the lateral control rod is located on
the left hand side of the bridge.
j. Scissors
(1) There are two main rotor scissors located on the main rotor system attached at the rotating
swashplate and lower pressure plate.
(2) The main rotor scissors are connected to the swashplate and the lower pressure plate.
(3) The rotating scissors change non-rotational input into rotational output to the main rotor head.
k. Main Mast Extension
(1) The main mast extension raises the main rotor 15 inches above the top of the fuselage, reducing
vibrations caused by the main rotor.
(2) The main mast extension (shaft extension) is a tubular component that is installed on the main
XMSN mast.
(3) Removal of the shaft extension allows the main rotor to be lowered for transporting the aircraft on
a C-141, however, removal is not required when transporting the aircraft on a C-5A.
(4) The lower pressure plate and split cones, in conjunction with the main shaft nut, secure the shaft
extension to the main shaft. The lower pressure plate also provides attachment for the rotating scissors.
The main rotor hub is supported by the main rotor shaft extension. The upper pressure plate and split
cones secure the main rotor hub to the shaft extension.
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