Helicopter Vibration | Types of Vibration in Helicopter, Helicopter Track and Balance Methods
Helicopter Vibration
Mechanical failures and major
safety concerns might result from helicopter vibration. When rotating items
become uneven, or when one side is heavier than the other, vibration occurs in
helicopters. The rotation is uneven because the excess weight on one side
upsets the center of gravity. When the propellers rotate, the uneven weight
moves in a circle, wobbling the movement.
Manufacturers of engines and
propellers each have an own method for doing a vibration study for various
purposes.
Dangerous vibration levels can
lead to a number of problems, including:
- Makes the aircraft unsafe for flying
- shortens the service life of the rotor and other parts of the helicopter
- Can crack the sheet metal and component
- Creates a higher-than-normal oil leaks and light bulb failures
- Induces physical movement of the airframe (buzz in the seat, yoke, and rudder pedals) of helicopter
- Extreme Noise in the cabin
- Cause helicopter to break
- People who are using the helicopter become fatigued
- Reduces personal comfort to passengers & crew
Rotor Track and Balance to Reduce
Vibration in Helicopter
For an aircraft that endures vibration, helicopter rotor track and balance are crucial. The rotor blades rotate around a fixed point at extremely high speeds, and because they pitch the aircraft at different angles to control it and create lift, they are vulnerable to wear and damage while in flight. The rotor may fall out of balance because to the wear and strain they endure. Vibration brought on by rotational motion is lessened with the use of rotor blade tracking and balancing. Additionally, it can guarantee that a crucial component performs at the right level and under ideal circumstances.
All helicopters include rotor
height and weight changes for vibration reduction. A helicopter that is out of
alignment could potentially vibrate so much that it would shatter. By adjusting
the blade's pitch, blade height can be changed. Weight can be changed by adding
or removing weights at the blade end caps or on the rotor head. Most also
include height and pitch vibration dampers. Some people also detect and reduce
vibration using mechanical feedback systems. Typically, a mass is used as a
"stable reference" by the feedback system, and a linkage from the
mass activates a flap to change the rotor's angle of attack in order to dampen
vibration. Because measuring vibration is challenging and typically
necessitates sophisticated accelerometers installed throughout the airframe and
gears, adjustment can be challenging.
Use of a stroboscopic flash lamp
and observation of painted markings or coloured reflectors on the bottom of the
rotor blades is the method most frequently used to evaluate blade vibration
adjustment. Installing coloured chalk on the rotor tips and observing how it marks
a linen sheet is the conventional low-tech method. Vibration monitoring and
rotor track and balancing solutions are offered by Health and Usage Monitoring
Systems (HUMS) to reduce vibration. The
most frequent solution to gearbox vibration is a gearbox overhaul or
replacement. Vibrations from the drive train or gearbox can be very dangerous
for a pilot. Pain, numbness, and a lack of tactile differentiation are the most
serious side effects.
Types of Helicopter Vibration
Extreme Low Frequency Vibration
The only rock that exhibits
extremely low frequency vibration is pylon rock. The rotor, mast, and
transmission system all have an intrinsic tendency to cause pylon rocking (two
to three cycles per second). Transmission mount dampening is used to reduce the
vibration before it reaches audible levels.
Low Frequency Vibration
The rotor produces low frequency
vibrations (1/rev and 2/rev). There are two fundamental forms of 1/rev
vibrations: vertical and lateral. A 1/rev is simply generated when one blade
produces more lift than the other blade does at a given point.
Medium Frequency Vibration
Another vibration that is common
to most rotors is the medium frequency vibration (4 and 6 revs per revolution).
A decrease in the fuselage's ability to absorb vibration or a loose airframe
part, like the skids, shaking at that frequency, are the two factors that
contribute to an increase in the intensity of these vibrations.
High Frequency Vibration
Anything inside the helicopter
that rotates or vibrates at exceptionally high speeds has the potential to
create high frequency vibrations. When the tail rotor engine, fan, or shaft
assembly rattles or rotates at a speed equal to or greater than the tail rotor,
a high frequency vibration frequently results.
Rotor Blade Tracking
Blade tracking is the process of
locating the tips of the rotor blades in relation to one another as the rotor
head rotates and figuring out the adjustments required to maintain these
positions within predetermined tolerances. The blades ought to follow each
other as closely as they can. Blade tracking aims to align all blade tips along
the same tip path for the duration of each rotation cycle. Below are explanations
of various blade tracking techniques.
Flag and Pole
The relative positions of the
rotor blades are determined using the flag and pole method with chalk or a
grease pencil, the blade tips are marked. To make it simple to determine the
relationship between the other tips of the rotor blades, each blade tip should
be designated with a different colour. All helicopter models without jet
propulsion at the blade tips can employ this technique. For specific
procedures, consult the appropriate maintenance manual.
Electronic Blade Tracker
The Balancer/Phazor, Strobex
tracker, and Vibrex tester make up the most typical electronic blade tracker.
Blade tracking is possible with the Strobex blade tracker from inside or
outside the helicopter when it is on the ground or in flight. To create the
illusion that a stationary target at the blade tips is stopped, the system
employs a highly concentrated laser beam that flashes in time with the rotation
of the main rotor blades. An extended retroreflective number that is taped or
fastened uniformly to the underside of each blade serves as its identification.
The taped numbers will look normal when viewed from inside the helicopter. Tracking
can be done using a strobe light and tracking tip cap reflectors. Each blade's
tip has the tip caps temporarily fastened to them. In rhythm with the whirling
blades, a bright strobe light flashes. The electrical power supply of the
helicopter powers the strobe light. The track of the rotating blades can be seen
by looking at the picture of the tip cap that is reflected. Ground monitoring,
hover verification, forward flight tracking, and autorotation rpm adjustment
are the four processes that make up the tracking process.
Tail Rotor Blade Tracking
The next paragraphs outline the
marking and electrical methods of tail rotor tracking.
Marking Method
The following steps are required
to monitor a tail rotor using the marking approach.
• Check the tail rotor rigging
and keep track of the tail rotor blades after replacing or installing the tail
rotor hub, blades, or pitch change system. Before tracking, the tail rotor tip
clearance must be established, and it should be checked again after tracking.
If one is available, the tracking
device with strobes may be employed. There are usage instructions included with
the gadget. A six-inch piece of soft rubber hose should be attached to the end
of a flexible object, such as a one and a half to two inch pine stick. Spread
Prussian blue or a comparable colour that has been thinned with oil over the
rubber hose.
NOTE: Only authorized personnel
may execute ground run-up. In line with the relevant maintenance handbook,
start the engine. Put the pedals in neutral and start the engine. Reset the
marking device on the tail boom assembly's underside. Move the marking tool
into the tail rotor disc slowly, about an inch from the tip. When a blade is
close by, turn off the engine and wait for the rotor to stop. Extend the pitch
control link of the unmarked blade by one half a turn after repeating this
process until the tracking mark crosses over to the other blade.
Electronic Method
The Model 177M-6A Balancer, the
Model 135M-11 Strobex, the track and balance charts, an accelerometer, cables,
and mounting brackets are all included in the electronic Vibrex balancing and
tracking kit, which is kept in a carrying bag.
A helicopter's main rotor and
tail rotor vibration levels are measured and displayed using the Vibrex
balancing kit. The quantity and position of the rotor track or weight change
are calculated using the Vibrex's analysis of the vibration caused by
out-of-track or out-of-balance rotors and a chart that plots vibration
amplitude and clock angle. The Vibrex is additionally used for troubleshooting
by determining the rpm, vibration levels, and frequencies of unidentified
issues.
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