Helicopter Vibration

Helicopter Vibration | Types of Vibration in Helicopter, Helicopter Track and Balance Methods

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:

Affects on the health of flight crew
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

The four types of helicopter vibration are as follows:

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.