12.03 Blade Tracking And Vibration Flashcards
(30 cards)
What is achieved by correct rotor alignment?
Rotor alignment involves checking that all blades of the main or tail rotor are aligned correctly and have a satisfactory relationship to each other so that they are travelling on a common tip plane.
How are adjustments made for rotor alignment?
In most cases, the adjustment is carried out via the control rods on the rotor blade roots or by trimming the trailing edge of the blades.
When must rotor alignment be carried out?
The rotor must be aligned after a blade has been replaced or the rotor has had a major service.
On semi-rigid rotors how is blade alignment achieved?
The procedure involves moving the blades about the lead-lag axis held stationary during the operation by the drag brace or latch pins, depending upon the design of the head.
On metal blades what is added by the manufacturer to indicate an alignment point?
On metal blades only one pin exists, referred to as the alignment pin, located very close to the tip of the blade.
On large semi-rigid rotors what procedure is used in alignment?
A scope is placed on a special fixture attached to the yoke of the main rotor. The alignment points are then sighted from the scope to the blade pin in each direction, eliminating errors from using a string over a long span.
What other term can be used to describe blade alignment?
It is sometimes referred to as chordwise balance.
What is blade sweeping?
Blade sweeping is a procedure used after installation of new blades, head, or major components of the head, done only on semi-rigid rotor systems and only after blade alignment has been made.
What would be an indication of an unbalanced condition on the rotor?
A 1:1 lateral vibration from the main rotor is always an indication of an unbalanced condition.
What indications would a chord-wise imbalance produce?
Indications of chord-wise imbalance are nudging of the cyclic in flight and a heavy collective, indicating the necessity of sweeping the blades.
Before carrying out any adjustments for blade sweeping what should be done?
It is important that the original alignment setting be marked to ensure any adjustments can be reset to the original position.
Define what is meant by tracking?
Tracking is a procedure used to check that all blades are traveling within the same tip path plane.
What different methods are there to determine track on helicopters?
Stick Method, Flag Method, Light Reflector Method, Pre-track Method, Electronic or Strobe Method.
What is the first track you carry out?
All tracking procedures will begin with ground tracking.
How is pre-track achieved on blades, pre-installation on the aircraft?
The blade is matched to a master blade in the factory and pre-flown in a spin test, adjusting the pitch change rod to ensure it flies in the same tip path plane as the master blade.
After a pre-tracked blade is installed how is the blade set up?
The correct length is simply set on the pitch change rod and the blade is in track.
Describe how an electronic strobe system operates.
Reflectors are placed on the tips of the blades facing the cabin, and the strobe light is triggered by an interrupter and pick-up mounted on the swashplate, allowing the strobe light to be flashed each time the blade passes a certain point.
How is the ground track usually adjusted?
The ground track is adjusted by changing the angle of attack of the individual blade through the pitch change rod.
How is the in-flight track usually adjusted?
Most blades are limited to flight tracking only, using fixed trim tabs that are bent to change the angle of attack of the blade.
Describe what is meant by a Piezo-electric type accelerometer?
This device senses vibration through a special crystal that generates an electrical signal when it is strained or squeezed.
How is vibration indicated and what levels of reduction have been achieved?
Vibration levels are displayed on a meter in Inches Per Second (ips), with the goal to reduce this level to 0.1 ips or less, although 0.2 ips is a more realistic goal.
How is the phase angle determined during electronic balancing?
The phase angle is determined by a signal from a magnetic pick-up and the filtered accelerometer signal.
Describe what is meant by Vibration?
Vibration is a motion characterized by repeated oscillations about an average position.
Describe what is meant by a one-per-rev vibration?
An unbalanced rotating component causes vibration at the frequency that the component rotates, referred to as a one-per-rev vibration.
