THEORY: Blade Rate Vibration
By Mark Lester
Blade Rate Vibration
In a helicopter, the main rotor produces a Blade Rate (or N per rev) vibration as it passes through the air mass. This vibration is caused by the outward change in blade lift vector as each blade flaps in order to reduce lift when advancing with respect to the relative wind.
As the name would imply, the Blade Rate vibration occurs at a frequency of rotor speed times number of blades. As an example, a helicopter operating in forward flight with a Main Rotor speed of 390 RPM and having three main rotor blades will produce a Blade Rate vibration at 1170 CPM.
This Blade Rate vibration normally increases in amplitude with increased airspeed as the rotor moves faster through the air mass and the advancing blade flapping action increases.
Vibration Isolation Systems
Many helicopter designs attempt to reduce the effect of Blade Rate vibration on the aircraft occupants by isolating the Main Rotor and Main Transmission from the airframe.
Vibration isolation can be provided by simple elastomeric transmission mounting systems or by a complex suspension system which provides additional Main Rotor and Main Transmission isolation.
Dynamic Vibration Absorbers
In addition to the basic vibration isolation method described above, some helicopter designs incorporate a Dynamic Vibration Absorber (DVA).
The DVA is a resonant mechanical structure (usually composed of a beam and mass combination) which is attached to the aircraft structure and precisely tuned to the frequency of the vibration which is to be suppressed.
When the DVA is excited by vibration at its natural frequency, it momentarily absorbs the vibration energy and then feeds that energy back into the airframe structure shifted in phase by 180 degrees.
As a result of this behavior, the DVA will tend to suppress the unwanted Blade Rate Vibration and create a "sweet spot" in the airframe structure where the occupants will enjoy a comfortable ride.
Active Vibration Absorbers
In a few helicopter designs, an Active Vibration Reduction System (AVRS) is used to reduce Blade Rate vibration.
The AVRS incorporates a computer which constantly monitors the vibration level and then manipulates an active vibration control element (such as a hydraulic actuator) to actively suppress the unwanted vibration.
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