Vibration Control of Mode Shapes coinciding with the Blade Pass Frequency
This project required the manipulation of mode shapes and its natural frequencies to not coincide with the blade pass frequency (BPF).
Was it necessary? Or was there an easier solution.
Vibration Test Profile
The vibration test profile for rotary aircraft (Mil-Std-810) is shown within this section.
In the vibration break point diagram there is a random frequency profile with four discrete orders of the rotor blade pass frequency.
The vibration profile in detail
For a turbine powered helicopter, the profile between 10 Hz and 400Hz is random vibration, representing the vibration from the turbine engine during operation.
The four An (vertical lines) are the sinusoidal vibrations present on the helicopter. These frequencies vary from one helicopter to another.
As the turbine can operate at different speeds, so will the blade passing frequencies; therefore, these Ans have bandwidths.
These sinusoidal frequency bandwidths must be avoided to not excite natural frequencies in any structures of and in the helicopter.
Case study
Vibration control in this example pertains to designing the instrumentation and its structure not to have natural frequencies coinciding with these bandwidths.
As with any aerospace project, the lighter the better - less weight; consequently less fuel consumed, therefore less costly to operate.
Light weight design
The power distribution unit was fabricated out of 1.6mm thick aluminium sheet. This is light weight, and lacked stiffness in the vertical axis.
Inside the unit was a main contactor to bridge the power from the generator to the helicopter electrical system. This was mounted vertically
The problem
During the vibration test on the shaker using the vibration profile within this discussion, the contactor opened at the same frequency as the A1 BPF.
Control the vibration
With the enclosure being ultra light, stiffness was the only aspect on which the engineers could focus
By changing the geometry and topography of the enclosure, they were able to manipulate the natural frequencies and its corresponding mode shapes to ensure natural frequencies did not coincide with the blade pass frequencies.
But was there an easier way?
Yes!
The contactor that opened at blade pass frequency was mounted vertically in the distribution unit. The excitation in this direction was amplified by the base/floor of the unit, as it lacked stiffness, but the excitation in the longitudinal and lateral direction did have stiffness and much less amplification.
The quick fix was to reorientate the contactor to the lateral or longitudinal direction. These axes have less vibration than the vertical axis, as the vertical axis is predominantly excited by the blade pass frequency.
Further Information
For more information or help with vibration control in vibration environments, please call us on 01908 643433.
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