Vibration Control of Mode Shapes at Blade Pass Frequency

The project involved a turbine-powered helicopter subject to a demanding vibration test profile (MIL-STD-810). Within the 10–400 Hz range, random vibration from the turbine was overlaid with discrete sinusoidal components at blade pass frequencies (BPF). These frequencies varied with rotor speed and had bandwidths that had to be avoided to prevent structural resonance.

The lightweight aluminium power distribution unit, fabricated from 1.6 mm sheet to minimise weight, lacked stiffness in the vertical axis. During testing, its main contactor opened unexpectedly at the A1 BPF, jeopardising system reliability.

We analysed the vibration break-point diagram and mode shapes of the enclosure to identify the coupling between natural frequencies and the blade pass frequency. Structural testing on a shaker system replicated the MIL-STD-810 profile, confirming that the vertical axis response was the most critical due to low stiffness. The investigation compared geometry-based solutions with orientation-based alternatives to determine the most effective approach for shifting resonances away from excitation bands.

Two potential paths were identified:

• Structural redesign: Adjusting the geometry and topography of the enclosure to increase stiffness and shift mode shapes away from the blade pass frequencies.
• Component reorientation: Mounting the contactor laterally or longitudinally rather than vertically. These axes exhibited higher stiffness and significantly reduced amplification at the BPF.

The most efficient solution was to reorient the contactor, eliminating resonance without requiring extensive redesign or weight penalties.

• Contactor functioned reliably without opening under BPF excitation
• Vibration testing successfully passed to MIL-STD-810 requirements
• Lightweight design maintained, supporting fuel efficiency and operational cost savings
• Improved fatigue margins through reduced resonance amplification

• Vibration Analysis
• FEA
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Led by a Chartered Mechanical Engineer (MBA CEng MIMechE, MIoA) with 29 years’ experience in vibration, dynamics and acoustics, delivering projects internationally across aerospace, defence, automotive, energy and recycling.

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