Propeller-driven Vibration Testing (PVT)

• Propeller-driven Vibration Testing (PVT) is an output-only experimental methodology that uses the broadband vibratory forcing from rotating propellers for modal identification.
• It leverages operational modal analysis techniques to extract modal parameters such as frequencies, damping ratios, and mode shapes from structural response data.
• Its cost-effectiveness and flexibility are demonstrated in aerospace and marine applications, with advancements in excitation control, sensor optimization, and flow-conditioning strategies.

Propeller-driven Vibration Testing (PVT) is an output-only experimental methodology for modal identification in structures subjected to propeller-induced excitation. Characterized by its reliance on broadband vibratory forcing from rotating propellers, PVT has gained attention as a cost-effective and logistically flexible alternative to traditional input-controlled techniques such as Ground Vibration Testing (GVT). Deploying PVT, modal parameters—frequencies, damping ratios, and mode shapes—can be efficiently extracted from structural response measurements, often leveraging pre-existing propulsion systems, thus aligning PVT with the family of Operational Modal Analysis (OMA) approaches. Applications in aerospace and marine engineering demonstrate PVT’s utility for both modal model validation and flow-induced vibration/noise mitigation (Dessena et al., 13 Jan 2026, Cheng et al., 2024).

1. Conceptual Basis and Methodological Rationale

PVT operates as an OMA method, wherein the system’s excitation is not directly measured or controlled, but is assumed broadband and persistent, as provided by the rotation of a propeller. In contrast to GVT, which deploys instrumented hammers or shakers with precisely known inputs, PVT stimulates the test article via the unsteady aerodynamic and inertial loads generated by a spinning propeller mounted directly or indirectly on the test structure. This places PVT alongside Taxi Vibration Testing (TVT) and Flight Vibration Testing (FVT) as a non-intrusive approach suitable for ground or operational scenarios where direct input instrumentation is impractical (Dessena et al., 13 Jan 2026).

The motivation for PVT includes the reduction of campaign duration and cost, exploitation of existing propulsion architecture (notably in electric or hybrid-electric regional aircraft), and the potential for test repeatability without extensive infrastructure or operational constraints. For marine and hydrodynamics applications, PVT-style analysis extends to the study of flow-induced vibrations and noise in flexible hydrofoils subject to propeller/flow-generated forcing (Cheng et al., 2024).

2. Experimental Implementation

2.1 Structural Test Articles and Instrumentation

A prototypical PVT experiment features a cantilever beam, such as an Aluminium 7075-T6 wing spar, with a free span extending beyond a fixed root. The spar’s geometry provides a realistic stiffness distribution with managed complexity, and modal observability is enhanced by instrumenting spanwise positions with mono-axial accelerometers. Sensor placement can be optimized via the AutoMAC criterion to minimize mode shape correlation errors for the dominant bending/torsion modes. Sensitivities are typically stratified to accommodate varying amplitudes across the span (Dessena et al., 13 Jan 2026).

2.2 Excitation Mechanism

The central excitation system consists of an outboard electric motor (e.g., TMOTOR AS2317) powering a multi-blade propeller at prescribed distances from the root. Motor control is achieved using electronic speed controllers (ESCs) and radio receivers, allowing for both steady-state (constant throttle) and dynamic (throttle sweep) forcing cases. Throttle is typically commanded in percentage terms—representative test matrices span baseline (motor-off), fixed-throttle, and sweep scenarios over durations sufficient for spectral convergence.

2.3 Data Acquisition and Spectral Analysis

Acceleration responses are acquired at kHz-scale rates (e.g., $f_s = 1066$ Hz) across all sensor channels. Signal conditioning includes normalization and ensemble averaging to compute the Average Normalized Power Spectral Density (ANPSD), revealing dominant resonance peaks and the prevalence of propeller-induced harmonics—particularly at integer multiples of blade-pass frequencies. Throttle sweeps are vital for broadening excitation spectra, thereby dispersing narrowband lines associated with steady rotation and mitigating spectral masking of structural resonances (Dessena et al., 13 Jan 2026).

Empirical marine PVT employs similar principles, augmented by surface and field pressure probes, hot-wire anemometry, and particle image velocimetry (PIV) for flow quantification upstream of propellers (Sluchak, 6 Feb 2025, Cheng et al., 2024).

3. Modal Parameter Extraction and Analytical Frameworks

3.1 Post-processing Algorithms

Modal parameters are reconstructed using output-only system identification algorithms. In the referenced wing spar study, the Natural Excitation Technique with the Loewner Framework (NExT-LF) was employed:

• Cross-correlation estimation: For zero-mean acceleration data $x(t) \in \mathbb{R}^7$, the cross-correlation matrix $R_{xx}(\tau)$ is computed to characterize the pseudo-impulse response.
• Frequency-domain reduction: Loewner and shifted Loewner matrices are constructed at prescribed frequency samples and truncated using singular value decomposition.
• Eigen-analysis: Modal frequencies $\omega_k$, damping ratios $\zeta_k$, and associated mode shapes $\phi_k$ are extracted from the generalized eigenvalue problem.
• Mode shape correlation: The Modal Assurance Criterion (MAC) quantifies mode-shape consistency between runs (e.g., reference vs. sweep).

3.2 Representative Results

Natural frequencies and damping ratios for the first three modes are reported in the table below for both baseline and swept excitation (Dessena et al., 13 Jan 2026):

Mode	$\omega_n^{(i)}$ [Hz]	$\omega_n^{(vii)}$ [Hz]	$\Delta$ [%]
1	2.48	2.45	-1.21
2	13.37	13.30	-0.52
3	24.10	22.84	-5.23

Mode	$\zeta_n^{(i)}$	$\zeta_n^{(vii)}$	$\Delta$ [%]
1	0.008	0.014	+75.0
2	0.012	0.015	+25.0
3	0.028	0.031	+10.7

The MAC reveals near-perfect mode shape correlation for the first two modes ($>0.99$), with the third mode exhibiting reduced repeatability and a frequency shift suggestive of propeller-induced bending–torsion coupling and non-ideal sweep control ($\text{MAC}_3 = 0.827$).

4. PVT in Marine, Hydroelastic, and Noise-Vibration Studies

Cheng et al. performed high-fidelity numerical “PVT” on a flexible NACA hydrofoil in turbulent, potentially cavitating, flow using combined Large-Eddy Simulation (LES), mixture-model cavitation, and modal superposition. Tip vortex and trailing-edge vortex shedding, as well as sheet-cavitation, were found to synchronize with structural modes, resulting in amplified flow-induced vibration and radiated noise (Cheng et al., 2024). Key findings include:

• Tip-vortex shedding (Strouhal $St \approx 0.2$) drives low-frequency lift fluctuations and hydroelastic excitation.
• Trailing-edge shedding in flexible foils introduces additional spectral peaks (e.g., $St \approx 1.8$), correlating with higher-order structural modes.
• Cavitation-sheet detachment amplifies hydrofoil vibration, with flutter-like responses and hydrodynamic lock-in.

These mechanisms directly inform marine propeller-noise diagnostics and the design of mitigation strategies.

5. Vibration Mitigation and Flow Conditioning Devices

Passive flow-control devices such as thin ring wings function as vortex generators to reduce upstream propeller inflow irregularities, thus suppressing propeller-induced vibration and discrete-tone noise. Sluchak analyzed the linearized model of a thin ring wing, deriving closed-form expressions for induced vortex intensities as functions of flow irregularity and device geometry (Sluchak, 6 Feb 2025). The ring-wing’s circumferential vortex shroud generates radial velocity components, effectively “smoothing” the inflow velocity profile.

Key performance metrics from towing-tank tests demonstrate significant reductions in peak-to-peak inflow fluctuation ($A$) and discrete-tone sound pressure levels:

Metric	Pre-ring	Post-ring	Reduction
$A = \max V - \min V$	$0.15\,V_0$	$0.06\,V_0$	$-60\%$
$|a_1|$	$0.042\,V_0$	$0.018\,V_0$	$-57\%$
1R tone level	$102$ dB	$94$ dB	$-8$ dB

Design guidelines for effective ring wings include $R/D \approx 1.0$, chord-variation tailored to dominant hull-wake modes, clearance $c/D \approx 0.03–0.06$, and moderate azimuthal chord variation. Excessive chord or angle of attack leads to flow separation and reversal of mitigation efficacy (Sluchak, 6 Feb 2025).

6. Practical Observations, Limitations, and Future Directions

PVT reliably identifies the first few structural modes when forced by propeller-driven excitation, particularly at lower frequencies with high mode-shape correlation. However, narrowband harmonics associated with low-RPM propeller settings can mask structural resonances, and higher-order modes exhibit increased sensitivity to nonlinear effects and excitation non-stationarity. Throttle sweeps mitigate persistent harmonic overlap at the expense of added spectral noise and damping estimation variability (Dessena et al., 13 Jan 2026).

For more advanced implementations, automated throttle schedules with closed-loop RPM control and coupling-aware model reduction are being prioritized. In marine contexts, the integration of flow-conditioning rings has been experimentally confirmed to reduce vibratory and acoustic signatures, guiding future design and scaling of these devices for full-scale applications.

7. Cross-Domain Relevance and Integration into Modal and Vibroacoustic Analysis

PVT forms a methodological bridge between traditional structural dynamics, operational modal analysis, hydroelasticity, and flow-induced vibration/noise research. Its output-only excitation paradigm matches well with increasingly electrified propulsion architectures and enables routine in-place characterization essential for rapid prototyping and compliance processes (Dessena et al., 13 Jan 2026, Cheng et al., 2024). Passive flow-conditioning strategies derived from PVT-centric understanding, such as ring wings, offer validated pathways for reducing vibratory loads and radiated acoustic power in both aerospace and marine propulsion systems (Sluchak, 6 Feb 2025).