The continuous growth of unmanned cargo aviation is pushing manufacturers toward increasingly large, lightweight, and flexible airframes. In this context, Pipistrel Aircraft (https://www.pipistrel-aircraft.com/) is developing the Nuuva, a large, unmanned cargo aircraft designed to perform medium-range logistics missions with high payload capacity. The aircraft features a twin-boom configuration, a high-aspect-ratio wing system, and a distributed propulsion architecture that combines multiple electric motors for take-off and landing with a central internal combustion engine dedicated to cruise.
This configuration results in an aerodynamically and structurally efficient design, but also in a dynamically complex airframe, in which wings, booms, fuselage, and empennage are strongly coupled and elastic deformations play a significant role even at relatively low frequencies. Lightweight materials, distributed propulsion systems, and large-span lifting surfaces increase aeroelastic sensitivity, making accurate experimental modal data essential not only for numerical model validation, but also for the safe integration of flight control laws and for flutter clearance activities.
In this context, Ground Vibration Testing (GVT) remains a cornerstone experimental technique, providing a controlled environment to identify the dynamic behaviour of complex aircraft structures.
Vicoter (www.vicoter.it) recently carried out an extensive test campaign on the second prototype of the Nuuva cargo drone, with the objective of experimentally identifying its global structural dynamic behaviour over a wide frequency range and providing a reliable experimental reference for subsequent aeroelastic and multidisciplinary analyses. This campaign marked the second collaboration with Pipistrel Aircraft, who chose Vicoter again, based on the high quality and reliability of the GVT performed on the first Nuuva prototype. Drawing on extensive experience with full-scale aircraft testing, Vicoter delivered high-fidelity modal data, enabling a precise characterization of coupled structural responses and supporting both flight control integration verification and the refinement of predictive numerical models.
The first part of the campaign focused on acquiring a comprehensive set of dynamic measurements in terms of Frequency Response Functions (FRFs) from the complete aircraft under controlled excitation, following a classical Experimental Modal Analysis (EMA) approach.
The UAV was suspended in a dedicated test rig using a dual-point elastic rope system, specifically designed to minimize the intrusivity of boundary constraints on the aircraft dynamic behaviour while ensuring adequate structural stability. Preliminary measurements confirmed that the suspension system introduced minimal coupling between rigid-body and elastic modes, guaranteeing the reliability of the subsequent modal identification process.
Prior to testing, a pre-test analysis was performed using data from Pipistrel Aircraft’s preliminary finite element model which, although not fully correlated, provided valuable guidance for the optimal placement of sensors. Accelerometers were installed at critical locations across the front and rear wings, left and right booms, fuselage, engine, and empennages, covering both fixed structures and movable control surfaces in order to capture the full-field vibrational response. In total, over one hundred measurement points were employed, ensuring that all global elastic modal shapes were comprehensively recorded.
In addition, the pre-test activity was used to define the measurement directions of the employed single-axis accelerometers, allowing the total number of acquisition channels to be minimized, which resulted in 158 channels overall.
The main excitation was applied using an electrodynamic shaker positioned at a location identified during the pre-test phase as dynamically effective for the excitation of the global structural modes. Vertical and lateral input forces were applied simultaneously by appropriately tilting the shaker axis, allowing the dominant bending and torsional responses of the coupled wing–boom–fuselage system to be effectively excited. The input forces were transmitted through a low-stiffness stinger, specifically designed to avoid the introduction of significant local constraints at the excitation point. The input load was measured together with the acceleration at the driving point, enabling the estimation of modal masses and the proper scaling of the identified mode shapes. Random and burst-random signals were used to stimulate the airframe over a wide frequency band, up to 128 Hz.
In addition to shaker excitation, complementary impact tests were performed using an instrumented hammer. These tests served to confirm that no significant modes were missed due to reliance on a single excitation location and to enhance the excitation of in-plane modes of the front and rear wings, which, while not critical, were within the requested frequency range and of potential interest.
The adopted configuration ensured a high signal-to-noise ratio and excellent coherence across all sensors, providing the conditions necessary for reliable modal identification. The measured FRFs were processed using frequency-domain identification techniques, in particular the Polimax™ algorithm for primary modal extraction, with MLMM refinement applied as needed, allowing the extraction of natural frequencies, damping ratios, mode shapes, and modal masses associated with the global elastic behaviour of the airframe. The very low frequency step employed, combined with the high spatial resolution of the measurement grid, enabled clear separation between closely spaced modes and accurate reconstruction of complex, coupled deformation patterns involving multiple structural components.
The identified modal basis revealed strong coupling between the wings, booms, and empennage, confirming the highly integrated dynamic nature of the Nuuva’s airframe. Several modes exhibited significant participation of both lifting surfaces and tail structures, highlighting the importance of a full-aircraft experimental characterization rather than a component-level approach.
Following the identification phase, an intensive validation campaign was performed on the extracted modal data. This validation leveraged the comparison of acquired and synthesized FRFs, the cross-MAC matrix, mode complexity indices (MOOV, MPC, MPD, …), and the consistency of results across multiple datasets. These complementary checks provided confidence that the identified modes were both physically meaningful and robust, providing a robust experimental reference for the correlation and updating of the numerical models.
Following the characterization of the global elastic modes, the campaign also focused on identifying the vibrational behaviour of the movable control surfaces, including the rudders and the front and rear ailerons. Understanding the dynamics of these surfaces around their hinge axes is critical for defining appropriate spring parameters in the finite element model, ensuring accurate correlation with experimental data and a reliable assessment of potential flutter onset.
The control surfaces of the Nuuva are servo-actuated, and to accurately capture their dynamic response, targeted tests were performed under different control configurations. These included the “control off” case, where all surfaces were free to move, configurations with the flight control system engaged at various gain levels, and tests with input forces applied at different amplitudes to investigate potential nonlinearity in the response.
Specific test setups were implemented by temporarily adding accelerometers to the surfaces under investigation and connecting the shaker directly to individual control surfaces, enabling precise excitation of local rotational modes. This approach allowed the acquisition of reliable and high-fidelity data on the dynamics of each control surface.
Overall, the test campaign demonstrated the effectiveness of the adopted experimental approach in capturing the complex dynamic behaviour of a large, flexible, unmanned cargo aircraft. The quality and robustness of the identified modal parameters confirm the key role of full-scale experimental testing in supporting the development of next-generation UAVs, where lightweight structures, distributed propulsion, and advanced control systems increasingly demand accurate and reliable dynamic data.
Vicoter warmly thanks Usua Vegniory, Jure Marinko, and Alejandro Treceno Fernandez for their valuable support and technical contributions during the Nuuva GVT campaign.
Authors: Potito Cordisco, Senior Project Manager, Vicoter
Mauro Terraneo, Chief Technical Officer, Vicoter
