Authors: Eng. Potito Cordisco, Senior Project Manager, Vicoter
Eng. Mauro Terraneo, Chief Technical Officer, Vicoter
After the studies of the B8 and B23 Energic, Bristell (www.bristell.com) confirmed its trust in Vicoter (www.vicoter.it) assigning them the execution of the GVT (Ground Vibration Test) and flutter analyses of the new Bristell Classic LSA mounting the new ROTAX 915 iS and broaden fuel tanks.
In its constant care towards safety flights, BRM-Aero decided to repeat the test already made during the LSA certification, to be sure that the mass modification, due to the increment in the wing fuel capacity, and the possibility to exceed the VNE, given by the new engine, do not cause flutter.
Due to their decennial experience in the field of aeronautical structural dynamics, from an experimental and analytical point of view both, in June 2024 Vicoter engineers were called to carry out the verification of the modified Bristell Classic LSA.
The activity was divided into two successive phases:
- Experimental measurement of the elastic modes of the aircraft carried out at the customer’s headquarters in Kunovice, Czech Republic.
- Numerical coupling between structure and aerodynamics in order to estimate the V-g diagram and so the minimum flutter speed.
As usual in these kinds of tests, the airplane was suspended by calibrated springs in order to resemble free-free conditions encountered during the flight and to recover its dynamic behaviour, without biases introduced by the constraint conditions.
Test campaign was accomplished on three mass configurations: MTOW (Maximum Take Off Weight) and MZFW (Minimum Zero Fuel Weight) of the original aircraft, MTOW with added masses on the wings to simulate the weight of the fuel in conditions of enlarged tanks. For each configuration, the modes of the control surfaces were also identified in stick-free and stick-fixed.
Tests were carried out by MIMO (Multi Input Multi Output) methodology, using two shakers contemporary: the first one at the wing tip and the second one on the vertical tail, to guarantee to excite all relevant directions with sufficient energy. A total of 88 channels were simultaneously acquired with the accelerometers appropriately located on the lifting and control surfaces to obtain an excellent spatial reconstruction of the flexural and torsional modes of the various parts. Particular attention was paid to the control surfaces, given the important role they can play in the onset of flutter. In this regard, the database was enriched by the FRFs obtained by dedicated bonk tests carried out impacting each surface, including the tabs, using an instrumented hammer.
All the PCB-Piezotronic accelerometers and load cells were acquired by two connected Siemens-LMS SCADAS 316 front-ends.
The modes of the aircraft were identified in terms of frequency, shape, damping and modal mass up to 50 Hz, to feed the software used to evaluate the possibility of flutter instability insurgence. From the experimentally acquired FRFs (Frequency Response Functions) the needed values were obtained using the PolymaxTM algorithm, state of the art in the sector.
The second phase of the activity saw the calculation of the V-g diagram and consequently of the minimum flutter speed to compare with the maximum speed reachable by the Bristell Classic LSA with the new engine, enlarged of a 1.2 factor for safety purposes.
The analytical coupling between the aerodynamics and the structure was realized directly using the matrices of mass, stiffness and damping extracted from the experimental modes, without the need to develop expensive and time-consuming finite element models. This already employed methodology was possible thanks to the high number of sensors used and the consequent excellent spatial reconstruction of the modal deformation. This method was carried out using the NeoCass aeroelastic software developed by the Department of Aerospace Science and Technology of Politecnico di Milano. All the three mass configurations experimentally tested, both in stick-free and in stick-fixed, at different flight altitudes were evaluated. The ‘p-K’ calculation method was used for the solution of the aeroelastic problem given its proven reliability.
Thus the flutter free flight envelope of the modified Bristell Classic LSA was determined.
Vicoter thanks all the staff of BRM Aero for the courtesy shown during the stay in the Czech Republic and the permission granted to publish this article. Further thanks go to Luca Marchetti for the indispensable help given during the experimental activity and to Francesco Toffol for the high professional numerical work performed.
