Ground Vibration Test and flutter verification of the new Bristell B8 aircraft

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Vicoter (www.vicoter.it) is an Italian company that operates for more than ten years all over the world in the field of structural dynamics, from an experimental and analytical point of view. It operates mainly in the aeronautical sector carrying out both the GVT (Ground Vibration Test) and the numerical analyses necessary for the flutter certification of an aircraft, using state-of-art instrumentation and methodologies to ensure maximum reliability of its results.

In July 2021, Vicoter engineers were called to carry out the verification of the new Bristell B8 (www.bristell.com). The airplane is an elegant high-wing two-seater, completely made of metal, to be certified according to UL regulations in the Czech Republic, with a maximum take-off weight of 600 kg. The standard requires that the absence of flutter up to 1.2 VD be demonstrated by means of numerical analyses in which the modes of the main structural parts have been determined experimentally.

The activity was divided into two successive phases:

  1. Experimental measurement of the elastic modes of the aircraft carried out at the customer’s headquarters in Kunovice, Czech Republic.
  2. Numerical coupling between structure and aerodynamics, in order to obtain the V-g diagram and the minimum flutter speed.
B8 “High Wing” by BRM Aero

The aircraft was tested in free-free conditions to recover its dynamic behaviour as it is during the flight. To obtain this constraint, the B8 was suspended by means of a spring system sized and built by Vicoter, which allows to decouple the rigid behaviour of the airplane from the sought elastic modes.

B8 ready to be suspended and tested (left) and during tests (right).

The test campaign covered two mass configurations, MTOW (Maximum Take Off Weight) and MZFW (Minimum Zero Fuel Weight), both analysed with stick-free and stick-fixed.

The tests were carried out by MIMO (Multi Input Multi Output) method, by installing 64 accelerometers appropriately distributed between the lifting and control surfaces, in order to obtain an excellent spatial reconstruction of the modes. The aircraft was excited at the same time by two electromagnetic shakers installed one at the end of the left wing and the second on the vertical tail: in this way it was guaranteed to excite all relevant directions with sufficient energy. Particular attention was paid to the control surfaces, given the important role they can play in the onset of flutter: dedicated impact tests were carried out loading each surface, including the tabs, using an instrumented hammer.

The aircraft’s vibration modes have been identified in terms of frequency, shape, damping and modal mass starting from FRFs (Frequency Response Functions) using the PolymaxTM algorithm, state of the art in the sector. The band up to 60 Hz was investigated.

Example of identified modal shape.

The second phase of the activity saw the analytical coupling between the aerodynamics and the structure of the aircraft, in order to numerically determine the V-g diagram and consequently the minimum flutter speed. Thanks to the high number of sensors used and the consequent excellent spatial reconstruction of the modal shapes, it was possible to use the mass, stiffness and damping matrices extracted from the experimental modes and use them directly in coupling with aerodynamics. This methodology, developed and validated by Vicoter together with A-Cubed Technology
(www.acubedtechnology.com), avoids the expensive development of a correlated finite element model, significantly reducing the time required to obtain the results to be submitted to the certifier.

The simulation was carried out using the NeoCass aeroelastic software developed by the Aerospace Engineering Department of the Politecnico di Milano by analysing both mass configurations tested experimentally, both in stick-free and in stick-fixed configuration, at different flight altitudes. 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 Bristell B8 was determined.

Vicoter warmly thanks BRM Aero for the courtesy shown to him during his stay in the Czech Republic and the permission granted to him to publish this article.