Flutter analyses of Bristell B23E

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During the flight, an aircraft can be subjected to various vibratory phenomena induced by aerodynamics.

Generally speaking, such phenomena can be divided in two broad categories: forced responses and instabilities.

In the formers, the structure is loaded by aerodynamic forces that are independent from the motion of the system. Due to the lack of any coupling between excitation and response, the vibrations on the parts of the airplane can be evaluated without any iteration loop: once the exciting load is determined, a classical forced response analysis can be used to evaluate the oscillations, generally by a Finite Element Model. In this class of phenomena, fall the propeller wash excitation, the vortex shedding, the buffeting, …

Instabilities are, indeed, coupled aeroelastic phenomena, where the forcing function is dependent on the motion of the system. They are unstable, self-excited structural oscillations, where the energy is extracted from the airstream by the elastic deformation of the structure itself. The flutter instability (even whirl one), the divergence, the control reversal and, in some cases, the aileron buzz fall into such category. The assessment of insurgence of these events can be very complex, even remaining in linear field. Dedicated techniques are needed to interface structure and aerodynamics and to solve the problem in a reliable way. With the augment of the aircraft speed and the trend toward lighter and flexible structure, aeroelastic stability has become more and more important, especially because it can be catastrophic and can occur without warning. All the general aviation regulations, (LTF-UL, LSA, CS-VLA, CS-22, CS-23) require verifying the absence of instability phenomena to issue the certification. Even if the aeroelastic certification is based on the formal flight tests, for new or very performant aircrafts a preliminary numerical assessment of the possible flutter insurgence is required. Manufacturers are relatively free in the definition of the rational flutter analysis to use, as well as in the methodology to employ to perform a valid GVT (Ground Vibration Test), needed to characterize the dynamic structural behaviour of the aircraft.

Vicoter (www.vicoter.it) is a society operating in the field of the structural dynamics and, in particular, of flutter certification. Its purpose is to bring in the general aviation modern, but strongly qualified and reliable techniques, often exclusive of civil and military aviation, maintaining low costs and fast development time. About this, Vicoter, together with A-Cubed Technology (www.acubedtechnology.com), implemented and validated a methodology using the code NeoCASS, an aeroelastic software realized in an EU funded Clean Sky project. This code solves the flutter mathematical problem by the ‘PK’ algorithm, due to its proven reliability. The aerodynamics is modelled by Doublet Lattice Method (DLM), whose results in the unsteady environment had been granted in numerous test campaigns. NeoCass also has a special correction of the aerodynamics matrices to obtain reliable results, even for the T-tail design, where experiments showed that steady solution, in-plane motion and trim deformation affect aerodynamic unsteady forces and could dramatically modify flutter speed. An ad-hoc tool, specifically realized for the light airplane certification, permits to avoid the expensive development of a correlated Finite Element Model.

After its experience with the B8 aircraft, Vicoter was confirmed by Bristell (www.bristell.com) to carry out the flutter verification of the B23 Energic aircraft, which is the electrical powered modification realized by H55 (www.h55.ch) of the successful B23. The airplane needed to be certified in the Experimental category in Switzerland, with a maximum take-off weight of 850 kg.

B23 Energic.

At first, the experimental elastic modes of the aircraft were recovered performing a GVT analysis at the customer’s site in Sion, Switzerland.

The airplane was tested in free-free conditions, suspended by means of a spring system, sized to decouple the rigid behaviour from the first elastic modes. MTOW configuration was analysed with the control surfaces in free and fixed configuration. Tests were carried out using two shakers, one at the tip of the right wing and one at the tip of the vertical tail. Specific investigations of the control surfaces and tabs were performed by an instrumented hammer, to improve the quality of the acquired FRFs. Responses were recovered in 73 locations spread on the aircraft using high sensitivity accelerometers, with a total of 82 signals simultaneously acquired. Special care was given to install micro, low-mass accelerometers on the two tabs, to avoid mass loading effect.

GVT instrumentation setup.

Modal frequencies, shapes, masses, and dampings were identified by the complete dataset using Polymax™, the most advanced algorithm in the sector. A frequency band up to 60 Hz was investigated. A prove of the quality of the obtained results has been furnished by MAC, complexity check and synthetized FRFs deviation. 

Examples of identified modes. Tail torsion and ailerons.

The absence of flutter was demonstrated up to 1.2 VD, directly using the experimental modal data coming from GVT, accounting, in this way, flexural-torsion flutter of all the lifting surfaces. MTOW mass configuration was considered and proven with stick-free and stick-fixed at three altitudes by the 3D method. By the investigation of the V-g diagram, the flight envelope of the B23 Energic was proven to be flutter free.

Example of a V-g diagram.

Vicoter warmly thanks H55 and Bristell for the permission granted to publish this article and the courtesy showed during the stay in Sion.