Dedicated tests for flutter certification of aircraft are prescribed by EASA CS-23, EASA VLA and German LTF-UL (ultra-light aircraft) regulations to verify that any airplane is flutter free in its flight envelope.
Vicoter realizes all the experimental tests and numerical analyses needed to comply with the paragraph 629 of the standards according to well established procedures and state of art instrumentation and algorithms.
The method used by Vicoter to determine the flutter speed is the following:
- To measure the experimental modes of the aircraft by a GVT (Ground Vibration Test).
- To analytically simulate the dynamic behaviour of the structure coupled with aerodynamic forces.
- To calculate the flutter speed of the lifting and control surfaces at various altitudes and in different mass conditions.
Step 1. Aircraft modal analysis
GVT (Ground Vibration Test) is the first part of the activity. It is carried out to determine the structural modes of the aircraft in a reliable way.
Vicoter performs GVT using the most modern instrumentation. Emploied methodologies complyi with all the directives expressed in the advisory circular 23.629-1B of the FAA, valid even for EASA. All the instruments used for the tests are certified and calibrated.
Experimental tests are carried out with the aircraft suspended by soft springs to decouple rigid and elastic modes of the airplane. Vicoter takes care of designing and manufacturing the suspension system most suitable for the specific class of the aircraft. Generally, two mass configurations are verified: MTOW and MZFW. The aircraft is tested both in stick-free and in stick-fixed configuration.
The transfer functions, the ratios of the output accelerations on the input forces, are acquired using a MIMO technique, i.e. exciting with two electrodynamic shakers and simultaneously reading all the accelerometers installed on the aircraft. Vicoter is able to simultaneously manage up to 110 channels, which makes it possible to perform tests even on unconventional aircraft configurations.
Vicoter realizes GVTs directly at customer’s premises, in order to avoid transportation difficulties and costs.
Results of such first step are the resonance frequencies, modal shapes, dampings and modal massess of all the modes related to lifting and control surfaces.
Step 2. Aeroelstic model assemby
Part 2 of Vicoter’s approach to flutter certification is the realization of a mathematical description of the aeroelastic behaviour of the airplane. Coupling between structure and aerodynamics is realized numerically and the possibility of flutter insurgence verified by P-K method.
Two solutions are available for the introduction of the structural contribution. According to the needs of the customer, Vicoter is able to perform the investigation using:
- A description based on mass, damping and stiffness matrices directly assembled by the modes identified during the GVT.
- A description based on a Finite Element beam model of the airplane. A structural model of the aircraft is realized in an ad-hoc free numerical code, NeoCass, or alternatively in Nastran. Once its dynamic behaviour is correlated with experimental results such model is used to furnish mass, damping and stiffness matrices.
The former procedure is easier and cheaper, but it limits the flutter analyses to configurations of the aircraft not so far from GVT-tested ones. It reduces the possibilities of any sensitivity analyses to structural modifications. In any case the method to use directly the experimental results proved to be more reliable for flutter assessment in the last tests made in a wind tunnel for flutter academic study. The latter procedure is the classical one used for flutter certification. It has the advantage that, basing on a correlated FEM model of the structure, it rapidly allows modifications to the aircraft to assess its effects on flutter performance. On the other side it is expensive and very time consuming other than lesser reliable due to the unavoidable lack of correlation of higher frequency modes.
The aerodynamic effect is simulated using a DLM (Doublet-Lattice Method) method, suitable for unsteady cases. By a system of spline curves pressures are tranferred from the aerodynamic mesh to the structural one and in the same way the displcements from the structure to the aerodynamics.
Step 3. Flutter numerical assessment
Futter analysis are so carried out on the aerodynamic/structural coupled problem by a P-K or pursit method.
Results of such calculation are the so called ‘V-f’ and ‘V-g’ diagrams in which the evolution of the modal frequencies and dampings can be followed when the flight speed varies. Flutter presence is identified by the crossing of one damping curve of the zero line.
Read more about some real cases afforded by Vicoter for flutter certification in the following articles:
