Flutter certification for ultralight aircraft

Ultralight aircraft, as well as all the structures on which it acts a non-negligible aerodynamic force, can be subject to flutter. Indeed, both the German LTF-UL (ultra-light aircraft), the EASA CS-VLA (very-light aircraft), and the EASA CS-VLR (very-light rotorcraft) regulations prescribe dedicated certification tests in order to verify that any aircraft 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 alghorithms.
The method used by Vicoter to determine the flutter speed is the following:

  1. To measure the experimental modes of the aircraft by a GVT (Ground Vibration Test).
  2. To analytically simulate the dynamic behaviour of the structure coupled with aerodynamic forces.
  3. To calculate the flutter speed of the lifting and control surfaces at various altitudes and in different mass conditions.

 

Step 1. Aircraft modal analysis

The first phase is called GVT (Ground Vibration Test) and is carried out to determine the modes of the aircraft.

Vicoter performs GVT using the most modern instrumentation and complying with all the directives expressed in the advisory circular 23.629-1B of the FAA, which is also valid for the CS-VLA. All the instruments used for the tests are calibrated.
Experimental tests are carried out with the aircraft suspended by soft springs. Vicoter takes care of designing and manufacturing the suspension system most suitable for 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 and 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.

 

Courtesy of Alpi Aviation

 

Step 2. Flutter numerical analysis

Part 2 of Vicoter’s approach is the realization of a mathematical description of the structural behaviour of the airplane.

Two solution are available, according the needs of the customer:

  1. Description by mass, damping and stiffness matrices directly assembled by the modes identified during the GVT.
  2. Description by a Finite Element beam model of the airplane. Tha model, correlated with experimental results, can be realized either using an ad-hoc free numerical code, or using Nastran,

The former procedure is easier and cheaper, but it limits the flutter analyses to the aircraft GVT-tested configurations, making impossible any sensitivity analyses to structure modifications. In this case, the aerodynamic effect can be simulated using a DLM (Doublet-Lattice Method) method splined on a mathematical description of the aircraft obtained using the mass and stiffness matrices identified in the GVT.

The latter procedure is more valuable, but 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. Also in this case, the aerodynamic effect can be simulated with a DLM method, but it works on a FEM model (in NASTRAN or in NeoCASS) correlated with the experimental modal results. On this model flutter analysis is carried out.