Pipistrel Case Study: About virtual testing and certification

Pipistrel Case Study: About virtual testing and certification

Read the case study of our partner – Pipistrel Vertical Solutions

Addressing certification requrements

The obligatory testing to determine the natural frequencies of the aircraft and its control elements with the associated structural damping is a very time-consuming and expensive procedure which can take months. The results of such testing are then used for flutter analysis (an unstable oscillatory aerodynamic condition with large amplitude ensuing from the fluid structure interaction), which defines the maximum safe airplane speed and also shows how to distribute weight in the control components to reduce vibrations and improve stability of flight.

“Thanks to SIMULIA Abaqus robustness and reliability, the final results of the simulations were recognized by EASA (European Aviation Safety Agency). This gave us a quicker path to definitive confirmation of the structural integrity of the aircraft.” - Dr. Tine Tomažič, CTO, Pipistrel Vertical Solutions

Creating detailed digital twin of existing plane

Pipistrel Vertical Solutions used Abaqus software to successfully complete the first part of the task, thus the determination of the aircraft's eigenfrequencies, when certifying the new two-seater electric plane, the Virus SW 128. Abaqus uses the finite element method to perform realistic simulations. Since extensive physical testing had already been carried out a few years ago for a similar Virus SW 121 aircraft, the company had the ideal opportunity to build and calibrate the FEM model of Virus SW 121 aircraft and transfer the knowledge and parameters to the FEM model of the new Virus SW 128 aircraft. When the computed natural frequencies were compared with the measured ones, the results for all observed values were almost identical. The detailed FEM model of Virus SW 121 aircraft was confirmed to be a precise digital representation of already in-service aircraft.

Considering years of experiences, which Pipistrel has in building accurate FEM models of their airplanes, simulating the natural frequencies of well-defined composite structures, whose individual parts are mainly glued together, did not present a particular challenge. The most demanding challenge involved modelling of the mechanisms of the flight control surfaces (rudder and pitch controls and flaperons) and determining the necessary elasticity and damping coefficients of joints. The mechanical behaviour of the flight control surfaces has a major impact on the global eigenfrequencies of the wings and fuselage, as well as on the fluid structure interaction, which is key in flutter analysis. It is therefore very important that the FEM model accurately captures the stiffness and mass properties of the entire mechanisms of flight control surfaces. In this regard Abaqus offers a wide selection of joints and connectors.

“The numerical models of aircraft are also an excellent tool for optimizing the design, verify new ideas, comparing the strength and stability characteristics of carbon and glass fiber structures etc.” - Miha Škof, FEM Analyst, Pipistrel Vertical Solutions

Successful certification of the new plane based on realistic simulation

The model of existing aircraft was modified to fit the new Virus SW 128 aircraft. The natural frequencies were then calculated and sent for further flutter analysis. 

Figure 1: Torsion of tail with a rudder deflection at 6.1 Hz and asymmetric torsion of wings with flaperons oscillations at 31.9 Hz.

The final results confirmed the structural integrity of the new plane from natural frequency and flutter perspective and were also recognized by EASA (European Aviation Safety Agency). This enabled Pipistrel a quicker path to definitive confirmation of the structural integrity of the aircraft, i.e. test in the air at the highest airplane speed. As expected, the test was successful.

About Pipistrel: Pipistrel Vertical Solutions is the subsidiary of the Pipistrel Group, which specializes in aircraft design, innovative electric and hybrid aircraft drives, propulsion batteries and their certification to the highest aviation standards. It is the holder of the planning authority for aircraft by both the European Aviation Safety Agency (EASA) and the Slovenian National Civil Aviation Agency (SI-CAA). Pipistrel aircrafts are known for their unique design solutions, the use of modern composite materials, high reliability and low operating and maintenance costs. Above average flying capabilities and aerodynamic and structural features allow Pipistrel to use innovative electric propulsion to achieve zero emission. The Pipistrel Group holds a 100% market share of electric aircraft at customers in all continents of the world and has delivered over 2000 aircraft to customers in more than 100 countries in its 30-year history.


About SIMULIA Abaqus: 
The high-end Abaqus Unified FEA product suite offers powerful and complete solutions for most demanding and sophisticated engineering problems covering a vast spectrum of industrial applications. Its high accuracy, robustness, and performance are combined with easy to use pre and post processor. Abaqus is used with high benefits in different industries, from traditional transportation and mobility to the hi-tech and biomedicine. For more information, click here.


You can also read Pipistrel case study in Slovenian language