Authors: Eng. Potito Cordisco, Senior Project Manager, Vicoter
Eng. Mauro Terraneo, Chief Technical Officer, Vicoter
Due to its experience and strong theoretical background, Vicoter was asked by APUS Zero Emissions GmbH (www.group.apus-aero.com) to verify that the I-2 prototype is free from flutter in the reduced flight envelope agreed upon EASA for the maiden flight.
The APUS I-2 is a T-tail four-seat aircraft with 2200 kg MTOW, a range of 500 NM and a maximum cruise speed of 160 KTAS. With a wingspan of 13.2 m, a length of 8.86 m and two engines collocated on the wing, it is characterized by the propulsion system employing a hydrogen fuel-cell as its primary source of energy (APUS_i-2_Broschuere_20231027_Web.pdf).
The use of such innovative engine and related power devices brought to a re-design of the classical wing structure, almost in the inner part, and a different distribution of the inertias, with a possible strong impact on the dynamic behaviour of the aircraft. In cases like this, the use of information gathered from the test becomes essential to drive reliable aeroelastic analyses.
Structural modes of the airplane were identified during a ten-days test campaign conducted at the APUS laboratories of Strausberg, Germany, where a portal frame was previously prepared to suspend the aircraft. Two pairs of soft springs, with stiffness specifically designed to guarantee an acceptable decoupling of the rigid movements with respect to the elastic modes, were used to reproduce free-free constraint conditions (as in flight).
Sensors were installed on the airplane to recover the first bending and torsional modes of the lifting surfaces: wings, horizontal tail, vertical tail. They were also installed on the control surfaces to evaluate even the participation of ailerons, elevator, rudder and tab. Some sensors were located on the fuselage to improve the shape recognition of the tail modes. In total 109 channels were acquired contemporary by two Siemens/LMS SCADAS 316 front-end connected in series.
Single axis, low mass PCB piezoelectric accelerometers, measuring perpendicularly to the installation surface, were used during the test, paying attention to use miniaturized ones on the control surfaces (tab in particular) to avoid mass loading effects.
FRFs (Frequency Response Functions) were acquired exciting the structure by two modal shakers simultaneously: the first one at the tip of the left wing; the second one, at the tip of vertical plane, near the trailing edge. For both installations, the force is transmitted from the shaker to the airplane by a stinger connected to a load cell, screwed to the structure in a stiff point. At each input force location, an accelerometer, called “driving point” and directed as the input load, was installed, to have the possibility to calculate the modal mass, needed for the scaling of the modes.
Further datasets of FRFs, focused on the control surfaces and kinematic chains dynamics, were acquired using impact excitation realized by an instrumented hammer and further triaxial accelerometers on stick and pedals.
A single mass configuration, with stick-free and stick-fixed, was investigated experimentally.
Frequencies, shapes, dampings and modal masses in the band up to 100 Hz were identified in MIMO (Multi Input Multi Output) way using the FRFs obtained driving the shakers with a stepped sine signal. Software Siemens/LMS-Test.Lab was employed, using the algorithm Polymax followed by an MLMM (Maximum Likelihood Modal Model) to refine the estimation of the poles and the participation factors.
Starting from the description of the structural dynamic behaviour experimentally obtained by the identification of the modes of the aircraft, the real flutter estimation was realized using NeoCASS (https://www.neocass.org/). This is an open-source framework developed by the Department of Aerospace Science and Technology of Politecnico di Milano able to perform aeroelastic calculations, analytically introducing the aerodynamics, interfacing it with the structure and solving the flutter problem for different subsonic speeds and altitudes.
In NeoCASS, the aerodynamic unsteady forces are modelled with the doublet-lattice method (DLM). The displacements, calculated in the modal condensed way, are transferred to the lifting surfaces through the usage of the radial basis functions techniques; with the same method, the aerodynamic forces are transferred to the structure.
Mass, stiffness and damping matrices, needed for the mathematical description of the structural behaviour, were directly derived from experimental modes without the use of a FE model. This method, EASA accepted, was already used by Vicoter for certification improving the reliability of the results, bypassing the absence of a numerical model and reducing developing time. Thanks to the use of the driving points in every input load location, which allows to scale the modal shapes, the mass [M], stiffness [K] and damping [C] matrices of the structure can be identified.
They are all diagonal and, using the unit mass normalization, have the following shapes:
Numerical rigid modes of the flying airplane, based on mass, inertia, and CG position, are furthermore added to the deformable modal base to improve reliability of the calculation.
The NeoCASS flutter solver implements both the P-K (or British) and continuation method, that are the state of art algorithms in terms of flutter identification. They provide, for each complex aeroelastic mode, the real and imaginary part, allowing to obtain the modal frequency and the modal damping of the aeroelastic model. Evaluating the change of sign in this last parameter, it is possible to estimate if the flutter speed falls in the flight envelope.
With this methodology, it is possible to verify not only the flexural-torsional flutter of wings, horizontal and vertical tail alone, but even the flutter introduced by the coupling of different lifting surfaces due to aerodynamics, the flutter of control surfaces due to unbalancing, the body freedom flutter related to the coupling with the rigid motion of the aircraft and even the divergence instability (static).
Assessment of the APUS I-2 flutter speed was realized at three different altitudes up to 1.2 VD, as dictated in the paragraph 629 of CS-23, in stick-free and stick-fixed conditions both.
According to the requirements of the standard, even flutter behaviour in case of failure of any single element in the primary flight control system and any tab control system was studied. Five additional cases were run, in which ailerons, elevator, rudder and elevator-tab kinematic restraints were numerically disconnected, one at a time, in the modal base.
Due to the configuration with engines installed on the wings even the possibility of insurgence of whirl flutter has been considered.
Vicoter warmly thanks the staff of APUS, in particular Stefan Radek and Martin Koning, for the job made to organize all this project; Paolo Rubini for the invaluable support during the GVT; Francesco Toffol and Marco Morandini for the constant presence and help in the theoretical and numerical part.
Finally, special thanks go to professor P. Mantegazza, recently disappeared, without whom everything should have been more complicated. Ciao Paolo.
About Vicoter
Vicoter (www.vicoter.it) is an Italian company dealing in aerospace since almost 15 years, performing static and dynamic measurements and tests, both on-ground and in-flight. Vicoter is specialized in the execution of experimental GVTs (Ground Vibration Tests) and analytical flutter assessment, which are performed for first flights, problem solving or type certifications (up to CS23).
About APUS Zero Emission GmbH
APUS (www.group.apus-aero.com) represents a new era in aviation, with emission-free hydrogen aircraft, excellent aviation engineering services, and sustainable STC conversion kits. It takes both major innovations and many small improvements to finally make flying climate-neutral.
As an aircraft manufacturer, APUS aims to become the world’s first provider of certified aircraft powered by green hydrogen as an energy source. APUS is also an EASA-certified design organization, offering high-level aviation engineering services. The company’s research and development focus includes innovative propulsion systems for aviation, aero-mechanical design, structural design, CAD, and certification of aviation systems. Founded in 2014 by Phillip Scheffel as an aviation design office, APUS has been working on the future of emission-free flying ever since. Today, the company is headquartered in Strausberg near Berlin and employs 70 staff members.
