Authors: Eng. Potito Cordisco, Senior Project Manager, Vicoter
Eng. Mauro Terraneo, Chief Technical Officer, Vicoter
Autogyros are very special flying machine. Similar, at first glance, to the helicopters, they share with their more known cousins only the use of rotating blades to generate lift, whereas the differences are far more numerous.
The principal one is, for sure, that the main rotor is not motorized, but it is put in an autorotation regime by the relative air speed. Other, not less important, differences are the absence of a tail rotor to counteract the torque and the method to generate the force to move the aircraft horizontally. Indeed, while in a helicopter the movement direction is produced by opportunely orienting the tip path plane, in an autogyro it is obtained using an autonomous propeller connected to the engine.
The principal one is, for sure, that the main rotor is not motorized, but it is put in an autorotation regime by the relative air speed. Other, not less important, differences are the absence of a tail rotor to counteract the torque and the method to generate the force to move the aircraft horizontally. Indeed, while in a helicopter the movement direction is produced by opportunely orienting the tip path plane, in an autogyro it is obtained using an autonomous propeller connected to the engine.
The autogyro architecture causes the birth of dynamic problems specific of such category. The loading spectrum is strongly pitched, whereas the airborne broadband component is practically negligible. The main sources of vibrations are the 1x and 2x harmonics (gyroplanes have two blades) of the main rotor rotational speed, the 1x and Nx harmonics (where N is the number of blades) of the propeller and, in general, the tones of the engine and its equipment (i.e., for example, pumps).
Even if all these contributions can be strongly limited thanks to an opportune balancing of the rotating parts, the induced vibrations can be high and, potentially, catastrophic if one of their frequencies coincide with any structural resonance. Differently from helicopters, in gyrocopters the rotor mast is a tall and thin beam, with generally limited stiffness and with low frequency bending modes, which can be coupled with the main rotor. The tail is, from a dynamic point of view, a large mass cantilevered by a long and limited section beam. Such layout is well known to be prone to have low modes, that, in specific circumstances, can couple with tonal turbulence generated by the propeller.
To prevent such circumstances and improve the comfort in cabin, Vicoter (www.vicoter.it) performed a R&D test campaign on the M24 Orion autogyro produced by MagniGyro (www.magnigyro.it/). The activity, co-funded by the Lombardy Region within the DRIADE regional program (https://www.vicoter.it/index.php/progetti-di-ricerca/dafne/?lang=en), consisted in three activities covering all the aspects of the problems: GVT (Ground Vibration Test), choice of the most efficient engine dampers and flight test. Blade balancing was not considered due to the presence of an in-house consolidated procedure.
Modal analysis was realized by a MIMO impact methodology using more than 400 FRFs at the same time. Accelerometers were positioned on more than 50 locations, even on the rudder. The mast and the tail of the aircraft were excited in various points, along all the three directions to avoid nodes of the modes. Identification was performed by Polymax™ algorithm, state-of-art method. During tests, the autogyro was in free-free constraint condition, realized deflating the landing gear tyres and suspending the rear part by a very soft elastic rope.
The selection of the most performant engine damper was made evaluating the transmissibility of various products, different in terms of hardness. Two triaxial accelerometers were mounted on each of the four junction points of the motor, one on the chassis side and the other on the engine one. Comparing the acceleration levels of two homologous sensors, in fine and 3rd of octave band, it was possible to quantify the vibrations transmitted to the structure at different regimes of rotations. In this way, the damper guarantying the maximum cabin comfort was chosen.
To complete the investigation on the dynamic behaviour of the M24 Orion, flight tests were performed. Their purpose was, firstly, to measure the acceleration levels reached in different points of the structure (cabin included), during the various phases of the flight. Secondly, tests were useful to quantify the participation of some modes, with resonances close to the input frequencies. 16 accelerometers were installed and acquired in flight by an operator placed in passenger position. Data were recorded by a ROGA DAQ 16 expressly customized by Vicoter to be portable. Three VORU conditions at different speed and four manoeuvres were tested, in order to cover the whole flight envelope.
Vicoter warmly thanks all the staff of MagniGyro for its support and its capability to introduce to the specific problems of an autogyro, as well as for the credits on all the pictures of this article.
