PSI - Issue 16

Andrzej Gębura et al. / Procedia Structural Integrity 16 (2019) 184 – 191

187

Andrzej Gębura, Sylwester Kłysz, Tomasz Tokarski / Structural Integrity Procedia 00 (2019) 000 – 000

4

f i [s ]

4 rotations - 13 oscillations

Δ t = 1 s

t [s ]

Fig. 2. The waveform of the instantaneous frequency from Mi-24 – ‘positive standard’ .

f i [s ]

4 rotations - 51 oscillations

Δ t = 1 s

t [s ]

Fig. 3. The waveform of the instantaneous frequency from Mi-24 – ‘negative standard’ .

5. Selected operational problems of aircraft transmissions

5.1. General information on aircraft transmissions

Mechanical transmission is a mechanism or a system of machines used for transferring motion from an active element (driving element) to the passive element (drive element) with the simultaneous change of motion parameters, that is speed and force or the moment of force developed by Jakielaszek and Nowakowski (2014). The transmission may be: a) reduction gear – when the drive element rotates with the lower speed than the driving element, b) step-up gear – when the drive element rotates with the higher speed than the driving element. In modern aviation based on turbine engines, reduction gears with different velocity ratio are most frequently used. The highest reduction values have helicopter propulsion with turbine engines. The reduction gear is aimed at reducing the rotational speed of the main shaft of the turbine engine – Fig. 1, element 1 (this speed is maximised by construction engineers to obtain a high engine efficiency) to the rotational speed of helicopter rotor shaft – Fig. 1, element 3a (this speed is optimized by construction engineers to obtain a high aerodynamic lift i.e. to achieve a subsonic speed at the end of the main rotor blade developed by Barszewski (1956), Padfield (1998), Żurek (2006) . 5.2. Skews of shafts and their importance Skews of couplings manifest itself in the form of a sinusoidal modulation of the waveform of the instantaneous angular speed of driving element with the frequency of the second harmonic of the rated speed of individual shaft. It can be observed that skews of shafts result in dynamic excesses, which are two times more frequent (one full