5. Discussion
Propeller Rotation Frequency and its harmonics can be seen in
all analysed vibration spectrums. The existence of 1st, 2nd, 3rd
and 4th harmonic and appropriate sub-harmonics are obvious.
Also, vibration acceleration magnitudes are the highest on the fundamental
PRF and its harmonics in all flight modes. In theory andduring practical work of piston propelled planes the highest level
of vibrations and noise is expected from BPF. For installed six cylinder
engine during every crankshaft revolution there are three
internal combustions, also there is increase in 3 X PRF that require
dampening via engine rubber mountings/absorbers. The research
has been conducted on the prototype plane, with respectful working
time in extremely complex and discerning flying conditions,
and as such there is possibility for degradation of mechanical characteristics
on engine and its rubber mountings/absorbers (pistoncylinder
clearances; misalignments, mechanical wear and tear,
etc.). The above stated (possible) damages may increase vibrations
at subharmonics and higher harmonics, also they will require to be
looked at from an aspect for a high precision engine. However, this
aspect was not the research priority.
Lasta aircraft was built with a six-cylinder piston engine, and
therefore 3rd PRF harmonic coincides with the engine firing frequency.
The propeller is equipped with two blades, so the BPF coincides
with 2nd PRF harmonic. Matching 2nd harmonic crankshaft
rotation frequency with aerodynamic vibration from the blades
resulted in a significantly increased acceleration magnitude on this
frequency. Mechanical engine vibrations and aerodynamic propeller
vibrations overlap on all higher even harmonics too.
Fig. 11 shows the vibration accelerations on the seat in 1st test
without an absorber during three analysed flight modes.
Vibration accelerations on the seat, without an absorber, are the
highest at 2nd harmonic during all flight modes. Aircraft climbing
has the greatest contribution to vibration accelerations on the seat
in the direction of X axis. Dissipation of acceleration values in the
direction of Y axis, due to flight profile changes, is small, meaning
that impact of flight profile is practically negligible in the Y direc