Item 1059
OTHER:
Aerodynamics - Vibration - Rotor Induced - Overview
Vibration Acceptability:
[Source ~ ETF p.3 & 8]
The above figure is taken from British R & M Report 1637. This is a plot of the relationships between vibration frequencies, amplitudes, and accelerations and their effects on the human body. A curved line on this figure indicates the relationship of the threshold of constant discomfort. All areas below this line are considered satisfactory and all areas above this line are considered unsatisfactory.
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Comments in 1981 and Currently:
Sustained vibration levels greater than 0.10 g are not really comfortable.
If a level of 0.02 could be obtained, there would be no more discussion about helicopter vibration.
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Another Graph:
See;
A COMPREHENSIVE VIBRATION ANALYSIS OF A COUPLED ROTOR/FUSELAGE SYSTEM
http://www.enae.umd.edu/pdf/students/PhD/YeoPhD1999.pdf .pdf page 58
Hard copy page 31
This graph appears to show the worst range is between 4 and 8 cycles per sec (Hz).
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Additional Information and Graph:
See; 'Even more Helicopter Aerodynamics', Chapter 25.
Outside Helicopters
Frequencies of Specific Craft:
The vertical 2P vibration on the two-bladed Robinson R22 is about 18 Hz.
The
Flettner FL-282's vibration occurred at twice-per-rotor revolution and with the rotor turning at 180 rpm, vibrations occurred at the rate of 6 Hz.The
Sikorsky ABC's vibration must occur at three-per-rotor revolution and with the rotor turning at 240 rpm (aux. propulsion mode), vibrations peeked at 12 Hz.The SynchroLite will be (611/60)*2 = 20 Hz. in roll
and 41 Hz. in heave.The Unicopter will be
(700/60)*3 = 35 Hz. in roll and 70 Hz. in heave. (600/60)*3 = 30 Hz. in roll and 60 Hz. in heave._________________________________
A model synchropter was entered in the 1995 World RC Championships. It had vibration problems and tipped over. The web site w/ picture are no longer on the Web.
Possible Rotor Excitations:
Forces:
Longitudinal
Lateral
Vertical
Moments:
Rolling
Pitching
Yawing
Kinds of Vibration:
[Source ~ AH p.307]Ordinary:
See lists above.
Self-excited: (The following are not applicable to helicopters with Really Rigid Rotors. [Source ~ AH p.307])
Ground resonance.
Blade flutter. ~ involves the coupling of blade flexing and twisting with air forces.
Blade weave.
Vibration Alleviation Methods:
Alteration of the external (forcing) loads or moments (e.g. higher harmonic control, individual blade control, periodic flow control). Suppression of the dynamic loads at the source.
Accommodation:
Response control (e.g. frequency placement, isolation systems, absorbers, attenuators)
Related Web Pages:
See:
DESIGN: ~ SynchroLite - VibrationSee:
DESIGN: ~ UniCopter - Vibration
Noise:
Audible Sound (Noise):
When the rapid variations in pressure occur between about 20 and 20,000 times per second (i.e. at a frequency between 20Hz and 20kHz) sound is potentially audible even though the pressure variation can sometimes be as low as only a few tens of millionths of a Pascal. Movements of the ear drum as small as the diameter of a hydrogen atom can be audible! Louder sounds are caused by greater variation in pressure. A sound wave of one Pascal amplitude, for example, will sound quite loud, provided that most of the acoustic energy is in the mid-frequencies (1kHz - 4kHz) where the human ear is most sensitive. It is commonly accepted that the threshold of human hearing for a 1 kHz sound wave is about 20 micro-Pascals.
More basic information on acoustics:
http://www.campanellaacoustics.com/faq.htm
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Last Revised: February 3, 2007