Sobre balance Dinámico

Soy tecnico mecanico de helicópteros, recibi capacitación para realizar trabajos de balance dinámico con el equipo 8500C de Chadwick, tambien una breve y digamos concisa explicación del porque, tengo claro que en todo lo que gira hay dos componentes: el eje de giro y el centro de gravedad y que ha veces no coinciden y por lo tanto generan vibración.
Sin embargo me falta entender lo que se detalla en el capitulo 18 del BHT-212-MM.
Mi pregunta es si podrian ayudarme a traducirlo, me atrevo a enviarle una copia:
Vibration and noise analysis
18-1. GENERAL.
This chapter provides information and procedures required to maintain helicopter with respect to quality of the ride and control of rotating component generated vibration.
The instrument recommended to support the Model 212 is the Scientific Atlanta RADS AT.
For convenience, balancing of the tail rotor and main driveshaft are also included in this chapter. Maintenance procedures for these components will be found in the appropriate chapters.
Rotor smoothing and vibration analysis.
18-2. Vibration analysis.
18-3. Main rotor
Refer to Chapter 62 for the complete field maintenance instructions for the main rotor and rotating control components.
Most vibrations are always present in the helicopter at low magnitudes. The main problem is deciding when a vibration level has reached the point of being unacceptable. The only sources of any frequency are rotating or moving parts on the helicopter. Other parts vibrate only in sympathy with an existing vibration. Extreme low, low frequency, and most medium frequency vibrations are caused by the rotor or dynamic controls. Various malfunctions in stationary components can affect absorption or damping of existing vibrations and increase overall level felt by the pilot. A number of vibrations are present which are considered a normal characteristic of the machine. Two per revolution (2/rev) vibration is the most prominent of these, with 4/rev or 6/rev the next most prominent. There is always a small amount of high frequency present. Flight experience is necessary to learn normal vibration levels. Even experienced pilots sometimes make the mistake of concentration on feeling one specific vibration and conclude that the
Vibration level is higher than normal when actually it is not. For simplicity and standardization, vibrations are arbitrarily divided in general frequencies as follows:
Extremely low frequency Less than 1/rev pylon rock
Low frequency 1/rev or 2/rev type vibration
Medium frequency Generally, 4, 5, or 6 / rev
High frequency Tail rotor or faster (buzz)
18-4. Extreme low frequency.
Extreme low frequency vibration is limited to pylon rock. Pylon rocking two to three cycles per second is inherent with the rotor, mast, and transmission system. To keep vibration from reaching noticeable levels, transmission mount dampening is incorporated to absorb the rocking. Malfunctions in the dampening system will allow rocking to start and continue until it can be felt by the pilot. A quick check of the dampening system may be made by the pilot while in a hover. Moving cyclic fore and-aft at about one movement per second will start the pylon rocking. The length of time required for the rocking to die out after motion of the cyclic is stopped is indicative of the quality of the dampening. An abnormal continuation of rock during the normal flight is an indication something is wrong with the transmission mounts or dampers. This may be due to wear, parts loosening up, breakage, incorrect installations, or wrong type parts installed.
18-5. Low frequency.
Low frequency vibrations, 1/rev and 2/rev, are caused by the rotor. 1/rev vibrations are of two basic types; vertical or lateral. A 1/rev vertical is caused by one blade developing more lift at a given point than the other blade develops at the same point. A lateral vibration is caused by a spanwise imbalance of the rotor due to a weight difference between blades, alignment of the CG of the blades with respect to spanwise axis (which affects chordwise balance), or imbalance of hub or stabilizer bar. Rigidly controlled manufacturing processes and techniques eliminate all but minor differences between blades, resulting in blades which are virtually identical. The minor differences which remain will affect flight but are compensated for by adjustments of trim tabs and pitch settings. Initially, the rotor is brought into ground track by normal tracking procedures, using the pitch change link (rolling the grip) to make a blade fly higher or lower to bring both blade tips into the same tip path plane. A track is taken using a higher operating rpm to determine if one blade is climbing (developing more lift) more than the other as its speed increases. This climbing tendency is overcome by adjusting the trim tabs after a flight check is made, then flying again to determine the effect. Because of the physical differences in blades, it is sometimes necessary to roll a blade out of track slightly in order to get both blades developing the same amount of lift. Generally, verticals felt predominantly in low power descent at moderate airspeeds (60-70 knots) are caused by a basic difference in blade lift and can be corrected by rolling the grip slightly out of track. Verticals felt mostly in forward flight which get worse as airspeed increases are usually due to one blade developing more lift with increased speed than the other (a climbing blade). This condition is corrected by adjustment of the trim tabs. Smoothing of 1/ rev verticals is essentially a trial and error process. A basic straight forward procedure is used but the outcome of any adjustment is uncertain and requires flight-testing to determine need for further action. Because of the idiosyncrasies of the individual blades, it is occasionally necessary to attempt adjustment procedure not normally utilized, such as lateral procedures for a vertical, using roll when normally tab is used (and viceversa), or changing both tabs an equal amount. Associated with the 1/rev vertical is the intermittent 1/rev vertical. Essentially, this is a vibration initiated by a gust effect causing a momentary increase of lift in one blade giving a 1/rev vibration. This momentary increase is normal but if picked up by the rotating collective controls and fed back to the rotor causing several cycles of 1/rev, it is undesirable. Sometimes during steep turns, one blade will 'pop' out of track and cause a hard 1/rev vertical. This condition is usually caused by too much differential tab in the blades and can be corrected by rolling one blade at the grip and removing some of the tab (as much as can be done without disruption to the ride in normal flight). Should a rotor or rotor component be out of balance, a 1/rev vibration called a lateral will be present. This vibration is usually felt as a
vertical due to the rolling motion it imparts to the helicopter, causing the pilots seats to bounce up and down. It can be noted that the seats bounce up and down out of phase; that is, the pilot goes up while the copilot goes down. An unusually severe lateral can be felt as a definite sideward motion as well as a vertical motion. Laterals existing due to an imbalance in the rotor are of two types; spanwise and chordwise. Spanwise imbalance is caused simply by one blade and hub being heavier than the other (i.e., an imbalance along the rotor span). A chordwise imbalance means there is more weight toward the trailing edge of one blade than the...

2 Respuestas

Realmente no entiendo tu pregunta, me dices q eres tecnico mecanico de helicopteros y como bien sabes, en el ATA 18 te proporcionan informacion de cuales son los procedimientos de servicio y troubleshooting para mantener la calidad del montaje y control de los componentes rotativos q generan vibracion. No olvides q el nivel de vibracion sera estimado por el piloto al sentir q el helicoptero pierde estabilidad en vuelo o vibra mas de lo normal.
La seccion q me envias es la general (ATA 18-1) donde te explica generalidades de las vibraciones y cuales son los componentes q la causan, son cosas generales q debiste haber aprendido durante tu formacion.
Espero me aclares la pregunta y con todo el gusto te respondere.
estimado ingeniero Ruben
perdoname la insistencia, pero es que en mis cursos de capacitación, el tema de control de vibracion se vio como un tema práctico y en ella no tratamos sobre las frecuencias como se describe en la parte que le vuelvo a transcribir y te pido  por favor se pudieras aclararme o en todo caso descifrar en este parrafo
Vibration level is higher than normal when actually it is not. For simplicity and standardization, vibrations are arbitrarily divided in general frequencies as follows:
Extremely low frequency Less than 1/rev pylon rock
Low frequency 1/rev or 2/rev type vibration
Medium frequency Generally, 4, 5, or 6 / rev
High frequency Tail rotor or faster (buzz)
porque yo solo tengo en cuenta lo que se me enseño:
baja frecuencia, la vib se percibe como golpes y esta asociado generalmente al R/P
alta frecuencia, la vib se percibe como un zumbido a sociado a componentes que giran a altas rev,
pero a que vienen:
1/rev pylon rock ?
1/rev or 2/rev type vibration ?
1, 5, 6 rev ?
 gracias por tu atención
Gerardo A.
Te aclaro el parrafo.
El nivel de vibraciones es mas alto de lo normal cuando en realidad no lo es. Por simplicidad y estandarizacion, las vibraciones son arbitrariamente divididas en frecuencias generales como sigue:
Extremadamente baja frecuencia son menores a 1 vibracion x revolucion y pueden ser producidas en helicopteros bell con sistema de suspension pylon rock en la transmision.
Vibraciones de baja frecuencia son de 1 o 2 vibraciones x revolucion, las cuales son vibraciones tipicas del rotor
Vibraciones de media frecuencia 4,5 o 6 vibraciones x revolucion, estas se presentan generalemente en helicopteros multipala.
Vibraciones de alta frecuencia son las generadas por componentes q giran a gran velocidad o aproximadas a la velocidad del T/R.
Debes tener en cuenta q las vibraciones no solo se producen por descompensacion de las palas del rotor sino q pueden presentarse por inestabilidades o mal funcionamiento de los dampers de amortuguamiento, falta de servicio a estos componentes, etc.
Espero q esto aclare tu duda, pues es una traduccion casi textual.
estimado ingeniero Ruben
muchas gracias,  su respuesta  fortalece mi conocimiento sobre este tema.
gracias por su atención espero seguir contanto con su asesoria
un sincero abrazo desde Perú
Gerardo A.

Thanks for sharing. I found a lot of interesting information here. A really good post, very thankful and hopeful th at you will write many more posts like this one.

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