Item 1080

DESIGN: UniCopter ~ Vibration Analysis - Rotor Induced - Blades to Hub

The vibrations that a single rotor is subjected to.

Drawing:

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 Notes:

The pilot is located very close to the helicopter's center therefore rotational vibration should have very little effect on him.

Absolute rigidity is impossible but a close proximity to it should drive all localized perturbations to a very high and unobjectionable frequency. Large area perturbations, such as atmospheric turbulence, will effect the helicopter [Prouty says something on this], but perhaps not to the degree that they affect the airplane [CRAN].

The frequencies of the vibrations should be quite high and subsequently their amplitudes should be quite low. See: DESIGN: UniCopter ~ Vibration Analysis - Rotor Induced - Downwash on Fuselage ~ Frequency & Amplitude

 

Assumption:

The thinking on this page is initially based on the assumption that the helicopter has Absolutely Rigid Rotors. It is then relaxed, slightly.

Absolute rigidity implies that all six blades, the two hubs, the two final reductions plus the interconnecting frame have absolute stiffness.

Based upon the above:

Some of the vibration-induced acceleration will by resisted by the mass of the above assemblage.

Some of the vibration-induced displacement will be dampened by the six blades, particularly vertical motions.

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Small-localized perturbations (trailing edge vortices) are best handled by a rigid blade. The localized load is dampened by the much larger unaffected area of blade

Large perturbations (gusts) are best handled by a flexible blade. The blades flex and reduce the transmission of the vibration to the central hub.

Displacement:

Linear:

Longitudinal:

See Yaw, below..

Lateral:

Why should there be any?.

Heave:

Any alternating heave by the two rotors will behave like a centralized heave, at twice the frequency.

The mean center of thrust on both rotors will be located closer to the longitudinal center of the helicopter than the the center of the hub. This is because of the Advancing Blade Concept. This will place the center of heave of both rotors very close to the center of the craft. This means that alternate heving by the two rotors will not create a rotational vibration about the longitudinal axes but only a heave centered on the vertical axis. Because the rotor di(s) are absolutly rigid, the heave is in the center of this disk(s) and the rigidity will dampen the heave.

Angular:

Pitch:

Must subject the blades, at the same end, on the other rotor to the same vertical motion as well.

Must force a moment about the mass in the middle of the disks and the blades on the opposite side.

Roll:

Must subject the blades, on the same side, on the other rotor to the same vertical motion as well.

Must force a moment about the mass in the middle of the disks and the blades on the opposite side.

Yaw:

Alternating profile drag may cause oscillation. It might be mitigated a little by the fact that this drag is on the advancing blade and therefore the center of the blades drag is closer to the centerline of the craft.

Other Web Pages:

For sources of rotor induced vibration see: OTHER: Aerodynamics - General - Vibration - Rotor Induced

For general information see: OTHER: Aerodynamic - General - Vibration - Rotor Induced

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Last Revised: May 1, 2002