1. G ROUND R ESONANCE (H ELICOPTER ) Hrishabh Gupta 28/10/2010

2. Self excited vibration occurring whenever the frequency of oscillation of the blades about the lead- lag axis of an articulated rotor becomes the same as the natural frequency of the fuselage. Ground resonance is an instability that may develop on helicopters when the rotor is spinning on or near the ground, and that may result in the destruction of the structure.

3. Ground resonance only occurs on helicopters with a fully articulated rotor system such as the Chinook, TH-55, OH-6 and MD500. Ground resonance develops when the rotor blades move out of phase with each other and cause the rotor disc to become unbalanced. This is because the centre of gravity of the main rotor, acting as a flywheel, is displaced from in line with the axis of rotation, or the main rotor shaft,causing a “ wobble. ” In flight this causes no ill effect, however, if the skids or wheels are touching the ground, especially lightly, the wobble becomes exaggerated. If the frequency of these oscillations matches the helicopter’ s natural frequency, then ground resonance occurs. As each oscillation occurs, within seconds the amplitude increases until the aircraft has a hull breach.

4. Common factors that may cause or aggravate ground resonance are: Unbalanced rotor head or blades Faulty blade tracking Damaged or malfunctioning lead-lag dampers Uneven oleo struts or tire pressures One-skid/wheel landing Hard landing or running take-off /landing over rough ground Take-off from, landing on or lightly touching, a pitching ship's deck

5. the lead-lag dampers cannot compensate for excess in-plane movement of the blades and the centre of gravity of the rotor blades is no longer aligned with the rotor hub. This is particularly dangerous when a helicopter is in contact with the ground. The misalignment of the rotor centre of gravity and the rotor hub generates an unbalanced centrifugal force at a specific frequency. If this frequency be in phase with the natural frequency of the fuselage, the helicopter will start to rock on its landing gear Hard contact with the ground can send a shock wave to the main rotor head, resulting in the blades of a three-bladed rotor system moving from their normal 120° relationship to each other. This could result in something like 122°, 122°, and 116° between blades. When one of the other landing gear strikes the surface, the unbalanced condition could be further aggravated.

6. The figure includes elastic blades, a fuselage and discrete absorbers. The fuselage is modelled as a rigid body with pitch and roll rotations about the centre of gravity. The rotor blade is modelled as a slender elastic beam undergoing flap bending, lag bending, elastic twist, and axial deflection. The blade is embedded with discrete chord wise absorbers. Each absorber is modelled using a mass-spring pair and is assumed to move in the chord wise direction of the blade only. Blade element theory is used to compute the quasi-steady aerodynamic loads. The rotor inflow is assumed to be uniform. Rotor-fuselage-absorber model

7. Blade lag and absorber dynamic characteristics Blade and Absorber

8. For a blade with chord wise absorbers, the blade lag frequency increases, and the absorber frequency decreases as the rotor speed increases. At a certain rotation speed, the absorber is tuned to the blade lag frequency. The blade lag damping augmentation increases and reaches a maximum value as the absorber tends to the blade lag mode. The blade lag damping using the chord wise absorbers demonstrates an effective damping range in rotation speed. Blade lag damping augmentation using chord wise absorbers depends on the chord wise absorber mass, location, spring loss factor, and tuned frequency. Increasing absorber mass increases the blade lag damping augmentation. Increasing absorber loss factor decreases the blade lag damping augmentation. Blade lag damping augmentation increases as the absorber radial position moves from inboard to the tip of the blade. The blade lag damping augmentation also changes as the absorber natural frequency changes. Blade lag damping augmentation highly depends on the blade lag frequency. Increasing the blade lag stiffness increases the blade lag damping augmentation. The blade lag damping augmentation using chord wise absorbers improves the ground resonance stability of the rotor system.

9. 1. Implementation of shock struts, lead lag dampers, and properly inflated tires: Ground resonance is mitigated by employing the use of damping vibration isolators to attach the landing gear to the airframe. The Ground Resonance dampers are tuned to absorb energy at the proper frequencies of ground resonance and typically can prevent this destructive event. 2. Rotor Hub Vibration Absorber: A weight is located on the rotor hub axis and is held in place by 3 springs allowing it to vibrate. The weight/springs system is excited by the periodic cyclic loads on the rotor hub and responds at the excitation frequency by counteracting the excitation load.

10. 3. Cabin Resonators system : An engineering intervention is a Cabin Resonators system, used to cancel out vibrations at the aircraft natural frequency, which is normally near three (3) Hertz. The cabin resonator acts on a principle of physics called the resonator principle. It acts by damping the aircraft vibration at the attachment point, thereby reducing the vibration level. 4.Last intervention strategy the pilot must know: If the pilot has maintained the rotor RPM within the normal operating range after touchdown, breaking contact with the ground is the best way to break free of a ground resonance incident. If there is not enough rotor energy (RPM) present then shutting down the helicopter, lowering collective reduces lift and the tendency for the individual blades to “hunt” is reduced. Reducing this hunting may allow the blades to return to their normal in-phase position.

11. http://www.fsinfo.org Analytical and a numerical ground resonance analysis of a conventionally articulated main rotor helicopter by Eckert; Bernd. The theory of ground resonance of helicopters by R. F. Ganiev and I. G. Pavlov

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