Some time ago, news channels continued to broadcast a news about helicopter accidents, that is, a helicopter accidentally "disintegrated" accident occurred in Brazil. A rescue helicopter successfully landed, and suddenly the entire fuselage swayed, causing the entire helicopter to break and break. The insiders were lucky to escape and only suffered minor injuries. After the rescue helicopter returned to the scene after the end of the mission on the 23rd, in less than a few seconds, the fuselage suddenly swayed sharply, and the amplitude became larger and larger, causing the aircraft to disintegrate and the entire helicopter to be broken into three sections. The reason is that when the helicopter landed, the main rotor revolved and vibrated, and the ground resonated, causing the fuselage to shake violently and the structure could not bear, causing the helicopter to disintegrate.
Helicopter ground resonance is the self-excited vibration of the rotor and the body coupled when the helicopter is working on the ground. It must be avoided in the design of the helicopter. After the ground-operated helicopter receives the initial disturbance (such as rough landing, swaying, over-operation, etc.), the blades that make up the rotor vibration system swing more unevenly around the vertical, resulting in unbalanced centrifugal force of rotation. It stimulates the vibration of the structural system of the body supported on the rear landing gear; as the center of the rotor hub on the body, when the body vibrates together, the base vibration is used to invigorate the rotor in the plane of rotation to strengthen or weaken the original shimmy motion of each blade around the vertical hinge. If the frequency of the centrifugal excitation force generated by the rotor system is the same or close to the natural frequency of the vibration of the body on the landing gear, the natural vibration mode corresponding to the natural frequency can cause the center of the hub to vibrate in the plane of rotation; The damping of the blade damper and the damping of the landing gear buffer strut consume less work in the vibration than the above-mentioned excitation force, and the vibration of the blade and the vibration on the rear landing gear of the body will intensify each other. The vicious cycle can increase the amplitude to the extent that the helicopter is destroyed within seconds. Therefore, in the helicopter design, the parameters of the landing gear and the blade damper must be properly selected, and the normal values of the parameters should be maintained during maintenance to avoid ground resonance.
When the helicopter is subjected to external vibration when running or sliding on the ground, the rotor blade will move away from the stable position, such as the revolving motion of the rotor. At this time, the center of gravity of the blade deviates from the center of rotation, and the centrifugal excitation force of the center of gravity of the rotor causes the fuselage to rise. The vibration on the falling frame; the vibration of the fuselage is fed back to the swinging motion of the rotor, which supports the vibration of the rotor to form a closed-loop system, which makes the rotor swinging motion more and more, when the rotor retreats the frequency and the fuselage When the frequency of a model on the landing gear is equal or close, the damping of the system is not enough to consume the energy they mutually excite. At this time, the vibration of the whole system will be unstable, and the vibration amplitude will become larger and larger until the helicopter is destroyed. At the end of the day, there is a ground resonance.
Why do the oscillating and retreating type cause ground resonance, and the oscillating forward type cannot? The main reason is that the vibration phase relationship is transmitted to each other when the coherent retraction type and the fuselage vibration form coupling frequency are equal or close. The phase relationship of the oscillating forward type is energy consuming. If the damping of the blade damper and the landing gear buffer strut is sufficiently large, or the frequency of the centrifugal exciting force generated by the rotor system and the swing frequency of the whole machine on the landing gear are sufficiently large, then the above two vibration systems are due to external interference. The excited vibrations weaken each other until they disappear, that is, no ground resonance occurs.
Self-excited vibration generated by the coupling between the blade shimmy and the vibration of the helicopter when the helicopter is running on the ground. Although it is called ground resonance, in fact, this vibration is not a forced vibration due to an applied alternating force, but a self-excited vibration. The energy to expand the vibration comes from the engine that drives the rotor. Once this vibration occurs, the amplitude can reach a very severe degree within a few seconds, often causing serious accidents such as broken blades, broken tires, overturned fuselage, and even personal injury. Helicopter ground resonance was once a technical obstacle to the development of helicopters. When the articulated helicopter is running on the ground (including the ground interview car, taxiing, take-off, landing, etc.), due to the initial disturbance, each blade produces a front and rear swinging vibration around the vertical hinge phase, so that the total center of gravity of the rotor deviates from the center of rotation, resulting in no Balanced centrifugal force provokes vibration of the body on the landing gear.
The body is the support of the blade, so the vibration of the body will in turn stimulate the additional vibration of each blade around the vertical hinge in the manner of vibration of the support motion. When the rotor reaches a certain speed, under certain conditions, the two kinds of vibrations coupled by the inertial force will excite each other, and the amplitude will increase, making the helicopter unstable. This is the ground resonance unique to the helicopter. Helicopter ground resonance occurs within a certain speed range of the blade. This range is called the unstable ground speed of the helicopter ground. The main way to avoid ground resonance is to properly select the stiffness and damping of the landing gear, and configure an appropriate amount of damped damper on each blade.
Aircraft Group: Technical Specialist, Wu Jia-Kun