Liu Xu, Associate Professor of the Aircraft Teaching and Research Department of naval aviation university, explains to you——
How helicopters can prevent resonance
■ Liu Renfeng Duan Dongsheng
Rotor blade hubs fitted with oscillating shock absorbers. Profile photo
Historically, there is a story: in the early 19th century, when Napoleon's army in France passed the chain suspension bridge, the soldiers took the "step in step" way to advance, resulting in the suspension bridge vibrating violently, the chain breaking, and the soldiers fell into the water, suffering heavy casualties. Afterwards, scientists found that the tragedy was caused by the phenomenon of resonance.
Resonance also occurs during the helicopter landing phase. In February 2012, an As350B rescue helicopter in Brazil landed in Pará State when it broke off due to excessive amplitude in the fuselage.
So, why did the helicopter fuselage break down? It turns out that when the rotor rotates, affected by factors such as external strong winds, the blade will swing back and forth around the vertical hinge. If the vibration frequency of the landing gear or fuselage is similar to the frequency of rotor swing, the various components on the helicopter will "walk in unison", resulting in resonance. This process is like a breeze blowing leaves, the vibration of several leaves can drive the shaking of the forest, if the amplitude is too large, the leaves will break off the branches and fall down.
In fact, the pilot's way of operating to solve the problem of helicopter ground resonance is very simple - decisively pull up the total distance pole to take off or press the total distance to the end to implement emergency braking. However, the resonance phenomenon occurs without warning, and the amplitude increase is extremely fast, and in just a few seconds, serious accidents such as blade breakage, tire rupture, and fuselage overturning can occur. This is a great test of the pilot's psychological quality and reaction ability. In order to reduce the mental pressure of helicopter pilots when landing, helicopter designers have come up with 3 clever ways to prevent the occurrence of helicopter ground resonance.
Trick 1: "A slap" does not make a sound. In the helicopter design process, frequency avoidance is the most commonly used method to prevent helicopter ground resonance. When designing the structural shape of the helicopter rotor and landing gear, the designer will avoid the vibration frequency of the rotor and landing gear being too close, so as to ensure that the rotor and the landing gear will not resonate at the same frequency when the helicopter lands.
Tip 2: A journey of a thousand miles begins with "feet". The landing gear damping damping system, like putting a pair of "shock shoes" on a helicopter, prevents resonance by blocking or consuming the lateral shaking of the fuselage. Helicopter landing gear design is different, damping and damping damping methods are also different. The skid landing gear can increase the damping by adjusting the stiffness, stagger the resonance frequency, and reduce the probability of resonance phenomena. In contrast, the wheeled landing gear is equipped with a cushioning mechanism like a "shock absorbing air cushion", which can absorb most of the energy generated by the vibration of the landing gear, thereby reducing the vibration frequency and preventing resonance.
Trick three: install a "tight hoop" at the end of the paddle. The "tight hoop" is the oscillating damper set at the junction of the rotor blade and the propeller hub. Through this "tight hoop", the designers block the swing of the front and rear propellers of the helicopter, thereby limiting the swing amplitude of the propeller blades and ensuring that the range of the rotor blade swing is safe and controllable. At present, the helicopter propeller system is equipped with two kinds of dampers, springs and hydraulics, don't look at the "tight hoops" on these two kinds of paddles is not as big as a fist, but it can ensure the smooth landing of the entire helicopter, which can be called the "guardian spirit" of the safe flight of the helicopter.
Source: People's Liberation Army Daily