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AIRFRAME Rotorcraft Dynamics


This article will discuss the many technologies used in rotorcraft platforms to address vibration mitigation and motion accommodations. A general overview of each technology is summarized, where it is commonly used, and the advantages and disadvantages of each. A focus will be applied to elastomeric mounts with regards to their use, design considerations and inspection/life.


Overview Rotorcraft systems require compliant connections to accommodate dynamic motions, as well as isolators to mitigate vibration levels to occupants, electronics and structures. In doing so, these resilient elements are placed in the direct path of harmful environments. Selection of the right technology and careful design considerations are necessary when developing these types of mounts to ensure maximum performance and life. Early recognition of mount necessity and accurately defining the operating conditions will lead to the proper technology and envelope space needed to achieve the performance goals, all while maintaining life expectancy. Reactive solutions often limit the space available for an appropriate solution. The consequences include shortened life expectancy, increasing the interval of inspections, maintenance and repair.


Mount


Technologies There are several methods to reduce vibration levels or accommodate motion in rotorcrafts. These include both passive isolation and active systems. Passive isolation refers to a device having a fixed performance characteristic no matter the input. These include elastomeric mounts, tuned vibration absorbers and buffers, to name a few. An active system includes sensors and software that monitors the input to apply equal and opposite forces accordingly using actuators to optimize isolation. The table on page 17 summarizes commonly-used technologies with advantages and disadvantages for each.


Motion


Accommodating Mounts


Multi-module journal bearing


These types of mounts restrict deflection in certain axes while allowing motion in other axes. Tension-torsion straps (TT straps) have very high tension stiffness to resist centrifugal loads but allow compliance in twisting for the rotor blade. Centrifugal force (CF) bearings and spherical bearings have very high stiffness radially and axially, but allow torsion and cocking motions. These isolators are often critical to the performance and handling of the aircraft. Because they can experience large displacements during operation, they require frequent inspections, maintenance checks and replacement. Therefore, designing and testing a mount to maximize life is a key element. This includes defining the visual check criteria to determine the product’s life status, whether replacement is needed, and if the product has reached its end-of-life criteria.


Mark Ott | Technology Development Manager, ITT Enidine, Inc.


and How It Affects Performance, Maintenance and Repair


Vibration Isolation Mounts Rotorcraft systems create high levels of vibration due to primary rotor and blade by-pass frequencies, power and propulsion systems and flight dynamics. Untreated, these vibrations can affect the aircraft’s flight performance, component life, sensitive equipment, increased maintenance and operator comfort. The implementation of a device to either reduce the vibration levels or absorb the energy can improve this experience. These types of isolators typically target a specific stiffness and damping to achieve their goals. Lifespan is typically not an issue with these types of products unless the application is limited on space, or experiences loads unaccounted for during initial design and development. Although testing for fatigue life is common, it is not always necessary.


Multi-layer restraint


16


HelicopterMaintenanceMagazine.com February | March 2016


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