Five Questions about Shimmy
Who, What, When, Where and Why, but permit me to slightly modify this journalistic best practice a little.
Let’s apply, What, Why, When, How and Who?
What is Landing Gear Shimmy?
Landing Gear shimmy is a self-inducted oscillation of a wheeled landing gear assembly. Kinetic energy of a rolling pneumatic (elastic) tyre is converted into potential energy of the deflected assembly in either that yaw or lateral directions, with the energy converted back. The complex nature of this energy conversion can create stable, unstable, and asymptotic oscillations within the structure.
Why should Shimmy be investigated?
Their are many shimmy incidences with the history of aircraft, and it continues to a particular challenge within landing gear design. The results vary from conditions that are mildly irritating, to severe incidences resulting in failure of the landing gear.
When should the analysis be performed?
The primary route is to avoid shimmy, and where it cannot be avoided integrating technology or design features are added to suppress the vibration. It is commonly the case where design constraints such as space, weight, tire selection, perhaps steering system design, etc. leave little scope to alter the design into a shimmy stable zone. Therefore, the primary route is analysis, then adoption of a technology to suppress shimmy.
Preliminary analysis should be performed in preparation for the PDR phase, with a much more detailed a complete analysis for CDR. Additional validation analysis is required at later design phases as and when test data is available.
How is the Analysis performed?
The preferred and recommended approach is a detailed multi-body simulation model. This model can be high in model detail, however, a balance is needed between model fidelity, programme phase and deisgn risk, not to mention considerations regarding the complexity of the landing gear itself. An additional comment is needed, highlighting that specialised modelling tools are required to perform this task efficiently. As is experience to navigate the analysis challenges of this potentially complex analysis.
Who – Skynetics
In Skynetics’ experience, that oftentimes clients do not perform the shimmy analysis thorough enough unknowingly leaving considerable risk in the design. This is where Skynetics experience over multiple aircraft programs and with multiple international clients assists greatly. Skynetics has developed specialised tools over 15 years of investigating this specialist subject.
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