Modal tailoring and closed-form solutions for rotating non-uniform Euler–Bernoulli beams

• An inverse problem approach is used to find closed form solutions for a rotating beam.
• Many such solutions exist for boundary conditions simulating hingeless and articulated helicopter rotors.
• A method for finding a beam with a polynomial mode having a specified node is found.

In this paper, the free vibration of a rotating Euler–Bernoulli beam is studied using an inverse problem approach. We assume a polynomial mode shape function for a particular mode, which satisfies all the four boundary conditions of a rotating beam, along with the internal nodes. Using this assumed mode shape function, we determine the linear mass and fifth order stiffness variations of the beam which are typical of helicopter blades. Thus, it is found that an infinite number of such beams exist whose fourth order governing differential equation possess a closed form solution for certain polynomial variations of the mass and stiffness, for both cantilever and pinned-free boundary conditions corresponding to hingeless and articulated rotors, respectively. A detailed study is conducted for the first, second and third modes of a rotating cantilever beam and the first and second elastic modes of a rotating pinned-free beam, and on how to pre-select the internal nodes such that the closed-form solutions exist for these cases. The derived results can be used as benchmark solutions for the validation of rotating beam numerical methods and may also guide nodal tailoring.