Numerical and Experimental Modal Analysis of a Cantilever Beam Axially Loaded by a Tendon Which Is Attached in a Single Spanwise Location

It has been recently proposed to incorporate a tendon in a rotorcraft blade to introduce a means of controlling its dynamics properties. This has been shown as an effective resonance avoidance mechanism that should allow rotorcraft to operate with shape adaptive blades or with variable rotor speed, thereby increasing their performance and efficiency. In the previous studies, the tendon was attached to the blade’s tip, passed freely through its whole body and was fixed at the root of the blade. The tendon was therefore free to vibrate unrestrictedly inside the blade. This, despite delivering the required changes to dynamics, may not be the most optimal and viable design. In this paper, a modification of this concept is investigated. Unlike in the previous studies, the tendon does not pass freely through the blade, but it is connected to it in a single spanwise location using a mechanical attachment. This coupled blade-tendon system is studied both numerically and experimentally. The blade is modelled as the Euler-Bernoulli beam, the tendon as a taut string, and the attachment point as a concentrated mass. The boundary and connectivity conditions are used to ensure the required coupling between the beam and the tendon. Free vibration analysis is conducted using a boundary value problem solver and a bench-top experiment is used for validation of the numerical results. The variation of modal properties with the applied tendon tension and the location of the attachment point is investigated. It is found that many features observed in the previous studies, such as the frequency shift and frequency loci veering, are still exhibited by the modified system, but they are manifested under different loading conditions. In this way, the attachment points may influence the ability to control the beam’s dynamic properties. The implications of these phenomena for the application of an active tendon in rotorcraft are discussed.