An active tendon concept in rotorcraft with variable speed rotors: free vibration perspective: Free vibration perspective

Rotorcraft with variable speed rotors are known to offer performance improvement, fuel consumption efficiency and noise emissions reduction. However, significant dynamic complications may be encountered as for the various rotor speeds numerous natural frequencies of the main rotor blades can be excited. This paper investigates the use of an active tendon concept to control the dynamic properties of main rotor blades such that their resonances are adaptively avoided based on the current rotor speed. A coupled blade-tendon system is introduced and it is demonstrated that incorporating a tendon into a blade leads to two main effects-a shift of the natural frequencies of the blade, and a manifestation of tuned vibration absorber through frequency loci veering. These effects are explored using numerical simulations in context of their application in rotorcraft with a variable speed rotor. The main rotor of the Bo105 helicopter is used as a reference and it is shown how its dynamic properties may be controlled using a tendon. It is discussed that the reference rotorcraft equipped with the tendon can be operated in the range between 60-120% of its nominal rotor speed without any resonances. In addition, it is hypothesised that this concept could also be used to tune the dynamic properties of rotor blades employing shape adaptive or morphing technologies.