Bifurcation Analysis of Critical Flight Regimes for Light Helicopters
by
Giulio Avanzini
DIASP - Politecnico di Torino
Coauthors: Guido De Matteis and Federica Lucertini; DIMA - "Sapienza" Università di Roma

The paper aims at investigating the dynamic behaviour of a teetering rotor coupled with a stabilizing bar by means of bifurcation theory and numerical continuation methods. Both a Bell stabilizing bar equipped with passive dampers and a Bell-Hiller bar featuring aerodynamic appendages (paddles) will be considered in the study, where the effects of design parameters such as the Bell-Hiller mixing ratio or paddle size will be examined, together with the influence of the manoeuvre state.

Floquet theory will be used for determining the stability (and thus the stability threshold) of the rotor-bar model, which includes a simple dynamic triangular inflow model. Comparison with analytical results taken from the literature for simpler cases (linear aerodynamics, uniform inflow, etc.) will be considered for assessing the validity of the method, but also in order to demonstrate that it is possible to investigate a much wider range of operating conditions where the simplifying assumptions lose their validity and nonlinear effects become dominant.

The final objective of the paper is to investigate the occurrence of critical phenomena, such as the mast bumping, that are considered as possible causes of accidents to light helicopters which largely employ teetering rotors in order to provide design guidelines that push the critical thresholds out of the vehicle flight envelope.

Date received: March 11, 2008