Compounding has often been proposed as a method to increase the maximum speed of the helicopter. There are two
common types of compounding known as wing and thrust compounding. Wing compounding offloads the rotor at
high speeds delaying the onset of retreating blade stall, hence increasing the maximum achieveable speed, whereas
with thrust compounding, axial thrust provides additional propulsive force. The concept of compounding is not new
but recently there has been a resurgence of interest in the configuration due to the emergence of new requirements
for speeds greater than those of conventional helicopters. The aim of this paper is to investigate the dynamic stability
characteristics of compound helicopters and compare the results with a conventional helicopter. The paper discusses
the modelling of two compound helicopters, with the first model featuring a coaxial rotor and pusher propeller. This
configuration is known as the coaxial compound helicopter. The second model, known as the hybrid compound helicopter, features a wing and two propellers providing thrust compounding. Their respective trim results are presented
and contrasted with a baseline model. Furthermore, using a numerical differentiation technique, the compound models are linearised and their dynamic stability assessed. The results show that the frequency of the coaxial compound
helicopter’s dutch roll mode is less than that of the baseline helicopter and there is also greater roll damping. With
regards to the hybrid compound helicopter the results show greater heave damping and the stabilisation of the phugoid
due to the addition of the wing and propellers
