The dynamic inflow model is a powerful tool for
predicting the induced velocity distribution over a
rotor disc. On account of its closed form and
simplicity, the model is especially practical for
studying flight mechanics or for designing control
systems for helicopters. Scant attention has,
however, been paid so far in utilising the dynamic
inflow model to analyse an autorotating rotor, which
is different from a powered rotor in the geometric
relation between the direction of the inflow and the
rotor disc. Autorotation is an abnormal condition
for helicopters, but for gyroplanes it is the normal
mode of operation. Therefore the theoretical
discussion on an autorotating rotor is of importance
not only to improve the understanding of present
gyroplanes, but also in the development of new
gyroplanes and to analyse the windmill-brake state
of helicopters. Dynamic inflow modelling is reviewed
from first principles, and this identifies a
modification to the mass flow parameter. A
qualitative assessment of this change indicates that
it is likely to have a negligible impact on the trim
state of rotorcraft in autorotation, but a
significant effect on the dynamic inflow modes in
certain flight conditions. This is confirmed by
numerical simulation, although considerable
differences only become apparent for steep descents
with low forward speed. It is concluded that while
modification of the mass flow parameter is perhaps
mathematically accurate, for practical purposes it
is required only in a limited area of the flight
envelope of autorotating rotorcraft.