  We investigate the Sun-Earth dynamics of a set of eight well observed solar
coronal mass ejections (CMEs) using data from the STEREO spacecraft. We seek to
quantify the extent to which momentum coupling between these CMEs and the
ambient solar wind (i.e., the aerodynamic drag) influences their dynamics. To
this end, we use results from a 3D flux rope model fit to the CME data. We find
that solar wind aerodynamic drag adequately accounts for the dynamics of the
fastest CME in our sample. For the relatively slower CMEs, we find that
drag-based models initiated below heliocentric distances ranging from 15 to 50
$R_{\odot}$ cannot account for the observed CME trajectories. This is at
variance with the general perception that the dynamics of slow CMEs are
influenced primarily by solar wind drag from a few $R_{\odot}$ onwards. Several
slow CMEs propagate at roughly constant speeds above 15--50 $R_{\odot}$.
Drag-based models initiated above these heights therefore require negligible
aerodynamic drag to explain their observed trajectories.
