  The minimal sub-Planckian axion inflation model accounts for a large
scalar-to-tensor ratio via a spiralling trajectory in the field space of a
complex field $\Phi$. Here we consider how the predictions of the model are
modified by Planck scale-suppressed corrections. In the absence of Planck
corrections the model is equivalent to a $\phi^{4/3}$ chaotic inflation model.
Planck corrections become important when the dimensionless coupling $\xi$ of
$|\Phi|^{2}$ to the topological charge density of the strongly-coupled gauge
sector $F \tilde{F}$ satisfies $\xi \sim 1$. For values of $|\Phi|$ which allow
the Planck corrections to be understood via an expansion in powers of
$|\Phi|^{2}/M_{Pl}^{2}$, we show that their effect is to produce a significant
modification of the tensor-to-scalar ratio from its $\phi^{4/3}$ chaotic
inflation value without strongly modifying the spectral index. In addition, to
leading order in $|\Phi|^2/M_{Pl}^{2}$, the Planck modifications of $n_{s}$ and
$r$ satisfy a consistency relation, $\Delta n_{s} = - \Delta r/16$. Observation
of these modifications and their correlation would allow the model to be
distinguished from a simple $\phi^{4/3}$ chaotic inflation model and would also
provide a signature for the influence of leading-order Planck corrections.
