  We obtained 13 epochs of mid-infrared interferometry with the MIDI instrument
at the VLTI between April 2004 and July 2007, covering pulsation phases
0.45-0.85 within four cycles. The data are modeled with a radiative transfer
model of the dust shell where the central stellar intensity profile is
described by a series of dust-free dynamic model atmospheres based on
self-excited pulsation models. We examined two dust species, silicate and Al2O3
grains. We performed model simulations using variations in model phase and dust
shell parameters to investigate the expected variability of our photometric and
interferometric data. The observed visibility spectra do not show any
indication of variations as a function of pulsation phase and cycle. The
observed photometry spectra may indicate intracycle and cycle-to-cycle
variations at the level of 1-2 standard deviations. The best-fitting model for
our average pulsation phase of 0.64+/-0.15 includes the dynamic model
atmosphere M21n (T_model=2550 K) with a photospheric angular diameter of
7.6+/-0.6 mas, and a silicate dust shell with an optical depth of 2.8+/-0.8, an
inner radius of 4.1+/-0.7 R_Phot, and a power-law index of the density
distribution of 2.6+/-0.3. The addition of an Al2O3 dust shell did not improve
the model fit. The photospheric angular diameter corresponds to a radius of
520^+230_-140 R_sun and an effective temperature of ~ 2420+/-200 K. Our
modeling simulations confirm that significant visibility variations are not
expected for RR Aql at mid-infrared wavelengths within our uncertainties.
  We conclude that our RR Aql data can be described by a pulsating atmosphere
surrounded by a silicate dust shell. The effects of the pulsation on the
mid-infrared flux and visibility values are expected to be less than about 25%
and 20%, respectively, and are too low to be detected within our measurement
uncertainties.
