  We investigate the dynamic response of flexible aircraft in low-altitude
atmospheric turbulence. To this end, three turbulence models of increasing
fidelity, namely, the one-dimensional von K{\'a}rm{\'a}n model, the
two-dimensional Kaimal model and full three-dimensional wind fields extracted
from large-eddy simulations (LES) are used to simulate ambient turbulence near
the ground. Load calculations and flight trajectory predictions are conducted
for a flexible high-aspect ratio aircraft, using a fully coupled nonlinear
flight dynamics/aeroelastic model, when it operates in background atmospheric
turbulence generated by the aforementioned models. Comparison of load envelopes
and spectral content, on vehicles of varying flexibility, shows strong
dependency between the selected turbulence model and aircraft aeroelastic
response (e.g. 58\% difference in the predicted magnitude of the wing root
bending moment between LES and von K{\'a}rm{\'a}n models). This is mainly due
to the presence of large flow structures at low altitudes that have comparable
dimensions to the vehicle, and which despite the relatively small wind speeds
within the Earth boundary layer, result in overall high load events. Results
show that one-dimensional models that do not capture those effects provide
fairly non-conservative load estimates and are unsuitable for very flexible
airframe design.
