Effects of atmospheric turbulence on large-span aircraft.

摘要

There is extensive interest in High Altitude Long Endurance (HALE) unmanned air vehicles (UAV), used for atmospheric research, as pseudo-satellite systems, and for military C3I. Typical of this class is the Helios, a solar powered UAV of 245 ft wingspan designed and built by AeroVironment, Inc. as part of the Environmental Research Aircraft and Sensor Technology (ERAST) project funded by NASA. Because of its spanloader design, the wing is lightly loaded under normal cruise and G-Loads and experiences its most severe loading cases in turbulence of spanwise scale comparable to the wing. Existing gust load and turbulence models are deceptively conservative because they do not deal with the spanwise varying excitation. This research is funded by a grant from NASA Langley Research Center.; An initial fundamental aero-elastic model (the unitary mode) has been developed for the case of flexural deformation only, ignoring pitch. This model is the basis of the design of the AeroVironment, Inc. flight vehicles. A more realistic model, incorporating torsional deformations as well as flexural (the binary mode) has been developed. Exact solutions for excitation of any wave number and reduced frequency case are derived using Sears and Theodorsen models for unsteady airfoil loads. The bending moment transfer function shows that the maximum bending moment occurs for spanwise waves approximately equal to span, and that the contribution of flightwise only turbulence is minimal.; Sensitivity studies are made using a representative spanloader, identifying the effect of altitude, turbulence length scale, and stiffness. They show that wing flexibility reduces the turbulence loads by about 20% compared with those on a rigid wing.; Predictions of the response have been plotted against actual flight test data taken from initial Helios Missions. The flight data involves tests of limited periods and relatively low altitude, so that is not comprehensive. Subject to the restricted data the theory developed here shows good agreement with the experiments and supports the assumptions of the present model.