Modeling for Control of Very Flexible Aircraft

The first investigations into the dynamics of a highly flexible aircraft came from the Daedalus Project, initiated in 1984. 3 The goal of this project was to push the limits of human powered flight while promoting engineering, science and education. Prior to the Daedalus Project the longest distance traveled by a human powered plane was 23 miles, a record set in 1979 with a flight across the English Channel. Over the course of the project the distance record was broken several times and in 1988 Daedalus 88 flew 73 miles over the Aegean Sea from Iraklion Air Force Base on Crete to Santorini. Key results from the project were: an attempt to identify stability derivatives for a flexible aircraft using flight data, modeling the aeroelastic characteristics of a highly flexible aircraft, and tools for analyzing aerodynamic and structural loads on flexible–high–aspect–ratio wings under large deformation. Van Schoor et al. identified the importance of including the the flexible states in the stability analysis as the flexible model predicted an unstable phugoid mode with the rigid model predicting a stable phugoid. Building on his work in [7] Drela designed ASWING, a software package used for the study and simulation of flexible aircraft undergoing arbitrarily large deformations. The Daedalus Project, by design, was a segway to High Altitude Long Endurance (HALE) vehicles. The Helios aircraft, depicted in Figure 1, was developed under the Environmental Research Aircraft and Sensor Technology (ERAST) as a HALE class vehicle. The aircraft had two configurations specifically tailored to: 1) high altitude flight and 2) long endurance flight. On August 13 2001 Helios configuration–1 climbed to a record breaking altitude of 96,863 feet. The second configuration did not have the same success however, and on June 26 2003 broke apart mid–flight during testing. Throughout the flight the aircraft encountered low–level turbulence. After approximately 30 minutes of flight time a larger than expected wing dihedral formed and the aircraft began a slowly diverging pitch oscillation. The oscillations never subsided and lead to flight speeds beyond the design specifications for Helios. The loading on the aircraft compromised the structure of the aircraft and the skin of the aircraft pulled apart. One of the key recommendations that came from the flight mishap investigation was to, “Develop more advanced, multidisciplinary (structures, aeroelastic, aerodynamics, atmospheric, materials, propulsion, controls, etc) “time-domain” analysis methods appropriate to highly flexible, “morphing” vehicles”. A large body of work on VFA modeling has come since the Helios flight mishap. Patil et al. studied the open loop dynamics of a flying wing structure similar to that of Helios and found that flap positions used to trim the flexible aircraft differ greatly from those used to trim the rigid aircraft. The authors also captured instability in the phugoid mode which is present during large dihedral angles. Similar studies