Flight Testing of Autopilot Control Laws: Fly the HELIX!

Abstract

This paper describes a benchmark manoeuvre for autopilot control laws in the form of a helical flight trajectory. During this manoeuvre, several challenging aspects of autopilot design are encountered, such as co-ordination of longitudinal and lateral modes, accurate tracking of a curved flight path, combined tracking of inertial (the flight path) and air massbased references (the airspeed), and the trade-off between behaviour in turbulence and wind shear. During a helix manoeuvre namely, even little constant wind results in continuous wind shears in lateral and longitudinal directions. For these reasons, the helix manoeuvre is a very interesting trajectory to test and analyse autopilot performance, both in simulations and in flight test. This paper describes the geometry of the helix trajectory and uses basic flight mechanics to derive generally applicable reference variables and high-level control strategies for use with typical autopilot structures. This allows the reader to fly the helix manoeuvre using his or her own autopilot design. Based on recent flight test results from DLR’s fly-by-wire test bed ATTAS, some interesting aspects will be discussed that are typically revealed by flying the proposed manoeuvre.