## Unmanned Air System Departure Resistance Using Nonlinear Two-Time Scale Tracking Control

Northrop-Grumman Corporation

13 December 2011 - 31 January 2012

Total award $16,713

Unmanned Air Systems (UASs) are routinely restricted to fly in low angle-of-attack flight regimes where the vehicle dynamics are predominately linear. While this restriction helps ensure survivability of the vehicle by avoiding nonlinear flight regimes which can lead to departure from controlled flight, or nonlinear regimes where precise tracking of trajectories or aircraft states can be difficult if not impossible, it also restricts both routine operation and mission flexibility. A motivating example is the approach flight phase to precision landing, such as an arrested landing on an aircraft carrier. In this situation an aircraft must track both fast states (angular rates and sink rate) and slow states (flight path and heading) simultaneously, and accurately and reliably. Flying at higher approach speeds and therefore lower angles-of-attack can largely mitigate this two-time scale dynamics effect and prevent departure due to stall. But higher approach speeds have long been known to lead to higher occurrences of landing mishaps or accidents. Another motivating example is an aircraft tracking a prescribed fast moving target, while simultaneously regulating speed and/or one or more kinematic angles.

This work develops nonlinear approach & landing control laws for a UAS that accomplish global tracking of both fast and slow states, using our recent results in geometric singular perturbation methods. The objective is to reduce the approach speed while accurately tracking flight path and velocity. The approach has been applied to simultaneously tracking both fast and slow variables for a desired reference trajectory that requires the aircraft to fly between linear and nonlinear flight regimes. The control laws were designed and implemented without making any assumptions about the specific nonlinearity of the 6-DOF aircraft model. Nonlinear simulation results we generated for a combined longitudinal lateral/directional maneuver of an F/A-18A Hornet, consisted of an aggressive vertical climb with a pitch rate of 25 deg/sec, followed by a roll at a rate of 50 deg/sec, all the while maintaining zero sideslip angle. The controller accomplished global asymptotic tracking while keeping all closed-loop signals bounded and well behaved.

Extensions to the work in a subsequent phase will consist of verification flight testing of the controller, using the Pegasus research UAS owned and operated by the Vehicle Systems & Control Laboratory.

Working with me on this program is Graduate Research Assistant:

**Anshu Siddarth**, Ph.D. student