Direct Lift Control (DLC) is the capability to directly and intentionally influence lift on a fixed-wing aircraft by means of aerodynamic control devices, with minimum change of its angle of attack. Although several definitions exist, with various degrees of ambiguity, the combination of DLC and pitch attitude control has unambiguously proven to reduce pilot workload and improve flying comfort considerably. DLC has historically seen several applications on so-called inflight simulators and, recently, this capability has been rolled out over several aircraft types of the US Navy fleet, massively reducing pilot workload during carrier landings. On the civil front, only one aircraft type has been equipped with this capability, despite its very positive reception by flight crews and passengers. The intention of this paper is to revive interest in civil DLC applications, by reviewing in-depth its basic principles, characteristic features, benefits, and implementations so far. Several modern aircraft and disruptive wing configurations appear to be inherently capable of accommodating DLC functionality from a flight physical, systems, and software point of view. The proven benefits of DLC are likely to well outweigh the cost of the added functionality.

The research presented in this paper focuses on the effects of structural failures on the safe flight envelope of an aircraft, which is the set of all the states in which safe maneuver of the aircraft can be assured. Nonlinear reachability analysis basedonan optimal control formulation is performed to estimate the safe flight envelope using actual aircraft control surface inputs. This approach uses the physical model of an aircraft, where the aerodynamic stability and control derivatives are calculated using Digital Datcom. Symmetrical damages to a Cessna Citation II are considered with 25, 50, 75, and 100% spanwise vertical tail tip losses, leading to gradual shrinkage in the safe flight envelope. Based on the estimated safe flight envelopes, a discussion on the effects of structural damages and different flight conditions on the safe flight envelope is presented. In particular, the interpolatibility of the resulting safe flight envelopes is demonstrated. This property is essential for a novel database-driven flight envelope prediction method, where a database of safe flight envelopes is created offline to be accessed later in real time.