"X-Plane" differentiates itself from other simulators by implementing an aerodynamic model called blade element theory. Traditionally, flight simulators emulate the real-world performance of an aircraft by using empirical data in predefined lookup tables to determine aerodynamic forces such as lift or drag, which vary with differing flight conditions. These simulators sufficiently simulate the flight characteristics of the aircraft, specifically those with known aerodynamic data, but are not useful in design work, and do not predict the performance of aircraft when the actual figures are not available.