oscillation. Lift Depending on the aircraft design and flight regime, its tailplane may create positive lift or negative lift (downforce). It is sometimes assumed that on a stable aircraft this will always be a net down force, but this is untrue. On some pioneer designs, such as the Bleriot XI, the centre of gravity was between the neutral point and the tailplane, which also provided positive lift. However this arrangement can be unstable and these designs often had severe handling issues. The requirements for stability were not understood until shortly before World War I - the era within which the British Bristol Scout light biplane was designed for civilian use, with an airfoiled lifting tail throughout its production run into the early World War I years and British military service from 1914-1916 — when it was realised that moving the centre of gravity further forwards allowed the use of a non-lifting tailplane in which the lift is nominally neither positive nor negative but zero, which leads to more stable behaviour. Later examples of aircraft from World War I and onwards into the interwar years that had positive lift tailplanes include, chronologically, the Sopwith Camel, Charles Lindbergh's Spirit of St. Louis, the Gee Bee Model R Racer - all aircraft with a reputation for being difficult to fly, and the easier-to-fly Fleet Finch two-seat Canadian trainer biplane, itself possessing a flat-bottom airfoiled tailplane unit not unlike the earlier Bristol Scout. But with care a lifting tailplane can be made stable. An example is provided by the Bachem Ba 349 Natter VTOL rocket-powered interceptor, which had a lifting tail and was both stable and controllable in flight. Some aircraft and flight modes can require the tailplane to generate substantial downforce. This is particularly so when flying slowly and at a high angle of attack (AoA). On some types, the demand in this flight mode has been so extreme that it has caused the tailplane to stall. On the Gloster Meteor T.7 a stall could be triggered by turbulence when the airbrakes were deployed. On the McDonnell Douglas F-4 Phantom II it initially occurred during takeoff and landing approach, and leading-edge slats were fitted to the tailplane upside-down in order to maintain smooth airflow and downforce "lift" at high AoA. The Pilatus P-3 trainer required a ventral keel to cure a similar effect when spun, while the McDonnell Douglas T-45 Goshawk suffered excess