Modeling of the A320 Landing Gear Drag During Their Extension and Retraction

Abstract

The modeling of the transient drag increase during the extension or retraction of landing gears does not seem to have been studied in details. The aerodynamics and aeroacoustics of fixed and retractable landing gears have been investigated by various groups with the aim to the reduce their drag, especially for fixed landing gears, or their noise. For aircraft designers, there is almost no need for investigating the comparatively short duration (about 20\,s) of the gear extension or retraction, which explains the lack of publications on this topic. For non-retractable landing gears, the contribution to the overall aircraft drag is often minimized by covering most of the gear leg and wheel with a fairing. In terms of drag models, this contribution is often not distinguished from the overall aircraft drag. For retractable landing gears, the contribution of the landing gear to the total drag can be determined experimentally by using system identification techniques to identify two separate models: one with the landing gear extended and one with the landing gear retracted, and by subtracting the obtained drag values. The experimental determination of the transient evolution of the drag during the transition between these two positions is difficult and only useful for very specific use-cases. For instance, for making very precise predictions of the deceleration of an airliner in final approach with low power, knowing the actual time evolution of the drag within the 15--20 seconds of the gear extension can improve the predicted airspeed at the end of the extension by about 1--3 knots. Leveraging a series of prior work by the authors on flight performance identification for the Airbus A320ceo and A320neo aircraft and a dataset of operational flights with A320neo, a methodology allowing the identification of the transient drag during the landing gear extension is proposed and demonstrated. Data from 804 gear extensions from three A320neo aircraft during regular operations were used for this work. The causes for the highly nonlinear evolution of the drag during the gear extension are explained. A simple model of the landing gear drag as a function of the gear legs and doors positions is derived. This model is valid for the A320neo and all other models of the A320 family which shares the same gear. Similar effects as those identified for A320 can certainly be expected for many airliners, as they often have similar landing gear kinematics and geometries as the A320.