A semi-empirical methodology for balanced field length estimation of jet-engined aircraft in early design phases
Field performance is one of the key aspects of airplane design. In the very competitive commercial aircraft business of today, field performance is subjected to narrower design margins and very stringent market constraints. In addition, great uncertainties characterizes the estimation of take-off field length in early design phases, due to inaccurate data about the the airplane under development and outdated methods for performance estimation. This way, incorrect sizing often takes place in the conceptual phase, leading to loss of competitiveness. Reviewing and updating these established methods for calculation of field performance, mainly by including new parameters, better calibrations or new inputs weighting could greatly contribute to the proper airplane sizing. During these early phases of the aircraft design, the use of numerical simulation and integration to calculate performance is not practical, considering that it involves several aerodynamic characteristics of the airplane, which have an error margin greater than the required precision. Also, some unpredicted or unconsidered effect, such as interference drag or aerodynamic efficiency, can lead to largely inaccurate results, which would not be noticed without an adequate way to validate the numerics. For this reason, semi-empirical methods are historically used to this purpose, assuming that a relatively conventional design will follow historical trend, usually providing smaller deviations from actual results. Concerning take-off performance, there are many well established methods that provide reasonable results and that have been used for a long time. Among them, the best known and widespread one is the “Take-off Parameter” (TOP). Due to its simplicity, it is the standard method of aircraft design courses, and it is used as well in preliminary studies in the aeronautical industry. However, TOP prescribes a fixed linear relationship between the involved quantities, and it is strongly dependent on calibration, which reduces considerably its efficacy. Methods based on TOP, using quadratic calibrations, for instance, were proposed, which results in increased compliance to calibration data, but not guaranteeing its uncorrelation to these inputs. A more comprehensive method for field performance estimation was proposed by Egbert Torenbeek in the 70s in his Synthesis of Subsonic Airplane Design book. More recently, Torenbeek proposed another method in the new Advanced Aircraft Design book, containing another method for field performance calculation, proposing an “upgrade” of TOP, by adding a component for take-off airborne distance, including a few parameters. The method outlined in Torenbeek first book includes several aircraft characteristics that are very sensitive to the field performance and are not addressed by other methods, but in a way that the empirical characteristics are not overseen, assuring reasonable results. Some of these improvements include engine related characteristics, second segment performance, reaction time to engine failure (considering a balanced takeoff) and friction coefficient of the runway, and it is possible to use empirical values for these parameters, if they are available in the current project stage. This feature also allows a level of customization of the method, in order to obtain better accuracy updating specific segments inside it. The proposed method for field performance calculation contemplates auxiliary methods to consider effects of characteristics that have been defined for the aircraft in design, already. Thus, airplane behaviour is better described, relying less on calibration. In addition, the new method replaces fixed values used in the original methodology for estimations specific for each design, working as a compilation of methods. Sources of these methods include diverse airplane design bibliography, ESDU material and experimental results. Including a more precise modelling of turbofan engines behaviour, or predicting second segment rate of climb, based on available parameters, for starters, has resulted in moderate improvements, even without the complete set of additions, suggesting a promising outcome for the final modified version of the method. Parametrization of fixed components of the original methodology, taking into account aircraft characteristics, has also demonstrated to be a secure source of improvements in accuracy.