This work deals with the development of a new preliminary design method for aircraft directional stability and vertical tail sizing. In the final part of the paper, critical issues in vertical tail design are reviewed, highlighting the significance of a good estimation of aircraft directional stability and control derivatives. Also, a preliminary investigation about rudder effectiveness, related to aircraft directional control, is presented. The investigation has covered both the linear and the non-linear range of the aerodynamic coefficients, including the mutual aerodynamic interference between the fuselage and the vertical stabilizer. A wind tunnel investigation involving more than 180 configurations has validated the numerical approach. Semi-empirical methods have been put in comparison on a regional turboprop aircraft, where the VeDSC method shows a strong agreement with numerical results. The authors performed RANS CFD simulations to calculate the aerodynamic interference among aircraft parts for hundreds configurations of a generic regional turboprop aircraft, providing useful results that have been collected in a new vertical tail preliminary design method, named VeDSC. The focus on aircraft directional stability is due to the significant discrepancies that classical semi-empirical methods, as USAF DATCOM and ESDU, provide for some configurations, since they are based on NACA wind tunnel tests about models not representative of an actual transport airplane. This work deals with a comprehensive review of vertical tail design methods for aircraft directional stability and vertical tail sizing. The comparison between the proposed method and the standard semi-empirical methods available in literature proves the reliability of the innovative approach, according to the available experimental data collected in the wind tunnel test campaign. Wind tunnel tests over a wide range of airplane configurations have been used to validate the numerical approach.
The investigation on the modular model has permitted an effective analysis of the aerodynamic interference effects by moving, changing, and expanding the available airplane components. From the results of RANS simulations on a modular model of a representative regional turboprop airplane layout, the authors have developed a modern method to evaluate the vertical tail and fuselage contributions to aircraft directional stability. The article continues with a concise review of the numerical methods for aerodynamics and their applicability in aircraft design, highlighting how Reynolds-Averaged Navier-Stokes (RANS) solvers are well-suited to attain reliable results in attached flow conditions, with reasonable computational times. Recent investigations made by the authors have shown the limit of these methods, proving the existence of aerodynamic interference effects in sideslip conditions which are not adequately considered in classical formulations. This paper reviews the standard semi-empirical methods usually applied in the estimation of airplane directional stability derivatives in preliminary design, highlighting the advantages and drawbacks of these approaches that were developed from wind tunnel tests performed mainly on fighter airplane configurations of the first decades of the past century, and discussing their applicability on current transport aircraft configurations. Stability and control are often evaluated, especially in the preliminary design phase, with semi-empirical methods, which are based on the results of experimental investigations performed in the past decades, and occasionally are merged with data provided by theoretical assumptions. If these characteristics are not well balanced, the entire aircraft design may fail. Specifically, the vertical tail is responsible for the aircraft yaw stability and control. The safety, performance, and flight qualities of an aircraft also depend on a correct empennage sizing. Aircraft directional stability and control are related to vertical tail design.
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