# Longitudinal Wind Tunnel Tests of the PROSIB 19-Pax Airplane
## 1 Introduction  
The relentless evolution of electrical systems technology has given rise to advancements in hybrid and electric propulsion aircraft [1; 2; 3; 4]. These advancements are intrinsically linked with the pursuit of energy efficiency and environmental sustainability in aviation. The Italian national PROSIB project was aimed at developing and investigating innovative configurations for hybrid/electric propulsion systems in both regional fixed-wing aircraft and vertical take-off and landing (VTOL) rotary wing platforms. The project's primary objective was to achieve a substantial reduction of approximately 20% in the energy consumption of air transport by 2035, compared to conventional, non-electric propulsion solutions existing in year 2000. By aligning with the ambitious sustainability objectives set by the International Civil Aviation Organization (ICAO) [5], the PROSIB project endeavored to mitigate the aviation industry's environmental impact. Several critical components formed the foundation of the PROSIB project's research. As concerns the fixed-wing aircraft, the project ascertained the technological requisites essential for the electrification of aircraft within the regional turboprop class, identifying challenges and constructing a sensible developmental roadmap [6]. Innovations in aircraft configurations spanned from the ATR-42 regional turboprop airplane to the smaller 19-passengers platform, addressing the unique demands of varying operational contexts [7]. This paper deals with the wind tunnel testing of a new 19-passengers airplane model [6], representative of the Small Air Transport (SAT) category identified by the FlightPath 2050 document [8; 9]. Although general aviation's emissions impact is relatively minor on a global scale, the SAT segment functions as a crucial bridge, spanning the chasm between experimental aircraft and full-scale commercial aviation. SATs offer a conducive platform for proving and demonstrating diverse technologies, often more effectively than larger passenger aircraft. This is exemplified by the adoption of alternative propulsion systems, initially introduced in light aircraft, which has garnered significant attention in recent years. As such, the SAT segment serves as an ideal avenue for developing comprehensiveaircraft systems, establishing best practices, and charting technological roadmaps for larger aircraft platforms and demonstrators [10]. The use of the wind tunnel for the estimation of aircraft stability and control is as old as the concept itself. Famous manuals and textbooks [11; 12; 13] were written by collecting a huge amount of wind tunnel data on aircraft components and configurations. Many of these reports were focusing on a specific aircraft or technique [14; 15; 16], while others investigated the effects of component installation on static stability [17; 18; 19; 20; 21; 22] or presented advice on test procedures, including propulsive effects [23]. Moreover, there were also experiments focused on the aerodynamic interactions between a scaled model and the walls of the test section, providing procedures to correct wind tunnel data with respect to free flight conditions [24; 25]. The wind tunnel is, nowadays, also used to present test cases to the community to validate numerical methods [26; 27]. However, most of the literature did not focus on the estimation of the downwash for different wing-tail arrangements, with a few exceptions [22]. Methods for the estimation of the downwash gradient were derived by [PERSON] and [PERSON] [28] and expanded in USAF DATCOM [12]. Yet, these methods implicitly assume that the tail does not operate within the wing wake, providing a decreasing downwash gradient with the tail distance from the wing, regardless of any wake interaction. In this regard, [PERSON] [29], which investigated the wing pitch-up phenomenon, provided a chart indicating the relative wing-tail positions to avoid tail blanketing due to the wing wake. Such a chart was resumed by [PERSON] [30] in his famous aircraft design book.