Design Evolution and Wind Tunnel Tests of a Three-Lifting Surface Regional Transport Aircraft

This paper deals with the experimental assessment of the aerodynamic characteristics of an innovative large turboprop aircraft. The configuration is a three-lifting surfaces airplane with rear engine installation at tail tips, conceived to carry up to 130 passengers and targeting a minimum economic and environmental impact, which is competitive with regional jets on short and medium hauls. The three-lifting surfaces layout is the output of previous research made by the authors, and it has been selected to fully comply with the market and design constraints. An experimental test campaign was required to validate the aerodynamics, stability, and control of this innovative configuration. From the results of the first campaign, it appeared that the aircraft had insufficient longitudinal and directional stability. Thus, the authors worked to improve these characteristics, updating the design and executing a second wind tunnel test campaign. The evolution of the design is described in the first part of the paper. In the second part, the authors discuss the aerodynamic interference effects among aircraft components, detailing how the combined downwash coming from both the canard and wing, as well as their wakes, affects the empennage aerodynamics. Experimental tests have revealed a significant reduction of the longitudinal stability due to canard additional downwash, especially at low attitudes. Furthermore, it was found that the canard generates a non-linearity on the aircraft directional stability derivative at moderate sideslip angles because of its tip vortex impinging on the vertical tail. Despite the detrimental interference due to the canard, the updated aircraft proved to be statically stable with sufficient margin at the most rearward center of gravity. Lessons learned in this research may be useful to aerodynamicists and aircraft designers facing similar issues.