Three-Dimensional Design for Natural Laminar Flow on Vertical Tails of Aircraft

Natural laminar flow (NLF) airfoil design techniques were implemented on the vertical tail of single-aisle subsonic commercial aircraft operating under cruise conditions. The vertical tail features a high leading-edge sweep angle of 40.18 deg, complicating the simultaneous maintenance of laminar flow on the port and starboard surfaces. Thus, designs that achieve a significant extent of laminar flow under these conditions have rarely been reported. This study presents a design employing an inverse method to achieve pressure distributions conducive to NLF. Near the leading edge, the pressure coefficient decreases rapidly, resulting in strong acceleration that suppresses crossflow instability. Immediately downstream of the leading edge, a near-zero pressure-gradient region is introduced, followed by a region of gradual acceleration to mitigate Tollmien–Schlichting instability. Based on these pressure-gradient characteristics, a novel three-dimensional design is proposed, enhancing overall instability suppression. The designed airfoil maintained a laminar flow coverage of 50% under high-Reynolds-number conditions. The maximum transition Reynolds number, determined by freestream conditions and transition location, reached 19.4 million. For the vertical tail, which is symmetric, laminarization was achieved on both sides. These findings show that NLF design principles were successfully applied to a highly swept, symmetric vertical tail, addressing the inherent challenges.