# Investigation and Design of the Transonic Laminar Flow Characteristics in a Laminar Aircraft
###### Abstract
Secondly, traditional flight verification aircraft use manned aerial vehicles generally, so the higher risk of test flights is compared to unmanned aerial vehicles. Finally, the cost of a traditional test flight is high. There is an urgent practical need to use new specially designed aircraft for the flight verification of laminar flow technology. Therefore, a special layout aircraft shown in Figure 1 was designed by the first aircraft institute of the Aviation Industry Corporation of China. The aircraft adopts a modular design that the central experimental wing can be replaced according to the experimental requirements. Only the central experimental wing was used for the laminar flow experiment, and the outboard wing section and airframe did not participate in the experiment. The outboard wing section provides lift, and the central experimental wing is responsible for conducting a laminar flow test. It is a UAV (unmanned aerial vehicle) with a lower flight cost. Therefore, the flight test design is more flexible to further study the laminar flow mechanism, provide more efficient and low-cost flight verification data and shorten the period of laminar flow wing design. This article mainly qualitatively studies the effects of various sensitive factors on laminar flow characteristics to provide a reliable reference for the design and test flight of the laminar flow verification aircraft. It also provides a source of data for future studies on the differences between flight tests and numerical simulations. As is known, the key to laminar flow technology is to control the boundary layer transition. The boundary layer transition has a significant impact on the development of the boundary layer, frictional drag and flow separation position. The characteristics of the transonic boundary layer were studied in the 1960s. [PERSON] et al. gave a review of NACA wind-tunnel measurements of the pressure fluctuations at the surfaces of aerofoils and in their wakes. These observations show that large fluctuations may occur under the conditions for which separation would be expected ([PERSON] et al., 2018). In 1980, A theoretical analysis was made to determine the real-gas effects on the simulation of transonic boundary layers in wind tunnels with cryogenic nitrogen as the test gas by [PERSON] et al. The results  
Figure 1: Aircraft model with a central experimental wing. indicate that the adiabatic cryogenic-nitrogen boundary layers are not substantially different from ideal-gas boundary layers, with the maximum difference of the various boundary layer parameters being on the order of one percent [27]. Additionally, in 1985, [PERSON] et al. conducted a series of experiments to explore transonic shock-wave/turbulent boundary-layer interactions. They found that the interaction depends very strongly on the Mach number, but the effect of the Reynolds number on the interaction is small [28]. These studies mainly focus on qualitative analysis and interpretation of the characteristics of the transonic boundary layer because the ability to extract test data from the boundary layer is poor. Until recently, Boeing proposed BLDS (boundary layer data system) for commercial jet aircraft surface flow measurements in the transonic flight regime in 2016. Evaluations of boundary layer data quality and comparisons between the in-flight measured boundary layer velocity profile data and numerical simulation results are shown for the wing and vertical tail survey locations. The measurements and computations agree closely at lower Mach numbers. For the highest Mach numbers, there is general agreement for overall thickness, but some differences between predicted and measured profile shape details remain for the wing boundary layer. In general, it provides a better data extraction method for the in-depth study of transonic boundary layer characteristics [29]. In 2022, [PERSON] et al. investigated controlling transonic shock boundary layer interactions over a natural laminar flow airfoil by vortical and thermal excitation.