# Investigation and Design of the Transonic Laminar Flow Characteristics in a Laminar Aircraft
## 5 Conclusions  
In this paper, a parameter sensitivity analysis for a special layout laminar aircraft with a central experimental wing is carried out to investigate the transonic laminar characteristics. The airfoil of the central experimental wing of the aircraft is designed for the flight experiment. Based on the comparison and analysis of the results, several important conclusions are drawn as follows:  
1. The transonic transition is affected by a pressure gradient, \(FSTI\), \(Re\), \(Ma\), \(\alpha\), etc. The effect of \(FSTI\) is not easily overridden by other factors. The pressure gradient affects local flow stability. In most cases, a larger favorable pressure gradient zone implies a larger laminar flow zone. Once the values of \(FSTI\) and \(Re\) are too extreme, the relationship between the length of the favorable pressure gradient and the laminar zone length is not clear. \(Re\) affects the flow stability indirectly by changing the characteristics of the boundary layer. Its increase leads to an earlier transition, but its effect is easily covered by the strongly favorable pressure gradient. The effect of \(Ma\) on the transition is highly nonlinear and affects the transition mainly by changing the pressure gradient. Its effect on the transition is divided into two phases. The first phase has no shock wave. In this process, as the \(Ma\) increases, the favorable pressure gradient zone gradually extends with the delaying of the transition and  
Figure 27: Comparison of the aerodynamic coefficients of the central experimental wing.  
Figure 26: Comparison of the skin friction coefficient contour of the central experimental wing.  
the appearance of a tendency to form a shock wave. In the second phase, the shock wave has been formed. The pressure gradient is more stable than the first phase, so it is difficult for the _Ma_ increase to affect the transition position. \(\alpha\) affects the transition location by changing the pressure gradient. Increasing \(\alpha\) will cause the transition to advance. In short, the essence of the transition is the disruption of flow stability caused by the increase in flow entropy. _FSTI_ affects global flow stability, while pressure gradient affects local flow stability. _Re_, _Ma_ and \(\alpha\) indirectly affect the flow stability, and among them, _Re_ is easily determined by the pressure gradient;
2. The design of weak shock waves and aerodynamic load on the rear part of the airfoil effectively improves the lift characteristics of the central experimental wing. \(C_{L}\) increases by 0.28, which reduces the load burden on the outboard wing without affecting the laminar flow characteristics of the central experimental wing. It should be noted that this design idea is not only for the _OLD_ laminar flow airfoil, but this conclusion can also be used as a reference for designing the same type of laminar flow experimental aircraft. Because real flight conditions are much more complex than wind tunnel tests, the aircraft designer must do everything possible to optimize the performance of the aircraft.  
These results can not only help designers obtain a better understanding of the relationship between laminar characteristics and these sensitivity factors but also improve the laminar flow design by referring to the above conclusions.  
Writing--original draft preparation, X.N.; funding acquisition, J.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (NSFC) under Grant Number 11972304.  
Not applicable.  
The authors declare no conflict of interest.