Investigation On The Mechanism Of Drag Reduction And Positional Parameters Optimization In Close-Formation Flight

Formation flight is a familiar mode for migrants in long range fly. Human beings have been interested in this phenomenon for a long time. As early as one century ago, foreign scholars studied birds in formation flight using lift line theory. They found that formation flight has aerodynamics advantage. Due to the development of science and technology, more and more formation flight activities such as fighter’s formation flight and transporter’s formation flight appear. Aerial refueling is the most typical formation flight. For a less fuel consumption and a less harm to environment, researchers turn to bionics for a new scenario—formation flight for drag reduction, and don’t concentrate only on improvements of isolated aircraft’s aerodynamics performance. Many investigations of theoretic analysis and experiments have been conducted by foreign researchers, for example linear theory analysis, potential flow technique, low speed wind tunnel test and flight test. Therefore they have well comprehension to drag reduction in formation flight. But inland research on this topic is less and much concentrates on control techniques of formation flight. The mechanism of drag reduction is rarely investigated and related model is rough.This dissertation is devoted to the investigation on aerodynamics coupling between the leading aircraft’s wake vortexes and the trailing aircraft, and on the mechanism of drag reduction in formation flight. Response surface models about the trailing aircraft’s aerodynamic coefficients have been constructed. By the trailing aircraft’s lift-to-drag ratio as an objective function and its positional parameters as variables, the trailing aircraft’s position is optimized. Response surface models are also used to analyze the influence of the positional parameters on the trailing aircraft’s aerodynamics performance, and to study positional stability in typical plane. Measurements of aerodynamic forces and PIV have been conducted in a low speed wind tunnel to validate the influence of positional parameters. The main contents are described as follows:1. Based on potential flow theory, a model about increments of lift and drag of the trailing aircraft is constructed. It is assumed that wake vortexes generated by the leading aircraft were a pair of semi-infinite reversed vortex filaments. Oseen vortex model is used to calculate the induced velocity and efficient angle of attack of the trailing aircraft. This model is used to study the influence of positional parameters on lift and drag coefficients of the trailing aircraft. Results are compared well with the data published.2. Evolution of wake vortexes generated by isolated flying wing configuration is investigated by CFD. Then aerodynamics coupling between the leading aircraft’s wake vortexes and the trailing aircraft is computed in a leader mode formation flight, which consists of three flying wings, to investigate the mechanism of drag reduction in formation flight.3. Using dynamic mesh technique, Kriging response surface model of the trailing aircraft’s aerodynamic coefficients is constructed. Based on El function, an EGO method is used to optimize the trailing aircraft’s position to achieve the maximum lift-to-drag ratio. The Kriging model is also used to analyze the influence of positional parameters and positional stability of the trailing aircraft.4. Measurements of aerodynamic forces and PIV of the flying wing configuration in formation flight are conducted respectively in a low speed wind tunnel to study the influence of positional parameters on the trailing aircraft, and to validate the results of numerical research.