# A Novel Aerodynamic Modeling Method Based on Data for Tiltrotor evtol
## 2 Analysis of Aerodynamic Characteristics of Tiltrotor evtol  
Tiltrotor evtol aircraft have unique aerodynamic characteristics, including interactions between numerous control surfaces, rotor-fuselage interactions, rotor-rotor interactions, and rapid changes in aerodynamics during the transition phase as the rotor pitch angle varies. Therefore, a suitable aerodynamic modeling strategy for tiltrotor evtol must combine aspects of traditional aircraft modeling strategies. Figure 1 shows the tiltrotor evtol Vahana that Airbus conducted research and testing on several years ago. Figure 1: Test evtol for Airbus A3: Vahana. Aerodynamic modeling for fixed-wing aircraft usually entails generating aerodynamic interpolation tables or representing dimensionless aerodynamic force and moment coefficients as functions of aircraft states and controls [8]. The aerodynamic force and moment coefficients, which are dimensionless, are commonly expressed as functions of various parameters, including angle of attack (\(\alpha\)), sideslip angle (\(\beta\)), angular rate (\(p,q,r\)), and control surface deflections (elevator deflection angle \(\delta_{e}\), alieron deflection angle \(\delta_{a}\), rudder deflection angle \(\delta_{r}\)). These coefficients are known as response variables, while explanatory variables include factors such as airflow angle. The dimensionless aerodynamic forces and moment coefficients of a fixed-wing aircraft, such as lift coefficient \(C_{L}\), drag coefficient \(C_{D}\), and pitch moment coefficient \(C_{M}\), can be expressed as follows:  
\[C_{L},C_{D},C_{M}=f(\alpha_{F},\beta_{F},U,V,W,p,q,r,\ \delta_{e},\delta_{a}, \delta_{r}\ldots)\]  
Rotorcraft aerodynamic modeling relies heavily on computational or flight test data due to the difficulty in scaling rotorcraft proportionally and equipment limitations for wind tunnel testing [8]. It is important to note that unlike fixed-wing aircraft, rotorcraft aerodynamic modeling requires a different approach due to their unique characteristics [18]. Since stability axes and wind axes become undefined in hover, modeling is generally only performed in the body axes for rotorcraft. The formulation in terms of body-axis velocity components, as opposed to airflow angles \(\alpha\) and \(\beta\), allows the state variables to be defined in hover and reflects the fact that fuselage angle of attack and angle of sideslip are less physically meaningful for describing rotorcraft aerodynamics [8]. Explanatory variables for rotorcraft modeling often include body-axis velocity components (\(U,V,W\)), angular rates (\(p,q,r\)), pilot control inputs, and rotor states such as flapping and inflow. Tiltrotor evtols currently have smaller rotor diameters and do not use cyclic pitch control, which makes rotor section modeling less critical, except for specific rotor states [19]. Therefore, the aerodynamic modeling of tiltrotor evtol includes most of the characteristics of fixed-wing aircraft. However, the aerodynamic characteristics of the rotor section are more evident in the fuselage-rotor interaction. Figure 2 shows the relationship between the dimensionless aerodynamic coefficients and moments of tiltrotor evtol and the control and state variables. Variations in the state and control variables of tiltrotor evtol can cause changes in airflow speed and direction, as well as induced velocity characteristics, across different parts of the fuselage. These changes can result in complex aerodynamic features, such as rotor wake and rotor-fuselage interactions, which ultimately affect the aerodynamic forces of different components. It is important to note that these aerodynamic characteristics are  
Figure 2: Coupling relationship of aerodynamic characteristics of tiltrotor evtol. interdependent, which means characteristics located in the second layer of Figure 1 can impact each other. However, most aerodynamic modeling work for evtol to date has used analytical or semiempirical models for research or application purposes.