Theoretical model proposal on direct calculation of wetted area and maximum lift-to-drag ratio

As measuring flight performance by experimental methods requires a lot of effort and cost, theoretical models can bring new perspectives to aircraft design. This paper aims to propose a model on the direct calculation of wetted area and L/D_max.

Model is based on idea that the wetted area is proportional to aircraft gross weight to the power of 2/3 (W_g^2/3). Aerodynamic underpinning of this method is based on the square–cube law and the claim that parasitic drag is related to the S_wet/S_wing. The equation proposed by Raymer was used to find the L/D_max estimate based on the calculated wetted area. The accuracy of the theoretical approach was measured by comparing the L/D_max values found in the reference literature and the L/D_max values predicted by the theoretical approach.

Proposed theoretical L/D_max estimate matches with the actual L/D_max data in different types of aircraft. Among the conventional tube-wing design, only the sailplanes have a very low S_wet/S_wing. The S_wet/S_wing of flying wings, blended wing bodies (BWBs) and large delta wings are lower than conventional tube-wing design. Lower relative wetted area (S_wet/S_wing) is the key design criterion in high L/D_max targeted designs.

The proposed model could be used in wing sizing according to the targeted L/D_max value in aircraft design. The approach can be used to estimate the effect of varying gross weight on L/D_max. In addition, the model contributes to the L/D_max estimation of unusual designs, such as variable-sweep wing, large delta wings, flying wings and BWBs. This study is valuable in that it reveals that L/D_max value can be predicted only with aspect ratio, gross weight (W_g) and wing area (S_wing) data.