Ingenious idea for improving aerodynamic moment character of airfoil

In wing designing, bad aerodynamic moment character of an airfoil leads to large trim drag and bad longitudinal stability. How to improve aerodynamic moment character while keeping desirable basic aerodynamic characteristics unchanged is extremely difficult. We now present an ingenious idea that can do it. Also we will present preliminary experimental evidence that show our idea is feasible. The idea is that we take an airfoil called G140 by us which has satisfactory basic aerodynamic characteristics, and changes part of its trailing edge into a flat segment, approximately 5-8% of chord, thus creating a new airfoil called T140 by us. Our calculations and experiments were done for a T140 airfoil, whose flat segment is 7.5% of chord. In the full paper, we explain the idea and its implementation in some detail. In this abstract, we just add some pertinent remarks to listing the three topics of explanation. The first topic is: the contrastive analysis of the calculated aerodynamic characteristics of G140 and T140 airfoils. The contrast results are shown in Figs. 3 through 5 in the full paper. The second topic is; the contrastive analysis of wind tunnel experimental results of G140 and T140 airfoils. In this topic, we use the experimental model illustrated in Fig. 6 and contrast the aerodynamic characteristics of the two airfoils with contrast results shown in Figs. 7 through 10. The third topic is: flow mechanism analysis. The analysis results are given in Figs. 11 through 15. Now we are in a position to state the core of our idea as follows: (1) the flat segment supplies positive aerodynamic moment at small angle of attack, and the positive aerodynamic moment makes the C_mo, small; (2) as the angle of attack becomes higher and higher, the flat segment produces negative aerodynamic moment, which keeps longitudinal stability unchanged.