To provide better longitudinal stability at cruise for one of the MAV configurations developed at CSIR-NAL, a modifiedudversion of Eppler-61 airfoil designated as SM-4308 was used. The airfoil geometry was obtained using inverse design method ofudXFLR5 and aerodynamic characteristics of the modified airfoil were computed using XFLR5 at cruise Reynolds number ofud160,000. To get aerodynamic characteristics at off design Reynolds number, an airfoil of chord 150mm was fabricated and testedudin a 0.55m wind tunnel in the incidence range of -4 to 15 degree at low Reynolds numbers of 46,000, 67,000, 87,000 andud120,000. Surface pressures on the airfoil and total pressures in the wake were measured. From these, the aerodynamicudcharacteristics in terms of lift, drag and pitching moment coefficients were computed and compared with those obtained onudEppler-61 airfoil earlier. Comparison shows lower pitching moment with the newer airfoil, associated with lower lift and dragudcoefficient. Variation of lift coefficient with incidence was observed to have lower lift curve slope with the stall occurring atudlower angle. Also during positive angle of incidence no sudden change in lift was seen at lower Reynolds number of 47,000 andud67000 as observed on Eppler-61 airfoil. Flow on the airfoil was investigated in a 0.2m tunnel using smoke flow technique atudlower Reynolds number of 67,000 and 87,000. Analysis of these photographs together with surface pressure variation on theudupper surface shows the presence of bubble like structure on the SM-4308 compared to flow separation on Eppler airfoil. Theudearly stall seen with SM-4308 airfoil was observed to be due to the bursting of bubble. Comparison of surface pressuresudcomputed using XFLR5 with free transition mode on SM-4308 airfoil showed nearly good agreement with experimental data.
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