Wind tunnel–flight correlation for laminar wings in adiabatic and heating flow conditions

The DoAL3 airfoil was designed for a laminar wing of a commuter aircraft with small leading edge sweep flying at moderate Mach numbers, M=0.45–0.6, and Reynolds numbers up to Re=14×106. Free transition measurements were executed in the Transonic Wind Tunnel Braunschweig (TWB) of the DLR, German Aerospace Center, at a Mach number of 0.48 and a Reynolds number of 3.2×106. To quantify the airfoil performance for flight Reynolds numbers a Navier–Stokes method coupled to eN transition prediction is applied. The limiting N factor for the TWB facility has been shown beforehand to achieve a value of six, in flight a mean value of twelve can be expected. The extrapolation to flight Reynolds numbers considers the aircraft flying in adiabatic and heating flow conditions. When exposed to extensive sunshine on the airfield the fuel contained in the wet wings will be heated and the aircraft is then flying a considerable time in climb and cruise with wings exposed to non negligible heat transfer. The effects of Reynolds number and heat transfer on the extent of laminar wing flow are investigated separately. The important findings of the numerical study are, firstly, that in adiabatic conditions the wing will fly with the same extent of the laminar bucket as observed in the wind tunnel. Secondly, in heating conditions laminarity is completely lost, the drag is almost doubled with respect to the minimum drag for adiabatic flows.