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The use of a high-molecular-weight test gas to increase the Reynolds number range of transonic wind tunnels is explored experimentally. Modi cations to a transonic wind tunnel for heavy-gas operation are described, and the real-gas properties of the example heavy gas (sulfur hexa uoride) are discussed. Sulfur hexa uoride is shown to increase the test Reynolds number by a factor of more than 2 over air at the same stagnation conditions and test section Mach number. Experimental and computational pressure distributions on an advanced supercritical airfoil at Mach numbers of 0.7 and 0.72 in both sulfur hexa uoride and nitrogen are presented. Transonic similarity theory is shown to be successful in transforming the heavy-gas results to equivalent nitrogen (air) results, provided the correct de nition of gamma is used and viscous effects are not dominant. When strong shocks are present on the airfoil upper surface, transonic similarity theory is shown to be less successful in the shock-boundary-layer interaction region, in agreement with computational predictions. NomenclatureA = similarity parameter; Eq. (11) a = speed of sound a i ; b i ; c i = constants in equation of state; Eq. (1) C l = lift coef cient C p = pressure coef cient; . p ¡ p ref /=q 1 c = airfoil chord c p = speci c heat at constant pressure c v = speci c heat at constant volume d = constant in equation of state; Eq. (1) HP = drive horsepower h = enthalpy M = Mach number MWt = molecular weight p = pressure p ref = static pressure at entrance to test section q = dynamic pressure R = speci c gas constant Re = Reynolds number S = entropy T = temperature T c = ref. temperature, 318.8 K; Eq. (1) t = airfoil thickness x = streamwise distance on model ® = angle of attack 0 = fundamental derivative; Eq. (8)°= ratio of speci c heats ¹ = viscosity Presented as Paper 98-2882 at the

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