Numerical studies have been conducted to investigate the airside thermal-hydraulic characteristics of bare tube bank and plain finned tube heat exchangers intended for use in aero-engine cooling. The exchangers use small diameter tubes (3.0 mm) with compact tube layout and operate at high temperatures with large temperature changes over the exchanger depth. Calculations are performed for frontal air velocities between 5 and 20 m/s, yielding tube diameter based Reynolds numbers from 4898 to 19,592. Calculations are implemented using the Realizable k-epsilon turbulence model with consideration of the air physical property variations caused by the air temperature changes. The results show that ignorance of the air property variability leads to overestimations of both heat transfer and pressure loss, with a smaller air velocity yielding a more serious overestimation. The numerical fin efficiency agrees with the Schmidt fin efficiency within 7.0%. The plain finned tube heat exchanger is superior to the bare tube bank heat exchanger if the exchanger performance is evaluated using the heat transfer rate to pressure drop ratio. (C) 2017 Elsevier Ltd. All rights reserved.