Abstract

The gas-turbine nacelle accounts for 10% of civil aircraft drag, which could be minimised by reducing nacelle size. However, reducing nacelle diameter often worsens engine performance at off-design conditions, creating a trade-off between operability and drag reduction. During off-design climb, a transonic shock wave–boundary-layer interaction (SBLI) tends to form on the windward intake surface (inner nacelle surface). A shock-induced boundary-layer separation may cause a loss of thrust and stability margin. Could an improved understanding of how intake surface shape influences SBLI behaviour enable smaller nacelles without this compromise in engine performance? A quasi-two-dimensional experimental study was conducted under representative off-design climb conditions. Three geometric parameters of the intake surface were varied across three operating points to provide 27 test-cases in a Design of Experiments structure. This work presents: (i) a novel characterisation of the relationship between an SBLI and intake performance, (ii) a recommended limit for shock strength to avoid significant performance reduction, and (iii) recommendations on the optimal shaping of the windward intake surface to enable smaller, lower-drag nacelles.