scope:

ESDU 88031 provides a simple semi-empirical method for estimating the free-air low-speed values of lift and longitudinal forces for an unswept wing/flap system submerged in the slipstream from multiple propellers. It provides for the separate estimation of forces on the propeller and on the nacelle/wing/flap and requires a knowledge of the geometry and of the power-off lift and drag of the nacelle/wing/flap. There is a basic method, developed using simple momentum theory, which applies to flaps continuous across the slipstream, circular section nacelles of diameter equal to that of the spinner, and propellers that do not overlap. Guidance is then provided on the effect of flap cut-out, nacelle size and propeller overlap. The wing may be fitted with any of wide range of single- or multi-element flaps, provided the flow is fully-attached. Estimates are provided over the full range of thrust coefficients (based on slipstream kinetic pressure and propeller disc area) from zero to unity and predictions of the lift and longitudinal force on the nacelle/wing/flap system are within 10 and 15 per cent respectively.

A computer program of the method is provided as ESDUpac A8831.