INTRODUCTION

This Item provides a means of estimating the free-air low-speed lift and longitudinal forces on unswept wing/flap systems fitted with multiple tractor propeller/nacelle combinations. The propeller slipstreams are assumed to flow over the wing/flap system and are therefore deflected by it, thereby providing a form of thrust vectoring which augments the lifting force. The method is best suited to cases with moderately large flap deflections, as in take-off or landing, provided that the flow is fully attached. It does not however account for ground effects.

The method (Section 3), which is semi-empirical, is based on the application of simple momentum theory (see Appendix A). In this respect it is similar to the method of Derivation 10 but overcomes the acknowledged deficiencies of that method. If the power-off values of lift and longitudinal force are known the method requires a knowledge of only two additional, slipstream related, parameters (Section 4) which are dependent on thrust coefficient. The form of the variation of these parameters with thrust coefficient for attached flow conditions is determined empirically (see Appendix B) and the power-off values are determined by using the known power-off forces. Therefore the variation of the slipstream parameters with thrust coefficient can be simply determined with a knowledge of their values under static (zero forward speed) power-on conditions (Section 5).

The basic method is restricted to (i) flaps which are continuous across the slipstream, (ii) nacelles which are circular in cross-section with a diameter equal to that of the spinner and (iii) propellers which are not overlapped. Sections 6 to 8 provide some guidance on the effects of flap cut-out, nacelle size and propeller overlap, respectively. The applicability and accuracy of the method, which is applicable to a wide range of flap types, are discussed in Section 9.

Detailed worked examples illustrating typical uses of the Item are given in Section 11.