Abstract:

Multi-engined propeller driven aircraft are known to suffer from a reduction in lateral stability due to sideslip (the "dihedral effect", L_v), particularly at a combination of high power and large flap angle. An example which arises during certification is the baulked-landing case. The origins of this effect are discussed in ESDU 06012, which shows it to be dominated by the propeller slipstream passing over the wing. A method of estimating its magnitude in the linear range is presented. Non-linearities due to flap end effects are illustrated and further general limitations are discussed. A worked example is provided.The method has been derived using data from conventional twin-engined, single-fin configurations. Certain geometric parameters have been ignored in the development of the method. These include the effect of wing taper, the presence of engine nacelles, the proximity of the fuselage and the possibility of the slipstream impinging on the tail unit. As a satisfactory correlation has been obtained with the simplified method it is concluded that these effects are of minor importance for such configurations. In principle the method is equally applicable to four-engined layouts, provided care is taken if the outboard slipstream is close to the outboard flap end, because in this situation non-linearities may well be encountered. The method is accurate to within about 0.003 on rolling moment coefficient increment.