A new method for the prediction of the downwash angle gradient due to aircraft wings across the span of an aft horizontal tail in the incompressible regime is presented. The approach is quite general and accounts for all practical wing geometric parameters except for wing twist where it is shown that this parameter has very little effect on the downwash angle gradient. The method consists of an empirical law which requires the prior knowledge of three constants. These have been computed either based on experimental data when available or using numerical data obtained using the vortex-lattice method. These constants are given in tables for a wide range of aspect and taper ratios.

First, the new method has been applied to experimental data and has shown much higher accuracy than existing empirical law. And since the published experimental data is rather limited in terms of the geometric configurations considered, the work has been extended to account for most practical wing configurations using numerically obtained data instead. Henceforth, this important parameter for the conceptual and preliminary stage of aircraft design is available for a much wider range of wing geometry.