T/W and alpha effect on the calculation of sustained turn performance from level acceleration

A way of determining the sustained turn performance of a jet is through the level acceleration technique. The concept behind it is determining the excess power/thrust using level accel and translating it to the equivalent increase of induced drag during a turn, resulting in a value of sustained load factor (n_z). Two main limitations are found in this method: i) engine stabilization during the run ii) effect of the thrust component due to alpha (ref. USAF TPS – Chapter 9 -Energy).

In order to understand better the second of the aforementioned effects, the standard derivation of the n_z formula as found in the USNTPS_FTM_108 was repeated without ignoring the thrust components due to alpha this time. The following capture concludes the findings and shows that the error using the standard formula is a factor of the T/W and the aplha_nz (AoA during the sustained turn). In order to apply this method however, both accurate knowledge of T and alpha_1g during the level accel are required, so it is not very practical for application and its derivation mostly serves the demonstration and understanding of the main method’s limitations. It could be used however to provide an % error estimate of the method for an extreme case, as the one of a high T/W, high AoA maneuverable fighter.
Any comments/feedback are welcome.


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