Hello again!!! A man states without reading any of the part of the NASA document here: https://www.nasa.gov/centers/dryden/pdf/88104main_H-1391.pdf Although I have not read it myself because of College (vacations are over), and also, it involves a lot of advanced Math. The summary page in the beginning does assume a "flat, nonrotating Earth" but the introduction page says, "The need for linear models of aircraft for the analysis of vehicle dynamics and control law design is well known. These models are widely used, not only for computer applications but also for quick approximations and desk calculations. Whereas the use of these models is well understood and well documented, their derivation is not. The lack of documentation and, occasionally, understanding of the derivation of linear models is a hindrance to communication, training, and application." Also this:"Whereas it is common to assume symmetric aerodynamics and mass distribution, or a straight and level trajectory, or both (Clancy, 1975; Dommasch and others, 1967; Etkin, 1972; McRuer and others, 1973; Northrop Aircraft, 1952; Thelander, 1965), these assumptions limit the generality of the linear model." So, I just sort of debunked the claim, but I am not convinced as to what can they gain to assume a flat Earth, I mean the model will be a lot simple but still why assume it??? Teaching for the purpose to understand the influences of flight should be rigorous. Thanks in advance!!!