In making mathematical models, physicists often remove real-world details that have little influence over the final results for simplifications. In flight-dynamics, it is often perfectly adequate to assume Earth is flat & non-rotating, even if the final aircraft will be flying over spherical & rotating Earth.
Flat-Earthers claimed to have exposed ‘a secret document’ from NASA, saying that Earth is flat & non-rotating. In reality, the document is simply a derivation of a flight dynamics problem, assuming flat and non-rotating Earth, which is a common assumption made to simplify flight models. It does not imply the Earth is flat and non-rotating.
Solving all problems in physics involves some simplification. As an example, a thrown ball is affected by many factors, including air resistance, Earth’s curvature, wind speed and direction, Earth’s rotation, Earth’s motion around the sun, the sun’s orbit around the galaxy, etc. If we want to calculate the path taken by the ball, a lot of these things can be left out, and the results will still be practically correct.
In the paper “Tensor Flight Dynamics”, Prof. Peter Zipfel listed the various flight models he has created. He used the flat model for simpler problems like fighter aircraft and air-to-air missile. For other models like a cruise missile, he used the spherical model. And for more complex problems like rocket booster & hypersonic cruise missile, he used the WGS84 model.
Using a flat and non-rotating model does not imply the Earth is flat and non-rotating, but the shape and motion of Earth exert too little influence over the problem, and we can afford to leave them out without affecting the final results. More complex and more accurate models are available if the problem warrants them.
Flat-Earthers fished for the keyword “flat earth” over NASA’s website and cherry-picked the documents where it can be presented as if NASA tells us the Earth is flat. In reality, they have many more documents containing the keyword “spherical earth,” too. But flat-Earthers are not as interested.
- Derivation and Definition of a Linear Aircraft Model – NASA
- Flight dynamics (fixed-wing aircraft) – Wikipedia
- Linearization – Wikipedia
- Why does NASA need an aircraft model flying over a flat and nonrotating earth? – Physics StackExchange
- Tensor Flight Dynamics – Peter Zipfel
- Flight Dynamics Principles – Michael V. Cook
- Overview of Flight Dynamics – Dr. Radhakant Padhi
- Explained: NASA documents stating a flat Earth??? Linear Aircraft Models – Metabunk
- Spherical cow – Wikipedia