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 aircraft is flying over spherical & rotating Earth.

Some flat-Earthers claimed to have exposed ‘a secret document’ from NASA saying that Earth is flat & non-rotating. They are wrong. 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.

Practically all problems in physics involve 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 ignored, and the results will still be practically correct.

In the paper “Tensor Flight Dynamics”, Prof. Peter Zipfel listed the various flight models he 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 the flat and non-rotating model does not imply the Earth is flat and non-rotating. It is just the fact that Earth is flat and non-rotating has little influence over the problem. The 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 documents where it can be presented as if they say the Earth is flat. In reality, there are many documents there containing the keyword “spherical earth,” too. But flat-Earthers are not as interested.

References

- 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