I have some questions regarding the document

which states, in its abstract, that it

documents the derivation and definition of a linear aircraft model for a rigid aircraft of constant mass flying over a flat, nonrotating earth.

Why would NASA even model anything that references a flat earth, if in fact, the earth isnt flat? Why would NASA even allow this to be published if it was false? Just because it says 'Model', doesnt mean 'Not for consumption'. Or are published models a class of documentation that means 'experimental' or 'not to be taken seriously?

Dont say-they made a mistake. If not true, for a cataclysmic mistake like this, an entire department and everyone involved should have been canned immediately. It questions the very existence of globe earth by referencing not only a flat earth, but a non rotating earth.

On its face, NASA looks really bad about space flight if they are modeling aircraft flight over a flat, non rotating earth rather than a rotating earth. Felix Baumgartners jump aside.

The document was written in CA so maybe its just trial input for a Hollywood movie.


2 Answers 2


I don't really see the problem here.

The publication details a model (which is to say: a specific mathematical description with the inclusion of some physical effects and the exclusion of others) for the movement of an aircraft through the atmosphere.

In some situations $-$ when the aircraft is moving very fast, over long stretches of time, and over vast distances $-$ effects coming from the curvature and rotation of the Earth might start to play a role. But I think it's fair to say that for most situations encoutered by aircraft, the effect of these two contributions will be somewhere between minimal and negligible. As such, when doing actual calculations, it does not make sense to include them.

The language in the report that you reference details exactly such a situation: they are presenting a model which includes a bunch of physical effects, coming from kinematic, dynamic, and hydrodynamic origins, and which neglects others, among which are the curvature and rotation of the Earth.

In that sense, it is no different from the following:

The motion of a ball that is dropped from a height $h$ at zero initial velocity must obey Newton's second law of motion under gravity, so that its height $z$ obeys the differential equation $$\frac{d^2z}{dt^2}=-g.$$ The solution to this equation of motion is $z(t) = H - \tfrac 12 gt^2$.

This is a model for the motion of a mechanical system which does not incorporate the curvature of the Earth, nor its rotation. As such, it can equally well be described as "a model for a ball dropping over a flat, nonrotating Earth".

Of course, when we're talking about falling sports equipment, we don't really contemplate situations where effects coming from the rotation of the Earth might apply, so there is never really any need to make that clear. For people looking for models about aircraft motion, however, those effects might be important, so those reports should be clear and up-front about whether they are included or not within the mathematics.

The language of the report is extremely dry, corresponding to the format: this is a technical report forming a reference publication, so it is likely not expected to spend much time justifying its existence or explaining what's going on $-$ it sets out exactly what it's going to do, and then jumps right in to do it.

In this sense, it is very much in character when compared with the next report in the sequence, Reference Publication 1208 (pdf), which wastes no time talking about lidar measurements without even taking the time to explain what the measurements are actually of.

To be clear: the Earth is not flat (and it does rotate), and everyone at NASA is very much on board with that fact. But this report was written in 1988, when there weren't any public-communications concerns about people misinterpreting technical language, because the nonscientific flat-Earth conspiracy theory was not a thing at the time. If the report were written now, the authors would likely take due care not to write language that can give rise to misunderstandings if it is attacked by people intentionally looking for things to misinterpret. But it wasn't written now: it is historical literature, and should be understood in its context.


The whole entire point of making a model is that you are intentionally leaving out details that make the analysis harder but that do not fundamentally change the results.

Asking "why would NASA publish something when they know it is false" is exactly the same as saying "why do we use Newtonian mechanics", "why do we use lumped-circuit models", "why do we assume incompressible flow", etc.

We use Newtonian mechanics to analyze how cars go around a curve or how machines move or how to build buildings that don't fall down. It is false! It's just an approximation of General Relativity! Why don't we use General Relativity!

Because using General Relativity would be really freaking hard and wouldn't gain us anything.

We use lumped-circuit analysis when we design electronics. We use magnetic circuits along with our lumped-circuit analysis when we design motors. It is false! It's just an approximation of Maxwell's full equations! Why don't we use Maxwell's equations any time we're designing circuits or motors?

Because in many cases, using Maxwell's equations would be really freaking hard and wouldn't gain us anything.

We use incompressible flow to model all sorts of problems in fluid dynamics -- aircraft flight, flow of sewage in pipes, wind turbine design, etc. It is false! It's just an approximation of fluid dynamics! Why don't we always assume compressible flow!

Because unless we need to, taking compressibility into account is freaking hard and wouldn't gain us anything.

So -- why did NASA choose a model that's much easier to do calculations with, and results in satisfactory answers in many cases?


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