Is there a symbols chart or legend for common aerospace equations?
Unfortunately, the nomenclature used can vary from source to source, and the same variables can even be used for completely different things depending on the context. It is generally good practice for the author to describe the variables they use, but we've all been guilty of skipping that step.

In any case, there are many helpful sites such as See How It Flies or the Aerodynamics Index from NASA that tend to use pretty 'standard' notation. BUT, there is no single cheat sheet that has every variable for every analysis and one should always be careful to double check.