Rather than repeat the stuff again, here, you might like to have a read through a couple of relevant PPRuNe threads on the subject where several of us got a tad carried away with it all ...

https://www.pprune.org/tech-log/10019-aob-rate-turn-question.html?highlight=radius+turn

https://www.pprune.org/tech-log/390973-formula-radius-turn-4.html

The constants come about from running the unit conversions and amalgamating the various calculations to end up with a single constant. You will see very minor variations in the actual constants depending on the specific conversion factors used in their figuring. If playing with unit conversions gives you the heebie jeebies, the following post from one of the linked threads should help you through the maze

https://www.pprune.org/tech-log/390973-formula-radius-turn-2.html#post5239820

(For some reason beyond my ken there are a few linked graphics in the thread posts which aren't showing up. That makes the posts a little harder to follow but you should still be able to figure out what's going on. Edit - now they are showing, after a short while - perhaps pause for a few seconds after clicking the hyperlink).

There's a ton of stuff on the net relating to these calculations if you really want to get into the tedious detail ....

Thank you John.