Originally Posted by
DaveReidUK
One of the simplest rules-of-thumb that I've seen (assumes limiting bank angle of 25°) is to square the groundspeed (in nm/min) and then divide the result by 9, to give radius in nm.
For your info - This is described in the link posted (in a non-link form) above
Turn Radius
with a full proof.
Also suggests dividing by 10 as it's clearly easier and "in most cases is close enough".
To get a better approximation of dividing by 9 you could divide by 10 and add 10%.
So - taking the example of 25
25 / 9 = 2.78
25 / 10 = 2.5
2.5 + 10% = 2.75
To see the difference between 1/9 and the new approximation
1/9 = 0.111111
Dividing by 10 and adding 10% can be expressed as
0.1 * 1.1 = 0.11 which is very close to 0.11111. It's about 1% different.