G'day Tinstaafl.
With the TAS/200 lead distance thing, this works pretty well around the 100+ knot mark, but for a more accurate number you can use the circular slide rule:
For a rate one turn:
TAS
Radius of turn
---+--------------------+--------
400 TAS
(TAS in knots, Radius is produced in Nautical Miles)
The radius of turn will also be your lead distance, although the above calculation assumes that you are already established in a rate one turn, so as it takes a few seconds to establish the turn you need to add a bit for higher accuracy. You also need to adjust a bit for a tail or headwind.
As you can see:
- at 60 knots, the radius is 0.09 NM (TAS/200 gives 0.3 NM)
- at 100 knots, the radius is 0.25 NM (TAS/200 gives 0.5 NM)
- at 160 knots, the radius is 0.63 NM (TAS/200 gives 0.8 NM)
- at 200 knots, the radius is 1 NM, (TAS/200 gives 1 NM)
- at 250 knots, the radius is 1.8 NM. (TAS/200 gives 1.25 NM)
The TAS/200 approximation is simple, and as such works well in your head. At the lower speeds it also allows that little extra to roll into the turn, and as the required performance on the DME arc is ± 2 miles the extra accuracy really is academic...
...but then I am an academic kind of guy
P.S. I actually worked this out a while ago and my notes (scribbled on a scrap of paper) are at work, so I cannot remember if I used a rate one turn, or a 25° angle of bank turn (and I suspect the latter).
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Tech Log forum moderator
[This message has been edited by Checkboard (edited 23 April 2000).]