Originally Posted by
hugh flung_dung
BEagle: "12. The hardest sum next! Time = Distance / Speed! Either mentally or on the back of the checklist. Write it down, then lookout."
Avoid the random number generator known as "hard sums" by simply marking the track line at intervals of 1/10 of groundspeed (equivalent to 6 minutes) and adding-up the 6s (e.g. groundspeed 140 equates to 14 miles in 6 minutes).
HFD
You can use any size intervals you want. Time between two intervals - let's say that took 3 minutes, there are 6 more - well that'll be 1.5 x 6 = 9 minutes. Job done.
You don't care about groundspeed in knots, you just need it in minutes per mark on the chart.
I learned to navigate flexwing microlights that way, and still use the system in an open cockpit.
G