Course Deviation in Order to Find Stronger Lift
In his Corse Deviations article in the December 2011 issue of Soaring,
John Cochrane writes that it’s a mistake to measure course deviations using
distance, and that “the only thing that matters is how many degrees you go
of course.” This is always true when the course deviation is made in order
to avoid sink or to cruise through lift. If, however, we are deviating off
course in order to find stronger lift, this will only be true in a special
case when we are starting the glide at the top of a thermal and the next
turn point is very far away. Otherwise, a number of additional factors come
into play: the initial altitude, the maximum and minimum altitudes (the
working band), the aforementioned distance to the next turn point and
whether or not the stronger thermal can be reached in a single glide at or
above the minimum altitude.
The total time difference dT can then be calculated as dT = Ta + Tb + Tc.
A negative dT indicates time saved while a positive dT indicates time lost.
If we only change the distance to the next turn point from 1000 to 8 miles, the break-even lines turn into closed loops, as shown in the next set of graphs. Note that the loops close at some distance past the next turn point. This means that it may be beneficial to go past the turn point in order to find a stronger climb.
If we also reduce the starting altitude from 8,000 feet down to 3,000 feet, the break-even curves become even more interesting, indicating that back-tracking in order to find a stronger thermal could save us time.
For those interested in further exploring this topic, you can download the Excel spreadsheet used to produce the above graphs. The spreadsheet is interactive and allows you to change any of the parameters, including the glider polar and wing loading and the units of measure. You can also download a pdf with basic theory behind the calculations and the instructions on how to use the spreadsheet.
For any questions and comments, please email me at email@example.com
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