Nyal Williams wrote:


It appears that if you draw a tangent to your glider's
polar beginning, not at zero, but at any given headwind
speed, the line will touch the polar at a point that
is best L/D plus half that headwind.

I was under the impression it was added to give you
a margin for gusts
and turbulence, which are usually less than the average
wind speed. The
'half' was likely chosen empirically, as something
that was adequate
almost all the time.

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Eric,

Chris already accounted for the safety factor (gusts
and turbulence) with his statement about plus 5 knots.

Unfortunately, +5 knots is not very good insurance against gusts and
turbulence, which typically increase with wind speed. Or was this
supposed to be added on top of the "1/2 the wind speed"? If so, I
suggest the +5 knots is redundant in general (specific sites [hill
sites, for example] may require much higher speeds, of course).

His second factor was best speed to fly if you have
to close spoilers and need the guaranteed best speed
to fly for maximum distance.

I doubt it was chosen this way, though the correspondence with the best
L/D in wind is appealing. Since we routinely fly final approach at well
above best L/D glide slope (typically, the moderately steep glide slope
that is achieved with half spoilers), having "best L/D speed" available
when the spoilers are closed doesn't seem like a good way to pick
approach speed.

I believe, but have no direct evidence for it, that it was chosen
empirically: over many years, people that used that value had it work
out well, so it became the recommendation. I suspect the origin is now
shrouded in the fog of history.


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Eric Greenwell
Washington State
USA