Eric Greenwell wrote:

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).

US flying handbooks seem to suggest 1.3 times stall speed for
approach. Perhaps the aircraft POH differ (so I won't address that,
as there are too many reasons for that and too many POHs to
speak clearly to). But at 1.3 times Vso, for the gliders I know
of that stall at 30kts+, this means 40kts, which is 10 kts over
stall speed, and seems pretty good. With a 20 knot headwind,
now one would be on final at (1.3 x 30) + (1/2 x 20) = 50kts.

That seems pretty good for a glider that stalls at 30 kts.

So yep, it's a SWAG (scientific wild arse guess), but it seems
ok. Now a thermal right at flare and touchdown is a much
more interesting problem, but hey, ya gotta land SOMETIME!

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.

Yeah, trying to figure out the polar during landing was probably just
too complicated. The GFH tries to simplify things to 7th grade math.
Yep, so a 14 year old can do it
--

------------+
Mark Boyd
Avenal, California, USA