On 2010/05/28 03:02 PM, Andy wrote:
On May 27, 10:49 pm, Tim wrote:

Anyone have a good set of equations or example of how to do this
simply?


The fastest speed through the air mass will give the fastest speed
over the ground. The wind does not change the speed to fly. It only
impacts best glide speed to a landing.

So add 0xW for a headwind and subtract 0xW for a tailwind.

Andy
I am nnot very good at this kind of arithmetic, so - Let's do a little
mental flying.
Your mount is a first generation standard class. (assume 1:37, L/D max
at about 85km/h, minimum sink at around 70km/h)

You are thermal flying in a fairly strong wind - say 30kt at 5000 AGL.
Thermals are consequently quite broken at lower height and quite
strongly angled downwind once they get organised at 3000AGL.
The turnpoint (an assigned area with a nn km radius) is upwind of you -
around 20km. You are at cloudbase with minimum VFR clearance.
There are cloud streets - producing an average 1kt climb. The best
embedded thermals are around 4kt.

If you fly Mc 1 into a 30kt head component you are flying ~95km/h into a
headwind of ~55km/h - so you are covering ground at 40km/h while
descending at around 125ft/m.

To cover the 20km is going to take 30 minutes in this situation. With a
height loss of 30*125 = 3750feet. This will get you into the turn area
at 1250feet AGL.

Maybe this is not such a good situation to be in - low down you are out
of the working band on the thermals, and lift is broken and difficult to
work. So you decide to thermal at the bottom of the working band - and
end up going backwards at ~40km/hour every 2000/125feet = 16min. In
those 16 minutes you have covered approximately 5km. Assuming you
connect one of those 4kt thermals it is going to take you about 6
minutes to centre and climb back up. In those 6 minutes you have
traversed back at least 4km. So at best you have a nett gain of 10km.
Turn in a 2kt thermal and you have made 5km, turn in 1tk and you are
back at start.

This is an example of there being a cross over on the MC/wind speed. At
some point your ground covering performance deteriorates to the point
that you have to go faster than theoretically economical to make headway.

Bad math?

Cheers
Bruce

--- news://freenews.netfront.net/ - complaints: ---