That's exactly what i wanted to know, it all makes sense now.
"flying_monkey" wrote in message
oups.com...
Slick wrote:
Can someone elaborate on the formula GAG=GAA(airspeed +/- wind)/airspeed

I'm a little confused on what GAA is and if it's just a constant number.
I
kind of derived it as being 30 degrees as a constant target of angle you
want to the touchdown point (?). it works for Tom Knauff's calculation
in
his book with 50mph a/s and 30 mph headwind, and using 30 for GAA. but
when
I reduce it to a 5 mph headwind the resultant doesn't come within the
cone.
Am I missing something here?

I think you're getting two things confused. TLAR is what the other
posters have already said - "That Looks About Right." It's about using
judgement to modify your planned approach depending on what you see
that conditions are doing to your glider.

The formula you quote has to do with the changes a headwind or tailwind
makes to your glide angle. GAG = Glide Angle Ground, GAA = Glide Angle
Air. GAA is a constant, the glide ratio or lift to drag ratio of your
glider.

Some examples. First, lets assume that you're flying a G103, L/D 36:1
at about 51kt. So your GAA = 36. The wind is 0, so the formula for
GAG is airspeed +- wind component, all divided by airspeed, then this
result times the glide angle in air. So, 51 plus zero = 51, divided by
51 = 1.0, times 36 (GAA), resulting in a GAG of 36:1.

Now, let's put in 10 kt of headwind component. That could either be a
10kt direct headwind, or a 20kt wind 60 degrees off the nose, it's the
component that counts. You can figure that with an E6B computer, or
with graph paper, or by multiplying the wind speed by the cosine of the
angle between a direct headwind and the wind angle. So we have 51-10 =
41, divided by 51 = about .8, times 36 = 28.9, your glide angle with
respect to the ground.

Same thing, except with a headwind: 51+ 10 = 61, divided by 51 = about
1.2, times 36 = 43, your glide angle with respect to the ground.

Does that help? You can read about this, put in better terms than I
can, in this book:
http://www.eglider.org/index.php?act...&productId=120

Ed





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