On Thursday, July 21, 2016 at 7:03:33 PM UTC+3, Martin Gregorie wrote:
On Thu, 21 Jul 2016 06:18:46 -0700, Bruce Hoult wrote:

On Thursday, July 21, 2016 at 3:18:23 PM UTC+3, Martin Gregorie wrote:
That assumes the end of the rope is anchored at the launch point, which
looks like a worst case scenario. If the anchor is somewhere along the
runway, in theory at least, you can do a lot better. Putting the anchor
at the midpoint with the car starting at the 3/4 mark should launch to
a height of half the rope length.

That doesn't make sense. The anchor point can be anywhere at all at or
behind where the car starts from, including next to the glider, or
behind it. It's only dead (unused, wasted) rope length from the car to
the anchor point.

Draw a diagram: I had to before I could work out what was what.

Lets assume a 4000 ft run and 4000 ft of rope.

With the rope anchored at the launch point, the car will start at the
midway mark because that is when the slack will be out of the rope. Its
equally sure that when the car has driven 1000 ft from there, the glider
will be 1000 ft from the car and so cannot be higher than 25% if the rope
length.

Now consider a 4000 ft run and 2000 ft of rope, with the rope anchored halfway down the runway and the car also starting from the midway point. The analysis is absolutely identical, but you've saved the expense of 2000 ft of rope.

Draw a diagram.


With the rope anchored at the 2000 ft mark, the car has to be 3000 ft
from the launch point for all 4000 ft of rope to be tight. If the car
then drives the 1000 ft to the end of the run, the glider still has 2000
ft of rope between it and the car, so in theory could be at 2000 ft.

The car stops (there's no space in front of it), the glider releases, and you now have 2000 ft of rope flopping to the ground uncontrolled. Doesn't sound like a good idea to me.

Once again: The anchor point can be anywhere at all at or behind where the car starts from, including next to the glider, or behind it. It's only dead (unused, wasted) rope length from the car to the anchor point.

What counts is the length of rope from the glider to the car at the start, and the distance the car has available to drive from there.

The length of rope from the car starting point back to the anchor is absolutely irrelevant to anything .. it just lays there and does not affect the launch in any way.

Assuming a standard winch launch gets 33.33% of the runway length in
height, and assuming the glider averages the same climb angle with a 2:1
auto launch with the car starting from the runway mid point, then
release will occur with the car about 75% of the way down the runway,
and the glider at a height 25% of the runway length.

Exactly so - and if you move the anchor point halfway down the run, the
car will be at the 3/4 point when the slack is out and 3/4 of the line is
still between the car and glider

My analysis didn't specify where the anchor point is. It could be next to the glider, it could be next to the car starting point. Makes no difference at all as long as there is exactly enough rope to go from the glider to the car to the anchor point.

There has to be an optimum anchor point, but, guessing wildly, I suspect
its less than half-way down the run from the glider.

The anchor point makes no difference, as long as it's at least vaguely behind the car starting point.