Pete Brown wrote:
Derek Piggot writes in his book "Ground Launches , 1996, pg 34.):

“The optimum speed on the launch depends to some extent on the wind
strength with a slightly higher speed giving the best launch height in
calm conditions. Best L/D speed is close to the optimum, with the
minimum cruising speed or speed for minimum sink being the lowest safe
speed.” Derek Piggott, Ground Launches , 1996, pg 34.

If the speed for best L/D is close to optimum, what is optimum?

The optimum is the best glide speed at the wing effective load factor
based on the takeoff weight of the glider and the cable tension. For
example, for an ASK-21 with a best L/D speed of 49 knots at a weight of
1300 lbs, if the cable tension is 1900 lbs, the effective load factor is
1950/1300 or 1.5, the optimum launch speed is 49*SQRT(1.5) or 60 knots IAS.

The American Soaring Handbook, Vol 3, Ground Launch, 1980, pg 32 says
the following about auto tow

“The factor which determines the towing speed of the automobile is the
indicated air speed (IAS) of the sailplane. It has been shown that the
launching forces on the sailplane are at a minimum just below the IAS
for maximum L/D of the sailplane”

Just how far below the best L/D IAS is the point of minimum stress on
the airframe?

What they appear to be discussing here is a rule of thumb for automobile
speed based on the non-adjusted best glide IAS of the glider. Given a
nominal climb angle of 45 degrees, if the automobile is driving at 50
mph (44 knots), the glider will be flying at 62 knots TAS. So, in this
case, once the glider has rotated to climb angle you'd want to drive 6
or 7 MPH slower than the gliders best L/D speed at sea level density
altitude. If the density altitude is higher, you'd want to drive faster.

Once we have derived the factors to max out launch height, how do you
then balance efficiency versus climb rate after that point in the launch
when you have enough altitude for a safe recovery from a low rope break?

Assuming the winch (or automobile) is providing a constant cable
tension, the max launch height will be pretty close to optimal if the
pilot maintains the optimum launch speed IAS as calculated above. This
should result in a climb angle for most of the launch of about 45 degrees.

Marc