I think I have given this too much thought (or not enough??). If anyone has any
additions/rejections to the following please let me know. Else flame away...
The glider leaves the ground before the tug and establishes itself in effectively
high-tow. Assuming the tug to glider weight-ratio being 3:1 then the combination
center of gravity is approximately the point of the rope 1/4(approx) of the
distance from the tug to the glider.
Now, the tug pilot will separate from the ground when he has reached a certain
speed. At this speed he will begin his climb. If we maintain station in high
tow then the combination C of G will rise at that time parallel to the tug's
motion. If however the glider pilot then elects to slide along the ground then
the tug will be able to climb a little better because he will have less drag in
the form of having to increase the altitude of the glider for that short period.
However the climb difference will be far less than the altitude loss of the
glider due to the combination C of G being much closer to the tug. Also the
flight of the glider through the tug's turbulence will increase the drag induced
on the tug, again lessening any energy gain through the glider maintaining
station relative to the ground a little longer.
The glider does have to change station at some point and the question being,
should it change station near the ground therefore lengthening the runway
required for the combination to leave the ground or should it take place once the
combination is well established in the ascent? At the later change in station
there will be a loss in energy due to the glider passing through the tug's
turbulence but by then the combination is further in the air with different
options.
Another comment on the above discussion is the fact the glider is changing
position relative to the tug while the tug is close to the ground and thus
changing the direction of the drag on the tail of the tug again altering the
dynamics of the combination and requiring corrective action on the part of the
tug pilot. Should this change in dynamic occur close to the ground when the
combination is still trying to achieve speed/altitude or when they are better
established in the climb?
Pete Siddall wrote:
> >As for the difference in elevator drag and climb performance of the tug with
> >high and low tow position: is there any published scientific evidence on
> >this or is it just "gut instinct"?
>
> I think can be justified using conservation of energy principles.
>
> If the sailplane climbs from low to high tow, it's converting some of (the
> combination's) kinetic energy to (its own) potential energy. That means the
> tug has less airspeed and/or height than it would have if the glider stayed
> in low tow.
>
> Having said that, high tow, just above the slipstream, is not a really big
> difference from low tow, just below the slipstream. I'm sure nobody's
> advocating "too-high tow" where the glider is much higher than the tug...
> that's definitely dangerous.
>
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Darian...
Darian Jenik [EMAIL PROTECTED]
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