Mike,

I agree that the sag of the rope is small and can probably be ignored.

My calculations do not show drag values as high as 75 lbs however, so the
sag may be (a little) more that your calculations.

I figure a modern glider will have something in the neighborhood of 30 lbs
drag, while an old "boat" of a glider will be in the 50 lb range. (Am I
missing something here?)

I calculated the sag effect in the rope a different way. I figure a
typical 200' poly rope weighs about 4 lbs. The tow plane supports one
end, while the glider supports the other. I figured in a 2 lb down force
on the glider. Since, during tow, lift is essentially the same as weight,
you have to get an extra 2 lbs of lift to compensate for the weight of the
rope. If you figure the weight of the glider in the 700 to 900 lb range,
it is a tiny percentage of extra lift needed due to the rope. (like 2/10
of one percent.)

Same is true if the rope pulls (slightly) upward or downward, the change
in lift is tiny.

Now consider free flight in a 45 degree bank. Lift is increased by 40%.
The glider still will fly pretty well at relatively slow speeds in a 45
degree bank. Now look at 60 degree bank where lift is doubled......

Just a couple more thoughts about the forces on that tow rope:

When the tug is flying without a glider, the rope does not hang straight
down. The "drag" on the rope causes the rope to fly at quite a flat
angle. We could say that some of the weight of the rope is being
supported by the air drag on the rope. But then when a glider is on tow,
and the rope is essentially level, with a sag, I guess the front half of
hte rope is being lifted by air drag, while the back half of the rope is
being pushed down by air drag. So I think that just adding the weight of
the rope as a downward component will work in calcualtions.

Long way to get to the point however. I don't see how the tow rope can
be the culprit in this "too slow tow" situation.

Cookie









At 23:30 16 March 2009, Mike the Strike wrote:
I ran a quick calculation of the expected catenary sag in a towrope.
With a 200-foot nylon/poly towrope and 75 pounds of tension, the sag
amounts to less than a foot, ignoring aerodynamic forces. Under
steady towing conditions, it can probably be ignored.

Mike