On Mar 15, 8:30*am, Bob Cook wrote:
Let me clarify:

The rope itself is in pure tension.

Yes,this tension force applied to the glider, combined with other forces
acting on the glider can form a moment. (on the glider)

If the rope were magically "rigid" like an I beam, and welded to the
glider, the it could impart moments to the glider. *But a rope is
flexible.

When forces applied to the glider act in a direction other than through
the CG, moments are formed in the glider.

Like your example, tow rope attached low, so the "thrust" acts on a line
below the CG. * Drag acts in the opposite direction but on *a line above
the CG. * (like a high winged glider) A nose up moment is formed.

But there is still not moment transmitted directly form the rope to the
glider since the rope is flexible and the tow ring would not allow it
anyway.

Cookie

At 11:00 15 March 2009, The Real Doctor wrote:



On 15 Mar, 05:45, Bob Cook *wrote:
For purposes of analysis, the angle at which the tow rope meets the
glide=
r
is the angle to consider that THRUST is acting on the glider. =A0A
rope
c=
an
only be in tension. =A0It cannot impart any rotational "moments" to
the
glider.

Unless the hook is on the centreline and the tow rope is pulling
stright ahead, it certainly can apply a moment. Simplest case: low
down belly hook, pull straight ahead, pitch up.

Ian- Hide quoted text -

- Show quoted text -

Maybe the moment caused by the towhook location relative to the center
of drag helps explain the elevator authority issue. At slower speeds,
elevator is less effective allowing this moment to overtake it. The
aileron issue may be similar to what we experience on take-off. High-
alpha (sitting up on gear with tail on ground) combined with
relatively low-speed airflow over the wings and the effect feels very
much like initial take-off roll. As someone mentioned, the pull of the
rope keeps the glider in this state, yet prevents the stall break.

Just a complete guess on my part. Interesting discussion. I had a slow
tow last year and experienced the same phenomenon.