motorgliders as towplanes

On 15 Mar, 05:45, Bob Cook wrote:
For purposes of analysis, the angle at which the tow rope meets the glider
is the angle to consider that THRUST is acting on the glider. *A rope can
only be in tension. *It 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

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

To clarify some more - we tow behind a Pawnee using a 120-foot rope
and my Discus 2 has a nose hook. On tow, the Discus has a notable
nose-up attitude, so the 20 degrees I report is the apparent angle
from my seat. I am positioned immediately behind the towplane just
above the wake. I did have one tow last year in which I was unable to
maintain low tow and had to release.

The pull of the towrope can exert a moment if the force vector is not
aligned with the center of mass of the glider.

The catenary depends on the weight of the rope, but is very notable.

Mike

Mike the Strike wrote:
To clarify some more - we tow behind a Pawnee using a 120-foot rope
and my Discus 2 has a nose hook. On tow, the Discus has a notable
nose-up attitude, so the 20 degrees I report is the apparent angle
from my seat. I am positioned immediately behind the towplane just
above the wake. I did have one tow last year in which I was unable to
maintain low tow and had to release.

The pull of the towrope can exert a moment if the force vector is not
aligned with the center of mass of the glider.

The catenary depends on the weight of the rope, but is very notable.

Mike

I would bet you are in the top of the wake, not just above it. Matches
the symptoms. I have done the same thing since I like to fly very close
to the top of the wake. If you fly to put the towplane the same place on
the canopy (not uncommon in mountainous areas with no well defined
horizon), a slower tow speed will have you riding low and in the wake.
Guess how I know. Flaps really help here.

By all the reports I've read on the D2, seeing the towplane from normal
high tow position below about 65 kts is a problem. So going to a little
higher tow position may not be a sane option.

Your shortish tow rope may exacerbate this since you will need to be
higher to be above the wake, the wake will be stronger and the angles
will all be exaggerated compared to a longer rope you may well be used to.

The best option may indeed be more speed on tow. Or get some stinking
flaps

-Dave

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.