CG hook on aero tows??

Andreas Maurer wrote in message . ..
On 8 Jan 2004 11:02:45 GMT, "Ian Johnston"
wrote:

My Pirat does the rotation all by itself on a winch launch, regardless
of pilot input.

Does this mean that if you push the stick forward, you are unable to
stop the rotation (in other words - the pilot in a Pirat has no pitch
control during the winch launch)?

In my experience, yes, but only during the initial rotation, and it
only happens at a reasonable speed. In the climb it's fine. And a very
gentle initial acceleration avoids the earlier problems, mostly.

Ian

(Ian Johnston) wrote in message . com...

In my experience, yes, but only during the initial rotation, and it
only happens at a reasonable speed. In the climb it's fine. And a very
gentle initial acceleration avoids the earlier problems, mostly.

Sorry to follow up to myself but ...

The reson, I think, is that the Pirat has quite a high-set wing and a
belly hook as low as it can get. As a result, the lever arm, and
therefore upwards pitching moment from the winch cable about the wing
is relatively high compared to the typical glass glider with mid-set
wing and belly hook offset and up a bit. So with a good fast winch
launch, yes, this moment can overcome anything the elevator is trying
to do.

However, this is an intrinsically stable situation. As the cable angle
relative to the glider's longitudinal axis increases, the lever arm
reduces (by approximately 50% when the glider is 30 degree nose up and
the cable is 30 degrees down). So as the glider pitches up the effect
of the cable reduces, the effect of the elevator (all other things
being equal) stays the same and a point of equilibrium is reached. To
put it simply:

Nose down: winch pulls it up again.

Nose up: elevator pushes it down again

That, I think, is why a good winch launch - "it's like going up on
rails" is so easy and comfortable: the glider is much more stable in
pitch than it is in free flight. I'd expect this to be more marked as
the hook-wing distance increases.

I know, by the way, that I have omitted things like the effect of the
changing AoA on the lift and drag (secondary effects in this case, I
think) and the slingshot effect of a short rope and the relatively low
tow forces behind a tug.

Ian

The reson, I think, is that the Pirat has quite a high-set wing and a
belly hook as low as it can get. As a result, the lever arm, and
therefore upwards pitching moment from the winch cable about the wing
is relatively high compared to the typical glass glider with mid-set
wing and belly hook offset and up a bit. So with a good fast winch
launch, yes, this moment can overcome anything the elevator is trying
to do.

However, this is an intrinsically stable situation. As the cable angle
relative to the glider's longitudinal axis increases, the lever arm
reduces (by approximately 50% when the glider is 30 degree nose up and
the cable is 30 degrees down). So as the glider pitches up the effect
of the cable reduces, the effect of the elevator (all other things
being equal) stays the same and a point of equilibrium is reached. To
put it simply:

I think you forgot to account for the increasing load that pitching up
produces. Sure, the lever arm is reduced, but the glider is at a higher
angle of attack, increasing it's lift. This increases the force much
more than the lever arm is reduced, and the pitch up continues.

This eventually stabilizes with the glider in the normal nose high
attitude of a winch launch, but this is not a good attitude for
aerotowing! For a nose hook, the lever arm is much less to begin with,
and a small pitch up reduces it to zero - quite a different situation.
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Eric Greenwell
Washington State
USA

Eric Greenwell wrote in message ...

I think you forgot to account for the increasing load that pitching up
produces. Sure, the lever arm is reduced, but the glider is at a higher
angle of attack, increasing it's lift. This increases the force much
more than the lever arm is reduced, and the pitch up continues.

Good point. I have along train journey today, and will have a ponder
on this. I suspect the outcome will be along the lines of "the glider
has stick fixed stability in free flight anyway: the effect of the
added winch cable force is to increase this stability".

This eventually stabilizes with the glider in the normal nose high
attitude of a winch launch, but this is not a good attitude for
aerotowing! For a nose hook, the lever arm is much less to begin with,
and a small pitch up reduces it to zero - quite a different situation.

Agreed completely.

Ian