poor lateral control on a slow tow?

On Jan 1, 10:34*am, Doug Greenwell wrote:
At 15:09 01 January 2011, Derek C wrote:





On Jan 1, 11:15=A0am, Doug Greenwell *wrote:
At 20:23 31 December 2010, bildan wrote:

On Dec 31, 1:06=3DA0pm, Todd =A0wrote:
I too agree with the real or perceived tow handling
characteristics.

Looking at things =3DA0from and aerodynamics standpoint (and I am
abou=
t
as
far from and aerodynamicist as you can get) it should seem that
part
of the empirical data would suggest an experiment where you fly a
glider equipped with and Angel of Attack meter at your typical tow
speeds and record the AoA at various speeds. =3DA0Then fly that
glider
on
tow at those same speeds and record the results.

Done that - and as nearly as I can see, there's no difference in
AoA.

I've flown some pretty heavy high performance gliders behind some
pretty bad tow pilots - one of them stalled the tug with me on tow.
If I'm careful not to over-control the ailerons, there's no problem
at
all.

Heavily ballasted gliders respond sluggishly in roll just due to the
extra roll inertia. =A0A pilot trying to hold a precise position
behind
a tug needs and expects crisp aileron response. =A0When he doesn't
get
it, he increases the amount and frequency of aileron with a
corresponding increase in adverse yaw. =A0If he's less than equally
crisp with rudder to oppose the adverse yaw, it gets wobbly.

Where did you mount the AoA meter?

It's not the angle of attack that's the problem, but the change in
local
incidence along the wing. =A0The overall lift may not change by very
much
when near to the tug wake, but its distribution along the wing does,
with
increased lift at the tips and reduced lift at the root - putting the
aileron region close to the stall and hence reducing control
effectiveness.

I agree that increased roll inertia due to ballast is a factor, but
since
the same factor applies to maintaining bank angle in a thermalling
turn
I
don't see how it can account for a significant difference in handling
between tow and thermalling?- Hide quoted text -

- Show quoted text -

What started the debate at Lasham was using a Rotax engined Falke as a
glider tug. This towed best at about 50 to 55 knots (c.f. 60+ knots
with a normal tug), but K13s with a stalling speed of 36 knots felt
very unhappy behind it, especially two up. In a conventional powered
aircraft you pull the nose up (to increase the angle of attack and
produce more lift) and increase power to climb, the extra power being
used to prevent the aircraft from slowing down. I don't see why
gliders should behave any differently, except that the power is coming
from an external source. As you try not to tow in the wake and
downwash from the tug, I can't see that this is particularly
significant,

Derek C

In a steady climb in any light aircraft the climb angles are so low (
10deg) that the lift remains pretty well equal to weight. *For example a
10deg climb angle at 60 kts corresponds to an impressive climb rate of
10.5kts - but that would only give Lift = Weight/cos(10deg) = 1.02 x
Weight. *You don't need to increase lift to climb - you increase thrust
to overcome the aft component of the weight, and the stick comes back to
maintain speed ... at constant speed the increased power input comes out
as increasing potential energy = increasing height.

I think a lot of people confuse the actions needed to initiate a climb
with what is actually happening in a steady climb. *

On your second point, if you are on tow anywhere sensible behind a tug you
are in its wake and are being affected by the wing downwash. *Wake is not
really a good word, since it seems to get confused with the much more
localised (and turbulent) propwash.

A (very) crude way of visualising the affected wake area is to imagine a
cylinder with a diameter equal to the tug wing span extending back from
the tug - that's the downwash region, and then in addition there's an
upwash region extending perhaps another half-span out either side.- Hide quoted text -

- Show quoted text -

"aft component of weight??"

Not that this adds anything to the discussion, but.....weight acts in
a "downward" direction toward the center of the earth.

In a climb, on tow, the "aft" forces are drag (mostly) and a small bit
of lift.

Anyway, interesting topic.......has been beat to death at our local
field...EVERY pilot seems to have had it happen, in all different
kinds of gliders......many explainations....not one all-encompassing
explaination yet.

Cookie

On Jan 1, 10:11*pm, "
wrote:
On Jan 1, 10:34*am, Doug Greenwell wrote:





At 15:09 01 January 2011, Derek C wrote:

On Jan 1, 11:15=A0am, Doug Greenwell *wrote:
At 20:23 31 December 2010, bildan wrote:

On Dec 31, 1:06=3DA0pm, Todd =A0wrote:
I too agree with the real or perceived tow handling
characteristics.

Looking at things =3DA0from and aerodynamics standpoint (and I am
abou=
t
as
far from and aerodynamicist as you can get) it should seem that
part
of the empirical data would suggest an experiment where you fly a
glider equipped with and Angel of Attack meter at your typical tow
speeds and record the AoA at various speeds. =3DA0Then fly that
glider
on
tow at those same speeds and record the results.

Done that - and as nearly as I can see, there's no difference in
AoA.

I've flown some pretty heavy high performance gliders behind some
pretty bad tow pilots - one of them stalled the tug with me on tow.
If I'm careful not to over-control the ailerons, there's no problem
at
all.

Heavily ballasted gliders respond sluggishly in roll just due to the
extra roll inertia. =A0A pilot trying to hold a precise position
behind
a tug needs and expects crisp aileron response. =A0When he doesn't
get
it, he increases the amount and frequency of aileron with a
corresponding increase in adverse yaw. =A0If he's less than equally
crisp with rudder to oppose the adverse yaw, it gets wobbly.

Where did you mount the AoA meter?

It's not the angle of attack that's the problem, but the change in
local
incidence along the wing. =A0The overall lift may not change by very
much
when near to the tug wake, but its distribution along the wing does,
with
increased lift at the tips and reduced lift at the root - putting the
aileron region close to the stall and hence reducing control
effectiveness.

I agree that increased roll inertia due to ballast is a factor, but
since
the same factor applies to maintaining bank angle in a thermalling
turn
I
don't see how it can account for a significant difference in handling
between tow and thermalling?- Hide quoted text -

- Show quoted text -

What started the debate at Lasham was using a Rotax engined Falke as a
glider tug. This towed best at about 50 to 55 knots (c.f. 60+ knots
with a normal tug), but K13s with a stalling speed of 36 knots felt
very unhappy behind it, especially two up. In a conventional powered
aircraft you pull the nose up (to increase the angle of attack and
produce more lift) and increase power to climb, the extra power being
used to prevent the aircraft from slowing down. I don't see why
gliders should behave any differently, except that the power is coming
from an external source. As you try not to tow in the wake and
downwash from the tug, I can't see that this is particularly
significant,

Derek C

In a steady climb in any light aircraft the climb angles are so low (
10deg) that the lift remains pretty well equal to weight. *For example a
10deg climb angle at 60 kts corresponds to an impressive climb rate of
10.5kts - but that would only give Lift = Weight/cos(10deg) = 1.02 x
Weight. *You don't need to increase lift to climb - you increase thrust
to overcome the aft component of the weight, and the stick comes back to
maintain speed ... at constant speed the increased power input comes out
as increasing potential energy = increasing height.

I think a lot of people confuse the actions needed to initiate a climb
with what is actually happening in a steady climb. *

On your second point, if you are on tow anywhere sensible behind a tug you
are in its wake and are being affected by the wing downwash. *Wake is not
really a good word, since it seems to get confused with the much more
localised (and turbulent) propwash.

A (very) crude way of visualising the affected wake area is to imagine a
cylinder with a diameter equal to the tug wing span extending back from
the tug - that's the downwash region, and then in addition there's an
upwash region extending perhaps another half-span out either side.- Hide quoted text -

- Show quoted text -

"aft component of weight??"

Not that this adds anything to the discussion, but.....weight acts in
a "downward" direction toward the center of the earth.

In a climb, on tow, the "aft" forces are drag (mostly) and a small bit
of lift.

Anyway, interesting topic.......has been beat to death at our local
field...EVERY pilot seems to have had it happen, in all different
kinds of gliders......many explainations....not one all-encompassing
explaination yet.

Cookie- Hide quoted text -

- Show quoted text -

Just looking at the vectors..........lift + drag + weight + thrust(tow
rope)... must = zero
Then.....if the tow rope provides a forward and Downward pull........
(which was pretty much proven in an earlier discussion, by virtue of
the 'sag" in the rope, the angle at which the rope meets the
glider) then lift has to be GREATER than what you might at first
think. A lot more than if the thrust(tow rope) was pulling along in
the direction of flight. So...the angle of attack has to be higher at
a given speed on tow than it would be in free flight at the same
speed.....

plus all that other stuff already mentioned..........


Cookie

Suffice to say the glider is being towed at an artificial angle of attack
compared to free glide so requires more speed on tow. Heavy standard class
probably the worst needing 70-75kts on tow but thermalling happily at
60kts.
Re low tow, we use it in Australia, it feels more stable [to me] and we
release in low tow with no problems.

At 03:29 02 January 2011, wrote:
On Jan 1, 10:11=A0pm, "
wrote:
On Jan 1, 10:34=A0am, Doug Greenwell wrote:





At 15:09 01 January 2011, Derek C wrote:

On Jan 1, 11:15=3DA0am, Doug Greenwell =A0wrote:
At 20:23 31 December 2010, bildan wrote:

On Dec 31, 1:06=3D3DA0pm, Todd =3DA0wrote:
I too agree with the real or perceived tow handling
characteristics.

Looking at things =3D3DA0from and aerodynamics standpoint (and
I
=
am
abou=3D
t
as
far from and aerodynamicist as you can get) it should seem
that
part
of the empirical data would suggest an experiment where you
fly
a
glider equipped with and Angel of Attack meter at your typical
to=
w
speeds and record the AoA at various speeds. =3D3DA0Then fly
that
glider
on
tow at those same speeds and record the results.

Done that - and as nearly as I can see, there's no difference
in
AoA.

I've flown some pretty heavy high performance gliders behind
some
pretty bad tow pilots - one of them stalled the tug with me on
tow.
If I'm careful not to over-control the ailerons, there's no
problem
at
all.

Heavily ballasted gliders respond sluggishly in roll just due to
th=
e
extra roll inertia. =3DA0A pilot trying to hold a precise
position
behind
a tug needs and expects crisp aileron response. =3DA0When he
doesn'=
t
get
it, he increases the amount and frequency of aileron with a
corresponding increase in adverse yaw. =3DA0If he's less than
equal=
ly
crisp with rudder to oppose the adverse yaw, it gets wobbly.

Where did you mount the AoA meter?

It's not the angle of attack that's the problem, but the change
in
local
incidence along the wing. =3DA0The overall lift may not change by
ve=
ry
much
when near to the tug wake, but its distribution along the wing
does,
with
increased lift at the tips and reduced lift at the root - putting
th=
e
aileron region close to the stall and hence reducing control
effectiveness.

I agree that increased roll inertia due to ballast is a factor,
but
since
the same factor applies to maintaining bank angle in a
thermalling
turn
I
don't see how it can account for a significant difference in
handlin=
g
between tow and thermalling?- Hide quoted text -

- Show quoted text -

What started the debate at Lasham was using a Rotax engined Falke
as
a
glider tug. This towed best at about 50 to 55 knots (c.f. 60+ knots
with a normal tug), but K13s with a stalling speed of 36 knots felt
very unhappy behind it, especially two up. In a conventional
powered
aircraft you pull the nose up (to increase the angle of attack and
produce more lift) and increase power to climb, the extra power
being
used to prevent the aircraft from slowing down. I don't see why
gliders should behave any differently, except that the power is
coming
from an external source. As you try not to tow in the wake and
downwash from the tug, I can't see that this is particularly
significant,

Derek C

In a steady climb in any light aircraft the climb angles are so low
(
10deg) that the lift remains pretty well equal to weight. =A0For
exampl=
e a
10deg climb angle at 60 kts corresponds to an impressive climb rate
of
10.5kts - but that would only give Lift =3D Weight/cos(10deg) =3D
1.02
=
x
Weight. =A0You don't need to increase lift to climb - you increase
thru=
st
to overcome the aft component of the weight, and the stick comes
back
t=
o
maintain speed ... at constant speed the increased power input comes
ou=
t
as increasing potential energy =3D increasing height.

I think a lot of people confuse the actions needed to initiate a
climb
with what is actually happening in a steady climb. =A0

On your second point, if you are on tow anywhere sensible behind a
tug
=
you
are in its wake and are being affected by the wing downwash. =A0Wake
is=
not
really a good word, since it seems to get confused with the much
more
localised (and turbulent) propwash.

A (very) crude way of visualising the affected wake area is to
imagine
=
a
cylinder with a diameter equal to the tug wing span extending back
from
the tug - that's the downwash region, and then in addition there's
an
upwash region extending perhaps another half-span out either side.-
Hid=
e quoted text -

- Show quoted text -

"aft component of weight??"

Not that this adds anything to the discussion, but.....weight acts in
a "downward" direction toward the center of the earth.

In a climb, on tow, the "aft" forces are drag (mostly) and a small
bit
of lift.

Anyway, interesting topic.......has been beat to death at our local
field...EVERY pilot seems to have had it happen, in all different
kinds of gliders......many explainations....not one all-encompassing
explaination yet.

Cookie- Hide quoted text -

- Show quoted text -

Just looking at the vectors..........lift + drag + weight + thrust(tow
rope)... must =3D zero
Then.....if the tow rope provides a forward and Downward pull........
(which was pretty much proven in an earlier discussion, by virtue of
the 'sag" in the rope, the angle at which the rope meets the
glider) then lift has to be GREATER than what you might at first
think. A lot more than if the thrust(tow rope) was pulling along in
the direction of flight. So...the angle of attack has to be higher at
a given speed on tow than it would be in free flight at the same
speed.....

plus all that other stuff already mentioned..........


Cookie


yes - the next question then is how big an effect the rope angle has.
There does seem to be a consensus that the effects on lateral control get
worse as you get closer (lower) to the wake/propwash, so I think there's
got to be more to it than just geometry.

On Jan 1, 8:29*pm, "
wrote:

Then.....if the tow rope provides a forward and Downward pull........
(which was pretty much proven in an earlier discussion, by virtue of
the 'sag" in the rope, the angle at which the rope meets the
glider) * *then lift has to be GREATER than what you might at first
think. *

I was not part of that earlier discussion and I certainly don't accept
that conclusion.

All I have read here is that the D2, because of its very low angle of
incidence, may have a downward pull on the nose (and even here
downward would mean below the glider longitudinal axis, not
necessarily below the horizon). I'm quite sure that my ASW 28 being
towed on the CG hook has no downward force on the nose.

When I do tow in gliders with a nose hook I'm quite sure there is no
significant downward pull from the rope. Maybe it all depends on what
you call high tow. I've seen may pilots tow tens of feet higher than
I regard as normal high tow.

Andy

On Jan 2, 10:38*am, Andy wrote:
On Jan 1, 8:29*pm, "
wrote:

Then.....if the tow rope provides a forward and Downward pull........
(which was pretty much proven in an earlier discussion, by virtue of
the 'sag" in the rope, the angle at which the rope meets the
glider) * *then lift has to be GREATER than what you might at first
think. *

I was not part of that earlier discussion and I certainly don't accept
that conclusion.

All I have read here is that the D2, because of its very low angle of
incidence, may have a downward pull on the nose (and even here
downward would mean below the glider longitudinal axis, not
necessarily below the horizon). *I'm quite sure that my ASW 28 being
towed on the CG hook has no downward force on the nose.

When I do tow in gliders with a nose hook I'm quite sure there is no
significant downward pull from the rope. *Maybe it all depends on what
you call high tow. *I've seen may pilots tow tens of feet higher than
I regard as normal high tow.

Andy

On Sun, 02 Jan 2011 07:38:29 -0800, Andy wrote:

All I have read here is that the D2, because of its very low angle of
incidence, may have a downward pull on the nose (and even here downward
would mean below the glider longitudinal axis, not necessarily below the
horizon). I'm quite sure that my ASW 28 being towed on the CG hook has
no downward force on the nose.

Hmmm, My Libelle glides at around 55 kts with the trim full forward so
should need its nose held down a bit when being towed at 60-65 kts on the
nose hook. Its possible that I am holding the nose down - all I can say
is that I'm not aware of doing so once I'm off the ground, stabilised
behind the tug and waiting for it to unstick, gain speed and start to
climb.

There is a noticeable catenary in the tow rope and, since that is a thin,
flexible rope the pull on the nose hook will be at the same angle as the
rope leaves the nose and not on the direct line between my nose-hook and
the rope attachment point on the tug. This probably puts the force line
above the glider CG and so is contributing a nose down moment.

FWIW I estimate that climbing at 600 fpm at 60 kts is a 5.67 degree climb
and that the tow rope tension is 37.62 kg for my glider (10 kg is drag
due to the glider and the rest is due to the glider hanging from the
rope).

However, I don't know rope weight or exact length or how to calculate the
sag in the rope and hence can't estimate the distance of the force line
above or below the CG.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

On Jan 2, 10:38*am, Andy wrote:
On Jan 1, 8:29*pm, "
wrote:

Then.....if the tow rope provides a forward and Downward pull........
(which was pretty much proven in an earlier discussion, by virtue of
the 'sag" in the rope, the angle at which the rope meets the
glider) * *then lift has to be GREATER than what you might at first
think. *

I was not part of that earlier discussion and I certainly don't accept
that conclusion.

All I have read here is that the D2, because of its very low angle of
incidence, may have a downward pull on the nose (and even here
downward would mean below the glider longitudinal axis, not
necessarily below the horizon). *I'm quite sure that my ASW 28 being
towed on the CG hook has no downward force on the nose.

When I do tow in gliders with a nose hook I'm quite sure there is no
significant downward pull from the rope. *Maybe it all depends on what
you call high tow. *I've seen may pilots tow tens of feet higher than
I regard as normal high tow.

Andy

Which part don't you accept? The part about rope pulling downward, or
the part about the required lift being greater if/when it does?

In the previous discussion we all seemed to agree that the tow rope
has a consicerable sag during tow, and that the pulling force of the
rope acts in the direction of the rope meeting the tow hook, which is
not along the long. axis of the glider, and not parallel to the
direction of flight of the glider.

Now, how significant? I dunno!

With a mid-mounted wing glider and a nose hook, the forces of the tow
rope and the drag all run pretty close to the CG.....so probably
little to no pitching effect.......On a 2-33 for instance, where the
tow hook is mounted low, and the wing is high, I believe there is a
nose up pitching moment created, and in fact the 2-33 needs full
forward trim and considerable forward stick pressure on tow. Where a
mid wing nose hook glider flys nicely with about neutral trim and
little stick force if any.

But if we were to agree that the tow rope does not pull in the
dircetion of flight of the glider, and in fact pulls somewhat
"downward" compared to the direction of flight, we need to balance
this force......the only way to balance this force is for lift to
become greater, since weight, and drag remain the same. More lift
comes from more AoA.

I am not saying this is the only factor in this mushy tow deal, but I
think it contributes along with the other factors mentioned.


Cookie

On Jan 2, 6:14*pm, "
wrote:
On Jan 2, 10:38*am, Andy wrote:



On Jan 1, 8:29*pm, "
wrote:

Then.....if the tow rope provides a forward and Downward pull........
(which was pretty much proven in an earlier discussion, by virtue of
the 'sag" in the rope, the angle at which the rope meets the
glider) * *then lift has to be GREATER than what you might at first
think. *

I was not part of that earlier discussion and I certainly don't accept
that conclusion.

All I have read here is that the D2, because of its very low angle of
incidence, may have a downward pull on the nose (and even here
downward would mean below the glider longitudinal axis, not
necessarily below the horizon). *I'm quite sure that my ASW 28 being
towed on the CG hook has no downward force on the nose.

When I do tow in gliders with a nose hook I'm quite sure there is no
significant downward pull from the rope. *Maybe it all depends on what
you call high tow. *I've seen may pilots tow tens of feet higher than
I regard as normal high tow.

Andy

Which part don't you accept? *The part about rope pulling downward, or
the part about the required lift being greater if/when it does?

In the previous discussion we all seemed to agree that the tow rope
has a consicerable sag during tow, and that the pulling force of the
rope acts in the direction of the rope meeting the tow hook, which is
not along the long. axis of the glider, and not parallel to the
direction of flight of the glider.

Now, how significant? *I dunno!

With a mid-mounted wing glider and a nose hook, the forces of the tow
rope and the drag all run pretty close to the CG.....so probably
little to no pitching effect.......On a 2-33 for instance, where the
tow hook is mounted low, and the wing is high, I believe there is a
nose up pitching moment created, and in fact the 2-33 needs full
forward trim and considerable forward stick pressure on tow. *Where a
mid wing nose hook glider flys nicely with about neutral trim and
little stick force if any.

But if we were to agree that the tow rope does not pull in the
dircetion of flight of the glider, and in fact pulls somewhat
"downward" compared to the direction of flight, we need to balance
this force......the only way to balance this force is for lift to
become greater, since weight, and drag remain the same. *More lift
comes from more AoA.

I am not saying this is the only factor in this mushy tow deal, but I
think it contributes along with the other factors mentioned.

Cookie

Maybe the disagreement is only what is meant by downwards. I disagree
that for a glider towing just above the wake, using a CG hook, and
with the tug in a full power climb at normal tow speed, that the rope
applies any force to the glider in a direction below the local
horizontal plane. All the qualifiers above describe a normal tow for
me.

Andy

On Jan 3, 11:10*am, Andy wrote:
On Jan 2, 6:14*pm, "
wrote:





On Jan 2, 10:38*am, Andy wrote:

On Jan 1, 8:29*pm, "
wrote:

Then.....if the tow rope provides a forward and Downward pull.........
(which was pretty much proven in an earlier discussion, by virtue of
the 'sag" in the rope, the angle at which the rope meets the
glider) * *then lift has to be GREATER than what you might at first
think. *

I was not part of that earlier discussion and I certainly don't accept
that conclusion.

All I have read here is that the D2, because of its very low angle of
incidence, may have a downward pull on the nose (and even here
downward would mean below the glider longitudinal axis, not
necessarily below the horizon). *I'm quite sure that my ASW 28 being
towed on the CG hook has no downward force on the nose.

When I do tow in gliders with a nose hook I'm quite sure there is no
significant downward pull from the rope. *Maybe it all depends on what
you call high tow. *I've seen may pilots tow tens of feet higher than
I regard as normal high tow.

Andy

Which part don't you accept? *The part about rope pulling downward, or
the part about the required lift being greater if/when it does?

In the previous discussion we all seemed to agree that the tow rope
has a consicerable sag during tow, and that the pulling force of the
rope acts in the direction of the rope meeting the tow hook, which is
not along the long. axis of the glider, and not parallel to the
direction of flight of the glider.

Now, how significant? *I dunno!

With a mid-mounted wing glider and a nose hook, the forces of the tow
rope and the drag all run pretty close to the CG.....so probably
little to no pitching effect.......On a 2-33 for instance, where the
tow hook is mounted low, and the wing is high, I believe there is a
nose up pitching moment created, and in fact the 2-33 needs full
forward trim and considerable forward stick pressure on tow. *Where a
mid wing nose hook glider flys nicely with about neutral trim and
little stick force if any.

But if we were to agree that the tow rope does not pull in the
dircetion of flight of the glider, and in fact pulls somewhat
"downward" compared to the direction of flight, we need to balance
this force......the only way to balance this force is for lift to
become greater, since weight, and drag remain the same. *More lift
comes from more AoA.

I am not saying this is the only factor in this mushy tow deal, but I
think it contributes along with the other factors mentioned.

Cookie

Maybe the disagreement is only what is meant by downwards. *I disagree
that for a glider towing just above the wake, using a CG hook, and
with the tug in a full power climb at normal tow speed, *that the rope
applies any force to the glider in a direction below the local
horizontal plane. *All the qualifiers above describe a normal tow for
me.

Andy- Hide quoted text -

- Show quoted text -

Just some real fast and dirty assumptions.........say your climb angle
is 5 or 6 degrees.......200' rope. Rope could easily sag 10' in the
middle........I eyeball this to be 10 degrees "off horizontal" at the
ends.......this would net 10 degrees downward using the level earth as
a reference......and 15 degrees compared to the flight path of the
glider.



But I gotta agree that the numbers and angles are kinda small.....so
significant? Maybe, maybe not......Very little vertical force at the
nose can make a big difference......with a cg hook.....probably not
anything noticable...

Cookie