poor lateral control on a slow tow?

On Jan 1, 3:00*am, Doug Greenwell wrote:
At 23:25 31 December 2010, Andy wrote:





On Dec 31, 1:47=A0pm, Martin Gregorie
wrote:
On Fri, 31 Dec 2010 12:09:08 -0800, Derek C wrote:
On Dec 31, 6:19=A0pm, bildan *wrote:
On Dec 31, 4:40=A0am, "Doug" *wrote:

As an aerodynamicist/flight dynamicist recently re-soloed after
25
years off, people keep asking me hard questions. =A0One that has
com=
e
up recently is why a heavy glider on tow feels horrible, but
thermalling in the same glider at lower speeds is fine? (see also
Mike Fox's article on aerotowing in the October issue of S&G).

I did some calculations, and I reckon it's probably due to the
tug
wing wake (tip vortices generating a downwash inboard, upwash
outboard) changing the lift distribution on the glider wing -
with
a=
n
increased angle of attack out at the tips reducing aileron
effectiveness. =A0There's possibly an interesting academic
research
project here, but it's always best to get a reality check first
..

Is poor handling at low speed on tow a common experience? =A0I'd
appreciate any thoughts/comments/war stories ... particularly bad
tug/glider/speed combinations, incidents of wing drop during a
tow
etc etc?

Doug Greenwell

I suspect, but can't know unless I flew with you, that you are
unconsciously trying to "steer" the glider with ailerons.
=A0Overuse
o=
f
ailerons is very common and it makes aero tow 'wobbly'. =A0If you
consciously use rudder to aim the nose at the tug's tail and just
keep
the same bank angle as the tug with ailerons, it might work better.

Wake effects are generally favorable if you stay at the right
height
relative to the tug. =A0Using a slightly higher tow position can
sometimes help a lot.

The tip vortices rotate inward above the propwash which, if allowed
to
do so, will drift the glider to the center position and help keep
it
there. =A0I haven't noticed any tendency for them to yaw a glider
towa=
rds
a tugs wing tip.- Hide quoted text -

- Show quoted text -

There was a debate on our club forum about why gliders feel
uncomfortable on slow tows that are still well above their normal
stalling speed. We think the answer is that the glider is being
asked
t=
o
climb with the tug providing the thrust via the rope. The glider is
still effectively in free flight and therefore has to fly at a
greater
angle of attack for a given airspeed to produce the extra lift for
climbing. Hence its stalling speed is somewhat increased.

If the tug's downwash field extends back far enough to include the
glider, its AOA will be relative to the downwash streamlines. Add the
downwash angle to the climb angle of the tug-glider combination will
make
the glider look quite nose-high to its pilot. =A0

I know that the downwash angle is roughly 1/3 of the wing AOA at 4-5
chords behind the wing, i.e. about where the tailplane is, but not
what
its angle might be at the end of a tow rope.

--
martin@ =A0 | Martin Gregorie
gregorie. | Essex, UK
org =A0 =A0 =A0 |

I'd be surprised if the flow field from the towplane wake is
significant for gliders in normal high tow position. I do wonder if
the "sluggish controls" effect is to some extent psychological because
flying formation requires much more precision than normal slow flight
off tow. I'm most uncomfortable when I find myself slow and below the
towplane and need to climb up.

Unless the glider is accelerating vertically, I'm pretty sure that
steady climb requires the same amount of lift as steady glide. Steady
climb is not the same as accelerating climb. (F=3DMxA so if the lifting
force exceeds the glider's weight by definition it accelerates
vertically).

The towplane provides thrust to overcome the frictional and lift-
related drag losses, but unless you are well below the towplane the
force on the rope is, for all practical purposes, horizontal. If you
have a cg hook you will get a modest nose-up pitching moment from the
rope, but this is a trim issue more than an AOA issue I believe. The
tension on the rope could also provide some counter-force to rudder
and elevator inputs, but I don't think you'd feel much for small
angular displacements.

9B

It is surprising, but part of the problem is the word 'wake' ... in
order to generate lift a wing has to move a fair amount of air around
(although let's not start the bernoulli argument now!), so its influence
on the surrounding atmosphere extends a surprising distance away from it.
Tip vortices are also a very stable flow structure, so don't begin to
break up or decay for a very very long way downstream.

The climb angles are too small to make a significant difference to the
lift required from the glider wing (assuming the tow rope is straight),
since the effect on lift goes with the cosine of the angle

On the other hand, if the tow rope is not straight then there could be a
significant lift component from the tension force (going with the sine of
the tow rope angle) ... but you would have to be quite a long way above
the tug to make a big difference.



Span differential, the nature of wake roll-ups, and effects in the
larger free stream.

An airfoil moving through a viscous media makes quite a disturbance.
Among other things, it results in relative upwash upstream in the flow
field, downwash aft in the flow field, and effects which are
vertically displaced in the flow field as well.

However, lateral influence in the flow field- outside the wake rollup
at the tips- is of special interest here. Wake rollups with vorticity
do not spread the energy spent in achieving pressure equilibrium very
efficiently. That is why displacing the event over a larger area such
as laterally (as in more span) or vertically (as in the case of
winglets) makes the wing more efficient. Since the wing doesn’t do a
very good job of inducing lift beyond the tip on the other side of the
wake rollup, the downwash immediately aft of the wing is significantly
greater than the downwash aft of the wing and a meter or two outboard
of the tips.

This lateral downwash differential is preserved in the aft flow field,
albeit to lesser degrees with increasing distance until the free
stream reaches unity. However, when being towed slow and heavy it
doesn't take much to create a noticeable effect. In the case of a tow
plane with 10-11 meter span towing a glider of 15 meter span, the
downwash aft of the towplane and inboard on the glider span is greater
than the free stream field meeting the tips and the ailerons. The
effect is that a glider under tow can behave more like a design with
wings geometrically twisted in the wrong direction- with ailerons
operating near the stall. The effect increases with increasing
downwash required of the towplane.

One way to check this effect would be tow behind a motorglider of
greater span than your glider. This should provide for a better match
of downwash angles across your span.Get all the climb you can for a
given airspeed. Time your roll rates. Then tow behind a conventional
towplane at the same speed and same climb rate as the first case. Time
and compare roll rates.

You can also check numerically by calculating the rolling moments and
taking into account the assymetrical lift distributions using the
methods of Multhopp and Redeker. However, arriving at the effective
angles of attack across the span, in a modified and vertically
displaced flow field 200 feet aft of the tow plane might be rather
difficult. Several angle of attack probes positioned in front of your
wing and distributed along the span would likely be the better
approach.

If a towplane could push rather than pull a glider, the effect would
be reversed and the aileron authority would increase.

Best Regards,

Gary Osoba

On Jan 1, 3:21*am, Doug Greenwell wrote:
At 06:24 01 January 2011, Anne wrote:


I've certainly sparked some interest here - considering it's New Year
:-)- Hide quoted text -

And I mignt add this is a very fast moving discussion too! While I was
loging in 2 messages were posted..

Concerning the Tow Plane position while on tow, two of my CFIs have
said to position yourglider as if you were going to Machine Gun the
pilot of the Tow Plane. this is equivelent of aligning the horizontal
of the TP with a portion of his foweward fuslage, like the wheels on a
Pawnee.

Works great in all conditions I've come accross in 15 years flying 8
different types from 2-33 to Duo Discuss. Never been criticized for it
either in BFRs.

Wayne

At 19:29 01 January 2011, Gary Osoba wrote:


Span differential, the nature of wake roll-ups, and effects in the
larger free stream.

An airfoil moving through a viscous media makes quite a disturbance.
Among other things, it results in relative upwash upstream in the flow
field, downwash aft in the flow field, and effects which are
vertically displaced in the flow field as well.

However, lateral influence in the flow field- outside the wake rollup
at the tips- is of special interest here. Wake rollups with vorticity
do not spread the energy spent in achieving pressure equilibrium very
efficiently. That is why displacing the event over a larger area such
as laterally (as in more span) or vertically (as in the case of
winglets) makes the wing more efficient. Since the wing doesn=92t do a
very good job of inducing lift beyond the tip on the other side of the
wake rollup, the downwash immediately aft of the wing is significantly
greater than the downwash aft of the wing and a meter or two outboard
of the tips.

This lateral downwash differential is preserved in the aft flow field,
albeit to lesser degrees with increasing distance until the free
stream reaches unity. However, when being towed slow and heavy it
doesn't take much to create a noticeable effect. In the case of a tow
plane with 10-11 meter span towing a glider of 15 meter span, the
downwash aft of the towplane and inboard on the glider span is greater
than the free stream field meeting the tips and the ailerons. The
effect is that a glider under tow can behave more like a design with
wings geometrically twisted in the wrong direction- with ailerons
operating near the stall. The effect increases with increasing
downwash required of the towplane.

One way to check this effect would be tow behind a motorglider of
greater span than your glider. This should provide for a better match
of downwash angles across your span.Get all the climb you can for a
given airspeed. Time your roll rates. Then tow behind a conventional
towplane at the same speed and same climb rate as the first case. Time
and compare roll rates.

You can also check numerically by calculating the rolling moments and
taking into account the assymetrical lift distributions using the
methods of Multhopp and Redeker. However, arriving at the effective
angles of attack across the span, in a modified and vertically
displaced flow field 200 feet aft of the tow plane might be rather
difficult. Several angle of attack probes positioned in front of your
wing and distributed along the span would likely be the better
approach.

If a towplane could push rather than pull a glider, the effect would
be reversed and the aileron authority would increase.

Best Regards,

Gary Osoba


Absolutely - I used a simple vortex lattice method (AVL) to come to the
same conclusion. For the relatively short spacing between glider and
towplane the lack of a wake roll-up model in this code probbaly doesn't
affect the adverse change in spanwise lift distribution on the glider wing
- however, modelling any more complex interactions between tug and glider
vortices would need a proper CFD study. The other interesting element is
the effect of bank - with 15m+ wing spans it doesn't take much of a roll
angle to put one tip right into the tug wake, leading to yet more
asymmetric effects.

Most experimental wake vortex interaction studies (eg recent Airbus A380
studies) have used models in big ship tow tanks to get long vortices, but
because glider and tug are so close, we could probably use a large wind
tunnel. Another possibility would be to fly a motor glider behind a tug
aircraft, in order to take out any effect of rope angle.

It would probably be difficult to get someone to fund it though, since the
solution is simple - tow faster :-)

basic conclusions towthis doesm

On Jan 1, 6:00*am, Doug Greenwell wrote:
At 23:25 31 December 2010, Andy wrote:





On Dec 31, 1:47=A0pm, Martin Gregorie
wrote:
On Fri, 31 Dec 2010 12:09:08 -0800, Derek C wrote:
On Dec 31, 6:19=A0pm, bildan *wrote:
On Dec 31, 4:40=A0am, "Doug" *wrote:

As an aerodynamicist/flight dynamicist recently re-soloed after
25
years off, people keep asking me hard questions. =A0One that has
com=
e
up recently is why a heavy glider on tow feels horrible, but
thermalling in the same glider at lower speeds is fine? (see also
Mike Fox's article on aerotowing in the October issue of S&G).

I did some calculations, and I reckon it's probably due to the
tug
wing wake (tip vortices generating a downwash inboard, upwash
outboard) changing the lift distribution on the glider wing -
with
a=
n
increased angle of attack out at the tips reducing aileron
effectiveness. =A0There's possibly an interesting academic
research
project here, but it's always best to get a reality check first
..

Is poor handling at low speed on tow a common experience? =A0I'd
appreciate any thoughts/comments/war stories ... particularly bad
tug/glider/speed combinations, incidents of wing drop during a
tow
etc etc?

Doug Greenwell

I suspect, but can't know unless I flew with you, that you are
unconsciously trying to "steer" the glider with ailerons.
=A0Overuse
o=
f
ailerons is very common and it makes aero tow 'wobbly'. =A0If you
consciously use rudder to aim the nose at the tug's tail and just
keep
the same bank angle as the tug with ailerons, it might work better.

Wake effects are generally favorable if you stay at the right
height
relative to the tug. =A0Using a slightly higher tow position can
sometimes help a lot.

The tip vortices rotate inward above the propwash which, if allowed
to
do so, will drift the glider to the center position and help keep
it
there. =A0I haven't noticed any tendency for them to yaw a glider
towa=
rds
a tugs wing tip.- Hide quoted text -

- Show quoted text -

There was a debate on our club forum about why gliders feel
uncomfortable on slow tows that are still well above their normal
stalling speed. We think the answer is that the glider is being
asked
t=
o
climb with the tug providing the thrust via the rope. The glider is
still effectively in free flight and therefore has to fly at a
greater
angle of attack for a given airspeed to produce the extra lift for
climbing. Hence its stalling speed is somewhat increased.

If the tug's downwash field extends back far enough to include the
glider, its AOA will be relative to the downwash streamlines. Add the
downwash angle to the climb angle of the tug-glider combination will
make
the glider look quite nose-high to its pilot. =A0

I know that the downwash angle is roughly 1/3 of the wing AOA at 4-5
chords behind the wing, i.e. about where the tailplane is, but not
what
its angle might be at the end of a tow rope.

--
martin@ =A0 | Martin Gregorie
gregorie. | Essex, UK
org =A0 =A0 =A0 |

I'd be surprised if the flow field from the towplane wake is
significant for gliders in normal high tow position. I do wonder if
the "sluggish controls" effect is to some extent psychological because
flying formation requires much more precision than normal slow flight
off tow. I'm most uncomfortable when I find myself slow and below the
towplane and need to climb up.

Unless the glider is accelerating vertically, I'm pretty sure that
steady climb requires the same amount of lift as steady glide. Steady
climb is not the same as accelerating climb. (F=3DMxA so if the lifting
force exceeds the glider's weight by definition it accelerates
vertically).

The towplane provides thrust to overcome the frictional and lift-
related drag losses, but unless you are well below the towplane the
force on the rope is, for all practical purposes, horizontal. If you
have a cg hook you will get a modest nose-up pitching moment from the
rope, but this is a trim issue more than an AOA issue I believe. The
tension on the rope could also provide some counter-force to rudder
and elevator inputs, but I don't think you'd feel much for small
angular displacements.

9B

It is surprising, but part of the problem is the word 'wake' ... in
order to generate lift a wing has to move a fair amount of air around
(although let's not start the bernoulli argument now!), so its influence
on the surrounding atmosphere extends a surprising distance away from it.
Tip vortices are also a very stable flow structure, so don't begin to
break up or decay for a very very long way downstream.

The climb angles are too small to make a significant difference to the
lift required from the glider wing (assuming the tow rope is straight),
since the effect on lift goes with the cosine of the angle

On the other hand, if the tow rope is not straight then there could be a
significant lift component from the tension force (going with the sine of
the tow rope angle) ... but you would have to be quite a long way above
the tug to make a big difference.

Winter has certainly arrived in the northern hemisphere.Shall we next
discuss how many angels can dance on the head of a pin?

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 03:11 02 January 2011, 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
tow
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
the
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
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. =3DA0The 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. =A0For example
=
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
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 =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
yo=
u
are in its wake and are being affected by the wing downwash. =A0Wake
is
n=
ot
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


it depends on your reference frame - lift and drag are perpendicular to
the direction of motion (relative to the air), which is inclined upwards -
so if you take 'aft' as relative to the glider flight path rather than
the earth, then there is an aft component of weight.

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, 5:27*pm, Doug Greenwell wrote:
At 16:43 01 January 2011, Derek C wrote:





On Jan 1, 3:34=A0pm, 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
tow
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
the
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
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. =3DA0The 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. =A0For example
=
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
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 =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
yo=
u
are in its wake and are being affected by the wing downwash. =A0Wake
is
n=
ot
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 -

So why did a K13 feel on the verge of a stall at 50 knots on tow? All
the classic symptoms of a stall were there, including mushy controls,
wallowing around and buffeting. If you got even slightly low it seemed
quite difficult to get back up to the normal position. Lack of
elevator effectiveness is yet another sympton of the stall!

Fortunately we have given up aerotowing with the Falke. It just seemed
like a good idea at the time because its flying speeds are more
closely matched to a glider; in theory anyway.

Derek C

good question - which suggests that something more complicated was going
on? *

Lack of elevator effectiveness is not really a symptom of stall as such
.. it's a symptom of low airspeed. *So for buffeting and mushy,
ineffective elevator to be happening at an indicated airspeed of 50-55
knots I'm wondering whether the tailplane was stalling rather than the
wing?

In this case you'd a tug with a wing span of a similar size to the glider
(14.5m to 16m), which would put the tug and glider tip vortices very close
together. *Two adjacent vortices of the same sign tend to wind up round
each other and merge quite quickly - if this happened with the two sets of
tip vortices it would generate an increased downwash near the tail and push
the local (negative) incidence past the stall angle.

I'd be the first to admit this is getting rather speculative - but these
possible interaction effects would be amenable to some fairly
straightforward wind tunnel testing *... a good student project for next
year!-

Actually the only totally reliable sysmptom of being stalled is that
the elevator will no longer raise the nose. The elevator should still
be effective at 50 knots, so it's more likely that the wing is close
to the stall. The stall is only strictly related to the angle of
attack. During a aerotow climb the wing has to support an additional
weight component as well as drag, so the effective wing loading may
well be increased, requiring a greater angle of attack for a given
airspeed. Going 10 knots faster seems to cure the problem.

Derek C