poor lateral control on a slow tow?

At 10:38 01 January 2011, John Vella Grech wrote:
At 06:24 01 January 2011, Anne wrote:
On Jan 1, 12:38=A0am, Tony V wrote:
Bruce Hoult wrote:
....Since then I tow with the Pawnee horizontal stabilizer in the
same
position against the forward parts of the tug no matter what glider
I'm in and just ignore the horizon.

Yes, use the tug as a reference. Using the horizon doesn't work on
real
hazy days and it doesn't work in the mountains.

Tony V.

John Cochrane has the answer right, at least for standard class ships
like the Discus 2. I can verify that you run out of elevator control
at tow speeds significantly lower than the free-flight stall speed.
The reason is that the tow rope applies a downward thrust at the nose
- I have wing tip-camera video that confirms the tow rope has a
significant downward pull on the nose. I always try to stay away from
tow plane wash, so don't think that's a major component. I've never
experienced as marked a behavior in flapped ships, so I put it down to
AOA.

Mike
Surely LOW TOW has many handling advantages and I have been very
comfortable with this method gliding in Aus. At least the tow rope has
an
upward componenet.
John




There do seem to be many advantages to low tow - I'm not sure why it's
not used much in the UK. On the Junior the rope apparently fouls the nose
in low tow, so perhaps its a problem with some hook positions?

In thermic conditions I release from tow as soon as I think I'm entering
soarable lift - I don't hang on until an arbitrary height like 2,000' is
reached. (I have been known to release at 700' - but my club now charges
for a minimum of 1,000' even if one releases earlier so I tend to go a
bit higher now - I might have become a bit wiser as well!)

Since I must release from the high-tow position to ensure adequate
clearance from the metal rings on the rope immediately after I release -
if I'm in low tow I must go up to high-tow first - by which time I'm
well past the lift and will probably fail to find it. Hence my preference
for high-tow during a launch into a soarable sky. During a retrieve I will
often go low-tow.


At 11:45 01 January 2011, Doug Greenwell wrote:

There do seem to be many advantages to low tow - I'm not sure why it's
not used much in the UK. On the Junior the rope apparently fouls the
nose
in low tow, so perhaps its a problem with some hook positions?

On Dec 31 2010, 1:23*pm, bildan wrote:

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. *A pilot trying to hold a precise position behind
a tug needs and expects crisp aileron response. *When he doesn't get
it, he increases the amount and frequency of aileron with a
corresponding increase in adverse yaw. *If he's less than equally
crisp with rudder to oppose the adverse yaw, it gets wobbly.

Bill, I use whatever aileron is required to establish and maintain the
bank angle I need. I also use exactly the right amount of rudder to
maintain coordinated flight. My low speed ballasted tows are not
"wobbly". I am momentarily out of control since the ailerons are on
the stops and I'm still not getting the roll response I need.

Please don't assume that the problem is caused by pilots not
understanding how to use the controls. The problem is caused by being
towed at a speed lower than that at which the glider is controllable
in rough air.

The solution is simple and entirely in the hands of the tow pilot.
The aerodynamic explanation is of little interest to me.

Your insistence on using the term "wobbly" to describe the problem
convinces me that you have never experienced it.

Andy

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





On Dec 31, 1:06=A0pm, Todd *wrote:
I too agree with the real or perceived tow handling characteristics.

Looking at things =A0from and aerodynamics standpoint (and I am about
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. =A0Then 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. *A pilot trying to hold a precise position behind
a tug needs and expects crisp aileron response. *When he doesn't get
it, he increases the amount and frequency of aileron with a
corresponding increase in adverse yaw. *If 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. *The 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

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.

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.


whoops - I should have said Lift = Weight*cos(10deg) = 0.985 x
Weight, since in a climb the thrust (or tow cable) is supporting part of
the the weight .... long night, early morning :-)

On Jan 1, 3:34*pm, 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 -

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

On 1/1/2011 3:40 AM, Doug Greenwell wrote:


PS I've only ever come one other Greenwell outside the North East of
England ... any relation?

My great-grandfather, Henry Nicholas Greenwell, left Greenwell Ford
around 1850 for Australia, finally settling in Hawaii. Anything sound
familiar?

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

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!

At 17:16 01 January 2011, Eric Greenwell wrote:
On 1/1/2011 3:40 AM, Doug Greenwell wrote:


PS I've only ever come one other Greenwell outside the North East of
England ... any relation?

My great-grandfather, Henry Nicholas Greenwell, left Greenwell Ford
around 1850 for Australia, finally settling in Hawaii. Anything sound
familiar?

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)


I had to look it up - I was born 15miles away in Sunderland and had never
heard of Greenwell Ford. Doesn't ring a bell, but I'll have a look in
the family history files my wife has put together. It's certainly in the
right part of the NE, not far from Easington and the Durham coalfields

Doug