poor lateral control on a slow tow?

At 23:11 02 January 2011, John Chapman wrote:
The discussion of forces surrounding a climb on high tow has been
interesting.
However, has anyone considered descent on tow?

A SGS 2-33 glider descending on tow behind a Cessna 182 has some
interesting lateral control issues.
* Descending behind a 182 with 10 degrees of flap at 65 mph in high
tow with full spoilers is reasonably stable.
* At 40 degrees of flap the 2-33 is almost uncontrollable. You need
full stick and some patience to recover from the frequent level flight
excursions.
The 182 high lift/high drag wake is the obvious difference.

Would anyone care to venture an analysis or opinion?

Cheers, John Chapman, 1DG


At what speed are you using 40deg of flap?

In a descent I would think you are likely to be closer to the tug wake
than in a climb

On Jan 1, 3:06*am, Doug Greenwell wrote:
At 21:47 31 December 2010, Martin Gregorie wrote:





On Fri, 31 Dec 2010 12:09:08 -0800, Derek C wrote:

On Dec 31, 6:19*pm, bildan *wrote:
On Dec 31, 4:40*am, "Doug" *wrote:

As an aerodynamicist/flight dynamicist recently re-soloed after 25
years off, people keep asking me hard questions. *One that has
come
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
an
increased angle of attack out at the tips reducing aileron
effectiveness. *There'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? *I'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. *Overuse
of
ailerons is very common and it makes aero tow 'wobbly'. *If 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. *Using 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. *I haven't noticed any tendency for them to yaw a glider
towards
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
to
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. *

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@ * | Martin Gregorie
gregorie. | Essex, UK
org * * * |

The downwash angle doesn't change much past the tail, and a half to a
third of the tug AoA is a good first guess.

My modeling suggest that there does seem to be an overall reduction in the
glider wing lift (downwash over the centre wing having more of an effect
than upwash over the tips), so the glider requires another degree or two
in AoA - so feeling even more nose-up to the pilot!

Many thanks to the aerodynamics folks for cogent replies. From a
structures and vectors standpoint, the greatest amount of downward
catenary force possible from the rope is the rope's own weight (in
other words, damn little). If the towplane and glider are at exactly
the same elevation the vertical component of the catenary force equals
half the rope weight. Any other vertical forces imparted to the
sailplane result from the vector generated by the relative positions
of the towplane and glider. Kudos to Doug for the stimulating
discussion.

Thanks,
Craig

At 19:12 03 January 2011, Craig wrote:
On Jan 1, 3:06=A0am, Doug Greenwell wrote:
At 21:47 31 December 2010, Martin Gregorie wrote:





On Fri, 31 Dec 2010 12:09:08 -0800, Derek C wrote:

On Dec 31, 6:19=A0pm, bildan =A0wrote:
On Dec 31, 4:40=A0am, "Doug" =A0wrote:

As an aerodynamicist/flight dynamicist recently re-soloed after
25
years off, people keep asking me hard questions. =A0One that has
come
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
an
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
of
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
towards
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
to
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
mak=
e
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 |

The downwash angle doesn't change much past the tail, and a half to a
third of the tug AoA is a good first guess.

My modeling suggest that there does seem to be an overall reduction in
th=
e
glider wing lift (downwash over the centre wing having more of an
effect
than upwash over the tips), so the glider requires another degree or
two
in AoA - so feeling even more nose-up to the pilot!

Many thanks to the aerodynamics folks for cogent replies. From a
structures and vectors standpoint, the greatest amount of downward
catenary force possible from the rope is the rope's own weight (in
other words, damn little). If the towplane and glider are at exactly
the same elevation the vertical component of the catenary force equals
half the rope weight. Any other vertical forces imparted to the
sailplane result from the vector generated by the relative positions
of the towplane and glider. Kudos to Doug for the stimulating
discussion.

Thanks,
Craig


It's been very interesting - and sparked off a few potentially very
interesting research topics (typical academic - always an eye to the next
journal paper!)

Good point on the rope forces - I hadn't looked at it that way, but as
you say any bow in the tow rope won't actually have a significant effect
on the static forces/moments on the glider .. just as well, because it's
quite difficult to calculate the shape once you take drag forces into
account!

Doug

It seems to me that increased AoA must be a very large part of the
cause.

Imagine you are flying free @55kt. You have a sink rate of, say,
1.5kt. Now you are on tow, again @55kt, but this time the combination
is climbing @5kt. Your wings are generating 6.5kt more lift than in
free flight, and must therefore be at a substantially higher AoA.

Additionally, the faster you are climbing (in still air) the greater
the AoA must be for you to keep station with the tug.

I fly an Open Cirrus, towing from the C of G hook without ballast, and
never experienced this at my previous club which had a Citabria tug.
My current club has a Pawnee, and I have from time to time felt the
tow was too slow because the controls felt mushy and the glider
wallowed about, feeling as if it was close to the stall. The Pawnee
climbs much faster than the Citabria.

If in addition the tug's slipstream imparts a downward flow to the
airmass, even more lift and higher AoA is required.

On Jan 3, 6:30*pm, ProfChrisReed wrote:
It seems to me that increased AoA must be a very large part of the
cause.

Imagine you are flying free @55kt. You have a sink rate of, say,
1.5kt. Now you are on tow, again @55kt, but this time the combination
is climbing @5kt. Your wings are generating 6.5kt more lift than in
free flight, and must therefore be at a substantially higher AoA.

Additionally, the faster you are climbing (in still air) the greater
the AoA must be for you to keep station with the tug.

I fly an Open Cirrus, towing from the C of G hook without ballast, and
never experienced this at my previous club which had a Citabria tug.
My current club has a Pawnee, and I have from time to time felt the
tow was too slow because the controls felt mushy and the glider
wallowed about, feeling as if it was close to the stall. The Pawnee
climbs much faster than the Citabria.

If in addition the tug's slipstream imparts a downward flow to the
airmass, even more lift and higher AoA is required.

Actaully, comparing climbing steeply say, 10:1 on tow, to gliding at
40:1, the lift vector is (a tiny bit) SMALLER during the tow!

During the 10:1 tow, lift would be 99.5% of the glider's weight, while
during a 40:1 glide, lift would be 99.97% of the glider's weight!
(the missing 0.5% on tow is made up by the thrust vector...the missing
0.03% in glide is made up by the drag vector.

Cookie


Cookie

On Jan 3, 3:34*pm, Doug Greenwell wrote:
At 19:12 03 January 2011, Craig wrote:





On Jan 1, 3:06=A0am, Doug Greenwell *wrote:
At 21:47 31 December 2010, Martin Gregorie wrote:

On Fri, 31 Dec 2010 12:09:08 -0800, Derek C wrote:

On Dec 31, 6:19=A0pm, bildan =A0wrote:
On Dec 31, 4:40=A0am, "Doug" =A0wrote:

As an aerodynamicist/flight dynamicist recently re-soloed after
25
years off, people keep asking me hard questions. =A0One that has
come
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
an
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
of
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
towards
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
to
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
mak=
e
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 |

The downwash angle doesn't change much past the tail, and a half to a
third of the tug AoA is a good first guess.

My modeling suggest that there does seem to be an overall reduction in
th=
e
glider wing lift (downwash over the centre wing having more of an
effect
than upwash over the tips), so the glider requires another degree or
two
in AoA - so feeling even more nose-up to the pilot!

Many thanks to the aerodynamics folks for cogent replies. *From a
structures and vectors standpoint, the greatest amount of downward
catenary force possible from the rope is the rope's own weight (in
other words, damn little). * If the towplane and glider are at exactly
the same elevation the vertical component of the catenary force equals
half the rope weight. *Any other vertical forces imparted to the
sailplane result from the vector generated by the relative positions
of the towplane and glider. Kudos to Doug for the stimulating
discussion.

Thanks,
Craig

It's been very interesting - and sparked off a few potentially very
interesting research topics (typical academic - always an eye to the next
journal paper!)

Good point on the rope forces - I hadn't looked at it that way, but as
you say any bow in the tow rope won't actually have a significant effect
on the static forces/moments on the glider .. just as well, because it's
quite difficult to calculate the shape once you take drag forces into
account!

Doug- Hide quoted text -

- Show quoted text -

Actually, 5 or 10 pounds of down force at the glider's nose would be
significant. Every loosen your shoulder belts and lean
forward?.....this little weight shift will change pitch and speed.

Now with a cg hook ...probably not significant.


Cookie

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

On Jan 3, 6:30*pm, ProfChrisReed wrote:
It seems to me that increased AoA must be a very large part of the
cause.

Imagine you are flying free @55kt. You have a sink rate of, say,
1.5kt. Now you are on tow, again @55kt, but this time the combination
is climbing @5kt. Your wings are generating 6.5kt more lift than in
free flight, and must therefore be at a substantially higher AoA.

Additionally, the faster you are climbing (in still air) the greater
the AoA must be for you to keep station with the tug.

I fly an Open Cirrus, towing from the C of G hook without ballast, and
never experienced this at my previous club which had a Citabria tug.
My current club has a Pawnee, and I have from time to time felt the
tow was too slow because the controls felt mushy and the glider
wallowed about, feeling as if it was close to the stall. The Pawnee
climbs much faster than the Citabria.

If in addition the tug's slipstream imparts a downward flow to the
airmass, even more lift and higher AoA is required.

I also disagree with you statement that the AoA must be greater if
you climb more rapidly......not so....

Assuming a constant airspeed....

The rate of climb is strictly a factor of the power available. More
powerful towplane = faster rate of climb......lift on the glider's
wing, and the towlane's wing stays practically constant, therefore
the angle of attack is just about constant.

It is the climb angle (direction of flight) which changes with power,
not the AoA.

Cookie

On Jan 3, 5:23*pm, "
wrote:
On Jan 3, 6:30*pm, ProfChrisReed wrote:



It seems to me that increased AoA must be a very large part of the
cause.

Imagine you are flying free @55kt. You have a sink rate of, say,
1.5kt. Now you are on tow, again @55kt, but this time the combination
is climbing @5kt. Your wings are generating 6.5kt more lift than in
free flight, and must therefore be at a substantially higher AoA.

Additionally, the faster you are climbing (in still air) the greater
the AoA must be for you to keep station with the tug.

I fly an Open Cirrus, towing from the C of G hook without ballast, and
never experienced this at my previous club which had a Citabria tug.
My current club has a Pawnee, and I have from time to time felt the
tow was too slow because the controls felt mushy and the glider
wallowed about, feeling as if it was close to the stall. The Pawnee
climbs much faster than the Citabria.

If in addition the tug's slipstream imparts a downward flow to the
airmass, even more lift and higher AoA is required.

I also disagree with you statement that the AoA *must be greater if
you climb more rapidly......not so....

Assuming a constant airspeed....

The rate of climb is strictly a factor of the power available. * More
powerful towplane = faster rate of climb......lift on the glider's
wing, and the *towlane's wing stays practically constant, therefore
the angle of attack is just about constant.

It is the climb angle (direction of flight) which changes with power,
not the AoA.

Cookie

Ugh?

The glider is flying, the towplane is not dragging the glider up an
incline. If the combination is going up faster (=steeper climb rate/
angle) then both aircraft wings are generating more lift and they get
this this from some combination of increased AoA and airspeed. The
more powerful towplane may allow both aircraft to fly at an increased
AoA and overcome the associated drag. The increased climb angle comes
from the increased lift. Assuming a constant airspeed means all the
increase is coming from an increase in AoA and the more powerful
towplane thrust is offsetting the increased drag. I'd be interested to
see an explanation of any other way of generating an increase in climb
angle without increasing the lift of the glider and/pr towplane.

Darryl

On 1/3/2011 8:10 PM, Darryl Ramm wrote:
On Jan 3, 5:23 pm, "twocoolglid...@juno. com

The rate of climb is strictly a factor of the power available. More
powerful towplane = faster rate of climb......lift on the glider's
wing, and the towlane's wing stays practically constant, therefore
the angle of attack is just about constant.

It is the climb angle (direction of flight) which changes with power,
not the AoA.

Cookie

Ugh?

The glider is flying, the towplane is not dragging the glider up an
incline. If the combination is going up faster (=steeper climb rate/
angle) then both aircraft wings are generating more lift and they get
this this from some combination of increased AoA and airspeed. The
more powerful towplane may allow both aircraft to fly at an increased
AoA and overcome the associated drag. The increased climb angle comes
from the increased lift. Assuming a constant airspeed means all the
increase is coming from an increase in AoA and the more powerful
towplane thrust is offsetting the increased drag. I'd be interested to
see an explanation of any other way of generating an increase in climb
angle without increasing the lift of the glider and/pr towplane.

Actually, I do think the towplane is pulling the glider up an incline!
The flight path is inclined, and the towplane is the only one that can
provide the force. In fact, I think the lift required *decreases* with
increased climb rate during tow! How could that be? The tow rope
provides some of the force needed to hold the glider in the air.

Imagine an extreme tow, a 50 knot airspeed, but climbing at 35 knots (45
degree angle). The tow rope is providing 70% of the force holding the
glider in the air, so the wing needs to supply only 30% of the force.

Or imagine a really extreme, vertical tow: all the force required to
keep the glider moving steadily through the air is provided by the
towrope/towplane, and none by the wing.

Let the games begin!

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