poor lateral control on a slow tow?

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 4, 12:58*am, "
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.

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- Hide quoted text -

- Show quoted text -

If you had a really powerful tug that was capable of climbing
vertically, then the glider would just be dangling on the end of the
rope and would not have to produce any lift. The tension in the rope
would be equal to the weight of the glider plus any drag components.
While this is not a very likely scenario, I do think that the thrust
vector must be greater in a 10% climb than you are claiming.

Derek C

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)

On Jan 3, 8:54*pm, Eric Greenwell wrote:
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)

I think you are trying to push this argument up an incline with a
rope. :-) But I'll take your points into consideration next time I'm
vertically towing behind a helicopter.

---

I think Chris Reed well nailed the (somewhat bleeding obvious when you
think about it) issue here with AoA and handling on slow tow.

Darryl

At 07:51 04 January 2011, Darryl Ramm wrote:
On Jan 3, 8:54=A0pm, Eric Greenwell wrote:
On 1/3/2011 8:10 PM, Darryl Ramm wrote:



On Jan 3, 5:23 pm, "
The rate of climb is strictly a factor of the power available. =A0
Mor=
e
powerful towplane =3D faster rate of climb......lift on the
glider's
wing, and the =A0towlane'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 (=3Dsteeper 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)

I think you are trying to push this argument up an incline with a
rope. :-) But I'll take your points into consideration next time I'm
vertically towing behind a helicopter.

---

I think Chris Reed well nailed the (somewhat bleeding obvious when you
think about it) issue here with AoA and handling on slow tow.

Darryl



That's the problem with aeroplanes of any sort - the bleeding obvious is
not always right. I mean, it's obvious that if I'm a bit low on
approach I can stretch the glide by pulling back a bit more ... and a bit
more ... and ....

On 1/3/2011 11:51 PM, Darryl Ramm wrote:
On Jan 3, 8:54 pm, Eric wrote:

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.

I think you are trying to push this argument up an incline with a
rope. :-) But I'll take your points into consideration next time I'm
vertically towing behind a helicopter.

I'm serious! But, let me add this constraint to make the idea easier to
absorb: the glider pilot flies the tow so the rope is always parallel to
the fuselage.

In level flight, the rope pull equals the drag; the lift equals the
glider weight. Rope force vector and weight vector are at right angles.

In a 50 knot airspeed, 35 knot climb (45 degree angle of climb), the
rope vector and the glider weight vector are now at an obtuse angle, so
some of the rope force is supporting the glider.

Stating it another way: we know the rope is pulling a lot harder, but
the glider is not accelerating, so what force is opposing the rope pull?
It can't be additional drag (glider is still going only 50 knots
airspeed); it can't be the lift (regardless of it's value), because
that's acting almost entirely perpendicularly to the rope. So, what
force is opposing all that extra rope pull? I say - it's the weight of
the glider (about 70% of the weight).

Another way to imagine the situation, using the helicopter to provide a
50 knot airspeed tow, rope always parallel to the glider fuselage:
* level flight, wing lift = weight of glider
* vertical flight, wing lift = 0 (or the glider won't have right rope
angle)

So, in between level flight and vertical flight, there must be a region
where the wing lift is less than in level flight, right? I'm saying
there is a continuous reduction in the lift the wing must provide as the
climb angle increases.

Only two months till March flying starts...gotta solve this problem
while we still have time!

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