poor lateral control on a slow tow?

At 07:11 04 January 2011, Derek C wrote:
On Jan 3, 11:30=A0pm, 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 had a high tow behind a 180 hp Piper Super Cub (a fairly slow tug)
to practice aerobatics in a K21 on Sunday. The nosehook on the K21 is
situated under the nose, just in front of the nosewheel, and I was
flying it solo. For the last thousand feet of the tow the airspeed
dropped to about 56 knots and I got the same symptoms as described
above. The normal free flight stalling speed for a K21 is only about
39 knots, and I had no problems in the early part of the tow when the
airspeed was 60+ knots. If the glider feels as though it is close to
the stall, then it probably IS close to the stall!

Derek C



yes, but the critical question is is it the same kind of stall? A
straight stall in a K21 is pretty benign, with no particular tendency to
drop a wing.

At 04:54 04 January 2011, 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. 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)


stirrer! This will run and run ... please no-one mention Bernoulli

On Jan 4, 7:27*am, Doug Greenwell wrote:
At 01:01 04 January 2011, wrote:





On Jan 3, 3:34=A0pm, Doug Greenwell *wrote:
At 19:12 03 January 2011, Craig wrote:

On Jan 1, 3:06=3DA0am, Doug Greenwell =A0wrote:
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=3DA0pm, bildan =3DA0wrote:
On Dec 31, 4:40=3DA0am, "Doug" =3DA0wrote:

As an aerodynamicist/flight dynamicist recently re-soloed
after
25
years off, people keep asking me hard questions. =3DA0One
that
h=
as
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. =3DA0There'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?
=3DA0I'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.
=3DA0Overuse
of
ailerons is very common and it makes aero tow 'wobbly'.
=3DA0If
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. =3DA0Using 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. =3DA0I haven't noticed any tendency for them to yaw a
glide=
r
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=3D
e
the glider look quite nose-high to its pilot. =3DA0

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@ =3DA0 | Martin Gregorie
gregorie. | Essex, UK
org =3DA0 =3DA0 =3DA0 |

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=3D
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. =A0From 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). =A0 If the towplane and glider are at
exactly
the same elevation the vertical component of the catenary force
equals
half the rope weight. =A0Any 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

true - but it would take a very small elevator deflection to trim it out- Hide quoted text -

- Show quoted text -

Yeah.........Hey, I am not saying that this is the answer to the
question........I have yet to see any answer which fully explains the
phenomon........I am just bringing up this issue of the tow rope
because it was brought up in the earlier discussion. It is just one
of the "suspects" in the investigation.

We all agree that at a given speed, faily slow, that a glider handles
fine in gliding flight, and has troubles on tow at the same speed.

So there are obvious differences during tow.........the tow rope
hooked to the nose is one, along with all the others we have
discussed, like down wash, vortex, etc.......


Cookie

At 08:57 04 January 2011, BruceGreeff wrote:
Chris

As I understand things - you are confusing climbing with vertical
acceleration.

While flying without vertical acceleration within the performance
available to gliders (I.e. constant vertical speed, modest climb angles,

tiny descent angles) the wing has to support pretty much one glider
weight (1g*mass). This is true whether you are gliding at 1:nn, towing
behind an anaemic cub at 9000" density altitude, or screaming skywards
behind a turbo Cmellak (Zlin 37). All that changes is the angle your
flight path makes relative to the ground. While the rate of climb may
seem significant it has nothing to do with AoA.
So - the flight path angle to any given frame of reference is largely
irrelevant to the AoA required. The AoA required is dependant on many
things - as I understand it these include -
- the Cl of the wing,
- relative velocity of the air over the airfoil,
- relative density of the air,
- the surface area of the wing
- the force it must support.

The force it must support will vary slightly depending on the vector of
force applied by the propulsive device. This could be an engine, a tow
plane or gravity. But it is only significant for one propulsive tool I
am aware of - winching does involve a short period of significant
vertical accelleration. In the transition from the initial climb to the
steep climb part of a winch launch the accelleration changes from ~1g to

about 2g. So the wing has to generate enough lift to generate the force
to change the flight vector - for a short while the AoA is high, close
to the ground. If it goes wrong here the prospects for a stall are good.

In the steep climb the angle described by the flight path relative to
the ground can easily reach 45 to 50 degrees, but the AoA on the wing
remains constant. It will be supporting a constant about 1g after the
transition.

The bending moment on the wing root is higher for reasons related to
where the winch vector is applied, and to the direction and magnitude of

that force, but this is not the load the wing must support. As the
glider has no significant vertical acceleration, the wing is
aerodynamically supporting a constant ~1g. (It must be a little higher
because of the added component of the winch force vector normal to the
wing)

Of course - the angle that the flight path can make relative to the
ground is proportional to the excess power available - hence the low
rate of climb behind the cub, versus the extreme angle on a winch.

Aerodynamics guys - Am I confused?

Bruce


--
Bruce Greeff
T59D #1771 & Std Cirrus #57


No - that's a pretty good summary. Specific excess power (excess
power/weight) is a standard parameter used to define military aircraft
performance, characterising the ability to change energy (height/speed) as
fast as possible.

Winch launching is very different from aerotow because the flight path is
curved and hence you are accelerating all the time, even after transition.

On Jan 4, 7:23*am, Doug Greenwell wrote:
At 23:30 03 January 2011, 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.

In a steady climb (or descent) lift is very close to being equal to
weight, even taking account of tow rope inclination. *You don't need
extra lift to climb, you need extra thrust to increase potential energy.
(A pull-up or zoom climb is different, because in this case you are
trading speed for height).

The Pawnee is a significantly heavier aircraft than the Citabria, so would
generate stronger tip vortices at a given tow speed, and hence have more
effect on a glider behind it.- Hide quoted text -

- Show quoted text -

Yes, to be exact the lift, is the cosine of the (climb or descent)
angle x the glider's weight. This works out to 99 "point something" %
for reasonable climb or descent angles.............that tiny
difference may or may not be significant depending on what we are
looking at. But it is reasonlble to say "lift = weight" or lift
"just about = weight"

Cookie

At 12:40 04 January 2011, wrote:
On Jan 4, 7:27=A0am, Doug Greenwell wrote:
At 01:01 04 January 2011, wrote:





On Jan 3, 3:34=3DA0pm, Doug Greenwell =A0wrote:
At 19:12 03 January 2011, Craig wrote:

On Jan 1, 3:06=3D3DA0am, Doug Greenwell =3DA0wrote:
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=3D3DA0pm, bildan =3D3DA0wrote:
On Dec 31, 4:40=3D3DA0am, "Doug" =3D3DA0wrote:

As an aerodynamicist/flight dynamicist recently re-soloed
after
25
years off, people keep asking me hard questions.
=3D3DA0One
that
h=3D
as
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. =3D3DA0There's possibly an interesting
academi=
c
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?
=3D3DA0I'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.
=3D3DA0Overuse
of
ailerons is very common and it makes aero tow 'wobbly'.
=3D3DA0If
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. =3D3DA0Using a slightly higher tow
positio=
n
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. =3D3DA0I haven't noticed any tendency for them to yaw
a
glide=3D
r
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=3D3D
e
the glider look quite nose-high to its pilot. =3D3DA0

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@ =3D3DA0 | Martin Gregorie
gregorie. | Essex, UK
org =3D3DA0 =3D3DA0 =3D3DA0 |

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=3D3D
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.
=3DA0From
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). =3DA0 If the towplane and glider are at
exactly
the same elevation the vertical component of the catenary force
equals
half the rope weight. =3DA0Any 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. =A0 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

true - but it would take a very small elevator deflection to trim it
out-=
Hide quoted text -

- Show quoted text -

Yeah.........Hey, I am not saying that this is the answer to the
question........I have yet to see any answer which fully explains the
phenomon........I am just bringing up this issue of the tow rope
because it was brought up in the earlier discussion. It is just one
of the "suspects" in the investigation.

We all agree that at a given speed, faily slow, that a glider handles
fine in gliding flight, and has troubles on tow at the same speed.

So there are obvious differences during tow.........the tow rope
hooked to the nose is one, along with all the others we have
discussed, like down wash, vortex, etc.......


Cookie


Life's never simple! This thread has been really useful in highlighting
for me a number of factors that I hadn't really thought about when I
first started looking at this phenomenon.. All I need now is a bright grad
student to spend a year doing the sums while I go fly :-)

At 12:49 04 January 2011, wrote:
On Jan 4, 7:23=A0am, Doug Greenwell wrote:
At 23:30 03 January 2011, 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.

In a steady climb (or descent) lift is very close to being equal to
weight, even taking account of tow rope inclination. =A0You don't
need
extra lift to climb, you need extra thrust to increase potential
energy.
(A pull-up or zoom climb is different, because in this case you are
trading speed for height).

The Pawnee is a significantly heavier aircraft than the Citabria, so
woul=
d
generate stronger tip vortices at a given tow speed, and hence have
more
effect on a glider behind it.- Hide quoted text -

- Show quoted text -

Yes, to be exact the lift, is the cosine of the (climb or descent)
angle x the glider's weight. This works out to 99 "point something"
%
for reasonable climb or descent angles.............that tiny
difference may or may not be significant depending on what we are
looking at. But it is reasonlble to say "lift =3D weight" or lift
"just about =3D weight"

Cookie


'very close' or 'just about' leaves me enough margin for the uncertain
(but small*) effect of the rope inclination/bow on wing lift.
Whether the effect of the moments due to the rope tension on
pitch/roll/yaw stability are significant (and how important the hook
position is) is a much much harder question, so I'll pass on that for the
time being!

*worst case scenario ? say L/D = 15:1 on tow, rope angled off downwards
by 20 deg gives additional lift force required approximately as
sin(20deg)/15 = +2.3%, which would correspond to a 1% change in stall
speed.

On Jan 4, 7:38*am, Doug Greenwell wrote:
At 07:11 04 January 2011, Derek C wrote:





On Jan 3, 11:30=A0pm, 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 had a high tow behind a 180 hp Piper Super Cub (a fairly slow tug)
to practice aerobatics in a K21 on Sunday. The nosehook on the K21 is
situated under the nose, just in front of the nosewheel, and I was
flying it solo. For the last thousand feet of the tow the airspeed
dropped to about 56 knots and I got the same symptoms as described
above. The normal free flight stalling speed for a K21 is only about
39 knots, and I had no problems in the early part of the tow when the
airspeed was 60+ knots. If the glider feels as though it is close to
the stall, then it probably IS close to the stall!

Derek C

yes, but the critical question is is it the same kind of stall? *A
straight stall in a K21 is pretty benign, with no particular tendency to
drop a wing.- Hide quoted text -

- Show quoted text -

When I have my students practice "slow flight"in the '21, they have
trouble maintaining directional control, even though there is little
tendancy for wing drop. Zig Zagging around in free flight is not a
major problem, and maybe not even noticable, compared to what a little
zig zag looks like on tow.


Cookie

OK..........how about this for (simple) explaination?

"Climbing in descending air" (that's what I get from all of the
complicated explainations of down wash, vortex etc.)

I think that if we compared a motor glider climbing at say 50 MPH and
500 FPM to the same glider on tow at the same climb angle and rate,
and if we assume the air behind the tow plane is moving
downward..........

Then the glider on tow would have a larger AoA.....???


Cookie

Thanks Martin

I did use the "vector" word talking about the winch case - because the
cable has mass (our steel cable is ~150Kg so not insignificant) and
there is a pull at a downward angle. At top of launch cable angle
approaches 90 degrees to fuselage - If you want proof look at one of the
videos on you tube.

http://www.youtube.com/watch?v=v2Qh95I_YM0

http://www.youtube.com/watch?v=np8OGPZ2pvE

As Doug Greenwell points out - there is constant acceleration on winch
launch because the flight path is curved, describing a horisontal S. I
don't know how what magnitude the acceleration has, but subjectively it
is only significant in the brief rotation to steep climb, and possibly
on the level out if you are less than smooth...
Generally it is a relatively small change from a little over 1g to a
little under 1g at release.
Anyone have the maths capability to calculate for a known situation?

Cheers
Bruce

On 2011/01/04 1:43 PM, Martin Gregorie wrote:
On Tue, 04 Jan 2011 10:57:01 +0200, BruceGreeff wrote:

Of course - the angle that the flight path can make relative to the
ground is proportional to the excess power available - hence the low
rate of climb behind the cub, versus the extreme angle on a winch.

Aerodynamics guys - Am I confused?

Sounds fair to me except that you omitted two fairly significant forces:
- the weight of the cable
- the tension in the cable.

Both will add to the load carried by the wing. The tension should add a
fairly constant load to the wing once the glider has rotated into full
climb since the throttle setting remains fairly constant[*] from rotation
until the glider is near the top, but the effective cable weight will
increase as more of it is lifted off the ground and then as the whole
cable gets closer to vertical.

[*] this is true on a calm day but is obviously incorrect in the presense
of turbulence or a significant wind gradient.



--
Bruce Greeff
T59D #1771 & Std Cirrus #57