poor lateral control on a slow tow?

However, the D2 which does seem to climb well at low speeds seems to
be really sensitive to being towed slow, at least that I understand
from one of the local pilots. Maybe D2 owners could comment on that?

Andy

The D2 also has a very low angle of incidence and thus a high nose
attitude on tow. Could this be part of the issue -- it's just darn
uncomfortable to fly with the nose pointed above the towplane? That
would also account for why 15 meter seems easier. We fly with flaps;
they are interconnected to the ailerons so we're not getting great
roll rates on tow. But it does give a nose down attitude so we can see
the towplane.

John Cochrane

I too agree with the real or perceived tow handling characteristics.

Looking at things from 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. Then fly that glider on
tow at those same speeds and record the results.

I would suspect that we will find that the AoA is higher (e.g. closer
to the critical stall AoA ) and that this would explain the handling
characteristics. So the million dollar question is why? My guess is
that in soaring flight we are essentially coasting down hill at say
60Kt. In towing flight at 60Kt, we are being "dragged" up the hill
(plus, maybe, some effect from tow rope attachment point leverage) so
something, aerodynamically. has to be different and that has (???) to
be the AoA. As we all know from out stall training, as we approach
the critical angle of attack, we loose out roll (aileron)
effectiveness.

....or should I go back to learning to flip burgers at McD's?

On Jan 1, 8:07*am, John Cochrane
wrote:
The D2 also has a very low angle of incidence and thus a high nose
attitude on tow. Could this be part of the issue -- it's just darn
uncomfortable to fly with the nose pointed above the towplane? That
would also account for why 15 meter seems easier. We fly with flaps;
they are interconnected to the ailerons so we're not getting great
roll rates on tow. But it does give a nose down attitude so we can see
the towplane.

I had some really horrible feeling tows in a PW5. The thing felt
mushy, nose high, couldn't see the towplane, needed a lot of back
stick, afraid it was going to stall etc.

Then I realized that when you're going up at over 1000 fpm in still
air, keeping ANY part of the tug on the horizon (even wheels) is far
too high a position. I dropped down until I could start to feel the
wash and then came up a little. It felt much better but the tug seemed
WAY UP THERE.

Work it out ... at 65 knots and going up at 10 knots, the other end of
a 50m rope will be 7.7m above you if you're following the same path.
Even guessing 3m to get out of the wake, the tug should still be
nearly 5m above you.

And maybe it's 11 or 12 knots climb (I can't tell because the vario is
pegged), in which case that's another 1 or 1.5 m.

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. Even in the DG1000 two-up and
climbing at 700 fpm this still results in the tug's wheels being a
fraction above the horizon (and I've been criticized for this on
biannuals) but I'm still comfortably above the turbulence of the tug's
wake.

The worst towing experiences I've had has been during aero retrieves.
Once the tugs (I've seen this behind an L-19 and a Pawnee) reached
cruising altitude, it became extremely difficult to stay behind them.
I asked them to slow down (went from 85 to 65 kt), got into low tow,
slipped and even pulled on some spoiler, but my LS8 (unballasted) was
a handful in roll and pitch. Once on a turbulent blue day I
deliberately released 25 miles before I had final glide, preferring
the excitement of finding a means of staying up rather than continuing
on tow.

-John

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.

At 18:19 31 December 2010, bildan wrote:
On Dec 31, 4:40=A0am, "Doug" wrote:
As an aerodynamicist/flight dynamicist recently re-soloed after 25
years
off, people keep asking me hard questions. =A0One that has come up
recent=
ly is
why a heavy glider on tow feels horrible, but thermalling in the same
gli=
der
at lower speeds is fine? (see also Mike Fox's article on aerotowing
in
th=
e
October issue of S&G).

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

Is poor handling at low speed on tow a common experience? =A0I'd
apprecia=
te
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.


Certainly my early aerotows were a bit wobbly, although they've settled
down now - I (re-)soloed on an aerotow, which I think is relatively
unusual in the UK, but most of my launches since then have been on the
winch.

The problem is a bit more than wobbliness though - there does seem to be a
common theme of experienced pilots having real control difficulties when
heavy and a bit slow on tow (not a situation I've been in myself ...
yet).

Part of the difficulty in analysing this is splitting out the effects of
the aerodynamic wake/wing/fin interaction(s) and the mechanical cable
dynamics.

It just occured to me that it might be interesting to do this by flying a
motor glider in formation with a tug - no cable, so only the aerodynamic
effects to contend with. There has been a lot of work done on
aerodynamics of flrmation flight and towing, but everything published has
been on aircraft with either the same span or with the lead aircraft much
larger than the trailing aircraft.

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.

Derek C

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

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





As an aerodynamicist/flight dynamicist recently re-soloed after 25
years off, people keep asking me hard questions. Â*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. Â*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? Â*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. Â*Overuse of
ailerons is very common and it makes aero tow 'wobbly'. Â*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. Â*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. Â*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 |

On Dec 31, 1:47*pm, 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 * * * |

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

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

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

9B

At 23:25 31 December 2010, Andy wrote:
On Dec 31, 1:47=A0pm, Martin Gregorie
wrote:
On Fri, 31 Dec 2010 12:09:08 -0800, Derek C wrote:
On Dec 31, 6:19=A0pm, bildan wrote:
On Dec 31, 4:40=A0am, "Doug" wrote:

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

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

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

Doug Greenwell

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

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

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

- Show quoted text -

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

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

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

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

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

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

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

9B


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

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

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