poor lateral control on a slow tow?

At 21:47 31 December 2010, Martin Gregorie wrote:
On Fri, 31 Dec 2010 12:09:08 -0800, Derek C wrote:


On Dec 31, 6:19Â*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 |


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!

At 21:47 31 December 2010, Martin Gregorie wrote:
On Fri, 31 Dec 2010 12:09:08 -0800, Derek C wrote:


On Dec 31, 6:19Â*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 |


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!

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

On 12/31/2010 3:40 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 ...

The wake behind a climbing towplane should be well below a glider in the
high tow position. How can it lift it's weight and the glider, if the
wake isn't descending? Recall one of the exercises a student does while
learning to tow is to start in the high tow position, then move straight
down until the wake turbulence is felt. With the usual 150' to 200'
rope, it's way below the high tow position. So, I don't think it's wake
turbulence, IF the glider is in the high tow position.

I think a big part of the answer is the pilot's perception of
"attitude": the glider has an additional attitude (relative to the
horizon) over it's normal angle of attack because the glider and tug are
ascending at about a 6 degree angle. T His is perceived by the pilot as
a very nose high attitude, and makes him feel uncomfortable; that, along
with reduced control response, makes him feel it's worse than it really
is. Usually, this happens close to the ground, making the perception
even worse.

The unusually nose-high attitude can keep the glider pilot from using
enough up elevator, with the consequence that he does sink down into the
wake. That will make the situation actually worse, not just perceptually
worse. But, it's because he is far from the high tow position, not just
because the speed is slower than normal.

This analysis obviously assumes a high tow as the normal situation, and
may not apply to the situation where low tow is the norm.

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

At 23:35 31 December 2010, Eric Greenwell wrote:
On 12/31/2010 3:40 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 ...

The wake behind a climbing towplane should be well below a glider in the

high tow position. How can it lift it's weight and the glider, if the
wake isn't descending? Recall one of the exercises a student does while

learning to tow is to start in the high tow position, then move straight

down until the wake turbulence is felt. With the usual 150' to 200'
rope, it's way below the high tow position. So, I don't think it's
wake
turbulence, IF the glider is in the high tow position.

I think a big part of the answer is the pilot's perception of
"attitude": the glider has an additional attitude (relative to the
horizon) over it's normal angle of attack because the glider and tug are

ascending at about a 6 degree angle. T His is perceived by the pilot as
a very nose high attitude, and makes him feel uncomfortable; that, along

with reduced control response, makes him feel it's worse than it really

is. Usually, this happens close to the ground, making the perception
even worse.

The unusually nose-high attitude can keep the glider pilot from using
enough up elevator, with the consequence that he does sink down into the

wake. That will make the situation actually worse, not just perceptually

worse. But, it's because he is far from the high tow position, not just

because the speed is slower than normal.

This analysis obviously assumes a high tow as the normal situation, and
may not apply to the situation where low tow is the norm.

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


The wake does descend, although this is not necessary for a wing to
generate lift (otherwise wind tunnels would not work!) ... actually, the
downwash is a consequence of a reduction in lift and increase in (induced)
drag for a three-dimensional wing.

However, the turbulent prop wash also descends with it, so setting a tow
position on the basis of a reasonable distance above the prop wash would
automatically position you close to the tip vortices.

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

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


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

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

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

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


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

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

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


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

Doug

On Fri, 31 Dec 2010 11:40:53 -0000, "Doug"
wrote:

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?

Yes, it is common. I use to fly mainly at competitions, and among the
5-10 tow pilots, there's always at least one who, despite being
briefed by the towmaster, flies too slowly.

In my personal experience, it happened to me 3 times in a double
seater (Janus B and DuoDiscus). I don't remember any occurrence in my
single seater.
I can describe it as being unable to raise the nose. As the towplane
was flying below 100 km/h, I just couldn't match the climbing rate
with the glider, so I was more and more into the propwash. A gentle
pull up wouldn't work; pulling more hits the stop and the glider feels
like it's sinking.

I also cannot find an easy and believable explanation for this
phenomenon. I didn't recognize a lack of _lateral_ control, anyway.

aldo cernezzi

At 17:25 05 January 2011, cernauta wrote:
On Fri, 31 Dec 2010 11:40:53 -0000, "Doug"
wrote:

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?

Yes, it is common. I use to fly mainly at competitions, and among the
5-10 tow pilots, there's always at least one who, despite being
briefed by the towmaster, flies too slowly.

In my personal experience, it happened to me 3 times in a double
seater (Janus B and DuoDiscus). I don't remember any occurrence in my
single seater.
I can describe it as being unable to raise the nose. As the towplane
was flying below 100 km/h, I just couldn't match the climbing rate
with the glider, so I was more and more into the propwash. A gentle
pull up wouldn't work; pulling more hits the stop and the glider feels
like it's sinking.

I also cannot find an easy and believable explanation for this
phenomenon. I didn't recognize a lack of _lateral_ control, anyway.

aldo cernezzi


Interesting - most people are reporting problems with lateral control
(which seems to have a reasonably simple explanation), but running out of
nose-up pitch control also seems to occur ... and is harder to understand.


Did you notice any kind of change in elevator control force before you hit
the stops?
Did you experience this effect with any specific type of tug? Derek
Copeland has decribed a similar loss of elevator authority when towed by a
motor glider.

Doug

On Wed, 05 Jan 2011 17:52:28 +0000, Doug Greenwell
wrote:


Did you notice any kind of change in elevator control force before you hit
the stops?

i didn't notice, I was probably busy trying to keep altitude behind
the tug

Did you experience this effect with any specific type of tug? Derek
Copeland has decribed a similar loss of elevator authority when towed by a
motor glider.

Andreas says that it's much less likely to happen bihind a
motorglider, however.
In my case, I'm not sure, it may have been both times a Robin dr400.

I think the "descending airmass" theory by Andreas is the easiest to
understand and believe. It explains both the running out of elevator,
and the poor lateral control (when not perfectly centered behind the
tug, or because of the increased AoA at the tips)

aldo cernezzi
-
www.voloavela.it
the italian gliding magazine