poor lateral control on a slow tow?

Thanks Doug (am happy to learn from City as well as my own
institution!) and twocoolgliders.

So, if I understand you both correctly, the glider climbs on both
winch and aerotow because there is a force *pulling* it in (roughly)
the direction it is pointing, i.e. above horizontal. Once in a steady
climb, the lift generated by the wings balances the weight of the
glider + any other downward forces.

In a winch launch there are substantial downward forces from the
weight of the cable and the downward vector of the direction of pull.
Thus lift is higher than in steady free flight, and AoA is higher.

On aerotow the only additional downward force is from half the weight
of the towrope (pretty small), so the lift required is similar to that
in steady free flight (and in fact a little lower for other reasons).

_____________

This means that there are only two possible explanations for the
phenomenon on slow tow where the glider feels as if it is close to the
stall. Either:

1. It really is close to the stall, which means that the AoA is
greater than above, which means it must be flying in a continuous
downdraft (Andreas's explanation); or

2. Its AoA is as above, and the phenomenon has some other cause (such
as vortices acting on different parts of the wing) which replicate the
symptoms of approaching stall but do not in fact herald it.

Presumably we could test which is correct by taking a slow tow and
deliberately stalling the glider, monitoring the airspeed at which the
stall occurs. Volunteers to perform this experiment might be hard to
find!

Is there anyone who has actually stalled on tow unintentionally and
noted the airspeed when the stall occurred? I'd guess not, as the
pilot's attention would probably be elsewhere..

On Jan 6, 6:18*am, ProfChrisReed wrote:
Thanks Doug (am happy to learn from City as well as my own
institution!) and twocoolgliders.

So, if I understand you both correctly, the glider climbs on both
winch and aerotow because there is a force *pulling* it in (roughly)
the direction it is pointing, i.e. above horizontal. Once in a steady
climb, the lift generated by the wings balances the weight of the
glider + any other downward forces.

In a winch launch there are substantial downward forces from the
weight of the cable and the downward vector of the direction of pull.
Thus lift is higher than in steady free flight, and AoA is higher.

On aerotow the only additional downward force is from half the weight
of the towrope (pretty small), so the lift required is similar to that
in steady free flight (and in fact a little lower for other reasons).

_____________

This means that there are only two possible explanations for the
phenomenon on slow tow where the glider feels as if it is close to the
stall. Either:

1. It really is close to the stall, which means that the AoA is
greater than above, which means it must be flying in a continuous
downdraft (Andreas's explanation); or

2. Its AoA is as above, and the phenomenon has some other cause (such
as vortices acting on different parts of the wing) which replicate the
symptoms of approaching stall but do not in fact herald it.

Presumably we could test which is correct by taking a slow tow and
deliberately stalling the glider, monitoring the airspeed at which the
stall occurs. Volunteers to perform this experiment might be hard to
find!

Is there anyone who has actually stalled on tow unintentionally and
noted the airspeed when the stall occurred? I'd guess not, as the
pilot's attention would probably be elsewhere..

To the first part above...yes basically your are correct. But
remember is is not one single force acting on a glider to make it
climb. There are 4 forces acting......In fact the sum of all the
forces = 0 during steady climb. So the force of "thrust" need not act
in an upward direction for an aircraft to climb.

Thrust is simply where the energy comes from. More thrust = more
energy = more climb (rate and or angle)
we really have to use the term power or horsepoewer when it comes to
thrust. Horsepower is a rate of work. Lifting a certain weight to a
certain height in a certain time.

Part 1 and part 2 above seems to me both apply......"flying in a
continuous down draft" and that downdraft not being consistant over
the glider's wing span..........so extra angle of attack needed in
some of the wing and not so much needed in others.........the effect
is two fold "wash in"....and high AoA.........both bad for lateraly
control.



Cookie

At 09:50 06 January 2011, Bruce Hoult wrote:
On Jan 6, 9:50=A0pm, Doug Greenwell wrote:
A winch launch is very different because (a) the angle between the
cable
and the direction of motion of the glider is large, and therefore
unlike
=
a
tow the downwards component of the cable tension is no longer
negligible,
and (b) the motion is not steady. =A0

In this case the lift is greater than the weight because it is
partially
counteracting the cable tension and weight. =A0The precise balance
depend=
s
on pilot and winch driver technique. =A0Even so, it is still the
forward
component of the cable tension force that is doing the work required
to
raise the glider to its release height. =A0

It's pretty easy to show that in the early part of full climb on a
winch launch when the cable is horizontal and neglecting cable weight:

tension in the cable =3D glider weight * tan(climb angle)

lift required from the wings =3D glider weight / cos(climb angle)

The latter is identical to the lift required in a turn with the same
bank angle as the winch launch climb angle.

Thus at 45 degrees the tension is the same as the weight and the lift
is 1.4
At 60 degrees the tension is 1.73 times the weight and the lift is
twice.

When using a tension-controlled winch, what tensions are actually
used?


I don't know. There's a winch design group over on Yahoo who seem to
debate the question of how you control a winch on a regular basis. This
gets into the whole question of fast vs slow acceleration at the start and
the danger or not of over-rotation ... at which point a completely new
discussion starts!

Doug

At 09:50 06 January 2011, Bruce Hoult wrote:
On Jan 6, 9:50=A0pm, Doug Greenwell wrote:
A winch launch is very different because (a) the angle between the
cable
and the direction of motion of the glider is large, and therefore
unlike
=
a
tow the downwards component of the cable tension is no longer
negligible,
and (b) the motion is not steady. =A0

In this case the lift is greater than the weight because it is
partially
counteracting the cable tension and weight. =A0The precise balance
depend=
s
on pilot and winch driver technique. =A0Even so, it is still the
forward
component of the cable tension force that is doing the work required
to
raise the glider to its release height. =A0

It's pretty easy to show that in the early part of full climb on a
winch launch when the cable is horizontal and neglecting cable weight:

tension in the cable =3D glider weight * tan(climb angle)

lift required from the wings =3D glider weight / cos(climb angle)

The latter is identical to the lift required in a turn with the same
bank angle as the winch launch climb angle.

Thus at 45 degrees the tension is the same as the weight and the lift
is 1.4
At 60 degrees the tension is 1.73 times the weight and the lift is
twice.

When using a tension-controlled winch, what tensions are actually
used?


I don't know. There's a winch design group over on Yahoo who seem to
debate the question of how you control a winch on a regular basis. This
gets into the whole question of fast vs slow acceleration at the start and
the danger or not of over-rotation ... at which point a completely new
discussion starts!

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

At 11:18 06 January 2011, ProfChrisReed wrote:
Thanks Doug (am happy to learn from City as well as my own
institution!) and twocoolgliders.

So, if I understand you both correctly, the glider climbs on both
winch and aerotow because there is a force *pulling* it in (roughly)
the direction it is pointing, i.e. above horizontal. Once in a steady
climb, the lift generated by the wings balances the weight of the
glider + any other downward forces.

In a winch launch there are substantial downward forces from the
weight of the cable and the downward vector of the direction of pull.
Thus lift is higher than in steady free flight, and AoA is higher.

On aerotow the only additional downward force is from half the weight
of the towrope (pretty small), so the lift required is similar to that
in steady free flight (and in fact a little lower for other reasons).

_____________

This means that there are only two possible explanations for the
phenomenon on slow tow where the glider feels as if it is close to the
stall. Either:

1. It really is close to the stall, which means that the AoA is
greater than above, which means it must be flying in a continuous
downdraft (Andreas's explanation); or

2. Its AoA is as above, and the phenomenon has some other cause (such
as vortices acting on different parts of the wing) which replicate the
symptoms of approaching stall but do not in fact herald it.

Presumably we could test which is correct by taking a slow tow and
deliberately stalling the glider, monitoring the airspeed at which the
stall occurs. Volunteers to perform this experiment might be hard to
find!

Is there anyone who has actually stalled on tow unintentionally and
noted the airspeed when the stall occurred? I'd guess not, as the
pilot's attention would probably be elsewhere..

Chris

yes, that's about it. The danger on a winch launch is that although the
wing lift is much greater than the weight, the accelerations felt by the
pilot once in the climb are very small - so you've no physical indication
of a potential overstress.

As ever, it's a bit of both - there is a downdraft behind the tug, but if
this was constant over the whole span you would end up at the same AoA
*relative to the local airflow* as before, but at a higher pitch attitude
*relative to the tug flightpath* - so you might feel uncomfortably
nose-up, but shouldn't be any closer to the stall.

The explanation that I and others here favour is that you get closer to
the stall, and have poor aileron control, because the downdraft is not
constant in magnitude or direction - but varies from downwards over the
centre section of your wing to upwards over your tips, leading to a
different stall behaviour from free flight.
I did a calculation on a Discus2-like wing at 50knots, at which speed at
which the wing lift coefficient was pretty constant at about 1.1 across
the span in free flight. Put this wing behind a Pawnee and the lift
coefficient changes to about 0.9 at the root and 1.4 at the tip. Put in a
bit of aileron or bank angle and you've potential for early stall and wing
drop.

(not sure about the occasional report of reduced elevator authority though
... will have to think further on this one!)

We know the upwash is really there because flight tests (and watching the
birds) have shown you can get a significant reduction in power (= fuel
flow) required for cruise by flying just outboard of the tip vortices of
another aircraft. NASA did this with a couple of F18s - migrating birds
do it all the time. Interestingly, everyone wins in this scenario,
because the lead aircraft gets a push from the trailing aircraft - people
have looked seriously at flying airliners in formation across the Atlantic
to save fuel, but I'm not sure what ATC would have to say about it!

At 12:27 06 January 2011, cernauta wrote:
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



Yes - it's really difficult to explain in words, but Andreas'
description is good without being too technical.

On 06.01.2011 12:18, ProfChrisReed wrote:

Is there anyone who has actually stalled on tow unintentionally and
noted the airspeed when the stall occurred? I'd guess not, as the
pilot's attention would probably be elsewhere..

I never stalled a glider unintentionally in tow so far ....


.... but I stalled intentionally different gliders in tow behind
aircrafts, TMGs and Microlights in order to find limitations within tow.
And I noted well the differences in behavior and speed.

Doug and Andreas made the right observations with the correct
explanation. You may as well read the studies of Christian Ueckert, DLR
or the studies of DASSU/Stoeckl regarding use of TMGs for towing.

Did you ever look at the main wing of a canard aircraft, like the VariEze?

You may even see the built-in twist in the main wing due to the
downdraft of the canard wing on some pictures.
http://www.aero-auktion.com/angebotd...lectlotid=1786

In tow we have the overall fluid dynamics of a canard aircraft
(neglecting the two stabilisers).

On http://www.desktop.aero/appliedaero/...ardprocon.html
you may find
"Wing twist distribution is strange and CL dependent: The wing
additional load distribution is distorted by the canard wake."
as a inherent disadvantage of all canard aircrafts.

.... maybe we should start pushing our gliders into the air instead of
towing ....

PB

On Jan 5, 10:52*am, Doug Greenwell wrote:
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

I have also experienced the reduced of elevator authority effect while
towing a ballasted ASW-28 on a CG hook behind a Pawnee. Each time I
found myself in the top of the tug wake with a very loose feel to the
elevator and needing close to full back stick to get out of the wake
and wondering if that was gong to be effective. I favor a normal tow
position just above the wake rather than the super high position that
seems to be more common in U.S. That may mean it's more likely I'll
see this effect on a slow tow than others that tow higher. In smooth
air at a reasonable tow speed the ASW-28 on the CG hook will tow stick
free and maintain normal tow position for 5-10 seconds.

Andy

At 13:31 06 January 2011, Andy wrote:
On Jan 5, 10:52=A0am, Doug Greenwell wrote:
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? =A0I'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
hi=
t
the stops? =A0
Did you experience this effect with any specific type of tug? =A0Derek
Copeland has decribed a similar loss of elevator authority when towed
by
=
a
motor glider.

Doug

I have also experienced the reduced of elevator authority effect while
towing a ballasted ASW-28 on a CG hook behind a Pawnee. Each time I
found myself in the top of the tug wake with a very loose feel to the
elevator and needing close to full back stick to get out of the wake
and wondering if that was gong to be effective. I favor a normal tow
position just above the wake rather than the super high position that
seems to be more common in U.S. That may mean it's more likely I'll
see this effect on a slow tow than others that tow higher. In smooth
air at a reasonable tow speed the ASW-28 on the CG hook will tow stick
free and maintain normal tow position for 5-10 seconds.

Andy


This is interesting, because with a reasonable airspeed (even at a very
slow 50kts) the elevator shouldn't feel 'loose', since there should
still be airflow over it. Possibilities I can think of at the moment are
that either the airflow over the elevator is being disrupted due to an
upstream separation (wing or fuselage flow) or that the tailplane has
stalled due to the increased downwash from the tug.