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 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!

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

Please think with me -

The argument that the wing has the same AoA for a given speed, only
applies in a homogeneous airmass.

Consider that lift generated is integrated over the wing as a function
of the local AoA, Airspeed, density etc. The geometric angle of the wing
to the flight path is constant (ignoring washout) So - what happens when
the airmass is not homogeneous. According to this explanation - there is
a constantly varying vertical motion that has negative maxima either
side of the tug centreline and positive maxima some distance outboard of
the tug wingtips. This is consistent with the known vortex patterns - so
I think we can accept this is true.

Then we have a constantly varying effective angle of attack on the wing.
Some parts of the wing are at a lower, and others at a higher AoA than
for the "homogeneous airmass" case. So that would mean that on an
untwisted glider wing we are seeing the wing exposed to an angle of
attack varying by 4 or more degrees. The wing load distribution would be
distorted by these local variations in vertical speed of the airmass.

This means that at 1g, over the inboard section of the wing will be
producing less lift than in a homogeneous airmass, and the outboard
parts more. Given the normal load distribution for a glider, it is
reasonable to assume that the inboard section normally accounts for a
disproportionate amount of the lift. So it becomes plausible that the
entire wing may be at a higher aerodynamic AoA for the speed, to produce
the 1g lift required. (More lift coming from low lift sections of the
outboard wing) More importantly the geometric angle to the flight path
will be probably around 3-4 degrees higher than would be the case in
undisturbed air.

Some further thought on possible sources for the need for up elevator.
All the types I have heard mentioned in the thread have polyhedral wings
with an aerodynamic sweep back due to the multi trapezoidal shape. If
the lift distribution is moved outboard then one assumes that the centre
of pressure will also move aft due to geometry of the wing. If so - this
will introduce a nose down moment.
Similarly,if the glider is at a higher AoA and the vertical downwash of
the tug wing passes over the glider tailplane and it will result in a
lower relative AoA for the elevator. So needing more "up" elevator input
to balance.

So it is then possible that local but predictable variation in vertical
air mass movement is responsible for this effect.

So it looks like the wing MAY in fact operate at a higher angle of
attack for some of it's span, and this would be in the aileron portion
of the span, making all sorts of interesting things happen with induced
drag and local stalling etc. Which would in turn make the glider feel
unresponsive and "mushy" - while not being close to a stall inboard.

If that were the case then logic says we should use a little more flap
and unload the outboard part of the wing. Is there any empirical
evidence to support that?

Am I making sense here?

Bruce



On 2011/01/06 3:13 PM, Paula Bold wrote:
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

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

At 17:45 06 January 2011, BruceGreeff wrote:
Please think with me -

The argument that the wing has the same AoA for a given speed, only
applies in a homogeneous airmass.

Consider that lift generated is integrated over the wing as a function
of the local AoA, Airspeed, density etc. The geometric angle of the wing

to the flight path is constant (ignoring washout) So - what happens when

the airmass is not homogeneous. According to this explanation - there is

a constantly varying vertical motion that has negative maxima either
side of the tug centreline and positive maxima some distance outboard of

the tug wingtips. This is consistent with the known vortex patterns - so

I think we can accept this is true.

Then we have a constantly varying effective angle of attack on the wing.

Some parts of the wing are at a lower, and others at a higher AoA than
for the "homogeneous airmass" case. So that would mean that on an
untwisted glider wing we are seeing the wing exposed to an angle of
attack varying by 4 or more degrees. The wing load distribution would be

distorted by these local variations in vertical speed of the airmass.

This means that at 1g, over the inboard section of the wing will be
producing less lift than in a homogeneous airmass, and the outboard
parts more. Given the normal load distribution for a glider, it is
reasonable to assume that the inboard section normally accounts for a
disproportionate amount of the lift. So it becomes plausible that the
entire wing may be at a higher aerodynamic AoA for the speed, to produce

the 1g lift required. (More lift coming from low lift sections of the
outboard wing) More importantly the geometric angle to the flight path
will be probably around 3-4 degrees higher than would be the case in
undisturbed air.

Some further thought on possible sources for the need for up elevator.
All the types I have heard mentioned in the thread have polyhedral wings

with an aerodynamic sweep back due to the multi trapezoidal shape. If
the lift distribution is moved outboard then one assumes that the centre

of pressure will also move aft due to geometry of the wing. If so - this

will introduce a nose down moment.
Similarly,if the glider is at a higher AoA and the vertical downwash of
the tug wing passes over the glider tailplane and it will result in a
lower relative AoA for the elevator. So needing more "up" elevator
input
to balance.

So it is then possible that local but predictable variation in vertical
air mass movement is responsible for this effect.

So it looks like the wing MAY in fact operate at a higher angle of
attack for some of it's span, and this would be in the aileron portion
of the span, making all sorts of interesting things happen with induced
drag and local stalling etc. Which would in turn make the glider feel
unresponsive and "mushy" - while not being close to a stall inboard.

If that were the case then logic says we should use a little more flap
and unload the outboard part of the wing. Is there any empirical
evidence to support that?

Am I making sense here?

Bruce



I think so - flap should help, unless it's an integrated system - in
which case the ailerons droop as well and load the tips back up.

I can see there being an aft shift in centre of pressure as the tips
become more highly loaded, but the sweep angles are relatively small so
I'm not sure how big this effect would be.

At lower incidences any increased downwash over the tail should actually
help - 'up-elevator' corresponds to a downwards force on the tail, so
the tail is acting as an inverted wing. Downwash would make the tail
iangle of attack more negative and create more downforce, and hence more
nose-up pitching moment. However, if the downwash was large enough it
could possibly stall the tail - at which point you would lose elevator
authority and feel.
Just speculation though - the tug vortex/glider wing interaction is pretty
straightforward to model and predict, but a tug vortex/glider vortex/glider
tail interaction is much harder!