poor lateral control on a slow tow?

On Jan 2, 2:49*am, Doug Greenwell wrote:
At 03:11 02 January 2011, wrote:



On Jan 1, 10:34=A0am, Doug Greenwell *wrote:
At 15:09 01 January 2011, Derek C wrote:

On Jan 1, 11:15=3DA0am, Doug Greenwell =A0wrote:
At 20:23 31 December 2010, bildan wrote:

On Dec 31, 1:06=3D3DA0pm, Todd =3DA0wrote:
I too agree with the real or perceived tow handling
characteristics.

Looking at things =3D3DA0from and aerodynamics standpoint (and I
am
abou=3D
t
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. =3D3DA0Then fly
that
glider
on
tow at those same speeds and record the results.

Done that - and as nearly as I can see, there's no difference in
AoA.

I've flown some pretty heavy high performance gliders behind some
pretty bad tow pilots - one of them stalled the tug with me on
tow.
If I'm careful not to over-control the ailerons, there's no
problem
at
all.

Heavily ballasted gliders respond sluggishly in roll just due to
the
extra roll inertia. =3DA0A pilot trying to hold a precise position
behind
a tug needs and expects crisp aileron response. =3DA0When he
doesn't
get
it, he increases the amount and frequency of aileron with a
corresponding increase in adverse yaw. =3DA0If he's less than
equally
crisp with rudder to oppose the adverse yaw, it gets wobbly.

Where did you mount the AoA meter?

It's not the angle of attack that's the problem, but the change
in
local
incidence along the wing. =3DA0The overall lift may not change by
very
much
when near to the tug wake, but its distribution along the wing
does,
with
increased lift at the tips and reduced lift at the root - putting
the
aileron region close to the stall and hence reducing control
effectiveness.

I agree that increased roll inertia due to ballast is a factor, but
since
the same factor applies to maintaining bank angle in a thermalling
turn
I
don't see how it can account for a significant difference in
handling
between tow and thermalling?- Hide quoted text -

- Show quoted text -

What started the debate at Lasham was using a Rotax engined Falke as
a
glider tug. This towed best at about 50 to 55 knots (c.f. 60+ knots
with a normal tug), but K13s with a stalling speed of 36 knots felt
very unhappy behind it, especially two up. In a conventional powered
aircraft you pull the nose up (to increase the angle of attack and
produce more lift) and increase power to climb, the extra power being
used to prevent the aircraft from slowing down. I don't see why
gliders should behave any differently, except that the power is
coming
from an external source. As you try not to tow in the wake and
downwash from the tug, I can't see that this is particularly
significant,

Derek C

In a steady climb in any light aircraft the climb angles are so low (
10deg) that the lift remains pretty well equal to weight. =A0For example
=
a
10deg climb angle at 60 kts corresponds to an impressive climb rate of
10.5kts - but that would only give Lift =3D Weight/cos(10deg) =3D 1.02
x
Weight. =A0You don't need to increase lift to climb - you increase
thrust
to overcome the aft component of the weight, and the stick comes back
to
maintain speed ... at constant speed the increased power input comes
out
as increasing potential energy =3D increasing height.

I think a lot of people confuse the actions needed to initiate a climb
with what is actually happening in a steady climb. =A0

On your second point, if you are on tow anywhere sensible behind a tug
yo=
u
are in its wake and are being affected by the wing downwash. =A0Wake
is
n=
ot
really a good word, since it seems to get confused with the much more
localised (and turbulent) propwash.

A (very) crude way of visualising the affected wake area is to imagine
a
cylinder with a diameter equal to the tug wing span extending back
from
the tug - that's the downwash region, and then in addition there's
an
upwash region extending perhaps another half-span out either side.-
Hide
=
quoted text -

- Show quoted text -

"aft component of weight??"

Not that this adds anything to the discussion, but.....weight acts in
a "downward" direction toward the center of the earth.

In a climb, on tow, the "aft" forces are drag (mostly) and a small bit
of lift.

Anyway, interesting topic.......has been beat to death at our local
field...EVERY pilot seems to have had it happen, in all different
kinds of gliders......many explainations....not one all-encompassing
explaination yet.

Cookie

it depends on your reference frame - lift and drag are perpendicular to
the direction of motion (relative to the air), which is inclined upwards -
so if you take 'aft' as relative to the glider flight path rather than
the earth, then there is an aft component of weight.- Hide quoted text -

- Show quoted text -

Yes, this is true......but to me it is better to keep the vectors
simple. If you apply a component along the line of the fuselage (aft
vector) then you have to add in the other component too. What
direction? Remember aft is parallel to the glider, not the flight
path of the glider.

We could in fact break any vector up into any number of
components......but eventually you have to combine them again.

To me, using the Earth (as horizontal and vertical reference) is
best. Then we can easily see the climb angle of the glider, the
direction of flight if you will, and the speed. We can also easily
see the angle of attack. With this reference we need to apply only 4
vectors (forces) lift, drag, weight, thrust. IF we use the glider
itself, longitudinal axis as reference, we right away have 8 vectors
to contend with. On tow, if we know any three forces, we can
calculate the forth. In gliding flight its only three forces (thrust
= 0) so its even easier.

Ultimately, if in "steady flight" there is in fact no force acting on
the glider......because the sum of all of the components = 0.

I like your explaination of climbing aircraft above. Another way to
look at it: (speed kept constant) IF thrust is greater than drag, the
aircraft will climb, IF thrust = drag, the aircraft will fly level
with the Earth. IF thrust is less than drag, the aircraft will
descend. If thrust = 0 the aircraft will descent at its L/D angle.
(assuming thrust is applied along the direction of flight)

Oh yeah...yet another factor from the earlier version of this
discussion. The force of the tow rope(thrust) does not necessarily
act through the glider's center of gravity. Neither does the drag
vector. This can cause a pitching moment, which will require elevator
input to counteract. Another factor that can give a different "feel"
on tow.


Cookie