At 13:24 02 January 2011, wrote:
On Jan 2, 2:49=A0am, Doug Greenwell wrote:
At 03:11 02 January 2011, wrote:



On Jan 1, 10:34=3DA0am, Doug Greenwell =A0wrote:
At 15:09 01 January 2011, Derek C wrote:

On Jan 1, 11:15=3D3DA0am, Doug Greenwell =3DA0wrote:
At 20:23 31 December 2010, bildan wrote:

On Dec 31, 1:06=3D3D3DA0pm, Todd =3D3DA0wrote:
I too agree with the real or perceived tow handling
characteristics.

Looking at things =3D3D3DA0from and aerodynamics standpoint
(and=
I
am
abou=3D3D
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. =3D3D3DA0Then
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. =3D3DA0A pilot trying to hold a precise
positi=
on
behind
a tug needs and expects crisp aileron response. =3D3DA0When he
doesn't
get
it, he increases the amount and frequency of aileron with a
corresponding increase in adverse yaw. =3D3DA0If 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. =3D3DA0The 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. =3DA0For
examp=
le
=3D
a
10deg climb angle at 60 kts corresponds to an impressive climb rate
of
10.5kts - but that would only give Lift =3D3D Weight/cos(10deg)
=3D3D
=
1.02
x
Weight. =3DA0You 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 =3D3D 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. =3DA0

On your second point, if you are on tow anywhere sensible behind a
tug
yo=3D
u
are in its wake and are being affected by the wing downwash.
=3DA0Wake
is
n=3D
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
=3D
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 -

Lift is perpendicular.......drag is parallel........


... oh alright - I left a few words out :-)