At 13:20 02 January 2011, wrote:
On Jan 2, 6:01=A0am, Derek C wrote:
On Jan 1, 5:27=A0pm, Doug Greenwell wrote:





At 16:43 01 January 2011, Derek C wrote:

On Jan 1, 3:34=3DA0pm, 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
(a=
nd I
am
abou=3D3D
t
as
far from and aerodynamicist as you can get) it should seem
tha=
t
part
of the empirical data would suggest an experiment where you
fl=
y
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
som=
e
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
posi=
tion
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,
b=
ut
since
the same factor applies to maintaining bank angle in a
thermallin=
g
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
a=
s
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
powere=
d
aircraft you pull the nose up (to increase the angle of attack
and
produce more lift) and increase power to climb, the extra power
bei=
ng
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
exa=
mple
=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)
=3D3=
D 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
bac=
k
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
cli=
mb
with what is actually happening in a steady climb. =3DA0

On your second point, if you are on tow anywhere sensible behind
a
t=
ug
yo=3D
u
are in its wake and are being affected by the wing downwash.
=3DA0Wa=
ke
is
n=3D
ot
really a good word, since it seems to get confused with the much
mor=
e
localised (and turbulent) propwash.

A (very) crude way of visualising the affected wake area is to
imagi=
ne
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 -

So why did a K13 feel on the verge of a stall at 50 knots on tow?
All
the classic symptoms of a stall were there, including mushy
controls,
wallowing around and buffeting. If you got even slightly low it
seemed
quite difficult to get back up to the normal position. Lack of
elevator effectiveness is yet another sympton of the stall!

Fortunately we have given up aerotowing with the Falke. It just
seemed
like a good idea at the time because its flying speeds are more
closely matched to a glider; in theory anyway.

Derek C

good question - which suggests that something more complicated was
goin=
g
on? =A0

Lack of elevator effectiveness is not really a symptom of stall as
such
.. it's a symptom of low airspeed. =A0So for buffeting and mushy,
ineffective elevator to be happening at an indicated airspeed of
50-55
knots I'm wondering whether the tailplane was stalling rather than
the
wing?

In this case you'd a tug with a wing span of a similar size to the
glid=
er
(14.5m to 16m), which would put the tug and glider tip vortices very
cl=
ose
together. =A0Two adjacent vortices of the same sign tend to wind up
rou=
nd
each other and merge quite quickly - if this happened with the two
sets=
of
tip vortices it would generate an increased downwash near the tail
and
=
push
the local (negative) incidence past the stall angle.

I'd be the first to admit this is getting rather speculative - but
thes=
e
possible interaction effects would be amenable to some fairly
straightforward wind tunnel testing =A0... a good student project
for
n=
ext
year!-
Actually the only totally reliable sysmptom of being stalled is that
the elevator will no longer raise the nose.
The elevator should still
be effective at 50 knots, so it's more likely that the wing is close
to the stall. The stall is only strictly related to the angle of
attack. During a aerotow climb the wing has to support an additional
weight component as well as drag, so the effective wing loading may
well be increased, requiring a greater angle of attack for a given
airspeed. Going 10 knots faster seems to cure the problem.

Derek C- Hide quoted text -

- Show quoted text -


'Actually the only totally reliable sysmptom of being stalled is that
the elevator will no longer raise the nose.'

HUH? Many cases possible where we could have full elevator and not
be stalled. (I demonstrate this is 2-33 and grob 103 and ask-21.
All you need is heavy pilot (forward CG) and gentle stick back to the
stop. Glider will mush, but not stall. Elevator will not raise the
nose........wing does not have angle to stall.

On tow the only additional "weight component" would be a downward
component to the tow rope (thrust). Since the tension on the tow rope
is fairly low........it should not have a big effect, but there is
some effect.

But yeah, that extra 10 knots makes all the difference in the world.
(I remember occasionally getting a "slow tow" when flying a 2-32 with
three aboard..........what a handful!!!
Cookie



Depends on your definition of 'stall' - whether the nose drops or not
depends so much on the aircraft configuration, the aerofoil section and
the stall entry technique.

NASA did a 'deep-stall' test program in the early 80s with a modified
1-36 which was able to carry on pitching up to 70deg AoA ... look at the
incidence on this photo!
http://www.dfrc.nasa.gov/gallery/pho.../ECN-26845.jpg