poor lateral control on a slow tow?

On Jan 1, 5:27*pm, Doug Greenwell wrote:
At 16:43 01 January 2011, Derek C wrote:





On Jan 1, 3:34=A0pm, 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 -

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 going
on? *

Lack of elevator effectiveness is not really a symptom of stall as such
.. it's a symptom of low airspeed. *So 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 glider
(14.5m to 16m), which would put the tug and glider tip vortices very close
together. *Two adjacent vortices of the same sign tend to wind up round
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 these
possible interaction effects would be amenable to some fairly
straightforward wind tunnel testing *... a good student project for next
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