poor lateral control on a slow tow?

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





On Jan 1, 11:15=A0am, Doug Greenwell *wrote:
At 20:23 31 December 2010, bildan wrote:

On Dec 31, 1:06=3DA0pm, Todd =A0wrote:
I too agree with the real or perceived tow handling
characteristics.

Looking at things =3DA0from and aerodynamics standpoint (and I am
abou=
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. =3DA0Then 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. =A0A pilot trying to hold a precise position
behind
a tug needs and expects crisp aileron response. =A0When he doesn't
get
it, he increases the amount and frequency of aileron with a
corresponding increase in adverse yaw. =A0If 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. =A0The 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. *For example a
10deg climb angle at 60 kts corresponds to an impressive climb rate of
10.5kts - but that would only give Lift = Weight/cos(10deg) = 1.02 x
Weight. *You 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 = 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. *

On your second point, if you are on tow anywhere sensible behind a tug you
are in its wake and are being affected by the wing downwash. *Wake is not
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.

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