At 12:40 04 January 2011, wrote:
On Jan 4, 7:27=A0am, Doug Greenwell wrote:
At 01:01 04 January 2011, wrote:





On Jan 3, 3:34=3DA0pm, Doug Greenwell =A0wrote:
At 19:12 03 January 2011, Craig wrote:

On Jan 1, 3:06=3D3DA0am, Doug Greenwell =3DA0wrote:
At 21:47 31 December 2010, Martin Gregorie wrote:

On Fri, 31 Dec 2010 12:09:08 -0800, Derek C wrote:

On Dec 31, 6:19=3D3DA0pm, bildan =3D3DA0wrote:
On Dec 31, 4:40=3D3DA0am, "Doug" =3D3DA0wrote:

As an aerodynamicist/flight dynamicist recently re-soloed
after
25
years off, people keep asking me hard questions.
=3D3DA0One
that
h=3D
as
come
up recently is why a heavy glider on tow feels horrible,
but
thermalling in the same glider at lower speeds is fine?
(see
also
Mike Fox's article on aerotowing in the October issue of
S&G).

I did some calculations, and I reckon it's probably due
to
the
tug
wing wake (tip vortices generating a downwash inboard,
upwash
outboard) changing the lift distribution on the glider
wing
-
with
an
increased angle of attack out at the tips reducing aileron
effectiveness. =3D3DA0There's possibly an interesting
academi=
c
research
project here, but it's always best to get a reality check
first
..

Is poor handling at low speed on tow a common experience?
=3D3DA0I'd
appreciate any thoughts/comments/war stories ...
particularly
bad
tug/glider/speed combinations, incidents of wing drop
during
a
tow
etc etc?

Doug Greenwell

I suspect, but can't know unless I flew with you, that you
are
unconsciously trying to "steer" the glider with ailerons.
=3D3DA0Overuse
of
ailerons is very common and it makes aero tow 'wobbly'.
=3D3DA0If
you
consciously use rudder to aim the nose at the tug's tail
and
just
keep
the same bank angle as the tug with ailerons, it might work
better.

Wake effects are generally favorable if you stay at the
right
height
relative to the tug. =3D3DA0Using a slightly higher tow
positio=
n
can
sometimes help a lot.

The tip vortices rotate inward above the propwash which, if
allowed
to
do so, will drift the glider to the center position and help
keep
it
there. =3D3DA0I haven't noticed any tendency for them to yaw
a
glide=3D
r
towards
a tugs wing tip.- Hide quoted text -

- Show quoted text -

There was a debate on our club forum about why gliders feel
uncomfortable on slow tows that are still well above their
normal
stalling speed. We think the answer is that the glider is
being
asked
to
climb with the tug providing the thrust via the rope. The
glider
is
still effectively in free flight and therefore has to fly at
a
greater
angle of attack for a given airspeed to produce the extra
lift
for
climbing. Hence its stalling speed is somewhat increased.

If the tug's downwash field extends back far enough to include
the
glider, its AOA will be relative to the downwash streamlines.
Add
the
downwash angle to the climb angle of the tug-glider combination
will
mak=3D3D
e
the glider look quite nose-high to its pilot. =3D3DA0

I know that the downwash angle is roughly 1/3 of the wing AOA
at
4-5
chords behind the wing, i.e. about where the tailplane is, but
not
what
its angle might be at the end of a tow rope.

--
martin@ =3D3DA0 | Martin Gregorie
gregorie. | Essex, UK
org =3D3DA0 =3D3DA0 =3D3DA0 |

The downwash angle doesn't change much past the tail, and a
half
to
a
third of the tug AoA is a good first guess.

My modeling suggest that there does seem to be an overall
reduction
in
th=3D3D
e
glider wing lift (downwash over the centre wing having more of
an
effect
than upwash over the tips), so the glider requires another
degree
or
two
in AoA - so feeling even more nose-up to the pilot!

Many thanks to the aerodynamics folks for cogent replies.
=3DA0From
a
structures and vectors standpoint, the greatest amount of downward
catenary force possible from the rope is the rope's own weight
(in
other words, damn little). =3DA0 If the towplane and glider are at
exactly
the same elevation the vertical component of the catenary force
equals
half the rope weight. =3DA0Any other vertical forces imparted to
the
sailplane result from the vector generated by the relative
positions
of the towplane and glider. Kudos to Doug for the stimulating
discussion.

Thanks,
Craig

It's been very interesting - and sparked off a few potentially
very
interesting research topics (typical academic - always an eye to
the
next
journal paper!)

Good point on the rope forces - I hadn't looked at it that way,
but
as
you say any bow in the tow rope won't actually have a significant
effect
on the static forces/moments on the glider .. just as well, because
it's
quite difficult to calculate the shape once you take drag forces
into
account!

Doug- Hide quoted text -

- Show quoted text -

Actually, 5 or 10 pounds of down force at the glider's nose would be
significant. =A0 Every loosen your shoulder belts and lean
forward?.....this little weight shift will change pitch and speed.

Now with a cg hook ...probably not significant.

Cookie

true - but it would take a very small elevator deflection to trim it
out-=
Hide quoted text -

- Show quoted text -

Yeah.........Hey, I am not saying that this is the answer to the
question........I have yet to see any answer which fully explains the
phenomon........I am just bringing up this issue of the tow rope
because it was brought up in the earlier discussion. It is just one
of the "suspects" in the investigation.

We all agree that at a given speed, faily slow, that a glider handles
fine in gliding flight, and has troubles on tow at the same speed.

So there are obvious differences during tow.........the tow rope
hooked to the nose is one, along with all the others we have
discussed, like down wash, vortex, etc.......


Cookie


Life's never simple! This thread has been really useful in highlighting
for me a number of factors that I hadn't really thought about when I
first started looking at this phenomenon.. All I need now is a bright grad
student to spend a year doing the sums while I go fly :-)