CG hook on aero tows??

I don't think the positition of the horizon should
make any difference. In mountains it's irrelevant
and different tugs will climb at different rates -
a powerful tug will be more pitched up and adopt a
higher position relative to the horizon than a low
power one (relative to the thrust line, the prop-wash
and the best position of the glider just above the
prop-wash).
One of the advantages of being as low as possible is
not just the extra time that it takes to get to 'upset'
position (which is small) but the fact that you are
less likely to lose sight of the tug in the first place.
Once you can't see the tug, things can go wrong very
quickly.
I've flown in Australia with their low tow and whilst
I'm sure each method has its merits, I am personally
much more comfortable with the UK position, don't like
having to transition through the propwash at low level
and find that teaching a 'correct' position that looks
almost identical to the position on the ground before
All Out is easier too.
Rob

At 16:48 09 January 2004, Eric Greenwell wrote:
Andy Durbin wrote:

As a US instructor I have flown with many pilots that
received their
initial training from other instructors. I have often
been surprised
at the tow position taken by such pilots. I usually
urge them to keep
lowering the tow position until they feel the wake,
then to move just
high enough that the wake is not a factor. That tow
position may be
10 or 15 ft lower than that initially used.

Where is the towplane, relative to the horizon?

A pilot flying a high high tow has less time to react
to a potential
upset than one flying the UK recommended low high
tow. The fact that
US pilots seem to be trained to use high high tow
may explain why tug
upsets continue to happen even when a nose or forward
hook is used.

10 or 15 feet doesn't sound like it would give much
extra time, not like
the low tow position Australia uses.

--
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Eric Greenwell
Washington State
USA

In article ,
Ian Strachan wrote:
In article 3ffee053$1@darkstar, Mark James Boyd
writes

snip

If anyone decides to modify their experimental glider to
be towed tail first on a tailhook, I guess we have two
volunteers to be the tow pilots...

The main question is, in how many seconds would it be being towed
tail-first ......

Another question would be insurance, but we are in "reductio ad
absurdum" land, here, aren't we?

--
Ian Strachan
Lasham, UK

Ian,

Exactly right. So if we can all agree that "given the
appropriate level of experience and skill, a pilot can
fly any flyable aircraft in any conditions," we can avoid this
obvious truth and focus on the relative risks and
costs (including the cost of acquiring training and
experience).

The reson, I think, is that the Pirat has quite a high-set wing and a
belly hook as low as it can get. As a result, the lever arm, and
therefore upwards pitching moment from the winch cable about the wing
is relatively high compared to the typical glass glider with mid-set
wing and belly hook offset and up a bit. So with a good fast winch
launch, yes, this moment can overcome anything the elevator is trying
to do.

However, this is an intrinsically stable situation. As the cable angle
relative to the glider's longitudinal axis increases, the lever arm
reduces (by approximately 50% when the glider is 30 degree nose up and
the cable is 30 degrees down). So as the glider pitches up the effect
of the cable reduces, the effect of the elevator (all other things
being equal) stays the same and a point of equilibrium is reached. To
put it simply:

I think you forgot to account for the increasing load that pitching up
produces. Sure, the lever arm is reduced, but the glider is at a higher
angle of attack, increasing it's lift. This increases the force much
more than the lever arm is reduced, and the pitch up continues.

This eventually stabilizes with the glider in the normal nose high
attitude of a winch launch, but this is not a good attitude for
aerotowing! For a nose hook, the lever arm is much less to begin with,
and a small pitch up reduces it to zero - quite a different situation.
--
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Eric Greenwell
Washington State
USA

Robert John wrote:

I don't think the positition of the horizon should
make any difference.

It's the position I typically use, so I'm trying to get an idea of how
different it is from what I normally do. Unfortunately, I can't go out
and experiment right now, with a foot of snow on the runway!

So, with a 180 hp Super Cub or Pawnee, say, is the tug canopy on the
horizon, the wing root, wheels, or maybe the tug is an entire "tug"
distance above the horizon?

In mountains it's irrelevant
and different tugs will climb at different rates -
a powerful tug will be more pitched up and adopt a
higher position relative to the horizon than a low
power one (relative to the thrust line, the prop-wash
and the best position of the glider just above the
prop-wash).

--
-----
change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

As a US instructor I have flown with many pilots that
received their
initial training from other instructors. I have often
been surprised
at the tow position taken by such pilots. I usually
urge them to keep
lowering the tow position until they feel the wake,
then to move just
high enough that the wake is not a factor.

I also agree with Andy that the optimal tow location is just above the
wake. I like it because it's more efficient for the tow plane since less
up elevator is required to maintain proper airspeed. In the flat lands,
if the tug wings are on or above the horizon, you're too high, IMHO.

I've also been known to fly a little to the left on tow to give the tow
pilot's right leg a little rest.

Tony V.

In article 3ffee053$1@darkstar, Mark James Boyd
writes

snip

If anyone decides to modify their experimental glider to
be towed tail first on a tailhook, I guess we have two
volunteers to be the tow pilots...

The main question is, in how many seconds would it be being towed
tail-first ......

Another question would be insurance, but we are in "reductio ad
absurdum" land, here, aren't we?

--
Ian Strachan
Lasham, UK

Tony Verhulst wrote in message ...


In the flat lands,
if the tug wings are on or above the horizon, you're too high, IMHO.


I think you got something reversed. The lower the glider goes, the
higher above the horizon the tug will appear to be. When I tow behind
a 235 Pawnee all the tug is above the horizon.


Andy

if you are towing that low out here in the summer time... you are going to
find yourself in the wake from time to time.. and being in the wake
increases the drag on the tow plane and reduces the climb rate and increases
the down pull, forcing the tow pilot to push on the stick..

we "train".. tow plane on the horizon.. granted there are hills/mountains
all around so the horizon is a "relative" term... keep all 3 wheels of the
tow on the same horizontal line.. the tail wheel between the mains.. works
just fine for the Pawnee 235

if you are low.. "to just above the wake".. it is more difficult for the tow
pilot to see you in the mirrors.. and he may think your going down to box
the wake. Also, with summer thermals.. the tow plane enters the thermal
200ft before the glider, and a 400fpm rate of climb jump to better than
1000fpm for the tow plane.. and you are in the wake.. granted.. in a couple
of seconds or so.. you'll be in the thermal and going up just as the tow
plane exits it and hits the sink

it's hard enough fighting the thermal drafts.. but to add an unplanned
excursion into the wake? a couple of those.. and at 2K AGL.. I'm off...

BT

"Robert John" wrote in
message ...
I don't think the positition of the horizon should
make any difference. In mountains it's irrelevant
and different tugs will climb at different rates -
a powerful tug will be more pitched up and adopt a
higher position relative to the horizon than a low
power one (relative to the thrust line, the prop-wash
and the best position of the glider just above the
prop-wash).
One of the advantages of being as low as possible is
not just the extra time that it takes to get to 'upset'
position (which is small) but the fact that you are
less likely to lose sight of the tug in the first place.
Once you can't see the tug, things can go wrong very
quickly.
I've flown in Australia with their low tow and whilst
I'm sure each method has its merits, I am personally
much more comfortable with the UK position, don't like
having to transition through the propwash at low level
and find that teaching a 'correct' position that looks
almost identical to the position on the ground before
All Out is easier too.
Rob

At 16:48 09 January 2004, Eric Greenwell wrote:
Andy Durbin wrote:

As a US instructor I have flown with many pilots that
received their
initial training from other instructors. I have often
been surprised
at the tow position taken by such pilots. I usually
urge them to keep
lowering the tow position until they feel the wake,
then to move just
high enough that the wake is not a factor. That tow
position may be
10 or 15 ft lower than that initially used.

Where is the towplane, relative to the horizon?

A pilot flying a high high tow has less time to react
to a potential
upset than one flying the UK recommended low high
tow. The fact that
US pilots seem to be trained to use high high tow
may explain why tug
upsets continue to happen even when a nose or forward
hook is used.

10 or 15 feet doesn't sound like it would give much
extra time, not like
the low tow position Australia uses.

--
-----
change 'netto' to 'net' to email me directly

Eric Greenwell
Washington State
USA

With the supercubs and Robin DR400s at LGC the tug
wheels are generally on or a little above the horizon.
I wouldn't generally be so low as to be catching the
wake though in strong thermal conditions I can see
that you might. If I tow through a strong thermal
I'm going to release anyway!
Rob

At 18:00 09 January 2004, Eric Greenwell wrote:
Robert John wrote:

I don't think the positition of the horizon should
make any difference.

It's the position I typically use, so I'm trying to
get an idea of how
different it is from what I normally do. Unfortunately,
I can't go out
and experiment right now, with a foot of snow on the
runway!

So, with a 180 hp Super Cub or Pawnee, say, is the
tug canopy on the
horizon, the wing root, wheels, or maybe the tug is
an entire 'tug'
distance above the horizon?

In mountains it's irrelevant
and different tugs will climb at different rates -
a powerful tug will be more pitched up and adopt a
higher position relative to the horizon than a low
power one (relative to the thrust line, the prop-wash
and the best position of the glider just above the
prop-wash).

--
-----
change 'netto' to 'net' to email me directly

Eric Greenwell
Washington State
USA

Eric Greenwell wrote in message ...

I think you forgot to account for the increasing load that pitching up
produces. Sure, the lever arm is reduced, but the glider is at a higher
angle of attack, increasing it's lift. This increases the force much
more than the lever arm is reduced, and the pitch up continues.

Good point. I have along train journey today, and will have a ponder
on this. I suspect the outcome will be along the lines of "the glider
has stick fixed stability in free flight anyway: the effect of the
added winch cable force is to increase this stability".

This eventually stabilizes with the glider in the normal nose high
attitude of a winch launch, but this is not a good attitude for
aerotowing! For a nose hook, the lever arm is much less to begin with,
and a small pitch up reduces it to zero - quite a different situation.

Agreed completely.

Ian