motorgliders as towplanes

On Mar 14, 9:36*pm, "Mike Schumann" mike-nos...@traditions-
nospam.com wrote:
You are confusing a lack of elevator authority with stalling. *The two are
completely different phenomenon.

Mike Schumann

"Mike the Strike" wrote in ...

I took a good look at the geometry of my tow today. *The tow line
appeared to be pulling down on the nose at about an angle of twenty
degrees. At 65 knots, the tow line formed a catenary to the towplane
with significant sag in the line.

It should be possible to model the angles of the line and angles of
attack of the towplane and glider, but I suspect some of the
simplistic arguments have not explained the phenomenon because they
haven't taken full account of the complex geometry of the tow.

At around 50 to 55 knots, I am unable to maintain high tow and sink
into low tow with no elevator authority and reduced aileron control.
My free-flight stall speed is below 40 knots. *Gurus please explain.

Mike

Maybe so, but why is poor aileron control in a slow tow similar to
that experienced in an incipient stall? What are the symptoms of
running out of elevator authority and why does this happen on tow but
not in free flight?

No-one has satisfactorily explained this.


Mike

Mike,

If the tow line formed a 20 degree angle to the glider, (from tow plane
upward to glider) the glider would have to be 72 feet ABOVE the tow
plane.

Well, really allowing for the angle of climb, it would be 72 feet above
the "direction of flight" of the tow plane. If the tow plane could
climb at a 20 degree angle you would be level with the tow plane, but this
is not possible, If the tow plane climbed at a more reasonable angle of 5
degrees you would be 54 feet above the tow plane.

This, in my opinion, would be dangerous!

I like to tow with the tow plane's wheels sitting on the horizon. This
means that the glider is about 5 feet BELOW the towplane, and the angle of
the tow rope is about 1.5 degrees UPWARD from glider to tow plane.

I despise the term "high tow" as it implies positioning the glider
"high" and above the tow plane. I use the term, ""low tow" for
flying below the wake and the term "normal tow" for flying level with,
or slightly below, the tow plane, but above the wake.

In my opinion flying above the tow plane is forbidden!!

Cookie






At 04:07 15 March 2009, Mike the Strike wrote:
I took a good look at the geometry of my tow today. The tow line
appeared to be pulling down on the nose at about an angle of twenty
degrees. At 65 knots, the tow line formed a catenary to the towplane
with significant sag in the line.

It should be possible to model the angles of the line and angles of
attack of the towplane and glider, but I suspect some of the
simplistic arguments have not explained the phenomenon because they
haven't taken full account of the complex geometry of the tow.

At around 50 to 55 knots, I am unable to maintain high tow and sink
into low tow with no elevator authority and reduced aileron control.
My free-flight stall speed is below 40 knots. Gurus please explain.

Mike

Stall is a factor angle of attack, not airspeed.

You could say that your MINIMUM possible stall speed is under 40 knots,
but you could easily stall at higher speeds IF you get the angle of attack
high enough. Just pull back on the stick harder!

Cookie


At 04:07 15 March 2009, Mike the Strike wrote:
My free-flight stall speed is below 40 knots. Gurus please explain.

Has anybody actually stalled during an aero tow?


Cookie

At 04:36 15 March 2009, Mike Schumann wrote:
You are confusing a lack of elevator authority with stalling. The two
are
completely different phenomenon.

Mike Schumann

"Mike the Strike" wrote in message
...
I took a good look at the geometry of my tow today. The tow line
appeared to be pulling down on the nose at about an angle of twenty
degrees. At 65 knots, the tow line formed a catenary to the towplane
with significant sag in the line.

It should be possible to model the angles of the line and angles of
attack of the towplane and glider, but I suspect some of the
simplistic arguments have not explained the phenomenon because they
haven't taken full account of the complex geometry of the tow.

At around 50 to 55 knots, I am unable to maintain high tow and sink
into low tow with no elevator authority and reduced aileron control.
My free-flight stall speed is below 40 knots. Gurus please explain.

Mike

For purposes of analysis, the angle at which the tow rope meets the glider
is the angle to consider that THRUST is acting on the glider. A rope can
only be in tension. It cannot impart any rotational "moments" to the
glider.

If the tow rope has a big sag or belly in it, consider the angle at which
the rope meets the glider, not the angel of the glider relative to the tow
plane.

The sag in the rope is caused by the weight of the rope itself.

Cookie


At 04:07 15 March 2009, Mike the Strike wrote:
I took a good look at the geometry of my tow today. The tow line
appeared to be pulling down on the nose at about an angle of twenty
degrees. At 65 knots, the tow line formed a catenary to the towplane
with significant sag in the line.

It should be possible to model the angles of the line and angles of
attack of the towplane and glider, but I suspect some of the
simplistic arguments have not explained the phenomenon because they
haven't taken full account of the complex geometry of the tow.

At around 50 to 55 knots, I am unable to maintain high tow and sink
into low tow with no elevator authority and reduced aileron control.
My free-flight stall speed is below 40 knots. Gurus please explain.

Mike

On Thu, 12 Mar 2009 22:09:24 +0200, Bruce wrote:

Bruce wrote:

In case anyone thought that the MG/UL has too little grunt. Follow the
link below.

The story behind it is - experienced pilot in getting current again on
Janus forgot the thing has a drogue chute. On a check flight the Janus
is towed up from an airfield (Orient) at 5100" MSL at around 20
centigrade.
As you can see the Samba accelerates slowly - then the drogue chute
deploys. (one bump too many )
Tuggie happens to be enormously experienced CFI - uses his head and
abuses the Rotax a little. Combination manages to make a circuit and the
Janus gets dropped over the threshold. Samba engine did not over heat or
suffer any apparent damage, although it was kept in the time limited
maximum power range for the entire circuit. For information , low down
there are very few options straight out on 36 that would not include
reducing the Janus to kit form if he had been dropped before getting
back to the runway.

http://www.youtube.com/watch?v=4_w3n...eature=related

I actually witnessed that incident from a vantage point midway down the
runway. I did not notice at the time anyone with a video camera but you
can't be to careful these days!

I was not expecting to see two sets of wings climbing out from the dip
and the end of the runway. I realy thought we would be picking up broken
fibre glass that morning. One thing you missed out in the commentry, the
Janus' radio was also discovered to be not working. So the pilot did not
hear radio calls advising him of the situation. The tug pilot did hear
the radio excitement but became aware of the situation when he saw the
shadow of the drag chute. The glider pilot first became aware of the drag
chute when he climbed out of the glider after the landing.

The Samba, with its low mass, low drag airframe coupled with a VP prop
certainly has enough excess thrust to provide the aircraft at the end of
the rope a decent climb rate. The limitations a

- Low power to mass of glider + tug combination, leads to slow
acceleration.

- Low mass of tug relative to glider leaves little control authority for
handling out of possition gliders on tow.

- VP prop is a high maintenance item.

- The Samba airframe is designed to an AUM limit of 450kg (including two
pilots, engine and fuel). It is very light, has lots of carbon and is
strong in all the right places, but it has limited resilience for
handling club abuse which can test things in ways that the designer did
not anticipate.

Our club decided not to buy one. We did get feedback from other operators
including those at Gariep. Perhaps if you have an owner/tuggie operation
and you tow mainly experienced pilots in single seaters, it is a good
option.

For a club operation with a whole group of tuggies sharing the duties of
towing 2 seater trainers with a whole bunch of different instructors and
every level of student, I think the Samba may have a hard time.

Does anybody know what the certification situation is for towing with
ultralight catorgory aircraft (450kg AUW) is in Europe these days? How
many tow planes are certified for tugging and what mass of glider are
they permitted to tow?

Ian

On Sat, 14 Mar 2009 18:08:23 -0700, The Real Doctor wrote:

The behaviour of the Pirat on aerotow is rather different, by the way.
At low speeds it handles very nicely, but if the tuggy is a bit
enthusiastic the ailerons get horribly heavy and ineffective. Quite the
opposite of wallowing, really. At the same speed off tow they are light
and responsive. I'm guessing that the tug downwash affects the centre of
the wing more, effectively increasing the washout. Meanwhile, I just
cajole tuggies into flying with the CHT just below the red ...

How does the downwash angle vary with position along the wing span? Since
the lift distribution varies across the span its unlikely that the
downwash angle is constant. As an additional factor, even a single seat
glider's span is larger than the tugs span, so the glider's tips (and
ailerons) will be in a somewhat different airflow to the inboard sections
of its wing.

If the downwash angle behind the tug reduces as you move outboard along
its wing then that's effectively washin at the glider's tips, i.e. an
increased AOA, so during slow tow the glider may be nearly tip stalling,
which would account for the symptoms people have mentioned: poor aileron
response and apparent loss of lift. However, it doesn't explain the
behavior of Ian's Pirat, so I must have missed something obvious.


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

On 15 Mar, 04:07, Mike the Strike wrote:

At around 50 to 55 knots, I am unable to maintain high tow and sink
into low tow with no elevator authority and reduced aileron control.
My free-flight stall speed is below 40 knots. *Gurus please explain.

Type and hook position, please ...

Ian

On 15 Mar, 05:30, Bob Cook wrote:

If the tow line formed a 20 degree *angle to the glider, (from tow plane
upward to glider) the glider would have to be 72 feet *ABOVE the tow
plane.

Only if the tow rope were straight. He has already said that it was in
a clearly visible catenary curve.

Ian

Stick well back and won't further raise the nose, controls ineffective and
a feeling of 'wallowing'. All classic symptoms of being close to the
stall! If you tow on a belly hook (all my own glider has) the stick has be
further forward to prevent the glider trying to 'winch launch' and
directional control during the ground run is a bit more difficult, but the
other symptoms stay the same.

Derek Copeland

At 00:45 15 March 2009, Big Wings wrote:

Hmmm. In a glider with C of G in the correct place the tail plane will
be
producing a down thrust, hence the wing will need to generate lift
greater
than the weight of the glider when in more or less level flight. The
more
back stick, the more down thrust and the harder the wing will need to
work
to maintain level flight.

Now my Discus turbo stalls in free flight in the low 40s (kts). With
the
engine out and the thrust line well above the fuselage, thus pushing the
nose down, the best climb speed in the POH is given as 49 to 54 kts
depending on weight. If I let if fall to about 45 kts it is still
perfectly controllable - just less efficient. However aerotowing on the
nose hook at that speed would not be a happy experience. I have never
done a slow tow on a belly hook so don't know if the symptoms are the
same.

What I have observed on slow tows, and has been reported by others in
this
thread, is that the ailerons are ineffective, the glider tends to
wallow,
the stick is a long way back and the nose high - even though the speed
is
above the normal 1G stalling speed. These seem to be symptoms of an
accelerated stall or incipient spin - but the rope pulling ahead seems
to
stop the glider rotating into a spin with the associated wing drop.

I wish I knew the answer, but if I have the symtoms of an accelerated
stall in more or less straight level flight above the 1g stall speed
that
sugests that the wing is generating more lift than the weight of the
glider for some reason; if the stick is near the back-stop at 50 kts
there
is more elevator downforce so the wing will have to compenstate for that.

The pull from the rope may be slightly down if I'm in high tow,
especially with a long heavy rope with a slight bow in it, but to raise
the stall speed from say 42 to 50 kts (* 1.2) is equivalent to an
increase
in load on the wing from 1g to about 1.4g. At a take-off weight of say
475
kg this is equivalent to an additional 190kg or another 418 lb. Where
could this come from??? The downthrust from the elevator fighting the
tow
rope?