Safety of winch launch vrs. aero tow?

On Thu, 30 Oct 2003 10:11:18 +0000, Robert Ehrlich
wrote:

Bob Johnson wrote:

As long as we're sitting around the campfire and also to show you I can
go both ways, I 'member a time only a couple of years ago that I
experienced the dreaded aerotow line break at 200 ft and 60 kt over the
outbound fence.

I looked out front and it wasn't too exiting, so I gingerly turned
ninety to the left and the scenery looked some better, but not the
greatest, so I REALlY gingerly gave it another ninety to the left and
was really impressed this time as I found I was perfectly lined up with
the takeoff runway. And I recall I hadn't lost too much of my original
60 kt. Will wonders never cease!!

I thought "OK now God, you've made it possible for me to do this little
magic trick perfectly the first time I tried it in front of all my
friends and hangar bums, give me your hand again and let's try just one
more."

So I pulled spoilers, checked gear down and rolled up to my exact
takeoff spot.

And as I popped the canopy, my good friend who shall remain nameless,
said "S--t!, you've gone and lost us another Tost ring, somebody go back
to the hangar and see if they can find another tow rope". It was his
turn to tow, so I pushed back. Nobody else said a word.

Safety lecture from a dummy follows:

I don't remember to this day why I ninetied to the left. During the
previous year's biannual when Juan Batch pulled the plug on me over his
outbound fence, turning right was the correct choice because in that
direction lay the wind, which blows one back over the airport. This
improves the scenery like you wouldn't believe.

When I tried the trick for real solo, the wind lay to my left. I'd like
to think it was instinct. But I believe it was a coin toss.

Anyway, thank God. And Juan.

It Depends

BJ


This raises the interesting question of the height loss during a 180 degrees
turn in a glider or an airplane with a dead engine. I recently had a dicsussion
about that with a friend who is a power pilot and on this occasion made again a
small computation I had already made on this matter. As I never have seen
these results elsewhere, I think it may useful to show that here. Assume
you fly your turn wit an angle of attack which correspond to the speed V
when flying straight and wings level, and that the vertical sink speed
in the same conditions wuold be Vz, then during this 180 degrees turn
flown with a bank angle phi, the height loss is pi*V*Vz/(g*sin(phi)*cos(phi)),
and the turn is flown at speed V/sqrt(cos(phi)). The optimum (minimal
height loss) is when sin(phi)*cos(phi) is maximum, i.e. phi = 45 degrees,
and the product V*Vz is minimum. A glance on a typical glider polar will
show that this last thing is obtained with V just below min sink speed, but
as it is not easy to find how many below, let's assume the turn is done
at min sink speed, this is not very far from the optimum. For a typical
glider with min sink of .6 m/s at 80 km/h (22.2 m/s) the height loss is
8.5 m, for a typical airplane with min sink of 3 m/s at 120 km/h (33.3 m/s)
the height loss is 64 m. This explains why the 180 degrees turn back to
the runway over the outbound fence succeeds in a glider but not in a power
plane.

In the case mentioned above, the speed (60kt) was far over the optimum,
however the result is as expected not catastrophic. Assuming a bank angle
of 45 degrees, the equivalent speed in straight flight would be multiplied
by 1.18, this gives 26 m/s or 93 km/h. Assuming the sink speed is 1 m/s in these
conditions, we get a height loss of 16.6m. This is for a poor glider (L/D =
26 at 93 km/h).

Thanks for that. A most informative calculation and certainly matches
my most recent relevant experience.

The last time I was having a supervised aero-tow refresher in our
Puchacz I was doing a running commentary for the instructors benefit
and as soon as I said "400 ft - no problem now from a rope break" BANG
as he pulled the release. We had a touch over 60 kts and as soon as I
saw the rope go I pulled a 45 degree banked 180, keeping the 60 kts
just as Bob described, and was amazed at how easily we got in over the
fence. In fact, once I'd rolled out it looked like a normal approach,
so I opened the brakes and did a typical Puchacz approach and landing.

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

If someone wanted to do the world of ground launch a great favor, they would
start a web site where the collective wisdom of the world could be displayed
so that anyone wishing to undertake winch launch could go there and get an
education.

Ok it may not be the collective wisdom of the world. But there is
quite a bit of information on winch launching at:

http://www.northwestsoaring.com/sitemap.shtml

Let me know if I am missing anything really important about Winch
Launching here.

Brian Case
CFIIG/ASEL

"Brian Case" wrote in message
om...
If someone wanted to do the world of ground launch a great favor, they
would
start a web site where the collective wisdom of the world could be
displayed
so that anyone wishing to undertake winch launch could go there and get
an
education.

Ok it may not be the collective wisdom of the world. But there is
quite a bit of information on winch launching at:

http://www.northwestsoaring.com/sitemap.shtml

Let me know if I am missing anything really important about Winch
Launching here.

Brian Case
CFIIG/ASEL

Good site - congratulations.

About your 2003 procedures - you must have had a bad experience with gusty
wind conditions. This is why I have been advocating airspeed telemetry.

Bill Daniels

About 20 years ago I was doing an aerotow from the
front seat of a Janus at Lasham with Derek Piggot in
the back during a cross country course. We were just
off the deck within the airfield waiting for the tug
to start to climb and I heard Derek say '60 knots'.
Later I asked him why he said that out loud and he
told me that he always made a note of when the speed
reached 60 knots because he reckoned that at that speed
he could pull up and turn back without loss of height.

John Galloway


At 14:00 30 October 2003, Martin Gregorie wrote:
On Thu, 30 Oct 2003 10:11:18 +0000, Robert Ehrlich
wrote:

Bob Johnson wrote:

As long as we're sitting around the campfire and also
to show you I can
go both ways, I 'member a time only a couple of years
ago that I
experienced the dreaded aerotow line break at 200
ft and 60 kt over the
outbound fence.

I looked out front and it wasn't too exiting, so I
gingerly turned
ninety to the left and the scenery looked some better,
but not the
greatest, so I REALlY gingerly gave it another ninety
to the left and
was really impressed this time as I found I was perfectly
lined up with
the takeoff runway. And I recall I hadn't lost too
much of my original
60 kt. Will wonders never cease!!

I thought 'OK now God, you've made it possible for
me to do this little
magic trick perfectly the first time I tried it in
front of all my
friends and hangar bums, give me your hand again and
let's try just one
more.'

So I pulled spoilers, checked gear down and rolled
up to my exact
takeoff spot.

And as I popped the canopy, my good friend who shall
remain nameless,
said 'S--t!, you've gone and lost us another Tost
ring, somebody go back
to the hangar and see if they can find another tow
rope'. It was his
turn to tow, so I pushed back. Nobody else said a
word.

Safety lecture from a dummy follows:

I don't remember to this day why I ninetied to the
left. During the
previous year's biannual when Juan Batch pulled the
plug on me over his
outbound fence, turning right was the correct choice
because in that
direction lay the wind, which blows one back over
the airport. This
improves the scenery like you wouldn't believe.

When I tried the trick for real solo, the wind lay
to my left. I'd like
to think it was instinct. But I believe it was a coin
toss.

Anyway, thank God. And Juan.

It Depends

BJ


This raises the interesting question of the height
loss during a 180 degrees
turn in a glider or an airplane with a dead engine.
I recently had a dicsussion
about that with a friend who is a power pilot and on
this occasion made again a
small computation I had already made on this matter.
As I never have seen
these results elsewhere, I think it may useful to show
that here. Assume
you fly your turn wit an angle of attack which correspond
to the speed V
when flying straight and wings level, and that the
vertical sink speed
in the same conditions wuold be Vz, then during this
180 degrees turn
flown with a bank angle phi, the height loss is pi*V*Vz/(g*sin(ph
i)*cos(phi)),
and the turn is flown at speed V/sqrt(cos(phi)). The
optimum (minimal
height loss) is when sin(phi)*cos(phi) is maximum,
i.e. phi = 45 degrees,
and the product V*Vz is minimum. A glance on a typical
glider polar will
show that this last thing is obtained with V just below
min sink speed, but
as it is not easy to find how many below, let's assume
the turn is done
at min sink speed, this is not very far from the optimum.
For a typical
glider with min sink of .6 m/s at 80 km/h (22.2 m/s)
the height loss is
8.5 m, for a typical airplane with min sink of 3 m/s
at 120 km/h (33.3 m/s)
the height loss is 64 m. This explains why the 180
degrees turn back to
the runway over the outbound fence succeeds in a glider
but not in a power
plane.

In the case mentioned above, the speed (60kt) was far
over the optimum,
however the result is as expected not catastrophic.
Assuming a bank angle
of 45 degrees, the equivalent speed in straight flight
would be multiplied
by 1.18, this gives 26 m/s or 93 km/h. Assuming the
sink speed is 1 m/s in these
conditions, we get a height loss of 16.6m. This is
for a poor glider (L/D =
26 at 93 km/h).

Thanks for that. A most informative calculation and
certainly matches
my most recent relevant experience.

The last time I was having a supervised aero-tow refresher
in our
Puchacz I was doing a running commentary for the instructors
benefit
and as soon as I said '400 ft - no problem now from
a rope break' BANG
as he pulled the release. We had a touch over 60 kts
and as soon as I
saw the rope go I pulled a 45 degree banked 180, keeping
the 60 kts
just as Bob described, and was amazed at how easily
we got in over the
fence. In fact, once I'd rolled out it looked like
a normal approach,
so I opened the brakes and did a typical Puchacz approach
and landing.

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

In article ,
says...
I still fail to understand why any winch or cabel failure should lead
to an accident. With or without radio. With or without wind. With or
without water ballast.
Be prepared.

I seem to recall some launches where the cable became tangled in the
main wheel, when the winch jerked the glider forward, then paused very
briefly. This is caused the glider to pitch up too fast at the start,
and the pilot was unable to release, leading to a crash. Perhaps this
is not what you mean by a winch failure?

--
!Replace DECIMAL.POINT in my e-mail address with just a . to reply
directly

Eric Greenwell
Richland, WA (USA)

On 29 Oct 2003 23:17:50 -0800, (Marcel
Duenner) wrote:


I still fail to understand why any winch or cabel failure should lead
to an accident. With or without radio. With or without wind. With or
without water ballast.
Be prepared.

100% agree.

I sometimes get the impression that in case of a glider-winch radio
connection the glider pilot tends to rely on his radio commands
("faster" - "slower") to correct a sub-optimum situation. Which
obviously not works in all cases and then might lead to an even worse
situation.

In case of no glider-winch connection the glider pilot completely
relies on himself and takes the according action if a certain
parameter (e-g. speed) exceeds the (his) limits.
Bye
Andreas

What Eric describes has happened, but the way to eliminate that event is
to sheathe the cable with plastic hosepipe of suitable size at the
glider end, so it is too stiff to wrap round the axle (or even enter
the wheel box area). It is one of many things where the technology and
operational procedures have been modified and developed over the years
to reduce accidents and incidents to the minumum, leaving only pilot
error/failure to act as trained as the remiaining significant factor.

Chris N.


Eric G: "I seem to recall some launches where the cable became tangled
in the
main wheel, when the winch jerked the glider forward, then paused very
briefly. This is caused the glider to pitch up too fast at the start,
and the pilot was unable to release, leading to a crash. [snip]"

Hi Bruce --

That's a very welcome correction to my hazily-remembered version of a
second-hand report of what the 454 c.i. engine torque/hp/rpm chart looks
like. This is the kind of info I was looking for and thanks for providing it!

In all our past kicking around of the ideal winch prime mover, here's one
that sounds silly but might rate at least an engineering investigation -- a
recip steam engine! If I recall correctly, the steam engine develops max
torque at stall.

What got me thinking about that was the fact that the Navy gets 66,000 lb
Super Hornets flying in about the same three seconds that it takes us to get
airborne. And they do it with steam, not because it's handy, but that's
probably the only practical way to get it done.

BJ

Bruce Hoult wrote:

In article ,
Bob Johnson wrote:

Our engine is petrol fueled. Despite Google's best efforts, I have not
yet located a Torque/HP/RPM curve for our very common 7.4 L engine, but
have heard that it develops max torque and HP at about 3000 RPM and
further that the curves are fairly flat at this point.

You are making some totally contradictory and inconsistent claims there.

If max torque and max HP occur close together then they must both drop
off precipitously after that.

If the torque curve is flat then HP will be increasing linearly with
RPM, max torque and max HP will be very far apart.

It is quite likely that you do have maximum torque at around 3000 RPM,
but if for example the torque curve is flat enough that the torque at
the 5000 RPM redline is still 60% or more of that at 3000 RPM then that
(redline) is exactly where maximum power will be.

-- Bruce

On Thu, 30 Oct 2003 07:49:51 -0800, Eric Greenwell
wrote:

In article ,
says...
I still fail to understand why any winch or cabel failure should lead
to an accident. With or without radio. With or without wind. With or
without water ballast.
Be prepared.

I seem to recall some launches where the cable became tangled in the
main wheel, when the winch jerked the glider forward, then paused very
briefly. This is caused the glider to pitch up too fast at the start,
and the pilot was unable to release, leading to a crash. Perhaps this
is not what you mean by a winch failure?

That should be a recoverable situation provided that the signalling
channel between launch point and winch can convey three messages:

- take up slack
- all out
- STOP

If, as it appears to be the case at Torrey Pines, the headlamp signals
can't be used to signal STOP then you have an accident waiting to
happen. The launch marshal must ALWAYS be able to signal STOP and be
obeyed without question. Doesn't matter whether the channel is radio,
telephone, coded light flashes or signalling bat provided that it can
transmit those three commands unambiguously.

On the sites where I've winch launched an immediate STOP is signalled
if the glider overruns the cable for any reason. The reason we use the
three phrases listed above (repeated continuously) is that they have
three, two and one syllable and so can be distinguished despite noise
in the winch cab and/or wind noise in the launch marshall's
microphone.

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

Bob Johnson wrote:

Robert -
...
You may be using a which could account for your good performance
at lower revs.
...

Yes, it's a Diesel, with no gearbox, only a reduction box and and
hydraulic (or should be called "oilic" :-) couple converter.