SAFE Winch Launching

On Jul 19, 1:18*pm, johngalloway wrote:
On 19 July, 16:54, bildan wrote:

On Jul 19, 4:30*am, Derek Copeland wrote:

Just to emphasize John's point about a properly set up automatic gearbox
giving smooth and imperceptible gear changes during a winch launch, see if
you can hear the gearchanges in this video of a new Skylaunch winch in
action at Lasham?

http://www.youtube.com/watch?v=riqhzcXZqzg

Derek Copeland

Opening the throttle like a little old lady will mask shifts on any
transmission. *This obviously depends a great deal on the winch
operator opening the throttle very gingerly.

Bill,

You are implying that if the throttle of a Skylaunch winch is advanced
rapidly then the autobox changes will become obvious or a problem or
whatever. *This is simply not the case.

At various times I have test driven a Skylaunch, been inside and
outside the winch looking out for gear changes (because this was
something I was initially wondering about too), and been launched. *I
have seen the throttle advanced rapidly to maximum or advanced slowly
over 3 seconds and it makes no difference to appreciation of
gearchanges. *As a personal example I had a launch recently in which
the driver (watched by a friend in the cab) for some reason pushed the
throttle rapidly through the throttle stop guide for my type and gave
me a very overpowered initial acceleration. *There was absolutely no
detectable gear change effect during the initial (very rapid)
acceleration.

John Galloway

I am very familiar with many old Gerhlein winches that use exactly the
same engine and transmission as the Skylaunch - there is essentially
no difference. Unfortunately, I'm also VERY familiar with the General
Motors TH400 transmission used in the Skylaunch as a consequence of
the old Gerhleins.

Anyone can confirm everything I write below by calling a transmission
shop - or reading a shop technical manual. In my circle it's
considered common knowledge but since this is for the Brits, it's a
long post.

To get some data I borrowed a neighbor's GM Surburban with the view to
videotaping a portable repair shop electronic vehicle scanner display
which shows the shifts. The Surburban uses the same engine and
transmission as the Skylaunch. Unfortunately, the camera couldn't
read the LCD display with the lighting available. I plan to try again
tomorrow with different lighting. I'll post the video on YouTube.

To insure constant throttle, I monitored the throttle position sensor
with the scanner. At 24% throttle, the transmission made the 1-2 &
2-3 up-shifts at 11 and 23 MPH which is normal. On level ground and
with a lightly loaded vehicle, these shifts are VERY smooth and could
easily have been missed without the scanner. Using low power can mask
shifts. Not hearing or feeling them doesn't mean they don't happen or
they don't matter - please continue reading.

The old hydraulic transmissions like the TH400 don't have any "smarts"
other than what the GM engineers designed into them. They just do
what they do. Non-electronic automatic transmissions from all
manufacturers work(ed) essentially the same. Which is not to say they
worked well since we now have electronically controlled transmissions
which work much better.

I selected a stretch of high mountain road (9000 feet elev) with a
level section leading to a moderate hill with an equal descent on the
other side to simulate a glider hitting first a thermal and then sink
during a winch launch.

Holding fixed throttle (not speed), the transmission unlocked the
torque converter clutch just as the vehicle encountered the uphill
grade seen as a jump from 1700 RPM to 2000 RPM. This is effectively a
23% downshift since the torque converter is now slipping and
multiplying torque.

As the vehicle continued up the hill, the transmission shifted out of
overdrive into 3rd gear shown by a 1300 RPM increase in RPM. Finally,
near the top of the hill, the transmission downshifted to 2nd and a
further 1800 RPM increase. The 3-2 down shift is what most people
call "passing gear".

As the vehicle topped the hill and started down, the transmission
quickly up-shifted reversing its actions on the uphill grade.

The transmission was doing exactly what it was designed to do which is
to assist the driver in maintaining speed on a hilly road by changing
gear ratios to keep engine RPM in the power band. Each downshift
increases torque multipication which, in a winch, would be seen as
increasing rope tension. Each up-shift decreases torque multipication
which, in a winch, would be seen as decreasing rope tension - assuming
a fixed throttle.

In a winch, the transmission would see the increasing rope tension
caused by a glider encountering a thermal as a hill and start
downshifting increasing the tension further. This would INCREASE rope
tension in thermals. If the glider hit sink the slackening tension
would be seen as a downhill grade and it would up-shift and DECREASE
rope tension - exactly the opposite of what is needed for a smooth,
safe launch which is to hold tension constant.

That's why I think it is very poor engineering practice to use an
automobile transmission in a glider winch. It's designed to work in
an entirely different universe. The ONLY credible reason to insist on
using one in a glider winch is their low cost and availability. The
Hydrowinch people in Colorado Springs were able to build a
computerized diesel/hydrostatic tension controlled winch for LESS than
the selling price of a Skylaunch.

If people still insist on using automatic transmissions, there are two
or three modifications that can make them less bad. The first is a
"Manual Valve Body" which allows the operator to choose a gear and the
transmission will stay there regardless. This is NOT the same as
putting the gear selector in "2" since the 1-2 up-shift still occurs
in that case. Only the MVB actually prevents automatic shifting.
Unlike getting a heavy Surburban rolling, there is no need for lower
gears when launching a glider so the transmission can be put in
whatever gear is needed for the main part of the launch.

The second modification is to control the torque converter clutch. It
should lock up the torque converter - preventing torque multipication
- just after the throttle advance starts and not unlock until the
throttle is returned to idle.

An MVB opens the door to a third modification which is to replace the
stock gears with extremely close ratio planetary gear sets. These
allow the operator to choose small changes in winch gearing to suit a
range of wind and density altitude conditions.

TH400's can be ordered on-line with all three modifications already
installed for only 3-400 hundred USD more than a stock transmission.
If you're building a winch with an automatic, that would be money well
spent.

If you disagree call a transmission shop - preferably one that deals
with drag racers - and get the facts.