On Fri, 26 Mar 2021 13:37:25 -0400, Moshe Braner wrote:

On 3/26/2021 12:25 PM, ProfJ wrote:
On Thursday, 25 March 2021 at 08:33:54 UTC-6, Bob W. wrote:
On 3/24/21 8:56 PM, ProfJ wrote:
Hi Folks,

There's a lot of focus at the moment on wing dropping during the
ground roll phase. I did my primary training on winch launches. For
those who have never winch launched, it's quite an experience being
in a 900 lb. vehicle which is accelerated by a 300hp motor which is
fixed to the ground and connected by a cable, i.e. no slip anywhere.
One of my co-trainees was a large and apparently fearless
professional rugby player; some time later he admitted to me that he
had kept his eyes tightly closed for his first half dozen launches.

As a way of dealing with the sensory overload of the first few
seconds of a winch launch, I used a protocol which was based on what
I could control. It went like this:

In the first second of roll, there is only one control which has any
authority, and that is the cable release. You have to release if the
glider jerks forward and overruns the cable. So, have a good grip on
the release and the only thing you have to think about in the first
second is, should I release? None of the other controls have any
effect at this speed, so don't think about them.

In the second second of roll, the rudder starts to have authority.
The initial jerk probably pulled the glider offline. So get back on
line. Given the yaw/roll coupling, this will probably help raise any
falling wingtip.

In the third second of roll, the elevator has authority. So raise the
tail (or nose, depending) and get balanced on the main wheel.

You might question, why control yaw then roll? Even with two wheels
on the ground, there is some yaw control, and the initial launch jerk
probably has the tail bobbing up and down anyway, so don't worry
about pitch yet. Get in line.

In the fourth second of roll, the ailerons finally have some
authority and you can level the wings. By that time, with a winch,
you're probably airborne anyway.

This was helpful to me because it gave a structure for dealing with a
very fluid and rapidly evolving situation.

The main reason for describing this, is that it requires you to have
a grip on the release and understand that it's a primary control
during ground roll; certainly the only one that works in the initial
roll.

Just my two cents...

I see others have provided feedback related to the "weak point" in
your logical - and IMO fundamentally sensible - reasoning. My
knee-jerk mental response to your logic was along the lines of: It's
sensible in basic theory, but likely "only minimally applicable" to
most of the real (winching) world.

It would take some combo of a weak
winch,"leisurely-winch-operator"/lacking-headwind to yield a
4-second-to-liftoff winch launch in my direct/observational
experience...writing as one whose first experience with a winch was a
1948, Buick straight-8-powered, old/tired when our paths crossed
baby...and *IT* easily, consistently, bettered 4-seconds-to-liftoff
launches of loaded 2-33s.

In any event, if your logical/linear reasoning worked for you -
awesome!
Mebbe the problem lies in translating reality to words, I dunno. But
sensory-input overload is an inescapable hurdle teaching/experiencing
winch launching contains to be sure. There's no substitute for
experience...assuming a person survives it.

YMMV.

Bob W.

It's kind of fun to have all the experts invalidate my actual lived
experience, and also physics. Let's go watch some videos of winch
launches, shall we?

Here's Bruno Vassel launching
https://www.youtube.com/watch?v=VrUT...el=BrunoVassel .
Due to the wonders of actual video evidence, we see the glider taking
seven seconds from first roll to airborne.
Here's one from Oxford Gliding -
https://www.youtube.com/watch?v=-eTU...nel=GreatFlyer -
nine seconds from first movement to airborne.
Here's one from Mount Beauty -
https://www.youtube.com/watch?v=SE5J...=GeorgeSkarbek -
five seconds to airborne.
Here's one - called "Rather fast winch launch" ! -
https://www.youtube.com/watch?v=-eTU...nel=GreatFlyer -
five seconds to airborne.

I didn't cherry-pick these, they were the first four I found where you
could see the glider roll. If anyone can find one showing a two-second
roll, go ahead and show us.

Yes, it seems like it's two seconds. That's the problem of sensory
overload. Apparently even the experience of 1500 launches on a variety
of airfields doesn't overcome it.

Never let mere facts get in the way of a good theory!

....but all the same, all those launches look like those we had back in
2000 when I was learning to fly. That was a Supacat with steel cables,
which take much more power to pull in when the cable is still on the
ground than the plastic cable we use now. That winch had a 250hp aircooled
diesel V8 engine that I'm told was designed for pumping water to keep
mines dry.

Now we have a 400 hp Skylaunch (V8 truck engine running on LPG and fitted
with plastic cables, and initial acceleration is a LOT faster.

We use a 1km cable: when we were using steel cable even six men pulling
it couldn't pull the cable back after an aborted launch: we just told the
winch driver the wind it in and put the glider on the next cable.
However, with the plastic cable we use now, in the same situation two
blokes can pull the cable back the the launch point without difficulty.
That large difference in the effort needed to pull the cable along the
ground makes a huge difference to the initial launch acceleration.

I think the UK and Australian videos all showed winches using steel
cable. I can't see what winch or type of cable Bruno was using, but he
did mention 8000 feet of cable while the others all looked to be quite a
lot shorter.



--
Martin | martin at
Gregorie | gregorie dot org