How to get maximum height on a winch launch?

"Bert Willing" wrote in message
...
Ian,

winch launching my Calif over 8 years on a 280 hp winch was exactly "full
power until release", and that was the only way to go.
So, in some cases this type of instructions actually does work.

Bert

Full power for the whole launch works when the winch is severely
underpowered. As more powerful winches are introduced, you will find there
are points in the launch where the driver must reduce power to maintain
constant tension.

Bill Daniels

Should say that it's "full power all the way up" the main climb, of
course we back off at the top. But there was some suprise at the launch
point recently when a K21 waved off... the common thought was "K21s
should be able to handle it". Maybe it could if it was no wind, but
with a good headwind they just can't.

So far the sole answer to my original question has been "look at the
wingtips during the launch and find what stick position
will hold them at about 45 degrees to the horizon AND
give a safe speed. If you have a low powered winch
you may have to adopt a lesser angle" from Derek.

Does everyone agree with this?


Dan

Dan G wrote:
Should say that it's "full power all the way up" the main climb, of
course we back off at the top. But there was some suprise at the launch
point recently when a K21 waved off... the common thought was "K21s
should be able to handle it". Maybe it could if it was no wind, but
with a good headwind they just can't.

Headwind makes a big difference. Our underpowered winch can overspeed
the twins easily if the wind gets above about 15knots. If indicated
airspeed is above 60knots, you're getting diminishing returns from the
launch.

Frank Whiteley

Dan G wrote:
Should say that it's "full power all the way up" the main climb, of
course we back off at the top. But there was some suprise at the launch
point recently when a K21 waved off... the common thought was "K21s
should be able to handle it". Maybe it could if it was no wind, but
with a good headwind they just can't.

And that was a surprise? Find yourself another club, man, because if
headwinds on a winch launch are unexpected your current lot are going
to damage someone one day.

So far the sole answer to my original question has been "look at the
wingtips during the launch and find what stick position
will hold them at about 45 degrees to the horizon AND
give a safe speed. If you have a low powered winch
you may have to adopt a lesser angle" from Derek.

Does everyone agree with this?

No. I think it's bad advice. Yes, you should aim to have a proper climb
angle and air speed, but the stick position is irrelevant. Just use
enough to do what's needed.

How many inches of rudder movement would you use to balance a 20 degree
per second roll into a turn? Or would you just aim to us enough to keep
the string straight?

Ian

Hi Bill, Glad to see you are coming round to my way
of thinking, that the rotation into the full climb
should be gentle and controlled, and should not put
you in an unrecoverable position if the launch fails
for any reason.

The exact position to hold the stick during a winch
launch is very type dependent. Gliders with way back
c of g hooks, such as the K21, K8 and Pirat will require
the stick to be held neutral or even forward of neutral
during the full climb. Gliders with slightly further
forward hooks such as the K13 will need the stick held
fairly well back, although probably not on the back
stop. Gliders with single 'compromise' hooks such as
the Slingsby Skylark and the Bocian will need the stick
to be held on the back stop if they are to climb at
all steeply.

The best advice I can give to Dan G is to look at the
wingtips during the launch and find what stick position
will hold them at about 45 degrees to the horizon AND
give a safe speed. If you have a low powered winch
you may have to adopt a lesser angle.

Derek Copeland

At 15:18 13 December 2006, Bill Daniels wrote:
Dan, your questions concern me. Full up elevator is
dangerous on a winch
launch. All modern

At 15:18 13 December 2006, Bill Daniels wrote:
Dan, your questions concern me. Full up elevator is
dangerous on a winch
launch. All modern trainers like ASK 21's or Grob
103's will fly an almost
perfect launch without the pilot touching the controls
if trim and winch
power are right. Demonstrating hands off launches
is a good start to winch
training. (The take home lession is the launch will
be nearly perfect if the
pilot doesn't screw it up.)

You should be using just enough elevator to keep the
airspeed where you want
it. (Typically 55 - 60 knots, depending on the glider)
The basic principle
is that the winch driver controlls power and the glider
pilot controlls
airspeed. If it requires large amounts of up elevator
to control airspeed,
signal for less power. As you point out, some gliders
with high CG
locations like a K8 will need nearly full down elevator
from the start to
control the natural pitch up. It shouldn't need much
up elevator at all
later.

The winch driver should be slowly backing off the power
as the glider
approaches the top of the launch. Take a good look
at George Moores charts
that show cable tension - tension is a direct function
of winch power.

In any event, there's very little additional height
that can be achieved
late in the launch. Pulling hard near the top just
loads up the glider and
may break the weak link. (You are using the correct
Tost weak links,
right?)

Suggestion: Get a copy of a good glider flight simulator
like Condor and
practice lauches on a computer. There's a lot to learn
with a good sim. I
use Condor as an 'animated white board' when I teach
winch ground school.

Bill Daniels

'Dan G' wrote in message
roups.com...
Thanks Bill, that document is very interesting.

There is one thing in particular I am unsure about
however - how much
up elevator I should be putting in. With the K8, I
fly the launch at
50-55kts with the stick fully back for perhaps the
last 30% of the
launch after smoothly transitioning from full forwards
on the ground
run through neutral once in the full climb, until
I feel the glider
being pulled level, when I relax the aft pressure
in anticipation of
putting the nose down to reuce cable tension prior
to releasing. (This
later movement often results in quite an increase
in speed, which I
convert to height after releasing. Is this an indication
I'm releasing
back pressure too early, or the winch driver not backing
off the
throttle enough?)

With a K21, which winch drivers here (which includes
me) are told to
give 'full power all the way up' from our Oldsmobile
V8. Climb speed is
at lesst 65kts and I'm reluctant to pull back too
hard. Why? A K21
launches on a black weak link (1,000kg). I imagine
full up elevator in
a K21 would produce enough lift to break that at well
below max winch
speed, 81kts.

Is there a way to calculate how much lifting force
full up elevator
should produce for any given airspeed? Or should I
not be putting full
up in? If not, just how far should I bring the stick
back, and when?


Bill Daniels wrote:

If you join the Yahoo group 'winchdesign' and look
in the files section,
there are a series of truely excellent papers in the
folder 'Winch
Dynamics
and Performance' by George Moore. (soargsm)

Essentially what George determened is that the major
sensitivities a
1.Field length
2. Headwind
3. Cable tension as a percentage of glider flying
weight.
4. Weight & drag of the cable

There is very little extra height to be gained by
agressive technique
beyond
just hitting your target airspeed and holding it there
with smooth
control
inputs. This gives an angle of attack just slightly
greater than that
for
best L/D. If you want help with this, put some AOA
strings on the canopy
sides and calibrate them for L/D max and Vmin sink.
Of course, it goes
without saying that you should never put yourself
in danger by rotating
into
the climb so quickly that a recovery from a rope break
is impossible.
Smooth and accurate flying are what you want.

So, if you want to get higher, get a longer field
with more wind, replace
steel with high strength Dyneema and get a winch that
can accurately hold
tension at 80 - 90% of the weak link.

Bill Daniels

'Dan G' wrote in message
ups.com...
I'm interested in learning how to get the maximum
possible height on a
winch launch, without compromising safety in any
way.

This question has two parts really - how should the
pilot fly the
launch (pretty much how should he operate the elevator
during the
launch), how fast should the glider fly, and how
should the winch
driver feed the power in? For the sake of argument,
let's assume the
glider is a K13.

For reference, I fly from an airfield with a 3,300'
long cable, and the
maximum height I achieve repeatedly is 1,700'. Headwind
typically goes
from around 10kts at the ground to something like
(I estimate) 20kts at
height.

Derek Copeland wrote:

The exact position to hold the stick during a winch
launch is very type dependent. Gliders with way back
c of g hooks, such as the K21, K8 and Pirat will require
the stick to be held neutral or even forward of neutral
during the full climb.

My Pirat will fly a very nice winch launch which is almost completely
independent of anything the pilot chooses to do with the stick! It's
not so much the fore-and-aft position of the hook, I think, as the
vertical one: since it is at the bottom of a deep fuselage there is a
considerable nose up pitching moment during the ground run. As soon as
it takes off, therefore, it does a lovely rotation into full climb all
by itself: full forward stick has no effect at all.

Gliders with single 'compromise' hooks such as
the Slingsby Skylark and the Bocian will need the stick
to be held on the back stop if they are to climb at
all steeply.

All the Bocians I have ever flown had C of G and nose hooks. The Pirat
manual specifically permits winching on the nose hook: using full back
stick and forward trim from the "All Out", and warning that it will
only give 60% of normal release height. I have never tried this, for
several reasons, one being that I rather like having the back release
there ...

Ian

Dan G wrote:

how should the winch driver feed the power in?

I received a very detailed reply in the Yahoo winch design group. With
permission, I reproduce it below:

---------- Forwarded message ----------
From: andyh418
Date: Dec 14, 2006 12:01 PM
Subject: [winchdesign] Throttle during rotation
To:

Hi Dan

There are lots of ways to do this. After training lots of drivers,
and consulting with other experienced drivers of LPG V8 winches, this
is the method I teach. People operating diesel winches may have to
use a different technique, although I know at least some drivers of
diesel winches operate like this.

1. Before launch, pick a throttle position which you estimate is
correct for an initial climb setting for the glider and wind
conditions. (In a winch with throttle guides this is easy; without
them, use your experience. Marks next to the throttle lever help you
learn)

2. At "ALL OUT", accelerate smoothly to that position in 2-3 seconds.
Keep accelerating smoothly past your chosen position if the glider
does not rotate into the climb.

3. Hold throttle steady as the glider rotates into the climb and
assess airspeed from comparison with other launches and how
quickly/hesitantly the glider rotates (and of course any signals. Too
slow, lower nose, smoothly feed in power. Too fast, yaw the glider,
smoothly back off power). Most people use how quickly the glider
moves up the windscreen of the winch to assess speed. Listening to
the engine also helps. Make small corrections if necessary.

4. If the correct initial position is chosen, you will find you can
hold the throttle absolutely still for the first 1/2-2/3 of the
launch, then you will be gradually backing off the power towards the
top, otherwise the glider will accelerate.

5. Give a nice brisk cut to idle once you have assessed the glider is
at the top of the launch, to give a positive signal to the pilot.
After release, wind in the cable.

HOW THIS CAME ABOUT?

It was found that the existing method of giving a bit of extra power
initially, then backing off as the glider rotated to avoid overspeed,
then feeding the power back in once in the full climb was an
overcomplicated process with minimal gain, but often interpreted
incorrectly and not very standard between drivers. It was also found
that these power changes were centred about a common position - the
throttle position for the first half of the launch. The recommended
technique is much simpler, and easier to standardise drivers.
Standardising drivers has obvious benefits for pilot safety and
consistency of launch.

Also, during rotation, there are many things changing. The glider is
accelerating, cablespeed increasing. As the pilot rotates, the
geometry of the launch is changing the relationship of cablespeed to
glider airspeed and increasing the load on the winch. The glider may
be climbing into the wind gradient. Introducing another changing
factor - who is driving and what they do with the throttle - is a bad
idea. Constant throttle works.

BUT WE NEED TO SLOW THE CABLE DOWN AS THE GLIDER ROTATES, DUE TO
GEOMETRY!!

Yes, but that doesn't necessitate moving the throttle. This is why
I'm such a big fan of LPG/Gas/Petrol V8's as winch engines. If the
climb throttle position is set correctly for glider type and wind,
the engine will accelerate the glider to a safe speed for rotation,
but once the glider starts to rotate into the climb, the cable load
increases. This has the effect of reducing the cablespeed for a given
throttle position, in proportion to the increase in airspeed caused
by the changing geometry of the launch. Only minor correction are
required, but a novice who just holds the throttle in the suggested
position (throttle stops or marks next to the lever) and backs off
the power towards the top will achieve a good launch.

The video of the Tost record has the engine accelerate to a high rpm
and stabilise there briefly, before the rpm reduces and the load
increases (from the engine note) as the glider rotates into the climb
and loads up the winch. After that you only hear minor corrections,
with a trend of the power and rpm gradually reducing in the top 1/2
of the launch. We can't see the throttle lever or linkage. We can
make no assumptions about how the driver is handling the power. If he
was using the technique above, it would sound like that. If he was
adding abit extra then taking it off then feeding it back in, it
would also sound like that. These corrections are very minor which is
why we have learned they are unnecessary.

Are there any days when you need to modify this technique? Only one
scenario that I am aware of, and that is when operating in strong
headwinds. In these conditions, the climb power is relatively low,
but of course we need to get the glider through the wind gradient and
into the full climb before this is the correct power to set. The
solution is easy. Simply accelerate past your target climb throttle
setting, to a position about halfway between your target climb
setting and a no headwind setting for the glider you're launching.
Then gradually reduce back to your target setting as the glider
climbs through the wind gradient.

As for how much to back off in the top half of the launch? On a still
day you wont need to back off much. On a windy day this will be a
gradual continuous process as the glider moves into stronger wind
layers. Watch the glider and listen to the engine load. Pilot
feedback via radio etc is always useful, but particularly on windy
days. Marks next to the throttle or throttle stops helps make a
quantitative adjustment after feedback. They are an invaluable aid.

Andy

Dan G wrote:
Dan G wrote:

how should the winch driver feed the power in?

I received a very detailed reply in the Yahoo winch design group. With
permission, I reproduce it below:

Interesting ... but once more, beware of rules. Lots of factors can
make differences to technique: whether the drive is direct, fluid
flywheel or torque convertor, size of drums (and consequently inertia)
and turbocharged vs normally aspirated.

Ian

I have very little practical knowledge of winching and I
still do not understand what the theoretical factors are
that determine max launch height.

Neglecting wind speed and wind gradient for the moment, as
well as safety concerns relating to rope breaks down low,
what is the technique to max out height given the changing
climb and tow line angles and geometry?

Derek Piggot writes in his book "Ground Launches , 1996, pg
34.):

“The optimum speed on the launch depends to some extent on
the wind strength with a slightly higher speed giving the
best launch height in calm conditions. Best L/D speed is
close to the optimum, with the minimum cruising speed or
speed for minimum sink being the lowest safe speed.”
Derek Piggott, Ground Launches , 1996, pg 34.

If the speed for best L/D is close to optimum, what is optimum?

The American Soaring Handbook, Vol 3, Ground Launch, 1980,
pg 32 says the following about auto tow

“The factor which determines the towing speed of the
automobile is the indicated air speed (IAS) of the
sailplane. It has been shown that the launching forces on
the sailplane are at a minimum just below the IAS for
maximum L/D of the sailplane”

Just how far below the best L/D IAS is the point of minimum
stress on the airframe?

Once we have derived the factors to max out launch height,
how do you then balance efficiency versus climb rate after
that point in the launch when you have enough altitude for
a safe recovery from a low rope break?

Pete

--

Peter D. Brown
http://home.gci.net/~pdb/
http://groups.yahoo.com/group/akmtnsoaring/

Pete Brown wrote:
Derek Piggot writes in his book "Ground Launches , 1996, pg 34.):

“The optimum speed on the launch depends to some extent on the wind
strength with a slightly higher speed giving the best launch height in
calm conditions. Best L/D speed is close to the optimum, with the
minimum cruising speed or speed for minimum sink being the lowest safe
speed.” Derek Piggott, Ground Launches , 1996, pg 34.

If the speed for best L/D is close to optimum, what is optimum?

The optimum is the best glide speed at the wing effective load factor
based on the takeoff weight of the glider and the cable tension. For
example, for an ASK-21 with a best L/D speed of 49 knots at a weight of
1300 lbs, if the cable tension is 1900 lbs, the effective load factor is
1950/1300 or 1.5, the optimum launch speed is 49*SQRT(1.5) or 60 knots IAS.

The American Soaring Handbook, Vol 3, Ground Launch, 1980, pg 32 says
the following about auto tow

“The factor which determines the towing speed of the automobile is the
indicated air speed (IAS) of the sailplane. It has been shown that the
launching forces on the sailplane are at a minimum just below the IAS
for maximum L/D of the sailplane”

Just how far below the best L/D IAS is the point of minimum stress on
the airframe?

What they appear to be discussing here is a rule of thumb for automobile
speed based on the non-adjusted best glide IAS of the glider. Given a
nominal climb angle of 45 degrees, if the automobile is driving at 50
mph (44 knots), the glider will be flying at 62 knots TAS. So, in this
case, once the glider has rotated to climb angle you'd want to drive 6
or 7 MPH slower than the gliders best L/D speed at sea level density
altitude. If the density altitude is higher, you'd want to drive faster.

Once we have derived the factors to max out launch height, how do you
then balance efficiency versus climb rate after that point in the launch
when you have enough altitude for a safe recovery from a low rope break?

Assuming the winch (or automobile) is providing a constant cable
tension, the max launch height will be pretty close to optimal if the
pilot maintains the optimum launch speed IAS as calculated above. This
should result in a climb angle for most of the launch of about 45 degrees.

Marc