How to get maximum height on a winch launch?

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