How to get maximum height on a winch launch?

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.

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?)

We set the trim well forwards to give approach speed in the event of a
launch failure (I am not going to stop doing that, but hypothetically
where would you put the trim for best launch height?), and for K21s
winch drivers are briefed to give full throttle the whole way up (winch
is c.300bhp).

I've also heard several pilots be admonished by instructors recently
for "not pulling back" after somewhat low launches. Surely pulling back
increases the lift the aircraft is producing, and therefore gives more
height? If the airspeed is well below placard max winch speed, I would
have thought it would not result in enough force to break the link.

BTW we do, without fail, use the correct weak links (and not doubled-up
or anything stupid either). That said, we do break weak links with some
regularity.


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.

Heh, I have Condor but find it much harder to fly than "real life",
probably as my comp is rather weedy so it's a bit jerky. Thanks for the
suggestion though!


Dan


Bill Daniels

"Dan G" wrote in message
oups.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.