Hi Bruce,

I think we agree nearly 100%.
You describe the physics just as I would do, however, I'd like to add
a couple of comments.


On Tue, 02 Jul 2013 20:25:03 +0200, BruceGreeff
wrote:

Hi Andreas

200,000 winch launches without a single incident? Not one cable break,
dropped wing, weak link overload? I would be amazed if this were so.

200.000 launches without any uncontrolled pitch up caused by
insufficient elevator authority.

Of course plenty of the typical winch-launch incidents, fortunately no
bad crashes so far.


Glider hitting the tail on the ground on launch - you say no problem
ever. Well anecdotal evidence does not prove a theory. So let me
introduce a "Black Swan" for you.

Well... oldtimers are a different case, aren't they?

From an engineering point of view I doubt you would find any designer
who would agree that his glider's tail was designed to strike the runway
firmly hundreds of times. Maybe all German runways are soft deep grass,
but I doubt it.

I think that grass runways are an important factor here - I'd estimate
that 99.999 of the winch launches in Germany take place on grass
runways. I doubt that anyone who ever heard a Ka-8 tail crash onto a
concrete runway would not try to find a way to prevent that.

Anyway, winch drivers try to provide soft acceleration (we are using
the same standard 3 second 0.5g acceleration as you do), but from time
to time a catapult launch simply happens.

The tail always drops to the ground during initial acceleration, but
in nearly all cases this is relatively soft - definiteloy softer than
any landing.

I am not sure on your assertion that all German designed gliders are
designed to have full elevator authority at all design airspeeds. There
is a fair body of research on kiting on aerotow (at much greater speeds
than initial launch) and I recall it included a wide range of various
manufacturers aircraft. The same happens on winch.

They really are. Do the maths - even the extremely small tails of
an ASW-27 have plenty of lift to control any pitch-up momentum caused
by the tow rope. At least if the correct weak-link is used.
If the latter is (as it seems to become en vogue these days) much
stronger than certified, the bets are off.

Kiting on aerotow is an entirely different matter.
The problem there was not the inability to get the nose of the glider
down, the problem was the very quick climb that followed a nose-up
input on the stick.
If the pilot reacts halfways quickly, he is always able to arrest the
kiting (as proven by thousands of successful aerotows in belly-hook
only Ka-6s each year).

So - if the acceleration is such that the couple caused by the vertical
distance between the hook and the CG exceeds the available controlling
couple that the elevator can generate, the aircraft will rotate
uncontrollably. That is why many gliders drop their tail on the take off
run.

I disagree.
They all drop the tail, don't they?

As long as the glider is slower than liftoff speed, there is of course
insufficient aorodynamic force on the elevator, and of course the
glider drops its tail if there's a halfways decent acceleration. At
liftoff speed it would easily be possible to push the tail up with
forward stick.

I don't think I have ever seen a winch launch in person where the tail
was not on the ground.



If the acceleration through this uncontrollable part of the regime
happens entirely on the ground, then the glider reaches sufficient speed
for elevator authority with the ground contact preventing undesirable
rotation.

Indeed. This is what ahppens 100% of the time in my opinion.
And once airborne, it is the elevator authority that prevents pitch
up.



If the launch is (a little slower) close to the limit as the
glider leaves the ground, the reduction in drag as the wheel and cable
lift, can give the little extra acceleration. Then the glider is
climbing strongly, and the tail is rotating downward as the pilot moves
the stick forward.

Sorry, I cannot imagine the situation you are describing.

What I have seen is this:
- Airspeed too slow
- Winch driver gets notified and immediately increases power
- Increasing winch power causes nose-up rotation
- Pilot's reaction is too slow and too late
- So when he finally reacts, he has to arrest the nose-up rotation
first before he can lower his nose
- Things get interesting


If the tail hits the runway you have stressed the
airframe, and possibly damaged the rudder.

I dare to state that a proper held-off landing with the tail first
generates much greater forces in most cases.

How are you teaching the winch launch?
Here in Germany it is taught that the correct lift off occurs with
main wheel and tail at the same time.

All flying tails will easily stall - when elevators are at big
deflections and low airspeed (as will some other pre JAR22 gliders).

I think these flying tails (early LS-1's and Cirrus are typical
candidates) are the root of most myths of "uncontrolled pitch up
despite stick fully forward".

Perhaps we have to agree to disagree. One thing I am certain of though,
the process and experience has to be predictable and consistent. There
are enough other variables involved.

I absolutely agree.
But I honestly think that winch launching is not as difficult or
angerous as many people try to make it.


We prefer a controlled three second acceleration to maximum launch
speed. That gives a reasonably controlled 2s ground run and much lower
initial acceleration - and the correctly trimmed glider does as it is
designed to do, and flies off without pilot input.

This is exactly the same way we are doing things.


Cheers
Andreas