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.

If you have managed that many launches, with the wide variety of
equipment, and never damaged a glider, that is impressive and laudable.

I have to take issue with you - though it is only a personal opinion.

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. I have repaired a Zugfogel 3 that
returned from a nice cross country, with the rudder base split from end
to end. Pilot was warned that his tail had hit the runway on a snatched
launch, he was confident that it was no problem as it had never caused
damage before, and the strike was "not that hard". He got lucky, there
was still a couple of centimetres of fabric holding it together by the
time he landed.

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 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.

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. If you say you have gliders banging the tail down on launch then
you are saying that you are exceeding the controllable range of the
aircraft.

From a weight and balance point of view - light pilots tend to have
more problem that heavy - if the CG is aft the tendency to over rotate
is greater. From a physics point of view high wing + upright pilot tends
to have a higher rotational couple for any given longitudinal
acceleration.

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. 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. If the tail hits the runway you have stressed the
airframe, and possibly damaged the rudder. If the rate of climb is high
enough that the tail misses the runway, you are at a very high angle of
climb, at relatively low speed at very low height. If your equipment is
less than perfect and something goes wrong the consequences get a lot
more serious if you have too low energy.

Do agree on two points:

You should not have the stick in an extreme position during the launch.

All flying tails will easily stall - when elevators are at big
deflections and low airspeed (as will some other pre JAR22 gliders).
Then some time later when the AoA changes and the control un-stalls you
will get a big unexpected force.
Recipe for trouble. Again, anecdotes do not prove the physics, but I
have watched a light pilot in a Std Cirrus do this and overfly the cable
chute in the ensuing back release. Interestingly he was adamant that
this was the right way to do things despite evidence that it was not the
safest approach.

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.
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.


--
Bruce Greeff
T59D #1771