Winch Launch Safety Study

On Mar 20, 7:00*am, Derek Copeland wrote:
As I have tried to explain to Bill on numerous occasions, and on various
forums, the uncontrollable over-rotation you get from an over-powered
winch launch is caused by the pull line to the belly hook being below the
glider's centre of gravity and centre of pressure. This causes a rotation
for mechanical rather than aerodynamic control reasons. This is why high
winged gliders, such as the K6 and K8 are most at risk.

I understand this exactly and have wrote about it for many years.
Almost all gliders currently in use have no risk at all of
"mechanical" over-rotation. Does a G103 have "Uncontrollable Over-
rotation"?

The "Over powered" launches you speak of are strictly limited by the
weak link to slightly over one G which is a trivial value. Nothing
violent happens at one G unless the pilot initiates it.

No a Grob G103 is not prone to 'uncontrollable over-rotation' after
lifting off on a winch launch, because it has low set wings, hence a low
centre of gravity, a slightly forward set belly hook, and it's quite
heavy. Depending on the model, it either launches on a relatively weak red
or brown link, so it is prone to weak link failures, as is identified in
the Aerokourier report. So you need to be careful about overpowering it
for a different reason.

However there are many types that are prone to over-rotation, particularly
K6, K7, K8, K13, Standard Cirrus, and many other types of single seater.

In that it is very rare to break a weak link during the ground run,
however violent, it is likely that you can get much faster acceleration
than 1.2g. I think this is because of the fact that the glider is on
wheels and unloads the weak link as it rolls and accelerates forward. If
you where pulling against a concrete block set into the ground, you would
break the weak link every time at the same force. If the weak link does
break as a result of an overpowered launch, it usually does so sometime
during or just after the rotation, which is at the worst possible time! If
you drive a winch (which I do) you can feel the load on the cable increase
as the glider rotates, even if the throttle or tension setting remains
constant.

Have a look at the following video showing a K13 being winch launched:

http://www.youtube.com/watch?v=KUsqw0OwMZA

Note that after a slightly protracted 'take up slack' the tailwheel goes
down onto the ground when the winch driver opens the throttle and stays
there until the glider lifts off, if anything slightly mainwheel first.
This is despite the elevator being held slightly lower than neutral (stick
forward of centre), as it should be. If you have a K13 available, try
sitting two 180lb men plus parachutes in it and then push down down on the
tail until the tailwheel touches the ground. You will find that this
requires quite a lot of force, which illustrates just how powerful the
rotational couple must be.

Derek Copeland


At 16:48 24 March 2009, bildan wrote:
On Mar 20, 7:00=A0am, Derek Copeland wrote:
As I have tried to explain to Bill on numerous occasions, and on
various
forums, the uncontrollable over-rotation you get from an over-powered
winch launch is caused by the pull line to the belly hook being below
the
glider's centre of gravity and centre of pressure. This causes a
rotation
for mechanical rather than aerodynamic control reasons. This is why
high
winged gliders, such as the K6 and K8 are most at risk.

I understand this exactly and have wrote about it for many years.
Almost all gliders currently in use have no risk at all of
"mechanical" over-rotation. Does a G103 have "Uncontrollable Over-
rotation"?

The "Over powered" launches you speak of are strictly limited by the
weak link to slightly over one G which is a trivial value. Nothing
violent happens at one G unless the pilot initiates it.