Winch Launch Safety Study

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.

If you get such a launch, even holding the stick hard forward all the time
from 'take up slack' will not prevent the over-rotation, although it
might slightly reduce your chances of instant death from a flick spin.

Such events can be easily avoided by by controlling the winch so that the
ground run acceleration is not more than about 1.0g (or less for the types
mentioned above). This will still get you airborne and climbing within
about 3-4 seconds, which most pilots find quite fast enough! It will also
make very little difference to the achieved height.

Derek Copeland

At 22:41 19 March 2009, bildan wrote:
As the glider leaves the ground, the inertial rotation will begin but
if the acceleration continues, the elevator effectiveness will also
continue to increase with the square of the airspeed. Rotation can't
happen instantly because the glider has mass and rotational inertia.
In fact, the pilot has to start backing off the down elevator to allow
the glider to rotate into the climb.

Now, contrast this with a slow acceleration. The glider staggers into
the air and the nose-up inertial couple starts the rotation but the
low speed and acceleration doesn't provide adequate control. The nose
rises as the pilot struggles to control it with inadequate
airspeed.....

I've collected stories on this type of accident for decades and they
ALWAYS happen with slow acceleration.

Sorry, I forgot to add that there are a few types where holding the stick
hard forward to contain an over-rotation is not a good idea, as you can
stall the elevator/tailplane and make the situation even worse! These are
mostly gliders with all-flying tailplanes, such as the Standard Cirrus.

Again, all these unpleasantries can be avoided by just a slight moderation
of the ground run acceleration, if you have a powerful winch.

Derek C


At 13:00 20 March 2009, 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.

If you get such a launch, even holding the stick hard forward all the
time
from 'take up slack' will not prevent the over-rotation, although it
might slightly reduce your chances of instant death from a flick spin.

Such events can be easily avoided by by controlling the winch so that
the
ground run acceleration is not more than about 1.0g (or less for the
types
mentioned above). This will still get you airborne and climbing within
about 3-4 seconds, which most pilots find quite fast enough! It will
also
make very little difference to the achieved height.

Derek Copeland

Dick Johnson said in 2001:

In my opinion it is reasonably safe to ground launch a Phoebus A,B, or
C sailplane as long as the initial acceleration is not too great. The
all-moving horizontal tail on these sailplanes have more propensity to
stall than those of sailplanes equipped with conventional fixed
stabilizer/elevator configurations. When the all-moving stabilizer
stalls during an excessively high acceleration take-off, pushing the
control stick forward usually just puts the horizontal tail deeper
into its stall, and therefore it is even more ineffective.

At low airspeeds, the nose up couple produced by a strong tow rope
pull and a high sailplane CG location can easily overpower the
aerodynamic nose down capability of an elevator control system, and
that leads to an uncontrollable nose up pitch during take-off. In a
number of instances, this has lead to the sailplane stalling and
crashing; usually with fatal results. The Phoebus is not the only
sailplane that is susceptible to this fault, so be careful of high
initial tow acceleration - auto as well as winch. I have been there!
Dick Johnson

On Mar 20, 4:52*pm, wrote:
Dick Johnson said in 2001:

In my opinion it is reasonably safe to ground launch a Phoebus A,B, or
C sailplane as long as the initial acceleration is not too great. The
all-moving horizontal tail on these sailplanes have more propensity to
stall than those of sailplanes equipped with conventional fixed
stabilizer/elevator configurations. When the all-moving stabilizer
stalls during an excessively high acceleration take-off, pushing the
control stick forward usually just puts the horizontal tail deeper
into its stall, and therefore it is even more ineffective.

* At low airspeeds, the nose up couple produced by a strong tow rope
pull and a high sailplane CG location can easily overpower the
aerodynamic nose down capability of an elevator control system, and
that leads to an uncontrollable nose up pitch during take-off. In a
number of instances, this has lead to the sailplane stalling and
crashing; usually with fatal results. The Phoebus is not the only
sailplane that is susceptible to this fault, so be careful of high
initial tow acceleration - auto as well as winch. I have been there!
* Dick Johnson

Careful, Dick Johnson was a very old personal friend of mine. I
discussed this with him on many occasions and his opinion in no way
differed from mine.

As I have said many times, There are a very few gliders with control
difficulties and the Phoenix is one of them. Most of the other
problem glider were of that generation or earlier. There are very few
of them left.

Just because a few gliders have a problem is no reason to operate as
if they all do.

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.

If you get such a launch, even holding the stick hard forward all the time
from 'take up slack' will not prevent the over-rotation, although it
might slightly reduce your chances of instant death from a flick spin.

Such events can be easily avoided by by controlling the winch so that the
ground run acceleration is not more than about 1.0g (or less for the types
mentioned above). This will still get you airborne and climbing within
about 3-4 seconds, which most pilots find quite fast enough! It will also
make very little difference to the achieved height.

Derek Copeland

At 22:41 19 March 2009, bildan wrote:

As the glider leaves the ground, the inertial rotation will begin but
if the acceleration continues, the elevator effectiveness will also
continue to increase with the square of the airspeed. *Rotation can't
happen instantly because the glider has mass and rotational inertia.
In fact, the pilot has to start backing off the down elevator to allow
the glider to rotate into the climb.

Now, contrast this with a slow acceleration. *The glider staggers into
the air and the nose-up inertial couple starts the rotation but the
low speed and acceleration doesn't provide adequate control. *The nose
rises as the pilot struggles to control it with inadequate
airspeed.....

I've collected stories on this type of accident for decades and they
ALWAYS happen with slow acceleration.

And just as often I've challenged you to produce ANY engineering data
to support your mere opinion. You have not and I believe you cannot.

Bill,
Aside from your rude remarks and your inability to admit your mistakes
on several points (an automatic transmissions do not operate like a
cruise control), etc.....I have only posted that an article on winch
safety came out and was making people aware of it, nothing more. And I
am not the author either. However, the accident stats and their
carefully determined causes are what they are. So are the real world
experiences in a variety of gliders described in this thread suffering
from too strong initial launch acceleration. And they were not only
with types of planes that you think it can only happen to.

Dick Johnson's own words:

"...be careful of high initial tow acceleration..."

Now let me ask you a question for once: What rope tension, measured in
g do you recommend for the first seconds of the launch? Or do you
advocate RPM controlled winches over tension controlled ones?

Bill,
I also wonder if you have considered gusts and turbulence on elevator
authority. The winch RPM speed is not affected by this and will
continue accelerating if that 's what you want, when gusts or
turbulence affect the elevator. Even Tension controlled winches may
not be able to reduce tension quickly enough if this happens. That's
why the initial tension should not be too high to begin with.

I would like to hear your description of how winch settings should be
with some metrics. I know you advocate high initial tow acceleration
launches. But that doesn't really tell me the specifics.

On Mar 21, 9:29*pm, wrote:
Bill,
I also wonder if you have considered gusts and turbulence on elevator
authority. The winch RPM speed is not affected by this and will
continue accelerating if that 's what you want, when gusts or
turbulence affect the elevator. Even Tension controlled winches may
not be able to reduce tension quickly enough if this happens. That's
why the initial tension should not be too high to begin with.

RPM is of interest only as far as it is necessary to protect the
engine. It is of no interest in the launch profile. Properly
engineered automatic tension control winches can for all practical
purposes INSTANTLY reduce or increase tension in response to gusts.

I would like to hear your description of how winch settings should be
with some metrics. I know you advocate high initial tow acceleration
launches. But that doesn't really tell me the specifics.

Tension, and therefore acceleration, is limited by the prescribed weak
link strength to no more than about 1.2 G. George Moore has
extensively studied this and shows that a slight tension reduction at
rotation is needed although more actual measurements are needed to
validate that.

On Mar 21, 9:00*pm, wrote:
Bill,
Aside from your rude remarks and your inability to admit your mistakes
on several points (an automatic transmissions do not operate like a
cruise control), etc.....

I didn't say anything remotely like that. I said they ASSIST the
driver in maintaining speed by increasing torque multipication when
climbing hills.


Now let me ask you a question for once: What rope tension, measured in
g do you recommend for the first seconds of the launch? Or do you
advocate RPM controlled winches over tension controlled ones?

I advocate tension controlled winches and the tension should be the
maximum safe value.

Bill said:
"... the tension should be the maximum safe value."

OK, let me try this again. Maybe you'll understand this question:

1. In your opinion, what is the maximum safe value expressed in g, for
he initial launch phase?
2. Who else agrees with your position that the initial launch phase
should be aggressive? In this thread alone, nobody agrees with you.
Where are all these people? Mr. George Moore has also said he
advocates a slower launch than you do on the Yahoo groups site.
"Bill has advocated higher ground acceleration rates than I am
generally comfortable with."

I haven't heard a single person support what you advocate. Whatever
you find, please with verifiable quotes.

- And I don't know were your get your sense of what rude is, but
publicly and falsely claiming I am misrepresenting my identity without
any proof on your part what so ever, certainly is rude behavior.