Winch Launch Safety Study

Bill,
If the initial rope tension is too low, the pilot should just release
and not stagger around for long periods. Elevator authority should not
be a problem when rope tension is low. From experience, I have never
seen uncontrollable pitch ups on slow tows, only on fast ones.

On Thu, 19 Mar 2009 15:41:38 -0700, 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.

That depends on what you're flying and how lead footed the winch driver
is. I have an early (H.201) Std Libelle.

I remember having a fairly 'vigorous' launch on a calm day from a V8
Supacat. I started with full forward trim as usual but possibly with not
quite as much additional forward pressure as I normally use. The glider
started to pitch up shortly after lifting off. Applying full down
elevator maintained but didn't reduce the pitch-up rate until the full
climb attitude, when rotation eased off and I was able to ease back to no
stick pressure. This started to happen around 50 kts, the rotation rate
was acceptable and the full climb angle post rotation was steep but not
excessive. I had around 65 kts when rotation had stopped.

My Std. Libelle normally lifts off both wheels almost simultaneously,
even with full forward trim plus a bit of forward pressure. I usually
have no problems holding or reducing that attitude as the speed builds up
past 50 kts. However, on this occasion I want to emphasize that I went
fairly briskly to full forward stick as rotation started and REMAINED
THERE through rotation into full climb. At that attitude when I eased
back to the trimmed position as the rotation slowed. If the acceleration
had been higher I could have been in real trouble.

Now, I always ask to be launched "like a Junior but 10 kts slower", which
gives a nice lift-off and constant attitude until 50 kts and rising is on
the clock. At that point a reduction in forward pressure gives a nice,
controllable rotation into full climb. Easing the stick back as the
rotation slows gives a nice full climb attitude at just over 60 kts.
Vwinch is 65.

Caveat: I've never winched an H.201B, which has a bigger tail than the
H.201, so its winch behavior may be different.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

On Mar 19, 5:49*pm, Martin Gregorie
wrote:
On Thu, 19 Mar 2009 15:41:38 -0700, 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.

That depends on what you're flying and how lead footed the winch driver
is. I have an early (H.201) Std Libelle.

I remember having a fairly 'vigorous' launch on a calm day from a V8
Supacat. I started with full forward trim as usual but possibly with not
quite as much additional forward pressure as I normally use. The glider
started to pitch up shortly after lifting off. Applying full down
elevator maintained but didn't reduce the pitch-up rate until the full
climb attitude, when rotation eased off and I was able to ease back to no
stick pressure. This started to happen around 50 kts, the rotation rate
was acceptable and the full climb angle post rotation was steep but not
excessive. I had around 65 kts when rotation had stopped.

My Std. Libelle normally lifts off both wheels almost simultaneously,
even with full forward trim plus a bit of forward pressure. I usually
have no problems holding or reducing that attitude as the speed builds up
past 50 kts. However, on this occasion I want to emphasize that I went
fairly briskly to full forward stick as rotation started and REMAINED
THERE through rotation into full climb. At that attitude when I eased
back to the trimmed position as the rotation slowed. If the acceleration
had been higher I could have been in real trouble.

Now, I always ask to be launched "like a Junior but 10 kts slower", which
gives a nice lift-off and constant attitude until 50 kts and rising is on
the clock. At that point a reduction in forward pressure gives a nice,
controllable rotation into full climb. Easing the stick back as the
rotation slows gives a nice full climb attitude at just over 60 kts.
Vwinch is 65.

Caveat: I've never winched an H.201B, which has a bigger tail than the
H.201, so its winch behavior may be different.

--
martin@ * | Martin Gregorie
gregorie. | Essex, UK
org * * * |

Martin, your post has all the ingredients I'm talking about. If you
know your glider, you'll know if it will require down elevator and
roughly how much. It's probably a good idea to start the roll with
full down elevator to eliminate 'reaction time'. The problem occurs
when the nose is allowed to rise too far and the pilot then tries to
correct the situation.

This has been a hard fought discussion but I kept at it because there
is a lot of good things in it that winch novices need to think about.

On Mar 19, 5:04*pm, wrote:
Bill,
If the initial rope tension is too low, the pilot should just release
and not stagger around for long periods. Elevator authority should not
be a problem when rope tension is low. From experience, I have never
seen uncontrollable pitch ups on slow tows, only on fast ones.

Right, and you DON'T start the rotation. If you do, that's when the
problem starts and you run out of elevator control.

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?