Winch Launch Safety Study

"I think understand, Tommyto is Derek Copeland using one of his
hundreds of aliases. "

Let us get at least this part straight - my name is Thomas Vallarino,
not Derek Copeland and I do not even know the gentleman.

"My measurements suggest the exact opposite - that increasing the
acceleration REDUCES the maximum AOA."

You can believe whatever you want to. You are free to disregard the
calculations in the analysis, the accident data and the experiences of
those here in this thread who describe specific cases of
uncontrollable pitch up movements, despite full forward stick - from
Karl's ASW-17 incident to the same experience of others including
myself with a variety of other models. It's not always the pilot's
fault when this happens as the pilot runs out of elevator authority.
The article calculated the pitch arm moment of many gliders and what
tension would be necessary to exceed the elevator authority at the
beginning of the launch. That's why they recommend a range of initial
rope tension of no more than 0.5-1.0g for singles and 0.7-1.0g for
doubles at the beginning of the launch.

Just think about it. Why does the tail go down into the ground at the
beginning of the launch, despite full forward stick? Because there is
little to no air moving over the elevator to overcome the pitch up
moment produced by the rope tension when the glider fist begins to
move forward at 0.1 seconds into the launch. This effect can continue
after the glider leaves the ground if the rope tension is too high.

If there is no stall and accident, then yes the airspeed will continue
to build up very quickly and so will elevator authority and the AOA
can be moved back to larger margins quickly by the pilot, so long as
tension doesn't continue to increase. However, there is a specific
rope tension for any airspeed within the first phase of the launch
(initial angle vector of rope VS horizontal), after which the maximum
AOA will be exceeded due to the pitch up moment and limited elevator
authority to prevent a stall.

In all my years, I have never even seen an accident on the winch,
where anything was damaged. Plenty of incidents, but everything
always turned out well. So what we're talking about here is to make it
even safer than it already is.
Thomas Vallarino
Manhattan Beach, California

On Mar 19, 3:14*pm, wrote:
"I think understand, Tommyto is Derek Copeland using one of his
hundreds of aliases. "

Let us get at least this part straight - my name is Thomas Vallarino,
not Derek Copeland and I do not even know the gentleman.

Well, that's incredibly good news for many reasons.

"My measurements suggest the exact opposite - that increasing the
acceleration REDUCES the maximum AOA."

You can believe whatever you want to.

Hard engineering data ALWAYS trumps calculations and predictions no
matter how logical they seem. I'll believe measured data instead of
calculated results any day.

"Just think about it. Why does the tail go down into the ground at the
beginning of the launch, despite full forward stick?"

And why does it stop there? It hit the ground, right? Now you have
hard acceleration and NO rotation. As the speed increases, the pilot
continues to hold full down elevator which increases in effectiveness
with the square of the airspeed.

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.

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 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.

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.

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