Compare/Contrast: CG hook on aerotow vs. CG hook on winch

At 16:12 02 June 2012, Dan Marotta wrote:
All the concern about nose vs. CG release and the correcting force of the

nose release indicates, to me, poorly trained or lazy pilots. If you will

simply fly the aircraft and pay attention to your flight path, you'll find

that it makes no difference what type of release you have (unless you're
doing a ground launch).

[snip]

The concern in the UK about towing on CG hook when a nose hook is available
came from tug pilot fatalities. The first, and most of the other, such
accidents happened with CG hooks, and one or more of the other factors,
which led to kiting, tug upset, and tug hitting the deck. As a result, the
BGA issued a poster listing the 6 factors most closely associated with
upsets, and advised having not more that two (IIRC) such factors on any one
launch.

More recently we started to have upsets again, so far fortunately without
fatality. Unable to find the original poster, we issued another which had
the following:
-------------------
Tug Upsets

These happen when the glider suddenly zooms above the tug, putting it into
a steep dive requiring considerable height to recover.

The BGA ran a successful campaign to end these fatalities to tug pilots,
but several years without incident now appear to have ended. This year
there have been two reported upsets and at least one other not reported.
Fortunately none resulted in crashes.

Six factors make upsets more likely:

 Lightweight, low wing-loading

 C of G hooks intended for winch launching

 Short ropes

 Inexperienced pilots

 Near aft C of G.

 Turbulent conditions

If two of these are present the danger becomes significant. More than
two should be considered unacceptable.

-----------------
Chris N

On 6/2/2012 9:12 AM, Dan Marotta wrote:
All the concern about nose vs. CG release and the correcting force of
the nose release indicates, to me, poorly trained or lazy pilots. If you
will simply fly the aircraft and pay attention to your flight path,
you'll find that it makes no difference what type of release you have
(unless you're doing a ground launch).

Do some of you actually rely on the nose release to keep the nose
aligned rather than flying the aircraft?

I did in some conditions, such as cross winds and unassisted (no wing
runner) takeoffs. When the glider is moving slowly at the beginning of
the launch, there is no "flying" the glider - you are ballistic for a
length of time that depends on the wind and towplane acceleration.

Once aerodynamic control is available, then the differences between CG
and nose hooks are reduced, but in all the gliders I've flown, it was
still noticeable. How noticeable did depend on the glider type.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

At 16:12 02 June 2012, Dan Marotta wrote:
All the concern about nose vs. CG release and the correcting force of the

nose release indicates, to me, poorly trained or lazy pilots. If you will

simply fly the aircraft and pay attention to your flight path, you'll find

that it makes no difference what type of release you have (unless you're
doing a ground launch).

[snip]

The concern in the UK about towing on CG hook when a nose hook is available
came from tug pilot fatalities. The first, and most of the other, such
accidents happened with CG hooks, and one or more of the other factors,
which led to kiting, tug upset, and tug hitting the deck. As a result, the
BGA issued a poster listing the 6 factors most closely associated with
upsets, and advised having not more that two (IIRC) such factors on any one
launch.

More recently we started to have upsets again, so far fortunately without
fatality. Unable to find the original poster, we issued another which had
the following:
-------------------
Tug Upsets

These happen when the glider suddenly zooms above the tug, putting it into
a steep dive requiring considerable height to recover.

The BGA ran a successful campaign to end these fatalities to tug pilots,
but several years without incident now appear to have ended. This year
there have been two reported upsets and at least one other not reported.
Fortunately none resulted in crashes.

Six factors make upsets more likely:

 Lightweight, low wing-loading

 C of G hooks intended for winch launching

 Short ropes

 Inexperienced pilots

 Near aft C of G.

 Turbulent conditions

If two of these are present the danger becomes significant. More than
two should be considered unacceptable.

-----------------
Chris N

Given all the comments in the thread so far, I guess it's about time I gave
what follows another airing. It was written a few years ago in the
aftermath of a tow pilot fatality:

Whilst I was Chief Instructor at Booker Gliding Club, we conducted two
series of test on the phenomenon variously referred to as “Kiting�,
“Winch Launching behind the Tow-Plane� and “Sling-Shot Accident�,
one in 1978 and one in 1982; my memory of them is quite vivid.
Â*
Airplanes used were, for the first series, a Beagle Terrier (a side by
side, two place, high wing, tail-dragger), fitted with an Ottfur Glider
hook for towing (very similar to the Tost hook, dissimilar to the Schweizer
hook) with a 160 hp Lycoming engine; for the second series of tests a
PA18-180 with a Schweitzer hook was used. Gliders used were a Schleicher
Ka 8b and ASK 13. Tow rope initially used was a heavy (4000 lb breaking
strain) rope with a thinner rope weak link at the glider end (nominally 900
lb, but a well worn specimen could break at as little as 200 – 300 lbs
– laboratory tests, not opinion), the second series of test used the same
heavy duty rope with “Mity� links at each end, 1100 lbs at the
Tow-Plane end and 900 lbs at the Glider end – these links use metal shear
pins, one under load and a second unloaded, which takes over if the first
one fails. This eliminates failure due to fatigue and means that the links
always fail at close to their nominal load even after some time in service
– again laboratory tested, not just subjective opinion. Rope length was
around 180 feet in all cases.
Â*
I was the Glider Pilot on all tests; Tow-Plane Pilot was Verdun Luck (then
my deputy Chief Instructor) for the first series of tests and Brian
Spreckley (then Manager of Booker GC) for the second. The object of the
tests was to try to reproduce the “Kiting� under controlled
circumstances, with a view to developing a Tow-Plane release mechanism that
would automatically release the glider if it got dangerously high above the
Tow-Plane. All tests were conducted at about 4000 feet agl.
Â*
First test: Terrier Tow-Plane and ASK 13 on nose-hook. At about 4000 feet
I took the glider progressively higher above the tow-plane, eventually
reached about 100 feet above tow-plane (i.e. rope angle more than 45
degrees above horizontal). At about this point, the tow pilot, who had
been using progressively more back stick, ran out of back stick and the
Tow-Plane began to pitch nose down but not excessively violently. I
released at that point. It took a very positive control input on my part
to achieve the displacement, we both felt it was something unlikely to
occur accidentally, even with an inexperienced glider pilot, and there was
plenty of time for either party to release if it did occur.
Â*
Second test: Terrier Tow-Plane and ASK 13 on C of G hook. I pitched the
glider about 25 – 30 degrees nose up – the weak link broke immediately!
Tow pilot reported a sharp jerk, but no significant change to flight
path.
Â*
Third test: Terrier Tow-Plane, K 8b on C of G hook. I pitched the glider
about 25 degrees nose up. The glider continued to pitch up fairly rapidly
(as at the start of a winch launch) and substantial forward movement of the
stick only slightly slowed the rate of pitch. The glider achieved about 45
degrees nose up, speed increased rapidly from 55 knots to about 75 knots
and the glider was pulled back towards level flight (again as at the top of
a winch launch). I released at that point. The entire sequence of events
occupied a VERY short period of time (subsequently measured as 2 - 3
seconds). The Tow Pilot reported a marked deceleration and start of
pitching down which he attempted to contain by moving the stick back; this
was followed immediately by a very rapid pitch down accompanied by
significant negative “G�. The tow-plane finished up about 70 degrees
nose down and took about 400 feet to recover to level flight. We both
found the experience alarming, even undertaken deliberately at 4000 feet.
Our conclusion was that the combination of the initial pitch down and the
upward deflection of the elevator caused the horizontal stabilizer/elevator
combination to stall and the abrupt removal of the down-force it provided
caused the subsequent very rapid pitch-down and negative “G�.
Â*
Our first conclusion was that, in the event of this sequence occurring
accidentally as a result of an inadvertent pitch up by the glider pilot,
there was effectively no chance that either the glider pilot or tow-pilot
would recognise the problem and pull the release in the available time.
Â*
Attempts to produce a tow-plane hook that would release automatically were
unsuccessful for reasons that became apparent later.
Â*
These tests were repeated a few years later with a PA18 – 180 as the
tow-plane, Brian Spreckley flying it. The third test described above was
repeated and photographed from a chase plane using a 35 mm motor drive
camera on automatic (this took a frame every half second – video
camcorders of small size were not readily available then). The photo
sequence started with the glider in a slightly low normal tow position and
starting to pitch up, the second frame has the glider about 30 degrees nose
up and about 20 feet higher than previously in the third frame it is about
45 degrees nose up and has gained another 30 feet or so, the tow-plane is
already starting to pitch down, in the fourth frame the glider is about 100
feet higher than its original position and the climb is starting to
shallow, the tow-plane is about 50 degrees nose down, the final frame shows
the tow-plane about 70 degrees nose down and the glider almost back in
level flight , almost directly above it (that was about the point that I
pulled the release).
Â*
Sufficiently alarmed by events, Brian Spreckley had been trying to pull the
release in the tow-plane earlier and found that it would not operate until
my releasing at the glider end removed the tension from the rope.
Subsequent tests on the ground showed that the Schweizer hook fitted to the
tow-plane, whilst perfectly satisfactory under normal loads, was jammed
solid by the frictional loads when subject to a pull of around 700 lbs with
a slight upwards component – not something that a normal pre-flight check
would reveal.
Â*
We solved that problem on our tow-planes by replacing the bolt that the
hook latches onto with a small roller bearing. So far as I know, no one in
the UK has tested the Schweizer hook as fitted to a glider, but I would not
be surprised if it exhibited the same characteristics at high loads.
Â*
The photo sequence also showed that at no time was the glider at an angle
greater than 30 degrees above the tow-plane’s centre-line. However, of
course once the glider has pitched up, the wings generate considerable
extra lift and that extra lift provides extra load on the rope. With a
large, heavy glider it is easy to exceed weak link breaking strains and
with a lightweight machine the tension can easily rise to 700 lbs or so.
With that much load on the rope, quite a small upward angle provides enough
of a vertical component to produce the results described.
Â*
That of course is the reason that attempts to produce a hook that released
if a certain angle was exceeded were unsuccessful. The, quite small, angle
between the rope and the fuselage centreline needed to trigger the
“Kiting� when the glider is pitched significantly nose-up is not much
greater than the amount of out of position commonly experienced in
turbulent conditions. We did build an experimental hook and tried it, but,
set to an angle that prevented “Kiting� it occasionally dumped an
innocent glider in turbulence, and set to an angle that prevented that, it
didn’t prevent the “Kiting�. What was needed was a hook that
responded to the vertical component of the load, not the angle at which it
was applied, and that problem we decided was beyond us (at least in a form
robust and fool-proof enough to be attached to the rear end of a
tow-plane).
Â*
Our conclusions for preventing “Kiting� we
Â*
Don’t aerotow gliders, especially lightweight, low wing-loading gliders,
on C of G hooks intended for winch launching (I think the JAR 22
requirement for nose hooks to be fitted to new gliders for aerotowing was
at least in part a result of these tests).
Â*
Don’t use short ropes. The speed at which things happen varies directly
with the length of the rope.
Â*
Don’t let inexperienced pilots fly at anywhere near aft C of G.
Â*
Don’t let inexperienced pilots fly solo in turbulent conditions.
Â*
Replace or modify all Schweizer hooks fitted to tow planes. (So far as I
know there are none on gliders in the UK, so that question never arose).
Â*
We did also modify our PA18’s so that instead of the release cable ending
at a floor-mounted lever, it went round a pulley where that lever used to
be, and then all the way up the side of the cockpit, anchored at the roof.
This meant that grabbing any point on the wire and pulling it in any
direction could operate the release; considerably easier than finding a
floor mounted lever when being subject to about minus two “G�. We
never regarded this modification as being likely to prevent a worst-case
scenario, because, as stated earlier, it was the opinion of all involved,
that in a real “Kiting� incident, there was no realistic hope that
either pilot would respond in time.


At 02:54 25 May 2012, son_of_flubber wrote:
I've been told (and witnessed) that aerotow on CG hook is initially
difficu=
lt for someone who has trained for aerotow with a "nose hook" (aka
hook-for=
ward-of-CG). Suppose this CG-hook-newbie were to take winch tow training
(=
with CG hook). Would the winch training reduce the initial difficulty of
a=
CG hook aerotow?

Do the difficulties/danger of aerotow with CG hook go away completely
with
=
training and experience?

What sorts of misadventures are reasonably attributed to aerotow on CG
hook=
?

I understand why CG hook is superior for winch tow. No need to rehash
that=
explanation.

I normally fly with aft CG. I have never had a kitting experience flying on a CG hook, but I have had occasion where holding full forward stick for a few seconds was necessary to hold attitude. That has happened only when the towplane was pulling me slower than I prefer.

This leads me to think that a preventive measure that perhaps every tow pilot should consider is to be vigilent in keeping the speed up on tow and especially in the first 500 feet (the death zone). At higher speed, the glider has more elevator authority to keep his nose down and the tug has more elevator authority to keep his nose up. Lifting forces on the elevator go as the square of airspeed, so a little extra airspeed can buy a lot of elevator authority.

Besides exacerbating the kiting scenario, slow towing at low altitude presents an entirely different and especially severe hazard to ballasted gliders.. That scenario has nearly killed me. Pulling the release is not an option when your glider is slow at 100 feet with the nose pointed up at 30 degrees in a full stall.

Keep the airspeed up tuggies.

A good post.

Some tow pilots strive to get into the air as quickly as possible since it's
easier to keep the tug straight in the air. This is not so nice for the
glider.

The last time I carried a full load of water, and with my CG aft of 90%, the
tug leapt into the air leaving me on the ground with full forward stick
trying to get the tail up. A few more knots would have made this an easy
tow.

When I'm towing, usually in a Pawnee these days, I simply guard the stick
during acceleration and let the tail come up by itself. Then, upon breaking
ground, fly in ground effect until reaching full tow speed for the glider's
weight. Haven't heard a complaint yet...


"Steve Koerner" wrote in message
...
I normally fly with aft CG. I have never had a kitting experience flying on
a CG hook, but I have had occasion where holding full forward stick for a
few seconds was necessary to hold attitude. That has happened only when the
towplane was pulling me slower than I prefer.

This leads me to think that a preventive measure that perhaps every tow
pilot should consider is to be vigilent in keeping the speed up on tow and
especially in the first 500 feet (the death zone). At higher speed, the
glider has more elevator authority to keep his nose down and the tug has
more elevator authority to keep his nose up. Lifting forces on the elevator
go as the square of airspeed, so a little extra airspeed can buy a lot of
elevator authority.

Besides exacerbating the kiting scenario, slow towing at low altitude
presents an entirely different and especially severe hazard to ballasted
gliders. That scenario has nearly killed me. Pulling the release is not an
option when your glider is slow at 100 feet with the nose pointed up at 30
degrees in a full stall.

Keep the airspeed up tuggies.

Sounds to me like you're talking about aircraft limitations. I haven't yet
seen a case where the glider could safely takeoff with a nose hook but not
with a CG hook AND the tow pilot was willing to do the tow.

Your operation may vary but, where I fly, we have a 15 kt crosswind
component limit on the tow planes. I've towed and flown my CG hook equipped
LAK-17 with winds gusting up to 28 kts though the cross wind component was
under 15 kts. In these conditions, ground handling is the most difficult
aspect of the operation and we sometimes stand down simply because of the
risk of moving the tug.


"Eric Greenwell" wrote in message
...
On 6/2/2012 9:12 AM, Dan Marotta wrote:
All the concern about nose vs. CG release and the correcting force of
the nose release indicates, to me, poorly trained or lazy pilots. If you
will simply fly the aircraft and pay attention to your flight path,
you'll find that it makes no difference what type of release you have
(unless you're doing a ground launch).

Do some of you actually rely on the nose release to keep the nose
aligned rather than flying the aircraft?

I did in some conditions, such as cross winds and unassisted (no wing
runner) takeoffs. When the glider is moving slowly at the beginning of the
launch, there is no "flying" the glider - you are ballistic for a length
of time that depends on the wind and towplane acceleration.

Once aerodynamic control is available, then the differences between CG and
nose hooks are reduced, but in all the gliders I've flown, it was still
noticeable. How noticeable did depend on the glider type.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email
me)

On Jun 1, 6:09*pm, Andreas Maurer wrote:

Let me explain:
I've often seen gliders roll behind the towplane for ages, balancing
on the main wheel, creating lots of rolling drag and bouncing all over
the place with each bump they hit. Usually lots of control deflections
to balance crosswind (a tail wheel in the air doesn't really help to
stabilize against a crosswind...), and often multiple touchdowns after
the first lift off due to PIO.

Andreas - Note that I said nothing about "pinning" the aircraft on the
ground. I simply said "get the tailwheel off the ground" - there's a
difference (albeit one I may not have clarified). You can get the
tailwheel off the ground (so the tail surfaces are more effective and
you can use the rudder to stay behind the towplane better), without
increasing your ground-roll by a huge margin.

I've seen plenty of people try to take off in a 2-point attitude with
the stick and/or trim back and when they jump in the air they quickly
get into a PIO because they have to shove the stick forward to prevent
kiting (and/or fight the back-pressure of a trim spring).

creating negative lift until the pilot decides it's time to lift off.
Scary.

Scary? Only if they prolong the takeoff to an unsafe degree.
Choosing when to take off is, IMHO, better than having the aircraft
jump into the air on the pilot and then the pilot is "behind" in
correcting and trying to chase the controls. But again, I was not
implying that people should keep the aircraft stuck to the ground -
just get the tailwheel off the ground so that you have better
directional control/authority.

One additional benefit: If the glider lifts off close to its minimum
speed, control authority of the elevator is less agressive, therefore
it's far less likely to overcontrol the glider and enter a PIO.

The flipside of your argument is that at minimum control speed you are
also more likely to be upset and/or lose control in the event of a
thermal or wind gust. Where I fly, we sometimes hit 8-knot thermals
at the departure end of the runway. Hitting that with one wing at
minimum airspeed would really ruin your day (and probably the tugs, as
you roll and yank him with you). Plus, with less control-authority
you cannot prevent "weather-vaning" as easily. IMHO, what you want is
to take off in a reasonable amount of runway that yields a flying
airspeed that is sufficient for good control authority.

I'm a little bit puzzled why one should mess with the trim during
aerotow.
By setting the elevator trim to the recommended position for aerotow
before launch *(trim forward) you get a slight nose-down stick
pressure on all the (German built) gliders I've flown, which in my
opinion is extremely effective to prevent ballooning: The glider is a
lot more likely to descent behind the tow plane if you don't pay
attention for a moment. Better be too low behind the tow plane than
too high.

....Because the factory-recommended trim setting does NOT account for
YOUR situation! It is a "book" value that doesn't account for your
weight, your CG, your aircraft's history of repairs or modifications,
etc. But most importantly: Because proper trim is always a good
idea. I know a lot of people are hesitant about this because they
think "wow, I'm so busy trying to keep position on-tow and look out
and manage the tow, why would I want to distract myself with trim?"
But they don't realize that part of the reason the workload is so high
is BECAUSE they're fighting the glider's trim! The truth is that
trimming properly lessens your workload and makes precision flying
easier. You should be able to (in a well-designed aircraft)
manipulate the trim without looking at it or taking your hands off the
primary flight controls. And you can feel the effects of the trim
change by the feedback on the stick - so if you make a couple of
incremental changes you can easily feel when you've got the trim set
properly (or close enough). Therefore adjusting the trim on-tow
should be relatively simple, and will result in a much better
experience (and with a lower work-load you can stay more alert to
traffic around you and emergency-preparedness as the tow progresses).

That nose-down moment IMHO is not good at all (and like all trim
settings, its highly dependent upon your current CG *and* your
airspeed). Why do I think a nose-down moment might not be good?
Springs are used for most glider control systems. Think about the way
a spring can bounce or oscillate, and think about your arm constantly
fighting that force. Isn't it clear how this can lead to PIOs and
over-controlling? Fighting a nose-down force may lead to PIOs just
like fighting a nose-up force can. Certainly the problem of kiting or
ballooning on initial takeoff is something to avoid; but I refer you
to my earlier comments in this message.

Bottom-line: The better you trim the aircraft in ALL phases of flight,
the better you'll fly. I may have only been doing this for a few
years; but the ONLY people I've had who fight me on this point are
people who've never used their trim all that much. Just last weekend
I mentored several budding XC pilots and almost all of them had a
horrible time making a consistent thermalling turn. Once I showed
them how to re-trim the aircraft once they'd established their bank-
angle and thermalling speed, all suddenly did MUCH better and remarked
on how much easier it was to thermal.
(Of course, then I had to remind them to watch their airspeed and re-
trim the aircraft as they rolled out of the turn... One step at a
time, I suppose! *chuckle*)

DG-300 cannot be trimmed nose-down at all (as you have probably
noticed in your 300, it needs a significant stick push which isn't the
best idea if you want to keep your position behind the towplane).

Not true. I can hold the trim-release lever and apply a small amount
of forward force to the "trim indicator" (which is strong enough on my
ship to handle this load), while keeping the stick pushed to its
proper position to maintain attitude, and trim forward just fine.
Obviously this is not applicable to all aircraft; but I am pointing
out that the blanket statement is not, in fact, true.

BTW, you can also hold the trim release, briefly push the stick
forward while releasing the trim-lever, and then relax the stick aft
some. The nose will dip but if you practice this (NOT ON TOW) you can
get quite quick with the maneuver and the glider will only rise or
fall in relation to the tug by some 5 feet throughout the maneuver. I
do _not_ recommend this; but again just pointing out that its
possible. :-)

Make one mistake, and you have a neutrally or even nose-up trimmed
glider - one further mistake and you might create brown pants in that
tow plane in front of you that is just disappearing under the nose of
your glider.

Not sure I follow you... If you can keep pulling back on the stick to
offset your nose-down trim, why can't you keep pushing forward to
offset a nose-up trim? Either one is bad, and both can cause problems
if you leave tow position and don't correct for it. The glider
ballooning seems really bad and scary, but a seriously-low glider can
also cause the towplane to pitch up and stall or spin; its not like
one situation is "bad" and the other is "good". Both are bad!

Neutral trim on-tow and careful attention to tow-position should yield
the best tow.

Clear case of bad training. One needs to learn to actively fly the
glider during an aerotow, otherwise problems are pre-programmed.

Very true. But its not just training. Or rather, a lot of pilots get
complacent and don't think of themselves as needing "training" as they
age and gain experience. Bad habits form. Laziness happens. People
with nose-hooks can get used to always being pulled into position by
the tug; and subtly their rudder-use decreases over time. Then one
day they take a flight in a CG-hook aircraft and things get rough. :-P

Take care,

--Noel
P.S. If I were to summarize my typical aerotow (with my DG's CG-hook)
& my trim adjustments, here's how it goes:
1] I trim almost full-forward as part of my pre-takeoff check.
2] As the glider begins to roll, I keep wings level and try to track
behind the towplane.
3] As soon as the tail comes off the ground I give the trim-level a
quick squeeze & release (handy to do in the DG). This resets my trim
for a fairly flat attitude, giving me good rudder authority to track
behind the towplane.
(At my main airfield we have to stage off to the side of the runway
and begin the takeoff roll at an angle - rudder control is important
as we gently curve onto the runway and lift off flying down the
centerline)
4] As my airspeed climbs up about 5-10 knots above stall (around 40-45
knots in my DG), I apply a small amount of back-pressure to lift off
and fly in ground-effect as we continue to accelerate. I sometimes
give another quick squeeze-and-release on the trim, if I find that I'm
having to apply force on the stick to keep the plane flying or to keep
it from rising above the tug.
5] Then when the tug and I reach about 100 - 150 feet I re-trim one
more time as our airspeed approaches "normal" for the rest of the aero-
tow.
6] I may fiddle with the trim once more as we fly upwind and/or
crosswind; as the tug may retract flaps or otherwise change its speed
& attitude.
[Of Course, Your Mileage May Vary...]