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

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...]