Auto-Tow Ground Launching 



	   1.1	Safety. 
	   1.2	Auto vs. Winch.
	   1.3	Ground launching is "circuit" training.
	   1.4	Low altitude handling.
	   1.5	Safe Airmanship.
	      1.5.1  Become Skilled Before Flying Low.
	      1.5.2  Avoid Trafficked airports.
	      1.5.3  Plan carefully.
	
			   Daniel L. Johnson

Thanks to Steve Fischer, Minnesota Soaring Association Chief
Instructor, for review of this manuscript, and for suggestions, all of
which I pondered and many of which I adopted whole.

Note: This manuscript is specific to the Blanik L-13, but the general
principles will apply to other gliders.	The L-13 uniquely has a two-
piece ground launch bridle, and I address the management of this
bridle specifically...

This essay is a summary of what our club learned by ground launching a
Blanik L-13 for five seasons.  It is the distillation of our
experience, it is my own opinion, not instruction.  If this adds to your
knowledge, I will have accomplished my goals; if you act foolishly,
you have only yourself to blame.


1  Why learn to auto launch a glider? (Introductory Essay)


1.1  Safety.


Most glider accidents occur in the landing phase of flight.  Fatigue
and medical factors are often blamed.  But after watching hundreds of
glider flights involving a few score pilots, it is remarkable that few
pilots practice pattern work and landings after certification or
checkout.

Auto launching is a safe, inexpensive way to practice pattern flying.
That's it, plain and simple.  Yes, it's fun.  Yes, it's a great way to
have a party.  Yes, it can be thrilling.  Yes, a "real" pilot can "get
away" from a 1000-foot tow -- ummm, once or twice in a season.	But
above all other considerations, the autotow ground launch is the most
cost-effective way to achieve valuable experience in low-altitude
flying.


1.2  Auto vs. Winch


Its cost is lower than winch launching, and we have no experience with
winches, so we're going to leave that for someone else.

The cost is low because the additional cost is small, as someone is
likely to already own a powerful truck that they're willing to use, if
it's handled respectfully, in a small-scale auto-launch program.  So
the capital cost has already been made by someone for reasons not
related to gliding.  The gliding is just one more justification for
owning a truck, not that any justification is needed.

A winch, on the other hand, is expensive, and becomes "cheap" only
when amortized across many glider pilots and many years.  It is not
useful for trips to the lumberyard or the dump, cannot be used for
off-roading, and won't pull a boat and trailer.

On the other hand, if you're running a high-volume operation, the
winch launch is able to achieve higher altitudes and has a shorter
cycle time (delay between launches).

Auto towing is inherently safer than winch launching.

This does not mean that the actual accident rate is lower, as that is
related more to the care with which the operation is conducted and the
experience of the operators.  Inherently, there's simply less to go
wrong in auto-tow launching.

For example, the physics of a winch launch are slightly more daunting, as at
the top of the climb there may be destructively high loads on the glider's
wings that don't exist during the auto tow.  The techniques are similar for
both, but the acceleration is much greater on a winch launch due to the engine
not having to propel the vehicle as well, so the risk of vestibular
dysfunction is less with auto launching (See Derek Piggott, Sub-gravity
Sensations and Glider Accidents, which you should buy and read).

You should also purchase Derek Piggott's Ground Launches, which is
exclusively about winch launching, and you should suck up to an old
geezer like me and beg to be allowed to read or xerox American Soaring
Handbook, Volume 3: Ground Launch by William R. Fuchs and Francis P.
Bundy.

Their analysis of the forces on the glider's wing at the top of a
winch launch, on pages 69-74, is worth studying and thinking about.
In case you can't get a copy for yourself, or don't want to read it,
here's a precis of section 18:  
(The link to section 18 is a 1.2MB .jpg file; it will open in a new window;
take this into account if you are thinking about clicking on it.)

The physics of the operation are thus:	The tension on the rope (wire)
and therefore the extra lift that must be generated by the wings in a
winch launch is equal to the gross weight of the glider times the
tangent of the angle between the glider and the wire, which increases
to infinity as the angle approaches 90ø.  This is because forward
speed can be imparted to the glider only with downward tension on the
wire.  Unless the weak link is carefully chosen and the tension
monitored by the winch operator, this force can (and has) broken the
wings from gliders,  embarrassing the crew and bringing permanent to
the grief of the pilot's family and friends.  Properly designed
winches include transmissions which usually incorporate a pressure
compensated hydraulic pump which senses a preset tension on the wire.
And home-grown winches don't usually incorporate a fail-safe tension
relief.

In the auto launch, however, the tow vehicle is not stationary, but is
moving at about one half to two thirds of the glider's speed as it
reaches the apogee of the tow.	The forward speed of the glider is
created early in the tow, and the tension on the rope is only that
necessary to create lift.  An aggressive pilot can, however, pull the
back of the truck off the ground if the weak link is too strong, which
is undesirable.  In my experience, the back wheels are felt to lose
traction just before the rope breaks.  This is followed by
chastisement of the pilot for climbing too aggressively.  The ground
crew repairs the rope, however, because to force the pilot to do so
would punish the pilot next in queue.

The winch launch has another danger.  The wire is recovered while
still in the air (or has just fallen), and is rapidly taken up by the
winch operator.  Fatalities and severed limbs have occurred when
ground crew have inadvertently gotten in the way of the wire after it
has fallen.  This sort of accident is less likely with auto towing, as
the rope simply falls to the ground and lies there until it is
retrieved.  There is danger, however, from the hardware at the end of
the rope, which usually buries itself an inch or two into the ground,
and would demolish the cranium of any bystander it happened to strike.
And when the rope is dragged back to the launch area, it should not be
handled except with a metal hook, and crew should beware of a loop
getting around a leg or arm.  Better that no one is near the rope or
handles it while it is being freed from runway lights, fences, and
other nuisances.

Auto launching also does not require an aggressive climb out to be
useful.  This decreases the risk of an accelerated stall during the
climb.	We are taught to get to a 45ø climb attitude as soon as one
has sufficient altitude to recover from a rope break and stall, about
200 ft agl, and sufficient speed to make a stall unlikely.  With a
short runway and the felt need to attain 1000 ft or more off tow,
extremely aggressive climbs can indeed be done with autotow launches.
But this is not necessary.  A pattern can be flown from as low as 400
ft agl, and rope break practice from any lower altitude.  If there's a
long runway and a fast tow vehicle, higher speeds and shallower climb
angles can be used, with less strain on the tow rope, less risk of an
accelerated stall, and less fright to the new pilot or to passengers.


1.3  Ground launching is "circuit" training.


The goal of most pilots is to soar, to stay aloft as long as possible,
to achieve a distance goal.  The approach and landing simply marks the
end of a long day, perhaps is a symbol of failure to achieve one's
dreams.

Anything we do repeatedly, with attention to excellence, results in
improvement.  This is called "the practice effect" by psychologists.
Simply to repeat an action does not cause excellence:  mere iteration
causes boredom.  Iteration with attention to technique and results
yields mastery.

For the last few years I've attended a cross-country soaring camp.
There have been a few damage incidents, nearly all involving landing.
There have been more than a few exciting moments, mostly involving
landing.  One year we facetiously gave out several "diaper awards" to
the pilots who most memorably recovered from hair-raising landing
situations might have caused those watching to need a diaper.

It's been remarkable, especially in view of the dinged glass and the
scary bits, that not one pilot at these camps has taken a series of
pattern tows just to get used to the field, never mind getting more
adept at flying the rental glider.  This field has sloping runways and
frequent high winds.  It is surrounded by mountainous terrain that
fosters visual illusions.  Much of the flying is done on hot days, and
the field elevation is above 5000 ft. msl.  There are hazardous areas
at the approach ends of the runways.  It's a challenging field but not
a dangerous one.  Yet we've never taken a practice day, a practice
session, or seen individual pilots practicing extensively.  Is there a
connection between our annual incidents and the tendency not to "need"
practice pattern work?

The autotow ground launch, because it seldom delivers the pilot into
lift at a usable altitude, fosters pattern practice and permits the
pilot to gain experience and skill in low-altitude maneuvering.  This
is not intended to help you learn how it feels to make a 500-foot save
over unlandable terrain; it is valuable in learning how to feel
comfortable piloting a glider when faced with urgent situations while
low on approach.  This practice is useful in learning to recognize the
safe altitude and distance limits during an approach.

It is also useful in efficiently teaching pilots to make precision
low-energy landings, which is very useful preparation for outlandings.


1.4  Low altitude handling.


During emergencies, there may be times when it is necessary to make a
turn at very low altitude or to use ground effect to stretch a glide.
Deliberately making planned low-altitude maneuvers of this type, and
flying in ground effect deliberately and carefully in safe
circumstances engenders skills that will be in the pilot's repertoire
if needed emergently.

Low altitude maneuvering "feels" much different than high-altitude
maneuvering.  In this sense, "low-altitude" means "below the pivot
altitude."  Pilots who prepare for their airplane commercial rating
must practice making turns about a point.  In this maneuver, the
airplane is held at the pivot altitude during a turn while the pivot
altitude changes from the effects of changing ground speed due to
wind.  Making a 90ø turn one day at 75 feet AGL gave me a whole new
appreciation for what the glider is doing at altitude: it is not
tracking on rails; it's a great deal more like a boat, coming around
gradually even after it's pointed in the right direction.

While thermaling, the glider seems to pivot around its CG, the inside
wing sweeping back across the terrain far below.  Below the pivot
altitude, the wing sweeps forward across the ground; even though the
glider may be going slowly, its speed seems fast, and any turn seems
extremely wide in comparison to the same turn at altitude.  The effect
is startling, especially after we've spent three or four hours
circling at 4 to 6 thousand agl.

This sense of changed motion related to the apparent movement of the
wing over the ground is called by perceptual physiologists, the
"vection illusion," the illusion of false motion.  This illusion is
probably the main factor in producing the skid that may end in a spin
during the turn from base to final.

To diminish the frequency of approach-to-landing accidents, we should
conscientiously practice low-altitude turns and maneuvering, under
controlled conditions, in a safe environment.  This is accomplished
most efficiently by auto-tow launching to an altitude of 800 to 1200
ft agl.  From this altitude, most gliders can safely make several 360ø
turns and still be able to fly an abbreviated pattern before landing.

By beginning the approach from various low altitudes and from
different spots around the airport area, the pilot learns in a
familiar environment the "feeling" of operating close to the ground
and of planning and adjusting the pattern.


1.5  Safe Airmanship.


It is not inherently dangerous to fly below 1000 ft agl: the laws of
physics do not change, and the aerodynamics of the aircraft change
only in ground effect, within a wingspan of the ground.  But at low
altitude, there is little margin for error, and few alternatives, so:


1.5.1  Become Skilled Before Flying Low.


The pilot should fly low only after becoming completely comfortable,
at altitude, with the glider's handling and with its stall
characteristics in straight flight and in coordinated and skidding
turns, and skilled in making all types of turns.


1.5.2  Avoid Trafficked airports.


The field chosen for practice should be free of conflicting traffic.
It should have alternative opportunities to land that are within the
glider's limits, such as grass alternate runways, grass over-run
areas, landable taxiways, and so on.  Use of such areas for emergency
practice must be cleared with the airport manager, of course.  Quiet
rural airports are the only suitable fields for such practice.	The
best candidates are towns of 5,000 to 20,000 population without air
carrier or air taxi service, and with a paved runway about 5,000 ft
long with at least 75 ft width between landing lights.	If the local
gliderport is to be used, the early to mid morning is a wonderful time
for this, as the power pilots aren't out of bed yet, and the thermals
aren't ready for the hotshot XC pilots.


1.5.3  Plan carefully.


Low-altitude maneuvering must be carefully planned, changing only one
or two variables at a time: altitude, entry position, pattern route,
landing spot, rate of descent, speed, and point of entry into ground
effect.  Changing only one variable at a time has two benefits: it
limits the adjustments you have to make in learning to handle the
changed situation and limits the surprises, and it helps you to
clearly understand the feeling and significance of each change.  Does
this mean flying a lot of patterns?  Yes, it does.  This is another
argument in favor of doing auto launching.

Copyright © 2000, 2002 Daniel L. Johnson; all rights reserved.