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SET YOUR TRIMS NOW for a reasonable climb, once you get to higher altitudes
it will be difficult to do this. The tow pilot must judge his rate of climb
based on the glider he is towing, the glider guy who can see the flag
and if there is any slack, can direct the tow guy to climb, or level out
as the case may be.
After a few tows you will develop your own symbiosis between tug and
sailplane
pilot.When in doubt get off. No if and or buts. You can always do another
tow.
Do not fly higher than you can see. This sounds basic, But I have seen
beginners
forget that they have a release switch
in their hand, and blame the tug pilot for
towing him too high. Watch the flag, it will tell you if there is slack
in the line and also when you are released. The Germans don't use a flag,
but this because
they must see better than we do. They don't as a rule cover the bottom
of their wings dark for visibility either. I asked Theo Arnold at the Elmira
Aerotow how he
could see his white winged sailplane at 4500 agl. He said: " It is
an airplane, it is
either going right or left." OhhhhK. Makes sense to me.
The tug pilot should fly large gentle circles or figure eights. The
climb rate should be in accordance with thepower of
the tug and weight of the glider. Shallow is usually better than too
steep. If the glider overtakes the tug, then the tow pilot needs to increase
his angle of attack or the glider pilot should feed in a little up trim.
Spoilers should not be used as this just creates more drag. The direction
of at least the first turn should be established before taking off. The
sailplane pilot's job is to keep his wings level using ailerons only. Once
released you can
recouple ailerons and rudder if you like. Keep in mind some high performance
scale
sailplanes cannot be flown coupled. Learn now,and save yourself the
trouble later.
These are the basics. Please allow that there are many variations on
this technique.
Nothing is cast in stone, but by and large this system works and is
proven to work.
Be advised that while towing from the tail of the tug or the bottom
of the tug are
possible it is not practical for safe and efficient towing. .
Towplanes come in many shapes and sizes.
There are no hard and fast rules governing what tugs wil pull what gliders.
Sufice to say, more power, more size, more stability are good attributes
for a tug. If you want a scale like slow tow, then the lower power to weight
ratio tug/sailplane combination is what you want. This set up requires
that both pilots fly precisely to avoid marginal conditions or early releases.
If your desire is to get as many sailplanes into the air as possible in
a short amount of time, then a super tug with a big motor will allow you
to do this. Some guidelines:
First of all the tow release on the tug should be between the C/G and
the trailing edge of the wing, not critical but somewhere in that vicinity.
(On TOP of the plane.)
3-4 meter gliders weighing up to 10 lbs.
Telemaster with 1.08 O.S. two stroke at the bottom of the spectrum up to
a Scale Wilga with a Brison 4.2 at the other end. The important thing here
is that the static thrust of the engine prop combination be roughly equivalent
to the weight of the tug. An example would be the 14 pound Telemaster should
be pulling about 13 to 15 pounds of thrust with a 1.08 motor . One of the
best combos for this size of glider is a generic Telemaster type airframe
with an O.S. BGX-1 35cc two stroke. It is easy to fly, relatively inexpensive,
and very dependable. The one we use weighs about 17 lbs. and pulls around
17 lbs. of static thrust.
A note on props: You can spend alot of time(and money) trying different
prop combinations, believe me, I have succumbed to the... what if I tried
this prop scenario. A good rule of thumb for aerotowing is to use the highest
pitch prop that will let your motor pull the most static thrust. This translates
usually to an 8 or 10 pitch prop in the upward end of diameters for your
motor. I have tried multi pitch props also. Some work better, some do not.
The Germans insist on a 10 pitch prop,Theo Arnold (EMS test pilot)Says
that they generally fly their sailplanes at a higher wing loading than
we do, and they need to fly fast. A preference more than a neccessity perhaps.
Experiment, have fun. Remember the towplane does well if it has a 1to1
power to weight ratio or better.
4-6 meter saiplanes weighing 10-25 lbs.
The bottom end of this class would be the Telemaster with the 35cc motor
on up to tugs weighing 25 pounds with 30 or more pounds of static thrust.
(70cc gas motor)
6 meter and up gliders weighing 25-50 lbs.
Large tugs with 50 to 100 cc gas motors work well here.
The Towline
We make our towlines like this: Total length, 75-100 feet Material: planer
board line or 150 lb.braided winch line Makeup: from the tug end, 10 feet
of line then 2-4 feet of fabric covered 1/4 " bungee cord, the balance
of the line,then a flag 2-3 feet long with the line running through the
flag, then about a foot more of line follwed by a swivel and heavy duty
fishing
clevis to hook the loop to. We have recently added a badmitten birdie over
the sailplane end to keep it from tangleing after the glider has been released.
The German's do not use a bungee or a flag on their tow lines, but otherwise
their line is similar. They use a weak link instead of a bungee. The Australians,
according to Martin Simons, use a heavy duty monofilament line which has
a certain amount of stretch and for them eliminates the need for a bungee
link. I have also heard from Ian Moreland in Australia. He indicates that
they in addition use a line made from the braided winch line similar to
what we use.
Sailplanes appropriate for towing
Almost any glider can be towed aloft. I have towed Gentle Ladies, Spirit
100's,Weston Magics, and 2 meter Icare ASK-21's. It works. Polyhedral ships
should probably be avoided, as they tend to continually correct their attitude
on the roll axis making for an almost uncorrectable wallowing on tow. Slowing
the tow down helps, but these ships benefit real well from a bungee or
winch launch. They are not generally suited to aerotowing. The smaller
scale gliders (2meter) are somewhat trickier to tow for a beginner, everything
happens faster. The kind of sailplane that benefits most from a tow aloft
is a large scale glider. These planes do not winch well or bungee launch
to any great height.(nor would I trust a 1/3 scale glider on a high start)
They also, because of things like reynolds numbers and span loading, really
come into their own at the higher altitudes acheived with aerotowing. Some
of the above is based on opinion and personal preference, I don't want
to get involved in philosophical differences on launch techniques. To each
his own in that regard. That said, The best range of sailplanes for aerotowing
I would say, would be 3 meter and up aileron controlled ships. The larger
they get, the greater benefit they achieve, in practical terms, from a
tow versus an alternate launch method. Those who have the luck of living
near a large active slope not withstanding.
Dolly or cart
We have used a dolly to train newcomers to aerotowing. This is not really
neccessary in most cases with experienced pilots. What the dolly does,
is take away all axis of control until the glider has flying speed. It
lets the beginner get the feel of the plane while the tug is doing its
job getting things off the ground. They can become a crutch if one lets
them. They are generally a four wheeled affair with a lip on the front
to catch the leading edge of the wing until the sailplane lifts off.
What is a Towpilot?
A tow pilot is a power plane flyer who can fly large climbing circles while
towing a sailplane at a steady rate of climb until both planes are nearly
out of site. Sound easy? It's not. Practice. Hint. Initially downtrim is
usually needed for the towplane. Each sailplane tows differently and elevator
trim on the tug will have to be adjusted accordingly.
Loops
Not an aerobatic routine, loops are small circles of winch line or 80 lb.
test fishing line used to engage the release mechanism in the nose of the
sailplane. These are then in turn clipped to the clevis on the tow line.
Cheap, reliable, and easy to use.
Tow Releases
Any tow release which is simple, reliable and can engage the loop previously
described will work. A simple music wire U with another piece of wire crossing
it to trap the string will do. Commercial releases are also available from
Europe* which are more scale like and fit nicely in the nose of a scale
sailplane. Two piece mechanical releases with a captured ball are bad news,
as they are unreliable and you have to continually chase down the other
end.
*Sources for tow releases and related items:
Sailplanes Unlimited LTD.
Unbeaten Path Imports Page