This article was submitted by Jeff Carr. His dad wrote this for the club that he belongs to. It has also been printed in the Clarence Sailplane society newsletter. Thanks to both Jeff and his dad.
Last years' contest performance was disappointing so I set about looking for a fix. I'm very fortunate in that my oldest son is also a contest R/C sailplane pilot. While explaining things to him it helped me organize my thoughts and work though the problem. Jeff lives in Maine where thermals are hard to find. This made have been the reason he listened to me so attentively! The first step was to localize the problem. I decided that it could be split into four groups. These are:
The aircraft:
There are some very good articles on the Northeast Sailplanes website dealing
with sailplane trimming. I read these and others dealing with the Dive Test and
decided that much of it did not agree with my experience. I use an angle of
incidence meter to set the wing of the assembled sailplane to zero degrees. I
then adjust the flying stab to +1 degrees relative to the wing. The transmitter
elevator trim is set to middle-throw. I then balance the model to 30% of
average chord being careful to check the math for multi-taper planforms. Once
that's done I head out to the field for some hand-chucks. The idea is to adjust
nose weight for a good glide without adjusting the elevator trim. When that's
done it's time for the winch launch. Since hand-chucks are slow speed flights I
expect a change in trim at thermal search speeds and there usually is. This is
very true of cambered airfoils where there is increased downward pitch of the
airfoil as speed increases. After a bunch of test flights, adding and removing
nose weight, I find the "best" balance for overall performance for my
conditions and flying style. Since the stab was trimmed for +1 degree of
"up", it provides very little "up" or "down"
force on the wing in level flight. This should translate into minimum wing drag
as well as equal stab authority in either direction. However, in higher speed
flight, like coming back upwind, up-trim adjustment is needed to keep the nose
up while slow flight wants a bit of down-trim to keep the nose level. Since a
typical flight is a series of compromises of speeds and trim settings the
object is to find a stab and nose weight combination that handles them all. In
theory this combination should make the aircraft fall to the ground the slowest
in conditions where there is no lift.
The radio:
As you may know, I enjoy rebuilding radios. The goal is to make a transmitter
that is comfortable in my hands, has all the controls in positions where I can
operate them with minimum effort and are light-weight. I like to fly big
sailplanes out to the limits of vision and take particular care about TX-RX
tuning and battery care for solid range. The connections from the servos to the
flying surfaces take extra attention. Ailerons and flaps are easy as long as
the hinges have no play. The stab and rudder usually take some work both for
the mechanics and throws. Pushrods must be stiff and straight and well
supported through the fuselage. Many models call for a tape hinge at the
rudder. I try to use pin-type hinges because tape works loose in hot weather
and gives poor centering and reduced throw. This is also true of flaps and
ailerons but I do compromise and use tape while being careful to inspect these
hinge points frequently. The last item is receiver and battery installation.
I've used foam rubber, white foam, and EPP foam. EPP is my favorite because it
returns to original shape on impact (landing) while absorbing large amounts of
energy. These helps preserve the cell and wire connections during my normally
imperfect landings (ED - Pete is a true gentleman and modest to boot, I try to
emulate his landing abilities!).
The pilot:
I do just what you're doing now. I try to find every scrap of information about
every aspect of soaring flight and read it. The video tapes from RadioCarbonArt
are very good. Resource pages of club websites such as the Dayton Darts,
DownEast Soaring Society and the Charles River club are excellent.
Unfortunately, magazines available now are generally aimed at electric
sailplanes but occasionally have article paragraphs about the thermal search.
There is also information available using a Yahoo or Goggle search for
"r/c sailplane trimming" or "r/c sailplane flying" and the
like. Lastly, while it's fun to fly alone, never waiting for the winch or the
pin, you can learn more in a shorter time by flying with other guys. You can
pick the best of their methods and techniques and add them to your own.
The air:
Finding thermals should be about gathering information. There are certain fixed
pieces of information such as sun angle, amount of dew or water on the ground
and degree of sky overcast. Then there are the variables. These include texture
of the terrain (rocks, grass, forests, etc), asphalt streets and parking lots
shaded for part of the day by trees or buildings, wind speed and direction.
Rocks retain more heat than plowed fields, which hold more heat than grass.
However, if the sun angle doesn't shine on the rocks they won't pop very many
thermals. It's the ever-changing relationship of all these factors that make
each flight such a challenge. Many times I've heard the Timer ask the pilot if
he has a plan. Sometimes the response is a muttered cuss word and sometimes
it's something like "I think I'll go left." I had the chance to fly
with Larry Jolly at the Westover AFB Nationals a while back. He said that he
watches the conditions a full 20 minutes prior to launch in order to set a
plan. He also mentioned that in some conditions thermals pop every 15 to 25
minutes from the same source. From his legendary success in R/C sailplanes it's
hard to argue with his comments. I try to build on that by checking the
Accuweather.com web site for hourly forecasts of cloud cover, wind speed and
direction and temperature on flying day. I also have a thermometer on my
transmitter to help spot thermals blowing through the flying field. And
obviously, other sailplanes and/or birds in lift are a pretty sure sign of good
air. There is a vast amount of information that needs to be weighed and
analyzed to give the best quidance on where to find lift. Since all this
information changes as the day ages it means that the pilot must be focused on
the mission all the time. At contests where pilots are called to fly in flight
groups it can be difficult to concentrate on the decision process with so much
happening around you. This isn't quite so bad at contests where the CD calls a
"round-per-hour" or such. You then have the option of picking you
launch time for most favorable conditions. This is also a variable that must be
processed!
To wrap up:
You've made sure the radio is right, the airplane is right and that you made it
only a "two beer" night last night. Before leaving home or the hotel
you checked the weather forecast and made notes of the highlights. Arriving at
the field you swap lies with the other pilots, line up some timers and assemble
your ship. From then on, it's work, work, work. Take a walk to the landing
circles, check the likely flight paths into each one and the trees, poles or
wires that you will have to dodge. Then check the winches for different foot
pedals, retriever lines and line size. If possible, you should get a test
launch in order discover any surprises with the winches. Once that's done you
need to go sit down and watch the sky. Look for insects, blowing leaves, birds
and watch the tree tops. Large fields of tall grass are excellent indicators of
wind action. Survey the terrain for likely thermal generators like asphalt,
buildings and tree lines. Watch the sun angle, or try to gauge it from the
weather forecast you looked at earlier. Then try to be the last of a group to
launch. Every ship that launches ahead of you is a source of information about
the upper air conditions. Even sailplanes that are sinking miserably are
telling you not to go near them. Get the highest launch possible since this
translates into more search time. Once clear of the line set the ship to best
cruise and go get your thermal. After you've made the time and landed it's best
to check over the ship for any problems before turning the transmitter back to
Impound. If all is well, set the plane aside and go back to watching the
changing conditions for the next round.
Time spent tossing a HLG is excellent for learning to read air. Nostalgia ships fly differently than modern glass birds or HLGs. A thermal sensing telemetry system is also a great way to learn thermalling. Whatever you fly, every minute of flight time on any ship will add to your knowledge and make you a better pilot.