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:
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.
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!).
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.
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.