Thermal Tips

by Mark Howard

When someone asks to fly one of my sailplanes, I am always flattered. I want the experience to be a positive one, as I pride myself on the performance of my ships. Often, however, others experience difficulty controlling my planes, particularly pitch - usually in unstable air. I believe this is caused by a marginally stable setup on the plane and over-correction by the pilot. A rock stable setup is very easy to fly - but is just no good for signalling lift. An unstable setup is great for signaling lift - provided that the pilot flys with "kid gloves". A marginally stable setup is an excellent compromise for most advanced pilots. Such a setup signals lift quickly, but inattention can put the plane in an uncomfortable attitude very quickly.

I am a believer in the CG dive test, but I never set the CG so far back as to preclude a slow but positive dive recovery. Such a setup will tend to pitch the nose up when flying into lift, and nose down when leaving lift. A sharp turn on thermal entry may result in a dramatic loss of altitude after an initial rise. The instinctive reaction on entering a thermal is to immediately roll into a turn and pull back stick which may be the wrong thing to do. This over-anticipation usually results in a turn away from the core of the thermal - into the sink. It also makes reading the thermals' size and strength difficult. It is better to wait until the rate of climb just starts to decrease - or even until slight sink is encountered before initiating any turns. The thermals' size and strength become more obvious. After turning back into the general vicinity of the lift, establish a circular pattern. Adjust the circle towards the "high" side and away from the "sinky" side. When the plane is rising, and there is no "high" or "low" side - then the circle is centered around the core. If the lift is strong with a small core, a steeper bank and smaller turning radius is necessary. Weaker thermals require a flatter turn. Experiment to find the flattest turn which results in a similar rate of climb.

Sometimes the core is "where it's at". You must work these thermals tight and adjust quickly, or risk losing the thermal entirely. These are the kind of themals that "spit out" hand launch gliders. They sometimes do the same to unlimiteds. Use camber to slow the stall speed , and try using more rudder to hold the turn. If the nose pitches up wildly - PUSH a bit and flatten the turn to prevent a "wingover". If a pitch cycle results in a dive, use the energy to advanatge: let the speed carry you back farther into the sweet spot. The air close to the core is the most turbulent - and flying smoothly in the core is much harder than on the core perimiter - and smooth is what it's all about. Thermals tend to spread out as they rise. Flying the perimeter makes it easier to gage subtle shifts in the thermals' direction.

New pilots often can't tell the difference between a workable thermal and a teaser. A teaser thermal is one that is not of sufficient diameter AND strength to work within the performance envelope of the sailplane. I've seen pilots work these "thermals" losing several hundred feet until it is rather obvious that they are not going up. DA! Remember, if you're not going up - LEAVE. Put your plane on a straight course towards greener pastures and watch! Now is a good time to think as well! Feel the temperature. A rise in temperature means lift is nearby. A drop means nearby air is sinky. If the wind has calmed and the temperature is rising, there is probably lift to be found in the upwind direction of what was the prevailing wind. If the wind is suddenly stronger, then there's probably a thermal blowing thru - and the lift is downwind. Try to approach thermals from the downwind direction. For some reason, you'll encounter less sink approaching from downwind rather than upwind - the sink band seems to be narrower and weaker. If you are flying upwind, and feel a thermal push thru, think twice before chasing downwind to catch it. You may lose all your altitude in the effort. Surely, you'll have to fly through a good bit of sink to find it. Use the information to your advantage. If you know a thermal just pushed thru - and the cycle rate, you can predict the upwind run needed to catch the next upwind thermal.

Watch for debris in the air - dust, cobwebs, etc. Feeding birds are a sure sign - they swarm in thermals that contain bugs. Remember that a thermals' top is almost always downwind of the bottom. Tune in to conditions: wind speed, wind direction, and temperature. Try to visualize where the lift is - especially before launch. When sport flying - make a conscious guess as to where lift can be found. Fly over to that area and see if you were right. If you fly with your thumbs, make a concerted effort to fly with the index-finger/thumb method. When I first started flying f3b, I realized that this method provides much more fine control. It also improves smoothness.