Thermals, observations on a invisible phenomenon
Once you can launch and land your plane in one piece, most people want to stay up longer, and this means flying your glider in lift. As Orville said "birds soar by gliding down in rising air". So how do you fly your plane in rising air? At first you are happy to find lift. Then you want to find lots of lift to get really high. Last you want to find lift that gets you where you want to go. Kind of like fishing, first you want to catch a fish, then you want to catch lots of fish, and last you want to catch Moby Dick.
Flying in lift means controlling your plane, not just punching holes in the sky. Save the punching holes for when you are 2000 feet high, and it’s a lot more fun then. To fly well in lift, the plane must be in good control and doing what you want it to do.
Phase One, how to decide where to fly.
To find lift you have to know what it is, or more specifically how to recognize it. There are three sources of information about lift, all of which are important and various stages of your flight. Use all of them and be more successful. The first source of information is your own senses other than sight. Feel the wind or air temperature changing as you stand around, this gives you a pattern of the cycle of lift and no lift. Even the calmest days will have noticeable shifts in wind and air temperature. The second source is observing the signs of lift around you. Some are obvious like a dust devil, or someone else’s plane going up like a helium balloon. Some are more subtle, like seeing a cloud of mosquitoes and fluff flying upward, or heat waves increasing in the distance. The last is how your plane responds to the air it’s in. This is really the most subtle and has to be learned from experience. Lets take a flight from start to finish and go through the steps. It becomes subconscious after a while when you are flying regularly.
From the time you arrive at the field, notice the wind direction, cloud formation and air temperature. This is the background against which all other observations are referenced. OK, store that away. Five minutes before you flight, take a look around, note wind shifts, other planes, heat waves and birds. Figure out where you are in the lift cycle and plan your flight path accordingly. End of phase one, now fly.
On the launch line, figure out if you can get to any obvious lift source, check wind. If you wait to launch into the wind, you will be launching into sink and the thermal is now downwind of you. Now launch, watch your plane to read the air on tow. If the plane jumps or wallows when it shouldn’t, it just went through lift or sink. Histarts are great for reading air on launch. Have your timer watch the chute as it is coming down, this shows lift very well as it drifts downward. Hey, watch the chute of the guy (pilot?) launching before you.
General plane responses to lift
Now your plane becomes the biggest source of information, but don’t shut down the others. Knowing your plane means that you can tell if it does something that it shouldn’t have done, based on the stick inputs you gave it. Your plane must be reasonably trimmed to fly straight and level. The most obvious response to lift is the plane turning when it shouldn’t, but there is also speed, attitude (nose up or nose down) and wobbling. Unstable planes are harder to fly, but respond better to lift. My philosophy is to fly the most unstable plane you can handle. More on this next issue. This way the plane will show you when the air is rising or falling, this is what my planes do in response to lift and sink.
Turn responses - a plane will turn away from lift, unless you get though the turbulent edge of the thermal and then the plane will turn to stay in the lift. Look for a wobble before the turn, no wobble then turn the other way. If it wobbles then turns, keep turning the same way. Refer to toilet bowl section below.
Pitch responses- a plane will nose down, with a delayed increase in speed when it crosses the boundary into smoothly rising air. A plane will nose up and not slow down in sink. Otherwise known as draggin’ the ass. Get out of there. A neutrally balanced HLG will readily display these pitch changes as the plane transitions into the rising and falling air.
Speed responses - A plane's ground speed will increase as it approaches a thermal due to the air being sucked into the rising mass. The same is true of sink. You figure out which it is.
These responses are much easier to detect flying into the wind, but we shall see that we want to be spending as much time flying downwind as possible. Spotting lift responses when screaming downwind is a real challenge.
Flying in a thermal.
Now that you have found a thermal, how do you stay in it. Let’s talk about small thermals, not the giant ones that lift cows and transport them to Airdrie. You should have no trouble finding and staying in a thermal 300 yards (271.32 meters) wide. Let’s say that you have the luxury of height to fiddle around a bit to find the best part (core) of the thermal. At low altitude (under 100 feet), just fight stay in the lift until you get high enough to start fiddling around
Rule one - your plane will not want to stay in a thermal.
Rule two - once in a thermal, your plane will not want to stay in the best part of a thermal.
Rule three - these rules depend on the plane.
Brutally stable planes will circle in lift by themselves, without operator input. Free flight models are the obvious example. Any plane that is reasonable set up will have to be flown into lift, and flown to stay in lift. Define the edges of the thermal if you can so you know the size of the lift area, more on this later.
Type 1 thermal
Think of a toilet bowl flushing (or warping of the space-time continuum by gravitational mass if you are a physicist). A thermal is like an inverted toilet bowl drain, analogies about the finishing job on my planes are not permitted. These thermals are funnel shaped bent downwind, start at higher elevations, and have a particular direction of rotation. This is why you try right and left turns to find the core, one way will be better than the other so learn to circle both ways. Your plane will circle faster when flying in the same direction of thermal rotation (DUH), and you want to be going the opposite way. Kind of subtle, worry about this later. Steve Y could only circle to the left, and never flew in Australia. Circle in the same direction as other planes in the thermal, this is common courtesy to reduce collisions. If you lose the lift look upwind, for two reasons: Your plane drifts faster downwind than the thermal drifts and if you don’t find it upwind, it’s easier to chase a thermal in the downwind direction. You have to fly the plane in weird directions to lose a thermal downwind so it is usually upwind.
Type 2 thermal
Think of a rising soap bubble. The soap bubble thermals are smooth and have no core. They tend to be smaller, at low altitude and don’t last long. HLG fly in the soap bubble zone. When you lose this one , you have fallen out the bottom and without flapping your wings, you will never get it back. Soap bubble thermals quite often form in the same place at regular intervals, and will merge with other lift to form thermals that are more useful. It takes nerve to fly soap bubble thermals since they are marginal and close to the ground. Circle as efficiently as possible and hope the bubble breaks loose.
Where to fly
Finding lift means covering as much ground as possible. Flying into the wind covers less ground than flying downwind, so fly as far downwind as you dare in order to increase the odds of finding a thermal. Needless to say, exercise judgment with respect to your plane and ability when deciding how far out is acceptable. In a contest, lesser men (pilots?) will not follow you, which is a significant advantage if you do find lift.
Rules of where to fly:
My routine on a searching type flight is to fly into the wind to cover the lift cycle, turn either right or left and fly 100 yards to one side so the downwind leg isn’t over my head, turn downwind until lift is found, turn to return to the landing area when the plane will just make it back. So a flight with no lift has three turns and lasts about 3 minutes, covers about a ¾ mile. A pattern like this will do five things:
At the Red Deer lake field on a normal day, you will find usable lift more than 95% of the time using a good searching pattern. Other patterns include flying right or left off launch until the limit of visibility, doing one turn back to the spot for a crosswind landing, and other variations. Keep in mind the above points and fly a consistent search pattern. This is why I like planes with high L/D at the expense of low minimum sink rate, they find more lift. When you are confident that your plane will signal lift and that you will be able to core it, the chance of staying up increases as more ground covered. Sometimes you also have to be comfortable with being beyond the point of no return (PONR). Do not fly past the PONR over the lake or Bob’s house.
The searching patterns change when the wind is up and you need to make a BIG flight time. Since you will be drifting with the wind in a thermal, you are much better off finding a thermal upwind, so the plane stays in sight as it drifts and rises. In this case, come off the winch and fly diagonally upwind looking for lift. Keep going until you find lift or chicken out. Since you will always be upwind of yourself and the landing zone, you can keep searching quite low and still make it back home. This pattern is easy to fly since the plane is always heading upwind, but it does not cover as much ground as other patterns.
How to fly a thermal
OK, now you have lift and can recognize it as confirmed by numerous hours of practice. Once your plane signals that it has entered lift you must first determine if the lift is usable. Usable means that the lift is strong enough and a large enough area to circle the plane. Pilots who start circling immediately with the first lift signal will often do several sinking circles before determining that the thermal is not usable. You should fly the plane through the lift until the plane leaves the other side of the lift zone, then you know how big the lift area is. Assuming the thermal is circular, this also give you an idea as to how large to start the circle and where the center/core of the thermal will be. Determining that a thermal is large enough to use, and not wasting valuable altitude on weak/small thermals is a major difference between expert and intermediate pilots.
To maximize the height gain, you need to find out two things, what is the optimum speed to fly your plane at in the lift, and where is the best lift within the thermal. The two are not unrelated. A plane will have the minimum sink rate when the bank angle is a shallow as possible, since less energy is wasted in generating circling forces. On the other hand, if the fast rising core is very small, you will give up flying efficiency and minimum sink while circling tightly to stay in the best lift. How do you do this? The key is to vary the size of your circle. Once you have picked your circling direction, open up the size of the circle to define the shape of the good lift zone. When the glider rises more on one half of the circle than the other, re-center the circle and make it smaller. Let the plane drift along at slightly less than wind speed to stay in the core. If it doesn’t rise and fall within the circle, increase the size of the circle to fly at minimum sink. You want to fly the biggest circle possible while the plane is not rising on one half and falling on the other. Keep the speed up since one stall and all that work is wasted. Well trimmed gliders will stabilize in a circle about the right size. At some time, I'll describe the process and plane clues of coring the ideal thermal.
Since thermals get larger with altitude, most of this fiddling around is done at less than launch height. By doing this continuously, the plane will go up, up, up! This is the most important skill in thermalling, keeping your plane in the best part of the lift while flying efficiently. You might have noticed that as the plane gets higher, its easier to get really high, since lift is everywhere. As the thermals merge, they do seem to get stronger and bigger. By the way, when you have made your max time in a contest, sucker all the other flyers into sink by circling like you are really working a thermal, but are really dropping in for your landing pattern. This is another reason not to follow me around in a contest.
Things to practice:
Keeping the glider up is what the game is all about and if you master this, you will be far ahead of me. The prior page is all I know about lift, and it’s not nearly enough. Now go out and practice.
Copyright W. Man-Son-Hing – April 1998.
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