James Freeman
Thermaling Tips
1) Thermal Lore
As you approach a thermal you can expect to find quite strong sink. You often run into alternating short 1-2 second surges of lift and sink as you get closer. Next you will encounter lift.
In almost all thermals there is at least one and often several strong cores of rapidly rising air surrounded by areas of more moderate lift.
To find the core you must first LOOK FOR IT. Too many pilots are simply satisfied to be in lift and contentedly circle in 200 up when there is 800 up to be had nearby.
If you have been circling in a core and then lose it you should have a plan. The best one is to first look upwind first as you generally fall out the downwind side (see books for explanation). Next look downwind.
Finally look crosswind. If there is no sign of the thermal after this search move on, thermals can and do have bottoms that you can fall through but unfortunately searching up is not an option!
At the beginning of the day bubbles predominate. "Classic thermals" occur during the peak of the day which depends on location but as a rule peak ground heating (and hence thermal strength) is around 2.30pm LOCAL SUN TIME. Towards evening smooth, wide weaker thermals are the norm.Turbulence is generally worst during the peak of the day and often near cloudbase.
Thermals are generated by buoyant air. Air becomes buoyant because it is less dense than the surrounding air. Differential heating of the ground causes differential heating of the layer of air above the ground. The warmer air expands becoming less dense and thus buoyant. Perhaps surprisingly the addition of water vapour also makes air buoyant. This is because water vapour is 5/8th as dense as air. So in general we look for areas that will be hotter and or have moisture added as likely sources of thermals. Beware that to much moisture can have a detrimental effect, whereas a little can have a very beneficial effect. A good way to get a feel for this is just take a walk and observe the temperature - if the air feels hotter then the surface you are walking over is a likely THERMAL GENERATOR. Classic generators include dark ground, burnt areas, tarmac roads/carparks, etc. Sand reflects heat so is bad. Some crop paddocks get surprisingly hot, whereas others are cool. The vital point to remember is the concept of differential heating - what you want is a contrast. By a contrast I mean an area that will get hot ext to or better still surrounded by area which are cool. The edges of forests, river banks and lake edges are all potentially good. Areas which heat up fast are good at the beginning of the day. Areas which heat up more slowly can be good towards the end of the day - for example you often find thermals over forests later in the day.
Just because air is buoyant does not guarantee a thermal. Just as water can cling to a ceiling until a drip forms so buoyant air can cling stubbornly to the ground. Before anything happens it must be TRIGGERED to release. A good analogy is to imagine that the ground is the ceiling of a steam room. Anywhere you would expect water to drip from so you can expect thermals to trigger from. In practical terms look for high points. The flatter the round the less significant the high point. In the mountains ridge tops are good but in the flatlands treelines,houses, rockpiles and even lonely telegraph poles all act as triggers.
Wind complicates the picture. Buoyant air can drift with the wind along the ground until it is triggered far from where it was generated. In this case sloping thermals result and you will tend to fall out the downwind side unless you continually centre the core by flying upwind (due to the fact that the thermal rises ~200 fpm faster than you do because even though you are climbing up from the ground you are always sinking down through the air. Alternatively the internal turbulence of the moving mass of buoyant air may cause it to trigger independent of ground features - in this case the thermals will be, perhaps surprisingly, vertical because the source is moving with the wind.
Wind also influences the nature of thermals. Strong winds encourage thermal triggering resulting in short lived bubble type thermals. The air in areas which are protected from the wind can continue to get hotter for longer before triggering - this often results in strong lee side thermals. Crop paddocks often hold onto their heating airmass for longer and can be better thermal generators than the classic ploughed paddock in windy conditions.
Experience shows us that whenever the wind blows thermals will generally be far longer downwind than they are wide, often with several cores lined up downwind.
On any given day thermals tend to remain similar in nature, unless of course there is a large change in conditions.
2) Bank angle
Good thermal pilots do not necessarily bank more or less than average pilots. What they do do is bank as much as is required to position their gliders in the core of the thermal.
Although some authors labour on about optimum bank angles the rule is simple. Bank up enough to stay in the core! Experiment. More bank -> better climb? -> continue banking it up. If more bank leads to a slowerclimb then make shallower turns.
We expect small bubbles near the ground so expect to have to bank it up. Late in the day wider thermals are the norm so shallower turns are usually the go.
So how do you center the core? There are several methods, of which I will mention two.
The standard method is to tighten your turns when the lift drops off (to bring the glider quickly back into the best lift) and to flatten the turns as the lift increases (to fly into the best lift).
The pro method is to fly into the thermal, feel the glider react to the air and then crank (bank it up) when you hit the core - more on this next.
3) Feel
Some pilots have a better natural feel than others, but don't despair its really quite straight forward.
As you correctly point out variometers have some lag. Sure some are faster and more sensitive than others but as a tool FOR CENTERING thermals they basically suck. Heresy to some no doubt but still true.
In a big gaggle at any given comp you will see pilots circling around many different points. Why is it so? They can't all be in the core. The fact that some pilots climb much faster proves the point. These eccentric circles result from what I believe is a total over dependence on variometers combined with the standard method for coring a thermal described above.
OK, here is what happens. Consider a glider flying in a straight line at 24mph (36kmh/10 metres per second) straight across the centre of a thermal. It will take this glider 9 seconds to traverse a thermal 90m in diameter. Lets say this thermal has a 30m or 3 second wide core in the centre. The glider enters thermal and is accelerated upwards. After a lag of say 2 seconds the glider ascends far enough for the variometer to note a change in air pressure and indicate a climb. Military studies indicate it will take about 1 second for the pilot to process this information by which time the glider has entered the core. A further 2+1seconds elapse while the glider accelerates/pressure changes/pilot assimilates change. Just as the pilot notes he is in the core he in reality actually flies out of it. Using classical theory he decides to bank it up when the vario indicates a drop off in lift. This occurs 2+1 seconds later just as the glider exits the thermal. The pilot now banks up the glider which takes a further 2 seconds due to glider response lag. At this stage the pilot is actually 20m past the entire thermal!
You can continue this description on indefinitely however the point is this:
"The classical method of centering a thermal will only work if there is no lag in variometer response, pilot (processor) response, and glider response"
So now we come to the secrets of thermaling - visualisation and feel.
The mark 1 accelerometer.
All of us come equipped with remarkably sensitive accelerometers which are perfect for thermaling once we recognise both their power and limitations. We can sense very small accelerations but feel nothing once the acceleration ceases and we are moving at a constant velocity. Our experience in our cars or in a lift shows us this. We feel the initial acceleration but while travelling at constant velocity we feel nothing until we feel the deceleration as we slow down. Our accelerometer is excellent for thermaling.
Our second key ability is our power of visualisation. Just as we can build up a mental picture of a dark room by wandering around bumping into the furniture we can build a similar picture of the invisible currents of air by flying around and bumping into them.
Here's how its done. Consider our pilot again. The instant he enters the thermal he senses the acceleration. The instant he hits the core he uses all his senses to note the strong surge of lift causing a strong acceleration which combines with a tendency for the gliders nose to pitch up to signal to his brain CORE! The one second processing lag means he is still in the core when the message arrives. Two seconds later he exits the core which he notes as a deceleration (like falling) and the nose of the glider pitching down. One second for processing lag and he initiates his turn. Two seconds later after response lag the glider turns, but this time is still in the thermal.
OK so far so good but we are still going to be plagued by the dual problems of processor and glider response lag. Here is where visualisation takes over. The pilot now constructs a mental picture of the thermal, where he is in it, where he is going, and finally what he needs to do to centre his circle on the core. With each circle more information is added to this mental map until coring becomes as easy asdriving round a round about. In simple terms say you are flying south when you feel yourself fall out of the core. OK you think the core is nore to the north so after a 180 you flatten out your turn for a couple of seconds then resume your circle, you are now circling further to the north and should be closer to the core.
Now we come to refinements. The first improvement is this. Pilot hits core and processes it 1 second later. Knowing that the glider response will lag 2 seconds he initiates an immediate turn - presto he is turning in the core, admittedly perhaps not yet centered but still streets ahead.
The second improvement is to recognise the glider as the extension of your body that it really is. Just as you can feel if I come up and push you so you can feel if a thermal pushes your glider. But how do you tell if a wing is being lifted and differentiate this from a wing which is sinking on the other side, after all they will both result in a roll in the same direction? Lift will be associated with an upward acceleration, cause the gliders nose to pitch up, and if off to one side cause a wing to rise. Sink or less lift (relative sink) will be associated with a downwards acceleration (falling feeling), the gliders nose pitching down, and if off to one side a cause a wing to drop. The bottom line is that differentiating wing lift or drop doesn't actually make that much difference. Why? Because in either case the glider is heading AWAY fromwhere you want it to go! Be your own boss. Don't let yourself be sucked into sink and spat out of lift.
The next refinement is speed control. Linger in lift, speed through sink. This goes for thermals to. Sometimes the core may be too small to circle in. Sometimes the air is so bubbly there are no long lasting cores. We can maximise the time spent in the lift by slowing down as much as possible as soon as we sense lift. Our gliders make this easy for us as the nose pitches up automatically. Don't fight it relax and let it, depending on your speed and altitude (not at 50' please) slow some more. Stall? Oh well slow a little less next time. You will be surprised just how far you can push the bar out when banked up in a strong core. Make sure you have enough height to recover from an unintentional stall before experimenting.
So what is the role of the vario. Well once we are centred it will happily chirp a continuous tone which is good because now we will get limited feedback from other sources. It also remind us we are not centred by showing oscillating lift strength.
There is no substitute for practice and the best way to see how you're going is to go to competitions. You don't need a high performance glider to have fun. I flew 185km (~115miles) in a Moyes XT intermediate glider in my first comp. Ask questions. Read all you can. Buy a copy of Cross Country Soaring by the late sailplane world champion Helmet Reichmann from Amazon.com or the Soaring Society of America - it covers all of this plus speed to fly in great detail. Check out this web site, it contains classic papers on soaring:
http://www.iac.net/~feguy/soaring_symposia/index.html
Thermal Search Patterns
A few people have written to ask for a bit more info about search patterns so here it is.
There are a number of circumstances where a search pattern is particularly useful.
1 When you are low, desperate and in marginal lift
2 Whenever you lose the core
3 Even when you feel you are in a "core" to efficiently look for even better lift
The essence of the search pattern technique is to NEVER lose track of your known "good" lift. You maintain contact with this know lift by centring your search pattern around it. Imagine this lift is situated at the junction of an imaginary cross roads. The 4 imaginary "roads" which lead away from this cross roads represent your search directions. What you do is effectively explore a little way down each of the 4 "roads" which lead away from this crossroads. If, after you explore a little way down a "road" , better lift is not found you return to the crossroads, maybe do a few reassuring circles, then try another road. If better lift is found you circle in that then repeat you search using this new area of better lift to search out from.
Typically a low save might go something like this. First you usually hit a few bumps of alternating sink and lift (pilots usually refer to this air as feeling live). Crank a turn as soon as any solid surge is felt (using the MK I Accelerometer rather than the variometer). Consolidate for a turn or two moving towards area where best surge of lift is felt as acceleration up (not the same a best vario response due to lag).
Check variometer averager to see how you are going. Allow heart beat to return to normal if averager shows positive number but don't dawdle if you've only achieved 50 down. Flatten turn and head in one direction (say north) for say 3 seconds then do a 180 degree turn, fly south for 3 seconds then resume original circle. You have then explored ~100 feet to the north of your known good lift before returning to your circle in this lift. The same procedure can be used to search the other three main directions (E, W, S). You can explore greater or lesser distances by varying the time you fly straight for. Provided you fly the same number of seconds out and back and do an accurate 180 degree turn you should never lose track of your area of known lift. You search distance should be tailored to the expected size of thermals on the day, in the local area, and at your altitude. Initially I usually make fairly nervous little explorations before running back to circle in the center. If experience shows that the sink monster is not lurking nearby I get a little more adventurous. Sometimes their simply is nothing better nearby. If you already seem to be in the best available lift patience is required and endless searching will just lose you altitude so you need to use this technique with restraint.
When you're really low the direction of the first explorations can be critical as you simply don't have enough altitude to explore far. Typically this direction will be either:
1 A continuation of the direction I was going when I hit lift on the basis that I was desperate and probably started turning before I got to the thermal proper
2 Towards any wing lift or area where better climb is felt
3 Towards any circling birds, leaves, etc
4 Towards any likely trigger areas like tree lines, etc
5 Upwind as we tend to fall out the back side of thermals
6 According to the formula:
Turn Direction (in degrees magnetic)= [Dry adiabatic lapse rate + altitude (in feet) - barometric pressure (in hectopascals) / 3 *log (# fairies dancing on head of pin in local area)] + RND(n=360)
The benefits of adopting this search technique in low save conditions are:
2 To minimise the time taken to find the best climb going.
3 To minimise altitude loss and thus minimise the risk of decking it.
A search pattern is the most effective way of ensuring that low save. It is also a logical way to search for lost cores or look for better lift during general thermaling. You will find the core more often if you look for it. You are less likely to miss it if you do a logical search pattern rather than blundering around hoping for the best.
If you look at racing a glider, or long distance XC it is in large part a climbing contest. Inter thermal glides are definitely important (indeed vital) but the fastest pilots over the course are invariably amongst the fastest climbers. If you have ever been fortunate enough to watch the really top pilots like Tomas Suchanek and Manfred Rhumer in action you will see them continuously exploring for the best lift, but usually not for long because they find it, out climb you, and are gone.
I once heard a pilot say "As soon as Tomas flew into this 400 up thermal it changed into an 800 up". Witchcraft? I think not. The truth is that he led the other pilots into the core. Looking for and finding the core, at whatever altitude, is just one of the many secrets of the black art of thermaling.
Submitted by James Freeman 10/1999
