Articles

There just aren't any Thermals!
What do they look like anyway?

- by Peter Moseley

There have been a couple of discussions I've had lately on just how do you manage to spot those elusive thermals - especially those invisible ones!


Well, sometimes they are extremely invisible - none of those brightly coloured flags stuck in them like we might wish for - and then you find you just have to use your plane like some elevated divining rod to track them down. But, at other times, those coloured flags to guide you in are almost there -

I guess the most obvious clue is when you see a cloud of birds wind-milling around in the thermal's ascending column of warm air - either they are just enjoying the effortless lift the thermal affords, or they are feeding on the many insects which tend to get hauled aloft by the thermal - either way lots of birds circling close together points to the bit of sky to aim for. Launch - get up there quickly, before the wind drifts it out of range... you then just have to hope they're all glider friendly birds !

The other big, natural pointer to thermals are clouds, but not just any cloud will do - clouds go through cycles. They're born - they expand and then they disintegrate. You need to 'use them' before they're clapped out and start to fall apart, so if you are looking to use them for lift, you need to study them for a few minutes to see if they're growing or shrinking.

A thermal tends to form when the ground is differentially heated by the sun - sounds a bit mathmatical but pretty obvious really - an area of black tarmac, like a large car park, will get heated far more than a field of grass stood next to it. A dry hill-top will get heated quicker than a damp valley - and it is this differential heating and how it then warms the air just above the ground - heating up more in one area than another - that causes warm bubbles of air to rise up through the surrounding area of cooler air...
And so a thermal is formed.

As the thermal rises up it may have quite a low rate of ascent in the first 1000 feet or so, and be only a few metres, or tens of metres across. But like the roots of a tree, many small thermals tend to join together as they rise, forming wider, stronger thermal 'trunks' which are consequently spaced further apart than the many 'rootlet' thermals. These trunks lean in any wind, getting pushed further downwind as they rise, until they reach an altitude where pressure and temperature allow any moisture to start condensing out to form a visible cloud.

Once a cloud has been blown downwind and 'disconnected' from the 'ground hotspot' that gave it birth - that area of tarmac or buildings - it's life can be sustained for some time by it's moisture condensing into, either rain, or into bigger droplets which give the cloud a darker appearance - condensing releases heat and this maintains the thermal updraft, so worth trying for lift under dark cloud-bases, but remember the wind angling the thermal - at the heights we fly at, you will find the bottom end of the cloud's thermals are some distance into wind from a line drawn vertically down from the cloud's location.

You should avoid collapsing clouds as these in their act of becoming invisible are likely to be 'stealing' heat from the air to evaporate the water droplets back to their invisible form and so are likely to be cooling the air and causing sinking air.

The fact that at the relatively low level we fly at - which can be several thousand feet below cloud level - and that the thermals feeding a cloud may be several in number - be small and be 'bent' by any wind, it is not easy to directly use clouds to pinpoint where to fly to catch the lift - when it comes down to it you are most likely going to have use your plane as that aerial, thermal divining rod - you are going to have to know how to interpret subtle movements of your plane to piece together a mental picture of where the rising air is to be found.


So, what is it that we look for?

To begin you really need a thermalling glider of a reasonable size - 2 to 3metres span - firstly for efficiency - generally the bigger the better they fly - and secondly because too small a plane quickly becomes difficult to see when you start having success at finding lift and go up and up and up...

Also, if you are serious about this rather addictive pursuit of looking for lift, I would really recommend a plane with the ability to come down when you want it to - might sound a little strange when the idea is to try and go up and up - to climb on some fabulous thermal to the stars that you've painstakingly discovered. But believe me, joy can quickly turn to fear as your prized plane starts disappearing ever upwards and you know that your lightly built model is likely to break it's wings if you try anything too dramatic to bring it lower.

Devices to stay in control of this vertical dimension include air brakes, or flaps which will deploy to an angle where they effectively become airbrakes, or spoilerons, which are derived from ailerons, each driven by a separate servo and mixed to a switch on your Tx so that both ailerons can be switched to angle upwards and so produce a reverse aerofoil over that part of the wing. They thus generate downwards 'lift' which pushes the wing down like the 'wing' does on an F1 racing car.

Then you need to set the plane up - trimmed so it will fly straight and true - hands off the sticks it'll fly really straight and so that if you fed in a touch of up it would stall.

Next to effectively use a thermal you need the ability to fly a good circle with a constant bank angle and airspeed and without any surges, dives, spirals, or spins - not always easy when you're trying hard to wring the last bit of lift out of the aerofoil by pushing it to the highest angle of attack just before the stall.

Just practice and more practice gets you close to achieving this, but you will find that working close to the stall, the bits of turbulence in a thermal will often nudge you into a stall and you'll annoyingly loose the few feet of height that you spent the last minute gaining.

So, with plane and pilot ready and we're off to find those thermals.

Now, our flyable airspace may only be 0.5 of a mile or so out in each direction from the flight line and as thermals are pushed along by the wind, they are only going to pass through our flyable airspace for a limited time and the faster the wind the shorter the time that any particular thermal will be within range and be usable. It therefore makes best sense to try and pick up any thermal just as it comes into our flyable airspace, then we can stay with it for the maximum time before it drifts downwind and out of range.

Therefore, if there are no 'brightly coloured, fluttering flags' pointing to a thermal that you could go to - like circling birds - then best to head into wind, climbing steadily and making your way nice and straight to a point about as far away as you feel comfortable but not too far away that you can't see if it slightly lifts it's nose or a wing.

Try to fly steady and straight, because now we are wanting to see if there is any disturbance to the flight path - and you're not going to spot that if you are constantly changing your heading or your speed.

If you're lucky and contact some lift as you're flying away from you and headed for an up-wind position, then great - but if you don't find anything by the time you've got as far away as you feel comfortable then turn cross wind - across left then turn about and head across right, remaining at this distance and keep tacking on left and right runs, going across wind until some lift comes along - blown towards you on the wind...

But how do you spot when you are in lift, or close to lift? And what do you do when you find it?

Let's assume your plane is headed away from you and directly into wind - if you have a well trimmed plane it will maintain a straight line course, that is until it finds a thermal.

If, instead of maintaining a straight line, your plane slightly lifts it's right wing and starts turning a little to the left - this indicates one of two things (we will ignor atmospheric flatulence - I think we all recognise that turbulent wind conditions can conspire to cause a plane to rapidly twist and turn and generally buck around the sky) talking thermals not general turbulence, either there is a patch of sinking air on the plane's left thus the left wing goes down, or ( if you're an optimist) the right wing rises because there is a patch of rising air - a thermal - to the plane's right, pushing up the right wing and so causing the left turn.


If the plane is being steadily pushed to the left, we certainly don't want to let it continue turning left, especially if it's sink to the left that's causing the plane's disturbance. And if we take the optimistic view that it may be a thermal on the plan's right, we can either turn the plane immediately to the right and start a circling flight pattern hoping we connect with the thermal that's somewhere over there - or we could try and pinpoint a little more accurately it's position before we start circling.

True, an immediate right turn might hit it spot on but here it might just be the weak leading edge to the main thermal, in which case, if we delay turning a little then we will make better contact with it and so circle more in the core of the thermal and not at it's weaker edge.

So, let's straighten up back to our original course and fly on for another 3 seconds or so to see if the plane still tries to turn left - if it doesn't then we know any lift is in a position 3 seconds back and to the right (plus, of course, an allowance for the thermal drifting with the speed and direction of the wind during that time), but if the glider does continues to turn left as we progress for those 3 seconds, then by evaluating whether this turn is weaker or stronger than we previously experienced gives an indication as to where in the sky to aim for - ie the bit of sky that's trying to push us away the most - that's where we need head for to circle.

And just keep a steady circle going and allow the wind to drift the glider's circles - the glider will then drift downwind at the same rate as the thermal is drifting and should stay with it until it is flying as far away downwind as you feel comfortable - then stop circling and head directly into wind... looking for the next thermal coming your way.

Of course you may fly directly into a thermal ie it isn't to the left or right, so the planes still going to fly straight, but as you fly into the rising air you will notice the plane start to behave as if it is just slightly tail heavy - it's nose will rise fractionally and any controls you input will have a slightly more 'sloppy feel' to them. Try waiting maybe a second or two to allow your plane to get well into the thermal and then start your circling.

If when you are into a circling pattern and drifting with the wind in that thermal you've painstakingly found, but then notice that the plane is definitely not going higher, and not even maintaining it's height, then it may well not be a thermal that you discovered was giving you lift in the first place. Chances are it was in fact wave lift caused by the air oscillating down then up then down then up... on the downwind side of a hill - and it need only be a small hill. You may have found a height and location where, at today's particular wind speed and wind direction, the upswing part of this wave oscillation is to be found.


Of course if the wind direction and speed don't change then the location of the upswing will be at a fixed location and because you were allowing your plane's circling to drift with the wind - as would be necessary to stay with a thermal - then you've now drifted away from the wave lift location. Best to head back to where you first discovered the lift - if it's still in the same place at the same height it is wave lift, so explore around that height and location to find the 'sweet spot' with the greatest lift.

As can be seen from the drawing- wave lift is not to be found close to the ground - here the wind tends to follow the land's contours and any upswings of air are as the wind rises over obstructions, like here: our small hill and at this point the lift is slope lift - here you would technically be slope soaring.

At the other extreme, the wind flow high up is not disturbed to any degree by the small hill and so there are no usable waves in it.

So, we have to use our glider to find the height at which the hill produces the biggest effect - the greatest amplitude of airstream oscillations downwind from the hill. My best guess, with my current experience - is this is going to be at about 2 to 3 times the height of the hill. Always difficult to estimate heights but I would say that for Goosedale, where we have old pit heaps to the west and the south - both of which I would estimate at about 200 feet higher than Goosedale - I estimate that the 'sweet spot' heights for best wave lift have been experienced with my plane at maybe around 400 to 600 feet up. Of course, once you're at this sort of height, to find the upswing/lift locations, you then have to fly in a line from Goosedale flying field directly into wind and head as far towards the pit heap as you feel comfortable, or until you contact the lift location. If you don't find anything then give it another try at a different height.

And so, back to thermals...
Assuming you've found your thermal and are drifting with it as you circle, should you be doing anything besides trying to maintain a neat non stalling circle? Well, to make the best of the lift, yes. And if you don't want the thermal to slowly modify your position relative to the thermal and progressively 'slide' you out of it, then again the answer is, yes.

Here, your ability to maintain that accurate circling path that drifts on the wind with the thermal is your key to noticing if your glider is remaining centred around the thermal's core. Obviously, if your circling starts to slip away from being centred then parts of your circle will have your plane closer to the thermal core and at other parts, further away and this causes the plane's angle of bank to vary slightly. If your circles are around the core then that part of the circle passing closest to the core will tend to have the angle of bank reduced compared to that part of the circle which passes at a greater distance from the core. Noticing where these changes of bank occur enables you to slightly modify your circling position to get back to being centred around the thermal and in the position of best lift.

There are some days, though, when there just aren't any real thermals to be found. But on a surprising number of such days - even in the winter time - there is often areas of the sky where a good degree of 'buoyancy' is to be found - areas where if you are careful with the gliders turns - don't stall it and don't use much bank angle in the turns - then you can almost maintain the glider's height without the motor running.

To find such areas, you have to again allow the glider to fly straight and true and watch it very, very carefully for those slight turns it might start to make 'on it's own'. You can mentally note where it happens and then carefully turn the glider to criss-cross the suspected lift area 2 or 3 times from different directions - all the time, of course, making allowance for the fact that the lift patch will be drifting with the wind - steadily you're piecing together in your mind, where and how big this gently rising patch of buoyancy is - all mentally computed from the various turns the plane makes 'on it's own' - as you repeatedly send it on these criss-cross 'recky' runs.

I haven't seen this DVD, but looks like it could well be informative and interesting: R/C Soaring Secrets of Thermal Soaring Training Video Preview Trailer.

And an interesting Video, showing what those who are really committed to RC sailplanes can get up to: Brief description of GPS Triangle Contest.

A further interesting fact is how thermal sources change with time of day.

Throughout a sunny day, all those concrete and tarmac areas - all those buildings and short dry grassy fields are being heated by the sun. They, in turn, are heating the air above them which is then readily detached and set free for it's upward journey as a thermal.

These thermal sources are active while the sun is high in the sky, but as the sun sinks towards the horizon in the evening, these hot spots rapidly cool - they have given their heat to generate the thermals throughout the day - now it is the turn of the trees.

Through the day, areas of trees have absorbed the suns heat - warming much more slowly than than the daytime thermal locations, but now, in the evening, they turn out to have the warmest air around and so bubbles of it start to escape from the tree canopy.

So, now is the time, as the sun heads for the evening horizon, to try a bit of thermal hunting above the wooded areas - remembering, as always, to allow for thermal drift caused by any wind that's present.

Some good comments here from a thermal glider pilot: Magic of Soaring - Flat Field Thermal Soaring.

In this article, promoting the ParkZone Radian Sailplane, there are some interesting comments about finding thermals:

..." If the wind has a distinct change of direction, there is a good chance of a nearby thermal."...

..." If you feel the wind strength increase, yet it continues blowing straight into your face, then the thermal is directly behind you."...

..."Basically the thermal will be in the direction that the wind is blowing towards."...
Article and YouTube Video: Thermal Soaring


Now THAT'S a thermal.