Tutorials and Topics
If you are like I was when I began paragliding, your appetite for information is insatiable. You're goal may be to devour any hints and ideas available from any source you can find. The material here is meant to be one more access point for ideas. I welcome your feedback and will post your additions when appropriate.
New
Polars and flying Footprints
One of the challenging parts of paragliding is discovering the power of the invisible. We can't see the wind, but only its effects. We can't see the ground heating up, but we feel its thermal loft. This essay is about one more invisible aspect of our flight. Specifically, it is about what I have come to call our "footprint".
Footprinting is the ability to visualize your glide path over terrain and against winds of varying angles so that you always know your reach. When you are plotting a path across a gap, trying to reach a thermal source or trying to divine the best course to an landing zone you are footprinting. You don't necessarily know it and you may e able to do it better when you do.
Some of us are familiar with polar charts. They are the plottings of our wings in terms of how the glide ratio or reach changes with air speed. We have our best glide when we are flying at that optimal speed for our wing that maximizes lift relative to drag. At that best speed we have a glide angle that is say 7 to 1. Going faster or slower than optimal reduces our glide ratio, or reach.
The chart below shows such a glide angle when flying toward point A:
Now, keep in mind that as you fly against the wind or with a tail wind the plot shortens or lengthens based on a ratio of the winds effect on your speed divided by your optimal speed for lift. The following chart displays varying headwinds reducing your reach. When the headwind equals your glide speed. Your ground track becomes zero and you descend straight down.
This all makes sense to most of us. What is worth exploring, however is how turns shape your glide and reach. Recall that any deflection in your wing beyond its optimum polar airfoil increases drag's effect on the lift to drag ratio. This ultimately means that your reach is always shortened when you turn. When you turn a lot, your reach is greatly shortened- even if you factor out the new direction of the flight path. Again this is no surprise. We all know that just yanking on your wing for no good reason just gets in your way. But lets start plotting this out and you'll see that we can create a new tool for flight.
In the chart below we describe the compromises in flight performance based on the drag generated in turns. The stronger or tighter the turn, the more drag we generate and the crummier our glide ratio becomes.
Now when we take the same chart, and instead of making it a side view of flight we make it a top view we begin to see our foot print.
We are at the white circle in the chart and have some altitude. The inverted heart is our foot print. If we fly straight (upwards on the chart) we have maximum reach. As we turn toward the right or left our glide ratio decreases. If we try to do a 360 or glide becomes very poor, not only because of the drag but also because of the loss of forward momentum and the pendular effect swinging the canopy so that it is not generating its lifting thrust vertically but in a tilted fashion.
To see this with flight vectors look at the following chart:
Now if you keep this in mind you can project this three dimensional figure over the landscape and better predict for yourself what is reachable and what is not. Things directly in front of you are much more reachable than things off to the side or behind you, if all else is equal.
Now picture the effect of wind upon this footprint chart. Much like the earlier wind charts you can see that the footprint changes in two ways. First the footprint is enlarged or truncated proportionately to the ratio of the wind speed to your optimal air speed. Secondly, your pinpoint, that white speck representing you moves across the foot print proportionately. This is illustrated in the chart below.
The charts will truncate or elongate left or right in a similar way for cross winds.
Now the size of the footprint is not relative in this tutorial It will be bigger or smaller based upon your altitude. Furthermore the varieties in glider performance will also reshape the footprint, but surprisingly little. A more efficient turner, may have a wider footprint. A great racer might have a longer footprint. An all around dog will have a smaller foot print.
One consideration we have left out of the story so far is the effect
of lift and sink. We have also left out terrain effects such as a
sloping field beneath you. But think about it. What effects
should they have on your footprint? We'll explore this in a future
tutorial.
Rob Cureton, January 1999
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Yep, I know. You already know how to turn a paraglider. Pull a little on one side, let up a little on the other side and lean a little into the turn. But have we left something out. Depending on your wing and how efficiently you wish to fly you sure have.
If you read the previous tutorial, (even if you didn't) you are aware that every deflection in your glider reduces your glide efficiency. Of course, we also know that if we don't turn at some point our friends will simply replace us with a bag of flour.
The detrimental aspects of turning are threefold: increased drag, reduced forward momentum, and pendular displacement of your vertical lift vector. Now keep this in mind and ask yourself. If I am ready to change direction is it better to make a quick turn or a slow turn? How much weight shift is enough? How much do you modify lift by adding brakes to both sides?
Lets deal with the easy stuff first. Turn in lift. Easy. If the lift is broad, then turn as broadly as possible. If the core of the lift is narrow, turn in the core. But if the core of the lift is only slightly better than the outer portion of lift, then you will probably gain more altitude circling in the outer lift belt.
If we add pressure effects, you'll get another slant on this. Low pressure days lead to more mixing of lift and generally wider lift areas. Circle wider, if all else is equal. If the pressure is higher than usual, the core is likely to be tighter and the net gain from a tighter circle may pay off better.
Almost all of us have seen wings with a step across them because a pilot is leaning so sharply into his turns that the right wing is a step lower than the left. This lasts for just a moment before the wing pendulums over and takes out the deflection. As you can imagine this is not the most efficient way to turn because it robs some of the lift off the center cell of the glider. The leaning turn is not in itself bad, but an efficient turn requires the lean to work in gradual coordination just ahead of the pendular swing. I'd like to explore the whole concept of a leaning turn further in a future tutorial.
Bear in mind that if you are turning in lift or turning to avoid danger, the key point is safety not efficiency. If, though, you are on a long flight and you want to conserve altitude or maximize reach then turning efficiency is important whether you are in lift or not. So now lets explore the amount of brakes to use for a turn.
The best way to fly is to learn your specific wing. This comes from experience and information. We use experience to get a feel for what good lift and bad sink feel like. Unfortunately we cannot discriminate efficiency of lift and drag when we are turning. The small changes in efficiency are made impalpable when obscured by the compelling centripetal forces of a turn. So instead of experience you need information. Use your vario to plot your descent rate at varying degrees of turn. The best way to do this is with long, high flights in calm air.
More often then not you will find that the most efficient turn is one
where you allow the outside wing to reach its optimum camber for best lift
to drag. On many gliders (even those with a reflex airfoil) this
means a flat profile on the bottom of the wing. The inside
wing should be deflected as little as necessary to carry out the turn that
places you where you need to be. Read the Footprint tutorial above
to conceptualize this. Lastly, use very slight weight shift for most
wings as this prevents secondary deflection from pendular reaction.
Rob Cureton, January 1999
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Stubborn and Spiderly: Thermaling Fun
Working thermal lift is the most exciting part of our flights. It represents teasing magic out of thin air. Consider this: You take a 200 lb.. man and add 35 more pounds of equipment. You throw him off a cliff and instead of falling like a bag of nails, he actually goes up. That's magic. That's the magic of thermals.
But for thermals to work for the man or the woman (or what have you), we need to know how to extract all that is good from their buoyancy and very little of what is bad. This is where stubbornness comes in.
Be clear. Thermals don't especially like us. They often surround themselves in a caustic sheath of sink. If you catch them at the wrong angle they violently attempt to buck us. On hot high pressure days they threaten to twist our canopies into tortellini.
The most insidious of thermals distaste for us, however, is the least visible. Thermals act as glider repellers. Almost any thermal worth our while will commonly send us gently away. If we are enjoying the view or heading to a destination, we just make a little correction to our wing and we are back on track without knowing something wonderful has slipped by.
Basically what is happening is that as we fly near a thermal, its lift affects the near wing by raising it and steering us away. We feel this as a pendulum effect and by the time we stabilize our wing we are past the turbulence and the accompanying lift. Indeed we are possibly in some associated sink. The following figure illustrates the natural course of a glider that is steered away from thermals because of their lifting the inside wing and pointed the glider outward.
In fact, this is not a bad thing when you are learning. In the early stages of flying, you just want your wing stable and over your head. You may not have accrued the confidence necessary to enjoy and exploit thermals with all their associated turbulence.
When you are ready to fly your wing and not let it fly you, you will comprehend the turbulence as lift on the near wing. You can immediately turn into the lift and circle further inward scanning for a core of solid lift.
Turning into to this type of lifting turbulence quickly balances the wing and better insures that you don't hang in the shear zone between the lift and any associated (displacement) sink.
Now, here's the key. Think of a spider sitting in her web. She doesn't need to see a tasty fly land. She is strung in a web that functions like a nerve extender. She senses everything and can tell where it is. You are similarly strung when in your paraglider. The same breadth of wing that allows a 200 pound man to actually defy gravity also allows imperceptible drafts of air to be magnified into the perceptible. Your lines signal to you the strength, location and texture of the lift you encounter. Your task is to learn to read these signals and to take the right action.
If you are numb to their signals you will be sent away from most thermals. If you are sensitive to their signals you can stubbornly challenge their dismissal and tame the thermals for your own purposes.
Rob Cureton, February 1999
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Great Grates: Thermal Concepts
Let's remember, thermals are invisible. At best we can see or feel their effects but when we are also working some ridge lift even those signals become obscured. To find these elusive ghosts we must know their secret nature.
One metaphor for a thermal is to think of them as bubbles. This is handy in a lot of ways in that it captures the tendency of the heated air to rise. Unfortunately this metaphor is exceptionally misleading as well.
An alternate description of thermals is as a column. Again this is a convenient way to represent them and one that I use often. It too, however, leads to misconceptions that can blind you to great thermaling opportunities.
The problem with both the bubble theory and column theory of a thermal is that they misconvey both the manner in which thermals rise and the nature of the boundary between the thermal and the surrounding air.
A thermal, like a bubble in water, rises up through heavier material. The difference between a bubble and the thermals we try to find is that a bubble is usually a different substance than the medium it is rising through. A bubble forms a coherent shape that remains rather constant during its rise. Thermals are made of the same substance as the medium they rise through. They do not form a spherical or bubble shape and the boundaries of a thermal are not sharply defined as are the boundaries of a bubble rising through water.
Thermals are always mixing with the air around them creating a mushrooming effect when the outer layers of the thermal are cooled and displaced by the warmer inner layers. Watch smoke rise and you'll see this cauliflower pattern which reveals the constant blending of the thermal with the cooler air.
When you realize that the thermal and the surrounding air are in constant mix, you can also see why thermals always drift downwind when not constrained by a hill. They separate from their ground source and become a little mushroom floating upward and downwind. This break away reveals why thermals are best not thought of as columns, unless you are close to the ground source.
Because they are made of essentially the same material as the surrounding air, thermals initially tend to cling to their surroundings. Only when their heat becomes excessive or they encounter an obstacle do they break away entirely. This obstacle can be anything that shakes up the air mass at the ground: a tree, a rise, a gully, a ridge, even a moving car or a glider on final approach. The swirling constitution of the thermal and its relationship to the ground create a grating effect. Whatever can serve to agitate the air interacts with the thermal and facilitates its rise.
I believe it was Dennis Pagan who has written about an effective concept for identifying thermal grates. PIcture turning the landscape upside down. Now, if it were wet where would the drips be falling from. Water will drip from a stalactite like source that helps strip the water from its adhesion to the surface. A thermal separates from its sticky air mass the same way.
Knowing this means that an effective thermal search involves looking for grating sources. These will be sources of agitation to a pool of hot air. Look for a rise of any kind downwind from a dark field. On a ridge which is receiving a lot of sunlight, look at a vertical prominence that can separate tease apart warmer from colder air as a thermal scrapes against it. Anything that displaces air vertically or horizontally can liberate thermals.
Once you have found thermals you will want to know how to core them better than before. That will be the subject of the next tutorial.
Rob Cureton, February 1999
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Okay. Now you've got it. You know how to read the ground for thermal releasing protuberances. You know how to use your wing as an enlarged nervous system that signals to you the strength, location, and condition of the thermal you bump into. Now, you keep wondering why the thermals are so weak and so slippery. Why is it some pilots can peg the core of it and climb to 5000' why you skin the edge of it and fall by the wayside sinking back down to scratch for every inch.
First lets rule out some big factors. They really might have been lucky. At the right place at just the right time. But you'll know if its luck or not by the pattern. Do they regularly out fly you? If so you need a little guidance. Consider also their glider. Trust me it makes a difference. Sink rate is vital. Optimal airspeed for the wing is vital. Glide ratio is vital. Now I wouldn't trade safety for these factors, but if the safety is the same, then pick a low sink rate for a slow wind site and a fast great glide for a faster wind site. If you have such a glider already and you are regularly being out flown you need to sharpen your flying judgment. Just a little tightening here and there. No big deal.
First lets look at skinning. You felt the tug of lift, you know its there but you can't ride it up. If you examine the figure below you will see a cervical cross section of a thermal. It is shaped like a hot air balloon. Now remember when you turn you sink faster. If you turn sharply you sink much faster. You must always divine the optimal amount of turning to stay close to the core without turning so tightly that your increased sink rate exceeds the loft provided by the thermal.
You will also notice that the timing with which you enter the thermal makes a huge difference. Except close to the ground during the thermal lift off, thermals have already broken away and take the shape as defined in the following figure. If you turning diameter is say 60 feet then you can expect to core this thermal only if you enter at point Y or above . Below point Y you can dart into and out of the thermal without grabbing enough of it to overcome the increased sink rate that comes from your turns.
Take this a step further. When the thermal is weak and drifting it will take on a tilted aspect like the following figure. Your circles still has to fit within this elongated core without protruding at the periphery. This elongated core matches the elongation of your own circle over the ground, but within the air mass your circle is not necessarily elongated. Your task is to still fit your curving flight path into the thermal at point Y or higher.
If you can quickly discern that the thermal is too weak or that you have hit it below point Y, then you can quickly move on in your search for usable thermals. This quick screening is often the talent of the superior pilots. Do not waste time on thermals that suck the life out of you. Its better to return to the ridge and scan for better candidates.
Once you have found a decent candidate, and you are clear that you are above point Y, your work continues. In the thermal you do as much searching as you did to find it in the first place. Use your wing to sense the strongest section of lift and try to spend as much time their as possible. If it is big enough circle broadly. This keeps your wing more horizontally and induces less drag so your overall rate of climb is better. If the core is tight an oval can work because it concentrates your lowest sink rate (the straight sides of the oval) right over the core and has you turning in the diminished but acceptable lift zones.
Now, keep in mind that the more quickly you can find the core, the grater your advantage. I will help you with this, because it is not intuitive. The core is rarely at the center of the thermal.
If you read the previous tutorial you are aware that the thermal is always peeling. It is shedding its outer skin as it mixes with the cooler air surrounding it. Where the mixing is greatest the core is closer to the boundary. This will most often bee the windward side of the thermal. The thermal is rising with a vertical vector. The wind is blowing with a horizontal vector. the two meet with a shearing action that accelerates the peel off of the thermal. This almost always places the core of the thermal on the windy side.
Keep in mind also that the width of the thermal tends to be broadest at this same interface with the wind. The thermal shape when viewed from above looks similarly to its shape from the side. Keeping this in mind means that until you find the core your search will be more efficient if you begin at the windward boundaries of the thermal.
Lastly, it is important to qualify this information. In the real
world thermals take a fluid and changing shape that is influenced by wind,
other thermals, pressure, ridge and ground terrain. Think especially
about how altitude and its pressure differences changes the shape and flow
of a thermal and you'll begin to get the picture. You can never fully
know a thermal, but armed with skills of better anticipation your flying
can become more efficient and more successful in the long run.
Rob Cureton, February 1999
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