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How a Thermal is Formed

پنجشنبه 13 آذر 1393 06:34 ق.ظ

To start with, a thermal is formed rather like the shape of a balloon when the sun heats the ground and the ground heats the overlying air in turn. If the heat source is strong, the thermal bubble transforms into a narrow pillar shape or column which grows upwards in continuous fashion, provided the warm air supply is large enough. Some days can exhibit both balloon and column-type thermals, though usually one or the other exists in a certain time period.
A thermal actually comprises the internal upward draft and a cooler descending draft surrounding. In flight, a pilot should expect to encounter both sets of drafts, one shortly after the other. This phenomenon establishes an essential equilibrium in the atmosphere.

Estimating a Thermal Lift

Here is a guide to estimating the thermal strength you will encounter based on the wind speed variation at launch. Assuming: Thermal vertical flow speed = 9/10 of the variation in wind speed at launch. Your average sink rate is 1.2 m/s (240 FPM) and knowing that 1 km/h = 0.278 m/s (1 mph = 88 FPM), we can establish that 4.3 km/h (2.7 mph) of vertical flow is needed to give you sustained flight which is equivalent to 4.8 km/h (3 mph) of wind speed variation felt on launch (4.3/.9). Every 4.0 km/h (2.5 mph) beyond that will add another 1 m/s (200 FPM) of lift.

How to Work a Thermal

Thermaling, or soaring in thermals, consists of remaining as much as possible in rising parcels of air and avoiding descending ones. As long as you maintain a low speed inside the thermal you'll be able to stay in it longer and then pass quickly through the descending currents by speeding up. Remember, turbulence usually exists around thermals, so maintain good control and inflation pressure.

How to Work a Thermal

Working a thermal basically involves flying where the lift is either by circling within it or by flying a figure 8. A combination of techniques is also a good idea. Your climbing motion can be either upward circles or by hovering vertically in rare cases where the wind is right. It is all a matter of experience and instinct, which is why women pilots tend to excel at thermaling!
In more detail, this is what happens: When you encounter a thermal you may feel the sensation of lift on one side of the wing. You should turn toward this lifted wing to enter the thermal. A thermal is stronger at its center and weaker on its rim or edge. You should find the area with the strongest stable lift, which is near the core or center of the thermal and circle to remain within it. The stronger the lift, the more you may be able to widen your turn radius and vice-versa. The air within a thermal is constantly and randomly changing. While flying circles in the thermal, apply sufficient pressure on the brakes, shift your weight towards the inside of the turn and control the brake on the opposite side to produce a smooth, coordinated turn. You must apply only the appropriate amount of pressure on the brakes, which is a matter of experience. That's what makes the difference between pilots in the same thermal.
A basic asset of thermaling skill is the ability to concentrate well. Listen to the indications of your variometer and adapt your maneuvering accordingly. Many pilots tend to alter their turn radius improperly and as a result drop out of the thermal. Those pilots who can detect the thermal drift or core variations best will climb highest. When the degree of lift is steady it means that you are located close to the center of the thermal.
Thermals vary in size. Tighter turns must be made for small thermals, and indeed there are occasions when you will not be able to complete a turn within the thermal diameter. Very large thermals are wide enough to maintain a straight flight or figure 8s, and thus you will not to have to make turns in them.
Thermals vary in their degree of lift. Strong, large thermals can be best utilized with a shallow bank angle. In weak thermals you can climb better by applying weight shift opposite to the turn. Thermal cores may merge into one and as a result it is not uncommon to have two pilots in separate thermals converging at a higher altitude. After landing a typical exchange of words between them would ensue along the line of:

 Thermal
Thermal cores may merge into one.

"I was there first",
"I don't think so. It was mine"
"You followed me" and so forth. In fact, reality shows that they had each climbed in two different thermal cores that went on to merge into one.
A number of pilots claim they can smell the thermal's whereabouts. Indeed, a waft of soil, traces of dust and leaves can be seen, so it is not idle talk.
My suggestion, after some years of flying, is to concentrate on your wing and feel its center of pressure. When you enter a thermal the tip is lifted, thus the center of pressure is to the side of the wing. You must bring it to the center by turning to the lifted side. It is easy to feel the pressure on your canopy if you concentrate a bit. In order to locate the thermal's shape just keep the center of pressure in the center. In a weak lift this is an essential technique.

In Search of Your First Thermal

In Search of Your First Thermal
Thermal wind reaches the launch area in cycles.

ترمال در آسمان
Thermal downdraft gives the impression of downwind.

In Search of Your First Thermal
Common thermal behaviour

Spotting a thermal straight after launching is important, especially when hillsides and slopes are not very high. Sometimes thermals tend to stand still against a ridge before assuming their long slender cylindrical formation at a higher altitude.
If a thermal has not assumed its shape because it is expanding by the slope or hillside we get the impression that we are experiencing a general wind such as in ridge flying. This is not the case, but if we steer away from the slope it is likely that we will enter sink and have to make a quick landing. It is better to fly in this area and wait for a stronger thermal.

  termal What Should be Done?

What Should be Done?

After launching try to achieve as much altitude as possible, either by gradually ridge soaring upwards or performing figure 8s in areas of lift. Then find a stronger lift, fly circles once you have ample clearance and finally climb above the slope. The first circle in close proximity to the launching area will be the trickiest to execute, as it needs to be a tight turn to avoid getting too close to the hill.
Thermals can often be found further out from the slope so you will have to track them down after reaching adequate altitude. Should you lose altitude, return to the lift at the slope and try again. Be patient. It's the end result that counts. There will be times when flights are terminated solely due to one minor oversight or error, so do not despair or feel sorry for yourself. As an example, in Spain once there was a group of pilots stuck that was low on a hillside. We had to wait for an hour for a decent thermal, but it was well worth the wait. We got over 2000 m (6,500 ft) of altitude out of it. Many smaller thermals had passed before, but none were able to lift us away from the hill. Sometimes clouds cover the sky and thermal activity is stopped for a while. Try to keep your height by ridge soaring until thermal activity returns.


Convergence ,Thermaling

پنجشنبه 13 آذر 1393 06:31 ق.ظ

Convergence

When a moving air mass meets another, convergence results. Convergence often occurs:

پاراگلایدر
Sea breeze meets a light prevailing wind

1. At mountain peaks when upslope breezes rise up on both sides of the mountain.
2.
When a sea breeze meets a light prevailing wind.
3. When downslope winds encounter each other as they flow down opposite sides of a valley.
Flying in convergence areas is usually very smooth with widespread ascending currents, since such a phenomenon usually occurs in light weather conditions.

Thermaling

Let's deal with this very important topic in a concise manner: A thermal is a warm upward motion in the air. Downdrafts often exist around a thermal since the air must move down to replace the rising thermals.

 Thermals

Instability results in thermals. Unstable days with humidity produce cumulus clouds which are a good sign of the presence of thermals. The so-called "blue thermals" do not produce clouds, since they are low humidity thermals. All thermals of reasonable strength reach the same altitude the dew point or cloud base. Weaker ones dissipate at a lower altitude.

Sources of Thermals

Strong thermals form a column.

Sources of Thermals

Strong thermals reach cloud base.

ترمال

The formation of a thermal cloud

Sources of Thermals

The best way to spot a thermal is to watch cumulus clouds forming or birds circling under them. In addition, thermal sources can be assessed by studying the terrain. An example will illustrate where thermals can be found in general: if we turn the earth's surface upside down after having it thoroughly flooded and observe where the last droplets of water are coming from, we will see that it is from these points or ridges that thermals are likely to originate.

Here are more thermal source keys:

• Light-colored terrain: Especially when surrounded by dark terrain. This is a fundamental source.
• Hills: The ridges of hills are ideal producers of thermals, even in late afternoon.
• Rocks: Despite taking time to heat up, they are good producers of thermals, even in late afternoon.
• Trees and flora: Evening suppliers of thermals.
• Villages and houses: Excellent sources during the day.
• Lee side thermals: Also excellent. But remember lee side means downwind side, and in any significant wind it is advised not to seek them out as its is dangerous. If you lose a leeside thermal you will land in the turbulence of the lee side area.
• House thermals: Reliable thermals commonly known to local pilots, which almost invariably emerge from the same point on the terrain.
• Magic Lift: Common parcels of rising air in a valley late in the afternoon (sometimes in combination with wonder winds). They are steady thermal suppliers.


flying Ridge Soaring

پنجشنبه 13 آذر 1393 06:27 ق.ظ

As we all know, paragliders glide steadily and progressively downwards. To maintain or, even better, gain altitude there has to be an updraft or ascending air current as discussed in a previous section. For flight within thermals we can use the term "thermal soaring" or "thermaling". Winds also provide another opportunity to stay aloft as we shall see below. We call such a practice "ridge soaring" and it is a result of "orographic lift". The combination of "thermal lift" and "orographic lift" is called "convergence lift".

Two pilots glide in different conditions. [Demonstrated in the upper drawing gaining height due to thermal]

Ridge Soaring

When wind blows on to a mountain slope, it is forced to follow the mountain's contour towards the peak. An ascending air current, called "orographic lift", is produced. We can fly up alongside the slope and benefit from this lift. Flying in this lift is called ridge soaring or slope soaring.
The height reached once above the peak will not be that great since the wind flow will resume its horizontal course again once the obstacle of the slope has been passed. Rounded hills and other shapes which do not block the airflow are not very suitable for ridge soaring. Often hills with bowls and ravines are the most appropriate for slope soaring, especially if they help channel updrafts. Ridge soaring allows student pilots to get their first long duration flights. This result often occurs when the wind is stable but may present danger if there is thermal activity because of turbulence.

On days that present a lot of thermal activity all pilots should gain height and stay well above the slope. Student pilots should not fly under such conditions. The feeling of safety near the slope can bring trouble to the pilot. Remember that no one gets injured in the air but on contact with the ground.

Ridge Soaring in Practice:

1. Right of way rules must be strictly adhered to, especially if there are a lot of paragliders in the same vicinity.
2. Figure 8's should be performed into the wind and you must not be tempted to head toward the ridge. Always make your turns heading away from the ridge.
3. Make turns whenever you are in ascending air so that no altitude is lost. 4. Pull gently on the brakes and avoid maximum airspeeds. Reserve some speed and use it when needed.
5. When flying low and close, always shift your weight away from the hill. This way, if you have a one side collapse you will be less likely to turn into the hill as you are ready to correct before a problem develops. Note that too much weight shifting away from the hill will require extra hill-side braking that is detrimental to the wing's performance. Determine your own safety and performance compromise.
6. Do not fly straight ahead when descending air currents are ahead. Bear in mind rotor turbulence produced by trees, rocks and other wings and alter your flight path to avoid undesirable air.
7. Do not attempt to fly in windspeeds of over two thirds (2/3) of your canopy's maximum speed. On many occasions there will be an upward deflected ridge current combined with thermals, which are released on the mountain slope or drift into the mountain. In such a case we may decide to fly in stronger wind because thermals add to the wind felt at launch, but not the true horizontal speed. The windspeed should be measured on the hillside itself at launch and should not fluctuate beyond the range of 15 km/h (9mph) minimum to 30 km/h (18 mph) maximum.
8. The further away we are from the hill the more the windspeed and lift will diminish.
9. Keep in mind that when the wind crosses from perpendicular to the slope, less lift is produced. If it is blowing parallel to the hill, no updraft is produced. In a tail wind sink and dangerous rotor are usually present.
10. Wind tends to accelerate through gaps and close to the slope. There is increased wind due to the "venturi effect" combined with anabatic flow.
11. You should not fly into ravines, because wind tends to be come stronger and turbulent there.
12. Wind speed and direction can change as you go out to land due to the deflecting effect of the mountain lower down to or the release of thermals.
13. You should check your ground speed at regular intervals. One sign that the wind is increasing is a general increase in lift beyond what it was before at a given area of the ridge.
14. You may encounter thermals which change the flow when flying near the ridge on an unstable day.

 


A guide to paragliding

پنجشنبه 13 آذر 1393 06:23 ق.ظ

 

“Pocket Aviation: A guide to paragliding” was written out of love for the sport of free-flight paragliding.
The author, being a paragliding champion and instructor, has endeavored to pass on his knowledge meticulously and consistently. He enclosed all his experience and enthusiasm while writing the handbook. He was able to see its results in practice instructing his students and thus improve the content of his previous book written back in 1997. Sense of humour, the use of simple instructions and the ampleness of chapters involved show the willingness of its contributors.
In his effort, the author was fortunate to have an editing done by Dennis Pagen who embraced the handbookPocket Aviation

Navigate the E-Book page by page by clicking the arrows on each page.
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Reviews:
Dennis Pagen USA: I see the quality of the book and the professionalism of the layout and illustrations.
Cross Country magazine UK: A fantastic book
Mark Andrews Canada: The best paragliding manual I have read                                             See Intro

Preface to pocket aviation by Dennis Pagen

I first met Panayiotis Kaniamos in 1998 on one of my numerous trips to Greece. I was immediately struck by his love of
paragliding and his determination to share his extensive knowledge of the sport with his fellow pilots. He soon showed the Greek version of Pocket Aviation to me.
Even with my limited knowledge of the Greek language, I immediately could see the quality of the book and the professionalism of the layout and illustrations. It appeared that Panayiotis had come up with a good book that his countrymen could use as a learning (and teaching) tool.
When Panayiotis introduced the idea of an English translation of his book and a companion CD, I was happy to see that he wished to expand on the reach of this product through a different language and the widespread CD media.
We jumped at the chance to publish it and bring it to a wider audience. Panayiotis’unique perspective and his extensive research combine to render this book one of the best introduction book to the beautiful sport of paragliding.
I wish you all the best in your quest for adventure in the air. With safety in mind and this book-CD set as a guide, you are
sure to expand your horizons.
Dennis Pagen


Launching paragliding

پنجشنبه 13 آذر 1393 06:19 ق.ظ

Launching

paragliding
Paraglider towed launch, Mirosławice, Poland
Paraglider in Sopelana, Biscay, Basque Country

As with all aircraft, launching and landing are done into wind. The wing is placed into an airstream, either by running, being pulled or an existing wind. The wing moves up over the pilot into a position it can carry a the passenger. The pilot is then lifted from the ground and, after a safety period, can sit down into his harness. Unlike skydivers, paragliders, like hangliders, do not "jump" at any time during this process. There are two launching techniques used on higher ground and one assisted launch technique used in flatland areas:

Forward launch

In low winds, the wing is inflated with a ‘forward launch’, where the pilot runs forward with the wing behind so that the air pressure generated by the forward movement inflates the wing.

It is often easier, because the pilot only has to run forward, but the pilot can't see his wing, until it is above him, where he has to check it in a very short time for correct inflation and untangled lines before the launch.

Reverse launch

Paraglider reverse launch, Mam Tor, England

In higher winds a ‘reverse launch’ is used, with the pilot facing the wing to bring it up into a flying position, then turning around under the wing and running to complete the launch.

Reverse launches have a number of advantages over a forward launch. It is more straight forward to inspect the wing and check the lines are free as it leaves the ground. In the presence of wind, the pilot can be tugged toward the wing and facing the wing makes it easier to resist this force, and safer in case the pilot slips (as opposed to being dragged backwards). But the movement pattern is more complex than forward launch and the pilot has to hold the brakes in a correct way and turn to the correct side so he doesn't tangle the lines. These launches are normally attempted with a reasonable wind speed making the ground speed required to pressurise the wing much lower – the pilot is initially launching while walking forwards as opposed to running backward.

Towed launch

Paraglider launching in Araxá, Brazil

In flatter countryside pilots can also be launched with a tow. Once at full height (towing can launch pilots up to 3000 feet altitude), the pilot pulls a release cord and the towline falls away. This requires separate training, as flying on a winch has quite different characteristics from free flying. There are two major ways to tow: Pay-in and pay-out towing. Pay-in towing involves a stationary winch that winds in the towline and thereby pulls the pilot in the air. The distance between winch and pilot at the start is around 500 meters or more. Pay-out towing involves a moving object, like a car or a boat, that pays out line slower than the speed of the object thereby pulling the pilot up in the air. In both cases it is very important to have a gauge indicating line tension to avoid pulling the pilot out of the air. Another form of towing is ‘static line’ towing. This involves a moving object, like a car or a boat, attached to a paraglider or hanglider with a fixed length line. This can very dangerous because now the forces on the line have to be controlled by the moving object itself, which is almost impossible to do, unless stretchy rope and a pressure/tension meter (dynamometer) is used. Static line towing with stretchy rope and a load cell as a tension meter has been used in Poland, Ukraine, Russia, and other Eastern European countries for over 20 years (under the name "Malinka") with about the same safety record as other forms of towing.[25] One more form of towing is hand-towing. This is where 1-3 people pull a paraglider using a tow rope of up to 500 feet. The stronger the wind, the fewer people are needed for a successful hand-tow.[26] Tows up to 300 feet have been accomplished, allowing the pilot to get into a lift band of a nearby ridge or row of buildings and ridge-soar in the lift the same way as with a regular foot launch.[27]

Landing

Landing a paraglider, as with all unpowered aircraft which cannot abort a landing, involves some specific techniques and traffic patterns.

Traffic Pattern

Unlike during launch, where coordination between multiple pilots is straightforward, landing involves more planning, because more than one pilot might have to land at the same time. Therefore a specific traffic pattern has been established. Pilots line up into a position above the airfield and to the side of the landing area, which is dependent on the wind direction, where they can lose height (if necessary) by flying circles. From this position they follow the legs of an flightpath in a rectangular pattern to the landing zone: downwind leg, base leg, and final approach. This allows for synchronization between multiple pilots and reduces the risk of collisions, because a pilot can anticipate, what other pilots around him are going to do next.


Brakes Controls

پنجشنبه 13 آذر 1393 06:13 ق.ظ

paragliding

Brakes: Controls held in each of the pilot’s hands connect to the trailing edge of the left and right sides of the wing. These controls are called 'brakes' and provide the primary and most general means of control in a paraglider. The brakes are used to adjust speed, to steer (in addition to weight-shift), and flare (during landing).

Weight Shift: In addition to the manipulating the brakes, a paraglider pilot must also lean in order to steer properly. Such 'weight-shifting' can also be used for more limited steering when brake use is unavailable, such as when under 'big ears' (see below). More advanced control techniques may also involve weight-shifting.

Speed Bar: A kind of foot control called the 'speed bar' (also 'accelerator') attaches to the paragliding harness and connects to the leading edge of the paraglider wing, usually through a system of at least two pulleys (see animation in margin). This control is used to increase speed, and does so by decreasing the wing's angle of attack. This control is necessary because the brakes can only slow the wing from what is called 'trim speed' (no brakes applied). The accelerator is needed to go faster than this.

More advanced means of control can be obtained by manipulating the paraglider's risers or lines directly. Most commonly, the lines connecting to the outermost points of the wing's leading edge can be used to induce the wingtips to fold under. The technique, known as 'big ears', is used to increase rate of descent (see picture and the full description below). The risers connecting to the rear of the wing can also be manipulated for steering if the brakes have been severed or are otherwise unavailable. For groundhandling purposes, a direct manipulation of these lines can be more effective and offer more control than the brakes. The effect of sudden wind blasts can be countered by directly pulling on the risers and make the wing unflyable, thereby avoiding falls or unintentional takeoffs.

Fast descents

Problems with “getting down” can occur when the lift situation is very good or when the weather changes unexpectedly. There are three possibilities of rapidly reducing altitude in such situations, each of which has benefits and issues to be aware of. The "big ears" maneuver induces descent rates of 2.5 to 3.5 m/s, 4–6 m/s with additional speed bar. It is the most controllable of the techniques, and the easiest for beginners to learn. The A B-line stall induces descent rates of 6–10 m/s. It increases loading on parts of the wing (the pilot's weight is mostly on the B-lines, instead of spread across all the lines). Finally, a spiral dive offers the fastest rate of descent, at 7–25 m/s. It places greater loads on the wing than other techniques do, and requires the highest level of skill from the pilot to execute safely.

آموزش پاراگلایدر
Paraglider in "Big Ears" manoeuvre
Pulling on the outer A-lines during non-accelerated, normal flight folds the wing tips inwards, which substantially reduces the glide angle with only a small decrease in forward speed. As the effective wing area is reduced, the wing loading is increased and it becomes more stable. However the angle of attack is increased and the craft is closer to stall speed, but this can be ameliorated by applying the speed bar, which also increases the descent rate. When the lines are released the wing reinflates. If necessary a short pumping on the breaks helps reentering normal flight. Compared to the other techniques, with 'big ears' the wing still glides forward, which enables the pilot to leave an area of danger. Even landing this way is possible, e.g. if the pilot has to counter an updraft on a slope.
B-Line stall
In a 'B-line stall', the second set of risers from the leading-edge/front (the B-lines) are pulled down independently of the other risers; with the specific lines used to initiate a stall. This puts a spanwise crease in the wing, thereby separating the airflow from the upper surface of the wing. It dramatically reduces the lift produced by the canopy and thus induces a higher rate of descent. This can be a strenuous maneuver, because these B-lines have to be held in this position and the tension of the wing puts an upwards force on these lines. The release of these lines has to be handled carefully not to provoke a too fast forward shooting of the wing, which the pilot then could fall into.
Spiral Dive
The spiral dive is the most rapid form of controlled fast descent; an aggressive spiral dive can achieve a sink rate of 25 m/s. This maneuver halts forward progress and brings the flier almost straight down. The pilot pulls the brakes on one side and shifts his weight onto that side to induce a sharp turn. The flight path then begins to resembles a corkscrew. After a specific downward speed is reached, the wing points directly to the ground. When the pilot reaches his desired height he ends this maneuver by slowly releasing the inner break, shifting his weight to the outer side and braking on this side. The release of the inner brake has to be handled carefully to end the spiral dive gently in a few turns. If done too fast, the wing translates the turning into a dangerous upward and pendular motion.
Spiral dives put a strong G-force on the wing and glider and must be done carefully and skilfully. The G-forces involved can induce blackouts, and the rotation can produce disorientation. Some high-end gliders have what is called a "stable spiral problem".[24] After inducing a spiral and without further pilot input, some wings don't automatically return to normal flight and stay inside their spiral. Serious injury and fatal accidents did occur when pilots could not exit this maneuver and spiraled into the ground.

fly-ozone




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