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 Flying steadily, a glider is coming down through the air. Some people say it is due to  gravity, others that the earth sucks. However there are many ways of obtaining lift :-

 In all instances, except in the case of dynamic soaring, the difference between the  speed at which the air is rising and the glider descending determines whether the  glider is going up or down.


 Most people are aware of thermals, or rising hot air relative to the surrounding air  temperature. e.g. a patch of black tarmac will be much hotter than the grass  surrounding it. This in turn will heat the air which expands to a lower density and  ascends as a result. You will often see birds circling in a thermal - it conserves their  energy. There is normally sink associated with thermals.     This is thermal soaring.


 Stand on top of a hill with the wind in your face, and that large mass of air below the  horizon has to go up to get over the hill!  This form of rising air we call slope lift, and is  sufficient for most gliders to stay up if the wind exceeds about 8 mph if the hill is  reasonably steep.          This is slope soaring


 Most people have seen waves that appear stationary downstream of a stone despite  the water flowing past. A similar phenomenon occurs downwind of a range of hills.  Get in the correct place in this wave and heights of over 30,000 feet have been  achieved with full size gliders in the UK.(There is associated sink as part of that  wave).                          This is wave soarimg.


 Just like at the bottom of a waterfall where there is a tumble bay, at the top of a hill  there is the equivalent which is called rotor. This is a region of often unpredictable air  and is normally to be avoided, it has been the cause of many accidents over the  years for both models and fullsize.

 With some hills the rotor is poor, others strong and reasonably predictable.

 There are pressure differences in this rotating air, and a combination of extracting  energy from the rotating air and flying through advantageous pressure gradiant  regions* can produce very high speeds, even when many flying in slope lift are  struggling to stay up.  Best attempted only by experienced pilots. It is quite addictive!!                    This is dynamic soaring

 *( air pressure is about 15 lbs/sq.ins, or put another way, 15 x 144 x 2 (don’t forget  there are two surfaces) =4320 pounds over one square foot of wing, which, at 8  oz/sq.ft.loading, only needs lift of 1/2 pound to fly!! This represents a pressure  difference of only about 0.01%!!!  Considering air pressure varies about 10% from a  depression to an anti-cyclone, this is not much of a change, and such differences can  be found in rotor.


 Often thermals pop up even on quite cold days, all that is needed is a temperature  difference. If the slope lift is good, but a thermal pops up to one side it will suck the  air into it that otherwise would be giving you lift, and your lift will become weaker,  often disappearing altogether if the wind is light, and turbulant at the same time.

 If you are flying on the side of a valley, slope lift can often be quite good, but you can  also find that you will fly into wave from the other side of the valley, and will either be  in better lift or sink depending what part of the wave you are in.

 Sometimes you can fly in a given wind strength and direction with excellent lift, yet on  another day with a similar wind the lift is poor. This is possibly due to a ‘temperature  inversion’. As you go up, the air gets colder - hence snow on the top of mountains,  but under certain meteorological conditions, a pool of cold air can sit, particularly on a  valley floor, with the result that the air is not coming up the slope face, but at you. IT  IS ONLY RISING AIR THAT GIVES LIFT. On a typical day, a combination of all of the  above can be experienced, it is the pilot that can best read the air that appears to be  the luckiest, but the more you practice, the more you work hard at it, the luckier you  will get. If the lift is good, the model can be put into a gentle dive by using the trims  such that the increased sink rate matches the speed of the rising air, but the model  is flying faster, often enabling loops, stall turns, reversals or rolls to be performed  without the need to gain height and then dive to achieve the necessary excess speed  required to carry out the desired manoeuvre.

 The best pilots I know are the ones that  practice most!

( contact the BMFA - see links page)  




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