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CHOOSING A SLOPE SITE | Back to Article Index

By Stan Yeo

(sorry there are no diagrams - they take up too much space)

Slope soaring, whether it is done by the birds, full size gliders or radio controlled gliders, is using the upward component of the 'wind' as it travels over a hill to sustain flight without recourse to mechanical or physical effort. The 'wind' when it approaches a hill can do one of two things, either go around it or over it depending on its shape and size (Diagram1). If it is a 'mound' then the majority of the air will go around it, if it is a long ridge then the air has no choice but to go over the top. Sometimes as in the case of a 'bowl' the air is actually funneled inwards and upwards, but occasionally the air travels in the opposite direction at the slope edge i.e. if it is a cliff. Each hill has its own peculiarities and if you want to go it alone or live in a remote area then some idea of how the wind is likely to behave on a particular hill country is a definite advantage.

The Ideal Slope

Ardent slope soar-ing enthusiasts all have dreams of their ideal site, mine would be a series of long ridges all facing in different directions so that the wind would never be in the wrong direction. To start with we need a hill. This hill needs to be:

1. 80 metres plus high

2. Minimum of 100 metres plus wide

3. At an angle of 45 degrees plus

4. Have a clear area in front of it for at least 2 miles

5. Free from trees, hedges and other obstacles that are likely to break up the airflow over the hill

6. Have a landing area free from rocks and other obstacles etc behind the hill

7. A smooth entry to and from the hill for the wind

8. Easy access and parking

This is quite a long list of and quite difficult to satisfy consequently you will find that most slope soaring sites only meet a few of the above requirements. If you look at each of the criteria you will see that one often influences another. To start with some hills that are well over 80 metres high often do not work in strong winds because there are hills in front preventing the air following the contour of the land due to the suction effect created by the strong winds blowing across the valley (Diagram 2). On light wind days, in the absence of this 'suction' effect, the air can follow the contour of the land and produce lift (Diagram 3). Sometimes when there is a hill in front of another and the windspeed / distance between the hills is right a wave is set up that will reinforce any lift produced making for ideal soaring conditions. Unfortunately any slight change in wind direction / speed is likely to have the opposite effect!

Hills that are not very high frequently do not produce good lift in strong winds due the air above the hill travelling so fast that the air below does not have sufficient energy to push it out the way. This results in the air close to the slope speeding up due to the hill and the air above forming a venturi (Diagram 4). Most hills eventually reach this state but the wind speed at which they do will de-pend largely upon the shape and height of the hill.

Ideally the hill should be shaped so that the air is funneled upwards at the bottom and rounded at the top so as not to generate any turbulence that will make landing difficult. Cliffs, par-ticularly those facing out to sea, pro-vide some of the best and smoothest lift you are likely to find anywhere but the sharp edge often found at the top produces a lot of tur-bulence. It is possible to stand on the cliff edge and see the frequency pennant pointing out to sea (always fly with a piece of thin ribbon or wool on the end of your transmitter aerial to indicate wind strength and direction). This disturbs all but the bravest because cliff sites usually have inaccessible bot-toms! The turbulence can be likened to the tumbleweed, often seen in western films, rolling down the street, being blown end over end by the wind (Diagram 5). It has the same effect on the model as well on landing approaches!!! Trees, hedges, and large boulders, apart from being flying hazards, also pro-duce turbulence on the slope and can sometimes affect the suitability of the site and make landings difficult.

Some cliff sites do have round tops. St Agnes Head in Cornwall is such a site. Here we get the best of both worlds, strong clean lift and a turbulence free landing area. 'Bowls' also produce excellent lift when the when the wind is blowing directly into a bowl. Here the lift produced naturally by the slope of the ground is reinforced by air directed into the centre of the bowl by the 'bowl' sides (Diagram 6). The White Horse near Calne in Wiltshire is a typical bowl. This tunneling effect can continue to work to our advantage even when the wind is not blowing directly into the bowl although the main disadvantage of a bowl is usually the limited wind directions it can cater for. Another disadvantage is usually the restricted flying area leading to an increased risk of mid-air collisions.

Try to avoid flying on a 'point' or the end of a peninsula of land because unless the wind is directly onto the 'point' the lift will be poor, even when the wind is ideal the lift area is usually quite narrow (Diagram 7). The air is deflected around the sides of the hill instead of over it. This pro-duces another hazard because the air has now been speeded up due to the venturi effect mentioned previously. Consequently any models that inadvertently stray into this area are go-ing to have great difficulty making their way forward, back into the lift. These side areas are often referred to as 'Death Valley'. Olivers Castle near Devizes, Wiltshire is a 'pointed' slope in a westerly wind.

The landing area should, ideally, be a flat and obstacle free behind the ridge and out of lift. If the landing area is in lift this can make landing more difficult particularly when flying a slippery model because as soon as the nose is lowered the speed builds up very rapidly, which means of course the model lands with more energy and an increas-ed risk of sustaining damage. Some hills drop away sharply behind the slope; 'razor backs' - ideal for dynamic soaring. Here the danger is going too far back and failing to make the landing area due to the severe sink. The advice here is take along a 'foamie' (EPP model) to 'sus' out the landing technique required.

Summary

The best way to find the local slope soaring sites is to ask in the local model shop and enquire if there are any slope soaring clubs in the area and make contact with. Failing this the only other way is to study the local ordnance survey maps, make a list of potentially suitable sites and systematically visit them in turn. A number will be unsuitable but do not despair most areas have a slope site within 25 miles. When you do reconnoitre a site pick a day when the wind is blowing in the right direction for the site under investigation. If the lift produced is flyable you can almost guarantee that something will be airborne using that lift even if it is only an old crow! Another tip mentioned previously is take an EPP model along as well, if you have one, to test fly the slope just in case it has some nasty surprises up it's sleeve! Be 'sensitive' when investigating a site as the land may be privately owned and or the local club may have negotiated an agreement with the landlord and any unauthorized use could jeopardize such agreements.

Finally I hope the above has been helpful. There are dozens of sites in most parts of the country, all they need is a little leg work to find them. The majority will work, particularly with a light-weight floater on a calm summer evening.

 

 

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