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CHOOSING
A SLOPE SITE |
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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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