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
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.
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
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
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.
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.
hills the rotor is poor, others strong and reasonably predictable.
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.
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.
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
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.
pilots I know are the ones that practice most!
ENSURE THAT YOU ARE PROPERLY INSURED
contact the BMFA - see links page)
FLY AT ALL TIMES WITH CONSIDERATION
FOR OTHER MODELLERS,
THE GENERAL PUBLIC AND PRIOPERTY.
PLEASE DO NOT UPSET LANDOWNERS OR THEIR ANIMALS!