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How do Gliders Fly?

So how can a glider stay airborne without an engine? Good question.

Just as in any other plane, it’s the wings that produce the lift that keeps a glider airborne. In a powered plane the engine pulls you forward to generate airflow over the wings. A glider achieves the same effect by flying slightly downhill all the time. In modern gliders the downhill angle needed is very small indeed. They can go 40 or 50 feet horizontally for every one foot of height they lose.

If the glider pilot can find air that is going up faster than the glider is coming down through it, the net result is that the glider gains height. The challenge in gliding comes from finding and exploiting rising air and avoiding air that’s going downwards (“sink”).

There are three main sources of this precious commodity we call “lift”.

Hill Lift

Hill Lift
Hill Lift

If the wind blows against a hill, it is forced up and over the top.  This creates a band of rising air immediately in front of the hill. Provided the glider pilot keeps flying within this band of lift, he can stay up pretty much as long as he likes.

hill lift
Soaring the Bishop Hill at Portmoak. Photo: Phillip Morgan

Portmoak is very well situated for hill soaring as we have two nearby ridges – Benarty and the Bishop hill.  These 'work' in wind directions from north, through west to south. A moderate westerly is ideal. The lift is easy to use, even for beginners, giving us an excellent training environment where the instructor can choose the length of flight required.

Thermal Lift

Thermal Lift
Thermal Lift

On a sunny day a patch of ground may get warmer than its surroundings and heat the air above it, which then rises. Once started, this process can continue, forming a continuous column of air known as a thermal. The air cools as it rises and if it gets cool enough for the moisture in it to condense, we get a fluffy cumulus cloud. This serves as a handy thermal marker for the glider pilot.

By flying from thermal to thermal the glider pilot is able to make her way across country. She climbs in a thermal and then glides downhill to the next. On a good day, a glider can travel hundreds of kilometres in this way.

thermal lift
Cross-country flying in thermal lift

Birds use thermals in exactly the same way. On a fine summer’s day you may see large birds circling up without flapping their wings. The canny glider pilot will often follow the birds and sometimes a bird may return the compliment by joining you in the thermal you found.

Wave Lift

Wave Lift
Wave Lift

Under certain meteorological conditions, once the air has gone over the top of the hill or mountain, it cascades down the other side with considerable energy and then bounces up again. If the airflow is right, a “standing wave” can stretch for miles downwind of the obstruction. The wave peaks can also be many times the height of the hills that originally caused them. The peaks are typically marked by long clouds lying across the wind direction that appear to be stationary even in strong winds. You sometimes see these wave clouds on weather forecast satellite pictures, stretching right across the country.

By flying in the up-going parts of the wave system the glider pilot can climb to great heights or travel many miles along the wave.

wave lift
Wave soaring in Scotland

In contrast to thermal soaring where the glider has to keep stopping to circle, cross-country flights in wave can be amazingly fast because the glider can surf along the crest without needing to pause. Some of our pilots regularly achieve cross-country flights of 500 km or more at speeds of over 130 kph. Mind you, it takes a lot of practice to get that good. For the less expert pilot the sheer thrill of finding the wave and being carried smoothly upwards to several thousand feet is an unforgettable experience.

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