How Gliders Fly

When people first heard that gliders don't have an engine, they are all terrified. I believe that is because they don't realise even an aircraft as big as an airliner can fly without an engine. In fact when designing a new aircraft, the aircraft manufacturer has to design it so it will glide for a certain distance without any engine working. How can that be done? Read on

Theory Of Flight

When people first heard that gliders don't have an engine, they are all terrified. I believe that is because they don't realise even an aircraft as big as an airliner can fly without an engine. In fact when designing a new aircraft, aircraft manufacturer has to design it so it will glide for a certain distance without any engine working. How can that be done? Read on

Forces acting on a powered plane and a glider in flight. The lift produced by a glider acts in a direction angled slightly forwards, providing a force to counter the drag produced.

The attitude (how far up or down the nose points) and therefore speed of the glider is controlled by the elevator, which is the control surface on the horizontal tailplane. This is controlled by the forward and backward movement of the control stick in the cockpit. Pushing the control stick forward moves the elevator downwards, lifting the tail and lowering the nose. This will increase the speed. Pulling back on the stick does the reverse, pointing the nose upwards and slows the glider down.

Just like a powered aircraft, gliders can be turned in the air with the ailerons, which are the control surface on each of the wings. Moving the control stick to the right raises the right wing and lowers the left wing, rolling the glider to the right. Moving the stick leftwards does the opposite, and rolls the glider left.

The last primary control is the rudder, controlled by the pilot's feet. The rudder changes the direction that the glider is pointing, but isn't used in turning it. Turning is achieved with the ailerons and rudder being used in combination with each other.

Launching

Being unpowered, gliders need to get airborne somehow. There are 2 main launching methods we use, and some other methods that are used at other sites, which we may visit on an expedition.

Winch Launching

Perhaps the most common method of launching gliders is by winch launch. The idea is that at the opposite end of the airfield, there is a large winch, with a cable running all the way back along the field to the glider. The winch has a very powerful engine, capable of accelerating the glider from a standing start to 60mph in as little as 2 seconds! The glider's wings start producing lift, and the pilot can then point the glider upwards at a 45 degree angle, rapidly gaining height. A winch launch can get you to 1,000ft in as little as 10 seconds on some days. There isn't much in the world that will do this quicker on the right day, not even the Space Shuttle!

After a launch to typically about 1,300ft (but sometimes over 2,000ft when it is windy), the pilot releases the cable and continues the flight from there, hopefully catching a thermal to gain even more height.

Winch launches are popular because they are cheap, quick and can achieve a high launch rate if operated efficiently.

Aerotowing

Gliders are also commonly launched by aerotow. It is a simple idea, you get towed up behind a light aircraft, and then release the tow rope from the glider end and continue the flight as normal.

Aerotowing allows the pilot to choose where he gets dropped off (for example, under a nice thermal!), and at what height the launch is taken to. They are more expensive than winch launches, and have a slower launch rate. Using several tow planes at once can achieve a decent enough launch rate though, with most competitions being launched using this method.

Other Launching Methods

Some other launching methods you might come across are:

Auto tow - The glider is attached to a car by a length of rope, which then drives down the runway, giving the glider air speed and allowing it to fly

Self Launching - This one applies to motor-gliders, and is fairly self-explanatory - the motor glider launches using its own propeller to give it air speed

Bungee - The glider is pulled down a hill with the aid of a bungee rope, until it has enough speed to fly. The bungee rope is then released, leaving the glider to fly

Staying Airborne

If you've just bungee launched a glider off a hill, then? Do you have to go and collect it from the bottom of the hill to have another go? Luckily, there are ways for gliders to stay airborne for hours, or more in a lot of cases. The atmosphere is always shifting around, with air moving both horizontally (as wind) and vertically. If we can position the glider in the upwards moving air, and the air is moving upwards faster than the glider is sinking downwards, we can then gain height. Think of it as a person trying to walk down a fast escalator: if the escalator is moving fast enough, the person will be taken upwards, despite the fact that they are walking down it.

There are 3 basic types of lift that gliders use to stay airborne for lengths of time:

Ridge Lifts

In the case of the glider we have bungee launched off a hill, it can gain height using ridge lift. Bungee launching is only done when there is wind blowing against the hill, for 2 reasons: First, it makes launching the glider a lot easier - it already has some relative airspeed even when it is still on the ground (equal to the wind speed), so achieving flying speed is a lot easier. Secondly, when the glider is launched, it can use the ridge lift generated by the wind blowing against the hill to gain height, and land back on the airfield for another launch.

Ridge lift is the easiest to imagine - the wind blows against a hill, and the hill deflects the air upwards. The glider can fly in this upward moving air to gain height or stay at the same height. The closer to the hill you are, the stronger the lift will be.

Thermals

In the summer (or even in spring and autumn when the sun is shining) different areas of the ground will heat up in the sun faster than others (for example, dark fields). These warmer areas heat up the air around them, and this air eventually breaks away from the ground as a bubble of warmer, rising air. These are called thermals, and gliders can use them to gain height on good days. Since they are roughly cylindrical in shape, gliders will turn in reasonably tight circles in order to stay in them. On a summer's day, you can often see gliders turning in circles inside thermals to gain height.

As the air rises, it cools down, and eventually it will reach a height where the water vapour inside the air will condense, and form a cumulus cloud. These are the clouds that glider pilots look for in order to find where the thermals are located. The white, fluffy clouds you see on a summer's day are all indicators that there is a thermal underneath them.

Waves

This is probably the hardest type of lift to understand, not even the experts When a strong wind blows across a series of obstacles (usually a range of hills or mountains), the air will flow up one of the obstacles and down the other side, up the next one, down the other side and continue like this. This oscillation can continue for hundreds of miles past the obstacles, and can also grow in height far beyond the height of the original mountains or hills.

Many height records have been set in wave, up to the heights commercial airliners fly at, and sometimes even higher. The world record for height in a glider stands at over 50,000ft. Pretty impressive for something that is only using the natural currents in the atmosphere to gain height!

We occasionally get wave at Cranwell on a strong wind day, but the best places to look for wave without leaving the UK are Scotland, Northern England and Wales. We organise an expedition to Portmoak in Scotland every year to try and sample some of these unique conditions!

Landing

Eventually of course, you are going to have to land somewhere one way or the other. Since gliders are so efficient, without anything to help, landing them would be pretty tricky. To get them to land back on the airfield would involve lining up for landing quite a long way away, quite low, in order to reach the ground at the right point. A typical training glider will travel about 30-35ft forwards for every foot in height it loses - equivalent to a glide angle of less than 2 degrees.

Luckily, nearly all gliders have airbrakes fitted in the wings. The purpose of the airbrakes is to destroy some of the lift the wings create, lowering the performance and steepening the glide angle. The effect the airbrakes will vary with how much of them were used, which allows us to control exactly where on the ground the glider will arrive.

If the pilot has judged his approach correctly, it is possible to land the glider very accurately - within a few metres of where it launched in most cases.

Summary

Hopefully this page would have given you basic information on how a glider is launched, how it was controlled and how can it stay airborne. There is a lot more to it all than what has been covered here, but it is all easily picked up as you learn to fly with us. It isn't essential that you know everything here when you come to the airfield for the first time, but understanding the concepts is a good start and will enable you to learn the more advanced theory easily.