Tuesday, 30 June 2015
Friday, 26 June 2015
Monday, 8 June 2015
How do gliders fly?
Gliders are light, unpowered aircraft. They rely on thermals, wave lift and ridge lift to stay aloft. They also played a key part in the inventing of aircraft.
To get in the air, there are four methods of launching. Aerotowing is one of the most common. It is where a tow plane is attached to the glider by a long rope. The tow plane takes off, pulling the glider with it. The plane takes the glider to the height and location the glider pilot requested and the glider pilot unhooks the rope.
Another common method is the winch launch. Winch launching is where a ground based winch is mounted on a heavy vehicle. The winch quickly pulls in a 1000 - 2500 metre cable made of steel wire or synthetic fiber so the glider rises. When it has risen high enough the pilot undoes it and it sinks to the ground, supported by a parachute.
Auto tows are rarer these days. It needs a hard surface and a strong vehicle attached to the glider by a long cable. First, the driver takes up the slack, then accelerates hard, rapidly rising the glider to heights of about 400 metres.
Bungee launching is where the glider is launched from the top of a gentle hill into a strong wind using a "bungee".The gliders main wheel rests in a small concrete trough. The hook is attached to the middle of the bungee. Each end is pulled by 3 or 4 people. One group runs slightly to the left and the other group runs to the right. Once there is enough tension in the bungee the glider is released and the glider gains just enough energy to leave the ground and fly away.
Gliders don't have engines so they have to generate lift and thrust another way. Once they've been launched they have to find a way to go up, or their flight won't last long. So, to generate lift glider pilots use thermals, ridge lift and wave lift. They circle around in the thermal, which lifts them up. For example if the glider is dropping 1 metre a second, but the air around it is rising at 2 metres a second, it's actually going up. When they reach the desired height they leave the thermal, and soar away. The first glider to enter the thermal chooses the direction, all the other gliders must go that way. To measure the performance of a glider you use its glide ratio. Glide ratios measure how far the aircraft can glide to the amount it drops. For example some modern gliders can have glide ratios better than 60:1 (60 miles gained, 1 mile dropped).
Gliders were invented by Sir George Cayley in 1853 and the Wright brothers. The sport of gliding only emerged after WWI. Germany was, and still is one of the most successful places for gliding. So in the 1920s - 1930s while aviators and aircraft makers in the rest of the world were trying to improve powered aircraft, Germans were making and flying ever more efficient gliders. The first German competition was held in 1920. The record time was 2 minutes and a world record distance of 2 kilometres. Today, at last count there were over 111,000 active pilots, and around 32,920 gliders.
Gliders were one of the first successful aircraft forms. They have changed a lot since they were first flown. They are still widely used but usually for recreation and competition use.