Before you read this, I suggest you read post 17.16.
In post 17.16, we saw that a plane flies because, when it moves forward with a speed v, the pressure under the wing is greater than the pressure on top of the wing. The pressure difference, Δp creates an upward force, equal AΔp where A is the area of the wings. This upward force is called lift. It arises because the cross-sectional shape of the wing causes air moving over the wing to move faster than air moving under the wing (post 17.16).
A conventional plane moves because it is propelled by its engines.
A glider (or “sailplane” in American English) has no engines. It is pulled into the air by another plane or by a winch at the top of a hill. It then acquires a speed v. As a result of Newton’s first law of motion it continues to move forward with this speed in a straight line. The air moving around the wing then creates lift in exactly the same way as it does for a conventional plane.
But a glider does not go on flying forever – it gradually falls. Why? Newton’s first law assumes that there is no external force acting on the moving object (in post 16.2 I described this by saying “if nothing interferes” with it). But when any plane is flying through air, there is a drag force acting on it that opposes its motion (post 17.17). As a result, the glider loses mechanical energy (post 17.17).
We can think of this energy loss in two ways. If we think of the energy of the glider as potential energy, loss of energy means that it must lose height (post 16.21). If we think of the energy of the glider as kinetic energy, loss of energy means that it must lose speed (post 16.21) – loss of speed reduces the lift (post 17.16) so the glider loses height. Whichever way we like to think of the energy loss, we get the same result. I believe that both explanations are equally valid. When you see two different explanations for the same thing, don’t assume that one of them must be wrong.
The loss of the height is characterised by the glide angle, α, defined in the diagram above. A glider is designed to have a small value for this angle. One way is for it to have long wings and, therefore, a large wing area, so increasing the lift (see the first paragraph). Another way is to design its shape to reduce drag (post 17.17) – this is called streamlining. A glider has a glide angle of around 1o, so it stays in flight for a long time.
Can the pilot make a glider go faster? Yes! He/she makes the glider lose height (by adjusting wing flaps), so converting some of its potential energy into kinetic energy with the result that it moves faster (post 16.21).
The pilot can also make the glider gain height by moving into a current of air that is moving upwards. Making use of this upward-moving air is called soaring.
We can think of there being a collision between the moving air and the wings of the glider. As a result, the glider gains momentum, in the vertical direction, from the air – so it moves upwards (post 17.30). If the idea of a gas (air) colliding with a solid object (the glider) seems odd to you just think of a lot of air molecules (mostly a mixture of oxygen and nitrogen molecules; post 16.30) colliding with the glider’s wings.
What causes these upward-moving currents of air? Air absorbs very little sunlight – that’s why it’s transparent. So, the energy (post 16.21) of sunlight is absorbed mostly by the earth – as a result the ground gains heat (post 16.35). Some of this heat is spontaneously transferred to the layer of air just above the earth (post 16.34 and post 16.35). This heated air expands (post 16.35) – in other words, its volume increases, so its density decreases (post 16.44). The air rises because it is less dense than the air above – rather like an object floating in water (post 17.6). Beacause they consist of air at a higher temperature than the surrounding air, these regions of rising air are called thermal columns or thermals.
We will meet these ideas in a later post about how birds fly.