The engines make the plane go forward, by providing thrust. But as the plane is being pushed forwards, it's also being held back by air resistance, called drag. Notice how the thrust arrow is longer?  As long as the thrust is greater than the drag, the plane will move forwards. This is easy to maintain, because the plane has a smooth, aerodynamic shape, and slippery skin, so there isn't much air resistance. The engines are powerful, so there's a lot of thrust. It's easy to build a plane with more thrust than drag, so a plane can easily move forward. There are two other forces acting on a plane. One is its weight, which is the force of gravity on the plane. This makes the plane want to fall. Counteracting this force is the lift, which pushes the plane up.  In the picture above, notice that the lift arrow is longer than the weight arrow. This implies that the lift force is larger than the force of gravity, so the plane will go up. What causes the 'lift' force? It has to do with the shape of the wing of the plane. Lift is provided by the wings. As the plane moves forward through the air, the air moves over and under the wings, like this:  The wing's shape and slight upward tilt cause it to push down against the onrushing wind. Newton's Third Law describes how, for every push or force, there is an equal and opposite force.  The push downwards acts on the wind. But the 'equal and opposite' force acts on the wing, and pushes it upwards. This lifts the plane. What's happening here is exactly what you will feel if you put your flat palm out the window of a car as it is moving. You will feel the force of the wind pushing your hand upwards. When a plane is in a constant rate of climb, the lift force from the wings is actually less than the weight of the airplane. Added to the lift force is an upward component of the thrust, which balances the weight. The plane isn't climbing because the lift exceeds the weight, but rather because the upward component of the thrust, added to the lift, balances the weight, and the forward component of the thrust pushes the plane upwards. We should also point out that lift is sometimes incorrectly described as the result of a difference in pressure between the slower-moving air below the wing and the faster-moving air above (Bernoulli's principle). This effect is present, but is not the cause of lift. |