“There is an art to flying
The knack lies in learning how to throw yourself at the ground and miss.”
Douglas Adams
Many people these days have taken a flight in an aircraft and if not, they have certainly seen them making their way across the sky. We fully accept that planes fly; after all, that’s their job. But fully loaded, some airliners can weigh over three hundred tons. So how on earth can it take-off and remain in the air?
As with most technical subjects, a detailed analysis can be difficult, so let’s keep it simple and look at the basic principles of flight. An aircraft can fly because it can generate lift when it is moving forwards. And the reason for this lies in the shape of the wing. If you take a close look at the cross-section of a wing, you will see that it has a particular curved shape. It is more rounded on the upper surface and flatter on the lower surface. This is called an airfoil section.
The engine supplies thrust to move the aircraft forwards. As a consequence, air moves over the upper and lower surfaces of the wings. Because the upper surface is curved, the air has further to travel than the air flowing on the lower surface. This causes a pressure difference between the two surfaces.
The air below the wing is at a higher pressure than the air flowing over the upper surface of the wing. This has the effect of causing lift to develop. As the speed of the aircraft increases, so does the lift. When the correct take-off speed for that aircraft is reached, it leaves the ground.
Let’s now consider the forces acting on an aircraft once it is in the air. If we take the example of a plane in the cruise, we say that it is in straight and level flight. It is not climbing, descending or turning. It is in balance or equilibrium.
There are four forces acting upon the aircraft. They are;-
Weight – which acts vertically downwards.
Lift – which acts vertically upwards.
Thrust – resulting from the engine which powers the aircraft forwards and
Drag – which acts in the opposite direction to thrust and is caused by air friction.
So in straight and level flight;-
Lift balances Weight and
Thrust balances Drag.
If any of these forces change, as they will throughout a flight, the aircraft will move out of balance and the pilot will use the flight controls to bring it back into balance. Reasons for altering the balance of forces could be the application of more or less power, a change in height or direction or a change in wind direction or strength on the aircraft.
These forces apply to all aircraft from the single-engine light aircraft pictured here right up to a large airliner like the Boeing 747.
Happy Flying.
John Pullen 