RE Museum - Developing thrust

Thrust - the forward force that keeps the plane moving through the air - is produced by the engines.
Most aeroplanes of today have jet engines, which create thrust by burning kerosene fuel and ejecting the hot gases produced at high speed.

As the jet of gases shoots backwards, the plane is thrust forwards by reaction - like the kick of a rifle after a bullet is fired. The gases do not push against the air, as some people think indeed, the presence of air reduces the thrust by slowing down the gas jet. This is one reason why jet planes cruise at high altitudes, where the air is thinner.

Two main types of jet engine are in use turbojets and turbofans. In a turbojet, air is taken in and compressed by a fan, or compressor. Fuel is then sprayed into the compressed air and ignited. The hot gases that are produced spin the blades of a turbine before escaping at the rear through the engine nozzle. The turbine drives the compressor.

The turbofan engine works on the same principle but it has another large fan in front of the compressor. This drives air around the engine as well as through it. A turbofan engine pushes out more air at slower speed than the turbojet. It is more efficient for slower aircraft - such as passenger airliners. Turbojets suit faster aircraft such as fighters and Concorde.

Some planes still use propellers for propulsion, as all the early planes did. A propeller has a number of cambered blades that, like wings, also have an aerofoil cross-section. When the blades spin in the air, they generate a forward thrust in much the same way that a wing develops lift. The propeller is often called an airscrew because pioneer aviators thought it pulled itself through the air in something like the way a screw pulls itself through wood.

Modern propellers have a variable pitch. The angle at which they meet the air is changed automatically according to the conditions of flight. For takeoff the blades need a fine angle to build up maximum thrust at a slow speed, rather like a car in low gear. When in flight, the angle is broader to give the plane as much forward movement as possible for each revolution of the blades.

Many propeller-driven aeroplanes are powered by petrol engines that work much like car engines. Others are powered by a turboprop - a modified jet engine with an extra turbine in the jet exhaust to drive the propeller.

read on: Streamlining and stability