Engines
An engine produces a force which acts toward therear of the aircraft which "thrusts" the aircraft forward. For this reason, the force produced by the engine is called thrust. Thrust is the most important
force acting on an aircraft, because regardless of the type of aircraft, ALL need some type of thrust to propel them aloft. Even unpowered aircraft such as gliders need a tow plane to provide an external force to pull the aircraft into the air, where it can obtain airflow over the wings to provide the necessary lift to remain airborne. Hang gliders use foot power to initiate movement prior to "leaping" off a cliff. The most common means of developing thrust on powered airplanes comes from propellers or jets. Whether an aircraft has a propeller, a turbojet, or a turbofan, all of these produce thrust by accelerating a mass of air to the rear of the aircraft. The movement of this air to the rear creates an unbalanced force pushing the aircraft forward.
The Wright brothers made many important things come together for their historic first heavier-than-air flight. One of the most vital was an engine that efficiently produced thrust while not weighing too much. They used propellers - the only effective means available of transferring an internal combustion engine's output into push or pull for the airplane. Propellers are essentially revolving wings situated so that the lift they produce is used to pull or push the airplane.
Most modern high-speed aircraft use a very different type of engine - the jet engine. Jet engines not only look different from propellers, they operate in a very different manner as well. More like rocket engines, jets produce thrust by burning propellant (jet fuel mixed with air) and forcing the rapidly expanding gases rearward. In order to operate from zero airspeed on up, jets use enclosed fans on a rotating shaft to compress the incoming air (and suck it in if the airplane is not going very fast) and send it into the combustion chamber where the fuel is added and ignited. The burning gases keep the shaft turning by rotating a fan before exiting the engine.
Turbojet Engine
Some other jet engines differ from this basic pattern by the way they compress the incoming air. Instead of forcing it down a restricting tube, the tweet's centrifugal flow compressor literally flings the air outward into the compressor section exit, compressing it against the outside wall.
Centrifugal Flow Jet Engine
In a turbojet engine, the inlet area is small when compared to that of a propeller. As the air exits the compressor section of the engine, it enters the combustion chamber where fuel is added. This densely packed air/fuel mixture is ignited and the resultant "explosion" accelerates the gases out the rear of the engine at a very high rate of speed. This chemical acceleration of the air (combustion) adds to the thrust produced by the engine. Most jet fighters have a system called afterburners, which adds raw fuel into the hot jet exhaust generating even more thrust through higher accelerations of the air. The jet generates large amounts of thrust by chemically accelerating the air as the result of combustion. The fact that the jet compresses the air as much as 40 times (depending upon the number of compressor rings) allows the jet aircraft to fly at higher altitudes where the air is too thin for Since the fan is mounted to the same shaft as the core, the by-pass ratio of these engines is determined by dividing the amount of air flowing through the fan blades by the amount of air passing through the engine core.
The engine thrust is controlled by a throttle - one for each engine. As the throttle is moved forward, more fuel is added and the engine rotates faster and produces more thrust. Thrust is also directly related to engine revolutions per minute (RPM); the amount of thrust is often referred to as percentage RPM.
There is a price to pay for the ability to fly at higher speeds and altitudes. That price comes in the form of higher fuel consumption, or is more everyday terms, lower fuel mileage. As a propeller blade turns faster, the tips begin to reach supersonic speeds. At these tip speeds, shock waves begin to develop and destroy the effectiveness of the prop. It would seem, therefore that the most efficient engine would be a combination of the turbojet and a large, slow turning prop. In recent years, these engines have been developed and are called "high by-pass ratio turbofans." The engines use a turbojet as a "core" to serve two purposes: 1) produce a portion of the total thrust, and 2) to turn a huge fan attached to the main shaft. The engine can operate at higher altitudes because the jet core can compress the thin air. The thrust produced by the core is supplemented by having a VERY large fan section attached to the main shaft of the core. The fan draws in huge amounts of air and therefore can turn slow enough to prevent the flow at the blade tips from becoming supersonic. The overall result is: 1) the fan mechanically generates a little acceleration to a large amount of air mass, and 2) the jet core compresses thin air and chemically generates large accelerations to relatively small amounts of air.
The wings are not the only "lifting surfaces" on an airplane. The horizontal and vertical stabilizers are lifting surfaces as well and use aerodynamic lift for the purpose of changing aircraft attitude and maintaining stable flight. Some aircraft also use the fuselage to produce lift (the F-16 is a good example
An understanding or at least "intuitive feel" for the production of lift is essential for safe piloting. Many would-be pilots have been killed because, when encountering an unexpected stall fairly close to the ground, they did not act to get the wing flying again (stick forward to decrease the angle of attack below the stall angle of attack) before attempting to pull away from the ground.