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[49], 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.
Aircraft Performance
Performance[50] generally refers to the motion of the airplane along its flight path, fore and aft, up or down, right or left. The term «Performance» also refers to how fast, how slow, how high and how far. It may also refer, in general sense, to the ability of an airplane to successfully accomplish the different aspects of its mission. Included are such items as minimum and maximum speed, maximum altitude, maximum rate of climb, maximum range and speed for maximum range, rate of fuel consumption, takeoff and landing distance, weight of potential payload, etc. There are specific maneuvers which are used to measure and quantify these characteristics for each airplane. In many cases, flight testing takes place in a competitive environment to select the best airplane for accomplishing a particular mission. Since all of these performance measurements are strongly affected by differences in the weather conditions (that is, temperature, pressure, humidity, winds), there are some very specific and complex mathematical processes which are used to «standardize» these values.
One of the most important considerations in flight is the balance of forces maintained between thrust, drag, lift, and weight.
Balance of Forces
An aircraft in flight retains energy in two forms, kinetic energy and potential energy. Kinetic energy is related to the speed of the airplane, while potential energy is related to the altitude above the ground. The two types of energy can be exchanged with one another. For example when a ball is thrown vertically into the air, it exchanges the kinetic energy (velocity imparted by the thrower), for potential energy as the ball reaches zero speed at peak altitude.
When an airplane is in stabilized, level flight at a constant speed, the power has been adjusted by the pilot so that the thrust is exactly equal to the drag. If the pilot advances the throttle to obtain full power from the engine, the thrust will exceed the drag and the airplane will begin to accelerate. The difference in thrust between the thrust required for level flight and the maximum available from the engine is referred to as «excess thrust». When the airplane finally reaches a speed where the maximum thrust from the engine just balances the drag, the «excess thrust» will be zero, and the airplane will stabilize at its maximum speed.
Notice that this «excess thrust» can be used either to accelerate the airplane to a higher speed (increase the kinetic energy) or to enter a climb at a constant speed (increase the potential energy), or some combination of the two.
