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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 di-
viding 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 throt-
tle 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 effective-
ness 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 en-
gines 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 accel-
erations 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 pur-
pose 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 essen-
tial for safe piloting. Many would-be pilots have been killed because, when encoun-
tering 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 at-
tack) before attempting to pull away from the ground.
Aircraft Performance
Performance 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 ma-
neuvers which are used to measure and quantify these characteristics for each air-
plane. In many cases, flight testing takes place in a competitive environment to select
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