Aircraft operate continuously within the Earth's atmosphere, relying on this gaseous fluid in no less than three distinct ways. First, the atmosphere is essential in generating a lift over the wings to counter the downward pull of gravity. The difference in air pressure, above and below the wing, accounts for the upward force of lift. Second, air contains a critical ingredient, namely, oxygen, which sustains combustion inside any engine. Airplanes therefore need not carry their own oxidizer any more
than you would bother with an oxygen bottle for the family car. Third, and this point is easy to overlook, the atmosphere provides the "working fluid" for the thrust generated by the aircraft engine. Again, the aircraft has no need to carry its own propellant, since it makes use of the air all around it to propel it forward. Propeller-driven aircraft accelerate the oncoming air past an airscrew, which provides the required forward thrust. In the case of the jet engine, air is sucked into the front end, compressed, combusted with fuel in the turbine, and shot out the rear end. In a turbofan, bypass air is brought in and shunted around the engine core itself, and used as the major portion of the total thrust in the exhaust stream.
Four basic forces are involved in the operation of any aircraft (Fig. 3.2). These are lift, weight, thrust, and drag. They occur in oppositely directed pairs, so that aerodynamic lift counters weight, and engine thrust overcomes air resistance or drag. Lift depends in a complex way on airspeed, air density, and a coefficient of lift unique to each wing. It is computed by taking the difference in air pressure above and below a wing, and multiplying that number by the wing area. Large wings, therefore, generate more lift than small wings do. The gravity force, or weight, pulls the aircraft always toward the center of Earth, so lift must continually counteract gravity while the airplane is in flight. In a similar manner, thrust and drag forces act in mutually opposite directions on an aircraft in flight. The thrust vector is pointed forward, while the drag vector is directed backward with the relative wind. These two forces must also balance for stable flight conditions to prevail. If these two forces are not equal, then the aircraft will either speed up or slow down.
Should the all-important airspeed drop below a certain critical value, the wings will stop generating any lift at all, and the airplane will stall. This has nothing to do with the engines. A stalled airplane is simply one whose wings have stopped producing lift. All aircraft must continue to develop lift over their wings until they land, or lose control and plummet to Earth. This applies to the Space Shuttle the same way as it does to any aircraft, more so because the Shuttle is an unpowered glider on every landing (Fig. 3.3). How, then, can the Space Shuttle fly at all? Where does its thrust come from?
Just before landing, the Shuttle actually depends on the force of gravity for its forward thrust. Maintaining its nose-down attitude until just before the touchdown flair, the gravity vector can be divided into two mutually orthogonal vectors, one in line with the direction of travel, and the other normal (perpendicular) to the belly. The vector component normal to the belly is countered by the lift generated by the Shuttle's wing and body, and the vector component in line with the direction of travel effectively serves as the thrust, which is countered by the aerodynamic drag.
The basic airplane maintains positive control during flight by using aerodynamic control surfaces. The three axes of control are pitch, roll, and yaw. Pitch determines the angle of attack, and is controlled by elevators mounted on the horizontal stabilizer or by forward-mounted canards. These small control surfaces move up and down together, allowing the pilot to pitch the nose up or down as desired. Roll determines the degrees of bank, and generally allows a pilot to keep the wings level during cross-country flights, or execute coordinated turns. Stunt pilots may perform barrel rolls while maintaining a constant heading. Ailerons mounted on each wing deflect in opposite directions simultaneously, causing the aircraft to roll about a longitudinal axis. Yaw determines aircraft heading and is controlled by means of a rudder mounted on the vertical stabilizer or "tailfin." An airplane's rudder works exactly the same way as a boat rudder, and serves the same purpose. To make a coordinated turn, the skilled pilot uses both roll and yaw control at the same time.
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