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AVIATION TECHNOLOGY 12
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Theory of Flight How does an aerofoil produce lift? ■Air flows faster over the upper surface creating an area of low pressure. And air below the wing is compressed. The resulting, forces lift the wing. ■The camber and angle of attack force the air behind the wing down. And with Newton's third law of action(state; To each action there is an equal and opposte reaction), there is an opposite reaction which is to force the wing up. Angle of Attack ■Maximum lift is generated just prior to the stall, with a conventional wing. This is 16 to 18 degrees angle of attack. Drag ■Parasite drag - Form drag is the resistance created by the form or shape pf the airplane as it passes through the air. -Skin Friction referes to the tendency of the air to cling to the surface of a body over which it is flowing. ■Induced drag - is skin friction. The drag produced as a result of the wing producing lift. Wing tip vortices ■When the two airflows unite at the trailing edge, they are flowing contra-wise. Eddies and vortices formed which tend to unite into and large eddy at each wing tip. This is called wing tip vertices. ■How does it cause? - Air flowing inward over the top and outward from under the bottom of the wing cause wing tip vartices. At the tip, the air rolls over the tip in an inward cicular. ■If wing tip vortices are reduced, induced drag is reduced. An example of an aerofoil with little induced drag is the wing on a Glider. The opposite extreme is the Concord with its delta wing; its high angle of attack and high wing loading (weight) on take off produce vortices capable of breaking up a light airplane. ■The wing tip vortices by a Boeing 747 passenger-carrying aircraft are greater on take off then they are on landing. Because, the strength of vortices is directly proportional to weight. On take off, the Boeing 747 is fully loaded. On landing, it has burned off most of its fuel.
■The minimum wake turbulene separation specified in the regulations for a light airplane behind a heavy airplane is 6 miles. Lift/Drag ■The ratio of LIFT and DRAG is important. If the wing is producing sufficient LIFT to maintain altitude and, at the same time, the DRAG is reduced to a minimum, the wing will be operating at its most efficient angle of attack. This is known as the BEST ot MAXIMUM LIFT/DRAG RATIO. ■At A conventional wing will attain its Maximum L/D ratio at +4 degrees of angle of attack. In glide, you should fly at a speed to give the MAX L/D RATIO. ■The boundary layer is a thin of air over the surface of the wing that moves at less than the free stream speed. Its flow is smooth and follows the curvature of the wing. As it nears the trailing edge, it breaks away and a small turbulent wake is formed. ■As the aerofoil approaches its stalling angle: -What happens to the airflow over the top pf the wing? =The boundary layer separates from the upper surface creating a turbulent layer over the surface of the wing. -What happens to the Center pressure? =It moves forward. ■To reduce aileron drag, most light airplanes have both differental and frise capacity in the design of their ailerons. ■Engine failure causes the nose to automatically drop. The couple between Lift and Weight assist this movement. The angle of incidence -the angle the wing makes with the longitudinal axis. The angle of attack -the angle the chord maes with the relative airflow. ■Wash Out not only reduces the stalling angle at the wing tip thus retaining aileron cannot into the stall, it also reduces wing tip vorties and consequently, the induced drag. ■A flap increases the camber of the wing, inreasing the lift but also the drag. -The advantages of using flaps on approach and landing are; the use of flaps results in a shorter landing roll. By reducing the Vs and allowing a slower approah speed (with a sab margin above stall), the airplane lands at a slower touch down speed. Flaps allow a steeper approach, with improved visibility because the nose is lower. -The disadvantages of using flaps on approach and landing in a light airplane are; the increased lift makes the airplane more susceptible to gusts. The lower speeds, and the blanking effect that the flaps have on the rudder, reduce the crosswind capability. ■An aft Center of Gravity will cause Longitudinal instability. ■Slipstream and asymmetric trust tend to yaw a propeller-driven single-engine airplane to the left in a climb. Best Rate of Climb (Vy) -the most gain in height for the least time. Best Angle of Climb (Vx) -the most gain in height for the least distace. ■In a turn an airplane is not in a state of equilibrium. ■In a level turn at a constant airspeed, if the bank is increased, the load factor increses, the rate of turn increases, the radius of turn dicreases, an the stall speed (Vs) increases. ■The stall speed inreases with the application of G or load factor. If Vs at 40 knots is 1G, Vs become 56 knots when in a 60 degree bank level turn. -(40 times the square root of 2G = 40 x 1.4 = 56) ■The use of anti-spin aileron (right aileron in a left spin) aggravate the spin because in increases the angle of attack the downgoing wing, accentuationg its stall conditioin, increasing its drag and reducing its lift. The end result is that the spin tightens up; the airplane rolls about the spin axis daster. ■Lift acts through the Centre of Pressure. As the angle of attack increases, the C of P moves forward until the wing reaches the stalling angle, then it moves Backward. This movement tens to rotate the airplane and Decreases the angle of attack. ■For a particular wing, the angle of attack never varies t the stall? ■The inward component of lift makes the airplane urn it is bankes. ■How does frost on the top of the wing reduce lift and increase drag? - The roughness spoils the smooth airflow and causes separation of the boundary layer. When that happens, the low pressure on the top of the wings breaks down, reducing lift. Further, the rough surface amplifies the many little vortices on the trailing edge, increasing the drag. ■The procedure for recovery from a spiral drive - Close the throtte, apply full opposite rudder, contril olumn forward. When the spin breaks, centralize the rudder, level the wings with aileron an ease put the drive. ■Wheelbarrowing is a situation where the airplane runs down the runway on its nose wheel. -The end resulte could be- any slight disturbance to directional control will cause the airplane to ground loopa(pivot violently about the those weel). -Causes could be- the pilot holds the control colummn hard forward on take off so the main wheels become airborne, OR the pilot shves the nose own when attempting to land at too high an airspeed -How does one prevent- follow the recommended take off and landing speed and techniques. Ground Effect ■The wings of a high wing airplane are not so near the ground as those of a lw wing airplae, conseq2uently the reduction in the induced drag is not so prononced. ■With a low wing airlanne, approximately by about 48% could you reduce the induced if, on take of, you raised the nose well above normal. ■At height of one half of the wing span is the majority (about 85%) of the groud effect no longer effective. ■In winter, if you do not remove the frost from the wing and you attempt to take off, you might have those problems; the frost causes the boundary to break up so that lift is not being generated by the top the wing. If te plot raise the nose high enough, the airplane becomes airborne because of the ground effect but has insufficient lift to sustain flight when the ground effect disappears. ■In flight training, Soft Field Take Off is ground effect utilized to the pilot's advantage. |