PDF Aircraft Flaps and Slats PDF

38 FLAP & SLATS “Flaps are hinged surfaces mounted on the trailing edges of the wings of a fixed- wing aircraft to reduce the speed at which an aircraft can be safely flown and toincrease the a...

38 FLAP & SLATS “Flaps are hinged surfaces mounted on the trailing edges of the wings of a fixed- wing aircraft to reduce the speed at which an aircraft can be safely flown and toincrease the angle of descent for landing. They shorten takeoff and landing distances. Flaps do this by lowering the stall speed and increasing the drag. 6. Slats are aerodynamic surfaces on the leading edge of the wings of fixed-wing aircraft which, when deployed, allow the wing to operate at a higher angle of attack. A higher coefficient of lift is produced as a result of angle ofattack and speed, so by deploying slats an aircraft can fly at slower speeds, or take off and land in usually shorter distances. They are used while landing or performing manoeuvers which take the aircraft close to the stall, but are usually retracted in normal flight to minimize drag. FLAPS 7. The general airplane lift equation: where: □ L is the amount of Lift produced, □ is the air density, □ V is the indicated airspeed of the airplane or the Velocity of the airplane, relative to the air □ S is the platform area or Surface area of the wing is the lift coefficient which is determined by the camber of the airfoil used, the chord of the wing and the angle at which the wing meets the air (or angle of attack)? 8. Here, it can be seen that increasing the area (S) and lift coefficient ( amount of lift to be generated at a lower airspeed (V). Flaps also increase the drag of Aircraft.) SLATS 9. Slats. Slats are like flaps only but extended over the leading edge of the wings.  39 10. Types of Slats. (a) Automatic. T he Slat lies flush with the wing leading edge until reduced aerodynamic forces allow it to extend by way of aerodynamics when needed (b) Fixed. The Slat is permanently extended. This is sometimes used on specialist low-speed aircraft (these are referred to as slots) or when simplicity takes precedence over speed. (c) Powered. The Slat extension can be controlled by the pilot. This is commonly used on airliners. STALL 11. A Stall is a reduction in the lift coefficient generated by foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded. The critical angle of attack is typically about 15 degrees, but it may vary significantly depending on the fluid, foil, and Reynolds number. Stalls in fixed- wing flight are often experiencedas a sudden reduction in lift as the pilot increases angle of attack and exceeds the critical angle of attack (which may be due to slowing down below stall speed in level flight). A stall does not mean that the engine(s) have stopped working,or that the aircraft has stopped moving. The effect is the same even in an unpowered glider aircraft. 12. A Stall is a condition in aerodynamics and aviation wherein the angle of attack increases beyond a certain point such that the lift begins to decrease. The angle at which this occurs is called the critical angle of attack. This critical angle is dependent upon the profile of the wing, its platform, its aspect ratio, and other factors but is typically in the range of 8 to 20 degrees relative to the incoming wind formost subsonic airfoils. The critical angle of attack is the angle of attack on the lift coefficient versus angle-of-attack curve at which the maximum lift coefficient occurs. 13. Flow separation begins to occur at small angles of attack with attached airflow over the wing still dominant. As angle of attack increases, the separated regions on the top of the wing increase in size and hinder the wing's ability to create lift. At the critical angle of attack, separated flow is so dominant that further increases in angle of attack produce less lift and vastly more drag. 14. A fixed-wing aircraft during a stall may experience buffeting or a change in attitude. Most aircraft are designed to have a gradual stall characteristics that will warn the pilot and give the pilot time to react. For example, an aircraft that does not buffet before the stall may have an audible alarm or a stick shaker installed to simulate the feel of a buffet by vibrating the stick fore and aft. The critical angle of attack in steady straight and level flight can be attained only at low airspeed.  40 15. Stalling Speed. Stalls depend only on angle of attack, not airspeed, however, because a correlation with airspeed exists, "Stall Speed" is usually used in practice. It is the speed below which the airplane cannot create enough lift to sustain its weight in flight. In steady, unaccelerated (1g) flight, the faster an airplane goes, the less angle of attack it needs to hold the airplane up (i.e., to produce lift equal to weight). As the airplane slows down, it must increase angle of attack to create the same lift (equal to weight). As the speed slows further, at some point the angle of attack will be equal to the critical (stall) angle of attack. This speed is called the "stall speed". The angle of attack cannot be increased to get more lift at this point and so slowing below the stall speed will result in a descent. Airspeed is often used as an indirect indicator of approaching stall conditions. The stall speed will vary depending on the airplane's weight, altitude, and configuration (flap setting, etc.) THRUST 16. Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude in opposite direction. Propeller A propeller converts shaft power from the engine into thrust. It does this by accelerating a mass of air rearwards. Thrust from the propeller is equal to the mass of air accelerated rearward multiplied by the acceleration given toit. A mass is accelerated rearwards and the equal and opposite reaction drives the aircraft forwards 17. Thrust is the force which moves an aircraft through the air. Thrust is used to overcome the drag of an airplane. Thrust is generated by the engines of the aircraft through some kind of propulsion system. 18. Thrust is a mechanical force, so the propulsion system must be in physical contact with a working fluid to produce thrust. Thrust is generated most often through the reaction of accelerating a mass of gas. Since thrust is a force, it is a vector quantity having both a magnitude and a direction. The engine does work on the gas and accelerates the gas to the rear of the engine; the thrust is generated in the opposite direction from the accelerated gas. The magnitude of the thrust depends on the amount of gas accelerated and on the difference in velocity of the gas through the engine. 19. Acceleration of Gas produces thrust propelling aircraft forward.  41 PROPELLER 20. The propeller blade is an aerofoil and the definitions for chord, camber, thickness/chord ratio and aspect ratio are the same as those given previously for the wing. The propeller accelerates a large mass of air rearwards thereby propelling the Aircraft forward. SUMMARY 24. The flight cadets should thoroughly be conversant with the above basic concepts of level flight for better understanding of aerodynamics.