![]() The two-blade propeller has an efficiency of 0.83. Assume that the power of the engine is proportional to free-stream density. The airplane is powered by a single piston engine of 345 hp maximum at sea level. Consider the single-engine light plane described as follow: aspect ratio = 6.2, wing area = 181 ft2, Oswald efficiency factor = 0.91, weight = 3000 lb, and zero-lift drag coefficient = 0.027. When pitch is reduced, the glide angle increases and the distance traveled will reduce.Consider an airplane with a zero-lift drag coefficient of 0.025, an aspect ratio of 6.72, and an Oswald efficiency factor of 0.9. Crews therefore need to be aware that reduced drag makes speed control on the approach more difficult.ĭrag will also increase if the landing gear or flaps are extended and the airspeed will then decrease unless the pitch attitude is reduced. Both aircraft will cover the same distance but the lighter one will take a longer time to do so.įrom the practical point of view one should remember that although it is well known that winglets reduce drag and save fuel, their effect on speed control may not have been highlighted before. If two aircraft have the same L/D ratio but different weights and start a glide from the same altitude, the heavier aircraft gliding at a higher airspeed will arrive at the same touchdown point in a shorter time. The heavier the aircraft is, the higher the airspeed must be to obtain the same glide ratio. The only effect weight has is to vary the time that the aircraft will glide for. Since the glide ratio is based only on the relationship of the aerodynamics forces acting on the aircraft, aircraft weight will not affect it. Lift/drag ratio also determines the glide ratio and gliding range. Any AOA lower or higher than that producing the maximum Lift/Drag ratio reduces the Lift/Drag ratio and consequently increases the total drag for a given aircraft’s lift. If the aircraft is operated in steady flight at Lift/Drag maximum ratio, the total drag is at a minimum. The maximum lift/drag ratio occurs at one specific CL (Lift Coefficient) and AOA ( Angle of Attack (AOA)). The shape of an airfoil and other lift producing devices (i.e., flaps) affect the production of lift which will vary with changes in the AOA ( Angle of Attack (AOA)). Aircraft with higher L/D ratios are more efficient than those with lower L/D ratios. A ratio of L/D indicates airfoil efficiency. The lift/drag ratio is used to express the relation between lift and drag and is determined by dividing the lift coefficient by the drag coefficient, CL/CD. The lift to drag ratio (L/D) is the amount of lift generated by a wing or airfoil compared to its drag. Lift/Drag Ratioĭrag is the price paid to obtain lift. The decrease in speed will lead to a decrease in drag. In the reverse situation, when during steady and level flight thrust is reduced, the aircraft will start to accelerate in the direction of drag (the speed will start to decrease). The increase in speed will lead to increase in drag. If during steady and level flight, thrust is increased, then the aircraft will start to accelerate in the direction of thrust (will start to gain speed). at the same speed and in the same direction. The airplane will continue flying at the same velocity, i.e. Drag opposes thrust, and acts rearward parallel to the relative wind. It is produced by the dynamic effect of the air acting on the airfoil, and acts perpendicular to the flightpath through the center of lift.ĭrag is a rearward, retarding force caused by disruption of airflow by the wing, rotor, fuselage, and other protruding objects. Lift is a force which opposes the downward force of weight. Thus, the two couples generally cancel each other out. The thrust and drag forces constitute an opposite couple, the effect of which is to pitch the nose upwards. In addition, the line of action of thrust is normally below the drag’s line of action, because engines are normally positioned midway or lower on the aircraft body. This couple has a turning effect on the aircraft, pitching the nose downwards. Since lift and weight do not act in the same line, they make up what is known as a couple. The center of pressure lies a short distance aft of the center of gravity. Typically, the forces created during aircraft operation are arranged as illustrated below. It is the properties of the air, changed by the shape of the wing, that generate the required lift force. The atmosphere (the air) is the medium in which an aircraft operates. Lift and Drag forces and the relationship between them.
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