The lift then increases as the square of the airspeed. The aerofoil is often cambered and/or set at an angle of attack to the airflow. Lift can be created when an aerofoil-shaped body travels through a viscous fluid such as air. A glider flying faster or slower than this airspeed will cover less distance before landing. This airspeed (vertical line) corresponds to the tangent point of a line starting from the origin of the graph. It is the flattest possible glide angle through calm air, which will maximize the distance flown. Polar curve showing glide angle for the best glide speed (best L/D). Parasite plus Induced) Coefficients of drag C D and lift C L vs angle of attack. L/DMAX occurs at minimum Total Drag (e.g. The L/D ratios for hydrofoil boats and displacement craft are determined similarly to aircraft. The L/D ratio can also be used for water craft and land vehicles. This results directly in better fuel economy. The L/D ratio is inversely proportional to the energy required for a given flightpath, so that doubling the L/D ratio will require only half of the energy for the same distance travelled. It depends principally on the lift and drag coefficients, angle of attack to the airflow and the wing aspect ratio. The L/D ratio is affected by both the form drag of the body and by the induced drag associated with creating a lifting force. It is measured empirically by testing in a wind tunnel or in free flight test. The L/D may be calculated using computational fluid dynamics or computer simulation. In almost all cases the graph forms a U-shape, due to the two main components of drag. These vary with speed, so the results are typically plotted on a 2-dimensional graph. The term is calculated for any particular airspeed by measuring the lift generated, then dividing by the drag at that speed. For a glider it determines the glide ratio, of distance travelled against loss of height. The L/D ratio for any given body will vary according to these flight conditions.įor an aerofoil wing or powered aircraft, the L/D is specified when in straight and level flight. It describes the aerodynamic efficiency under given flight conditions. In aerodynamics, the lift-to-drag ratio (or L/D ratio) is the lift generated by an aerodynamic body such as an aerofoil or aircraft, divided by the aerodynamic drag caused by moving through air. If the pilot changes the throttle setting, or increases the wing angle of attack, the forces become unbalanced. The aircraft will move in the direction of the greater force, and we can compute acceleration of the aircraft from Newton’s second law of motion.Measure of aerodynamic efficiency Lift and drag are the two components of the total aerodynamic force acting on an aerofoil or aircraft. With a constant ground speed, it is relatively easy to determine the aircraft range, the distance the airplane can fly with a given load of fuel. The motion of the aircraft is a pure translation. The ground speed is equal to the airspeed plus the wind speed using vector addition. If we take into account the relative velocity of the wind, we can determine the ground speed of a cruising aircraft. The aircraft maintains a constant airspeed called the cruise velocity. While the weight decreases due to fuel burned, the change is very small relative to the total aircraft weight. In this situation the lift is equal to the weight, and the thrust is equal to the drag. The closest example of this condition is a cruising airliner. In an ideal situation, the forces acting on an aircraft in flight can produce no net external force. If there is no net external force, the object will maintain a constant velocity. From Newton’s first law of motion, we know that an object at rest will stay at rest, and an object in motion (constant velocity) will stay in motion unless acted on by an external force. If the size and direction of the forces acting on an object are exactly balanced, then there is no net force acting on the object and the object is said to be in equilibrium. A force is a vector quantity which means that it has both a magnitude (size) and a direction associated with it. There are four forces that act on an aircraft in flight: lift, weight, thrust, and drag. Home > Beginners Guide to Aeronautics Airplane Cruise – Balanced Forces
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