飞机螺旋桨工作原理（The principle of an airplane propeller）

飞机螺旋桨工作原理（The principle of an airplane propeller） 飞机螺旋桨工作原理(The principle of an airplane propeller) Comparison with flight technology in reality: the principle of aircraft propellers First, the working principle The propeller can be considered as a wing that rotates on one side. The air flow through each section of the blade is composed of the forward velocity along the axis of rotation and the tangential velocity produced by rotation. At the propeller radius R1 and R2 (R1 < R2) two, take a minimum section to discuss the airflow on the blade. V axial velocity; n propeller speed; Phi air flow angle; that is, the angle between the air flow and the propeller rotation plane; the angle of attack of the alpha blade section; the pitch angle of the blade section; that is, the angle between the chord of the blade section and the plane of rotation. Obviously, beta = alpha + phi. When the air flows through each section of the blade, the generating power, resistance, Delta D and lift L, and the total aerodynamic force is R. A R along the flight direction pulling force component for Delta T, opposite to the rotation direction of the rotating propeller force P stop propeller. The forces on each blade of the blade and the forces that prevent rotation are added to form the pull of the propeller and the torque that prevents the propeller from turning. It is necessary to make each section of the propeller work at a higher angle of lift and drag, so as to obtain greater tension and smaller resistance moment, that is to say, higher efficiency. When propeller works. The axial velocity does not vary with radius, while the tangent velocity varies with radius. Therefore, near the blade tip, the larger the radius, the smaller the air flow angle, and the corresponding blade angle should be smaller. In the vicinity of the oar root, the smaller the radius is, the larger the air flow angle is, and the corresponding blade angle should be larger. The blade angle of propeller should gradually increase from oar tip to oar root according to certain rule. So the propeller is a twisted wing, more precisely. The airflow angle actually reflects the ratio of the forward velocity to the tangent velocity. For a section of a propeller, the angle of attack varies with the ratio. As the angle of attack changes, both the tension and the resistance moment change. Using the torque ratio "J" to reflect the airflow angle at the blade tip, J = V/nD. Type D - propeller diameter. Theory and The test proved that the propeller force (T), power to overcome the propeller resistance moment required (P) and efficiency (ETA) of the following formula: T=Ct P n2D4 P=Cp P n3D5 =J Ct/Cp? Type: Ct - Cp - force coefficient; power factor; p air density; N - D - diameter propeller propeller speed. Where Ct and Cp depend on the geometric parameters of the propeller, the value of each propeller varies with the J. Figure 1 - 1 - 21 is called the characteristic curve of a propeller, which can be obtained by theoretical calculation or experiment. The characteristic curve shows the change of propeller tension coefficient, power coefficient and efficiency with advance ratio. It is one of the main bases for designing propeller and calculating aircraft performance. As can be seen from the graph and the calculation formula, the propeller is very inefficient when it is advanced. A light aircraft with a lower flying speed and higher engine speed is extremely detrimental. For example, the flight speed of 72 km / hour, engine speed of 6500 rpm, ETA = 32%. Therefore, the ultra light aircraft must use a reducer to reduce the speed of the propeller, increase the distance ratio, and improve the efficiency of the propeller. Two, geometric parameters Diameter (D): one of the important parameters that affect the performance of propeller. In general, the diameter increases, the tension increases, and the efficiency increases. So, as far as the structure is allowed, the propeller with larger diameter is selected as far as possible. In addition, we should consider that the air velocity at the propeller tip should not be too large (< 0.7 sonic speed), otherwise the shock wave may occur and the efficiency will be reduced. Number of blades (B): the propeller's pull coefficient and power coefficient are proportional to the number of blades. Ultra light aircraft generally use a simple two bladed paddle. Only when the propeller diameter is limited, the propeller is added to the engine in good order by increasing the number of blades. Solidity (sigma): the ratio of blade area to propeller rotation area (PI R2). Its effect is similar to the number of blades. The coefficient of tension and the coefficient of power increase with the increase of solidity. Blade angle (beta): the blade angle varies with the radius, and the change rule is the most important factor affecting the propeller performance. Traditionally, the blade angle at the 70% diameter is the nominal value of the blade angle. Pitch: it is another representation of blade angle. Figure 1 - 1 - 22 is the relation between screw pitch and blade angle in various senses. Geometric pitch (H): the distance at which the blade cross angle is zero, and the pitch of the blade rotates a week. It reflects the size of the blade angle, and more directly points out the characteristics of the propeller. The geometrical moment of each blade may be unequal. It is customary to make the name value of the geometric screw at the 70% diameter. Foreign countries can order propeller according to diameter and pitch. For example, "64/34" means that the diameter of the oar is 60 inches and the geometric screw is 34 inches. Actual pitch (Hg): the blade rotates about a week and the plane moves forward. The actual screw moment of the propeller can be calculated by Hg = v/n. It can be roughly estimated by H = 1.1 to 1.3Hg to calculate the geometrical screw moment of the propeller used by the aircraft. Theoretical screw moment (HT): when designing a propeller, it is necessary to consider that the velocity of the air passing through the propeller increases, and that the velocity of the flow over the plane of the propeller is greater than the speed of the flight. Thus, the propeller travels with respect to the air, and the theoretical screw moment will be greater than the actual screw moment. Three changes in propeller tension during flight Variation of 1. blade angle of attack with speed At the same speed, the speed increases, the tangential velocity (U) increases, the distance ratio decreases, the blade angle of attack increases, and the propeller tension coefficient increases (Figures 1 - 1 - 20). And because the tension is proportional to the square of the speed, so when increasing the throttle, it can increase the tension. Variation of 2. blade angle of attack with flying speed: Under the condition of constant speed, the flight speed increases, the inlet pitch ratio increases, the blade angle of attack decreases, and the propeller drag coefficient decreases. As shown in Figures 1 - 1 - 20, the tension decreases. When the flight speed equals zero, the tangential velocity is the velocity, and the blade angle is equal to the blade angle. When the plane is running on the ground, the speed of flight (V) is equal to zero, the blade angle of attack is the largest, and some sections due to the angle of attack is too large, more than the stall angle of attack, aerodynamic performance worse, so the propeller generated tension is not necessarily the greatest. 3. propeller tension curve: According to the law that the propeller tension decreases with the increase of flight speed, the pull curve of propeller can be drawn. Four The change of propeller tension with speed and flight speed: In flight, fix after increasing the gas. The change of the tension of propeller with speed and flight speed is as follows: As the output power of the engine increases, the speed of the propeller (tangential speed) increases rapidly to a certain value, and the propeller tension increases. As the flying speed increases, the angle of attack of the blade starts to decrease gradually, and the tension decreases gradually. The resistance of the aircraft increases gradually, and the trend of speed increases gradually. When the tension is reduced to a point (i.e., tension equals resistance), the speed of the aircraft is no longer increased. At this point, the flight speed, speed, blade angle and propeller tension do not change, the aircraft is maintained at a new speed flying. Four, the rotation of the propeller: When the engine stops in the air, the propeller will continue to rotate in the same direction as the windmill. This phenomenon is called the propeller rotation. The rotation of the propeller is not driven by the engine, but is pushed by the oncoming air of the blade. It not only can not produce tension, but increase the resistance of the aircraft. When the propeller rotates, it forms a large negative angle of attack. The total aerodynamic direction and effect of the blade has undergone a qualitative change. It's a component (Q) and tangential velocity (U) in the same direction, becoming the blade automatically rotating power, forcing the blade along the direction of rotation: continue another component (-P) in contrast with the velocity, resistance plays a role in flying. When the engine of some ultralight aircraft is stopped in the air, the propeller will not rotate because of the smaller speed of the flight, the moment of spin, and the moment of resistance of the propeller that can not be overcome. At this point, the blade resistance is greater, and the lift drag ratio (or glide ratio) of the aircraft will be greatly reduced. Five, the effective power of the propeller: 1. definition: the propeller generates tension, pulls the aircraft forward and works on the aircraft. The work done by the unit time of the propeller is the propeller's effective power Formula: N paddle = PV Type: N propeller propeller effective power - Propeller pull; V - flight speed Variation of effective power of 2. propellers with flight speed: (1) when the ground test run, the aircraft has no forward speed (V = 0), and the pulling force does not work on the aircraft, so the propeller's effective power is zero". (2) the effective power curve of the propeller measured from the actual flight speed: In the OA speed range, the effective power of the propeller increases with the increase of the flight speed; after the speed range is greater, the effective power of the propeller decreases with the increase of the flight speed. In the OA speed range, when the flying speed increases, the tension decreases slowly, and the effective power of the propeller increases gradually with the increase of the speed. When the flight speed increases to A, the propeller has the most effective power. When the flight speed increases, the effective power of the propeller will decrease as the flight speed increases as the tension decreases rapidly. The propeller is driven by the engine, and the propeller is used to change the power of the engine to the effective power that pulls the plane forward. The ratio of propeller effective power to engine output power is called propeller efficiency. ETA =N propeller /N