直升机教员手册 Helicopter Instructor’s Handbook
时间：2014-11-10 08:35来源：FAA 作者：直升机翻译 点击：次
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To view this page ensure that Adobe Flash Player version 9.0.124 or greater is installed. 2. Drag—a rearward, retarding force caused by disruption of airflow by the wing, rotor, fuselage, and other protruding objects. Drag opposes thrust and acts rearward parallel to the relative wind. 3. Weight—the combined load of the aircraft itself, the crew, the fuel, and the cargo or baggage. The earth’s gravitational force, which creates the weight, pulls the aircraft downward. 4. Lift—overcomes the downward force of weight to allow flight to occur and is produced by the dynamic effect of the air acting on the airfoil and acts vertically through the center of gravity. Lift A very easy way to confuse new flight students is to throw a lot of obscure information at them with no concrete references or examples. Aerodynamics can be very difficult for the new student to understand because it is difficult to visualize what is happening to the rotor blades or tail rotor in flight. When teaching the student about lift and how the helicopter is able to obtain lift, the instructor must be creative and find ways to explain the theories, such as Bernoulli’s Principle and Newton’s Laws of Motion, in direct relation to the helicopter and how every flight control movement affects lift. Bernoulli’s Principle Instructors should introduce Bernoulli’s Principle to the student in simple terms and attempt to relate the theory directly to the production of lift that is created from the main and tail rotor blades. The discussion should begin with Bernoulli’s initial discovery that air moving over a surface decreases air pressure on the surface, and show the student an example of the differences in air pressure when an object moves through the air. Further discussion should include the following points and examples: 1. Show the student a picture of an airfoil and how the air pressure changes when the air is disrupted. A picture of an airfoil is usually a small cutout or slice of the entire wing or rotor blade. The instructor should explain that the entire rotor blade(s) are essentially one large airfoil. 2. As airspeed increases, surface air pressure decreases accordingly and this difference in pressure around the airfoil is directly related to the flight of an aircraft. 3. As an airfoil starts moving through the air, it divides the mass of air molecules at its leading edge. The distance over the top of the blade with the angle of attack is greater than the distance along the bottom surface of the rotor blade. Air molecules that pass over the top must move faster than those passing under the bottom to meet at the same time along the trailing edge. The faster airflow across the top surface creates a low-pressure area above the airfoil. 4. Air pressure below the airfoil is greater than the pressure above it and tends to push the airfoil up into the area of lower pressure. As long as air passes over the airfoil, this condition exists. It is the difference in pressure that causes lift. When air movement is fast enough over a wing or rotor blade, the lift produced matches the weight of the airfoil and its attached parts. This lift is able to support the entire aircraft. As airspeed across the wing or rotor increases further, the lift exceeds the weight of the aircraft and the aircraft rises. 5. Not all of the air met by an airfoil is used in lift. Some of it creates resistance, or drag, which hinders forward motion. Lift and drag increase and decrease together. They are affected by the airfoil’s angle of attack in the air, the speed of airflow, the air density, and the shape of the airfoil or wing. Newton’s Laws of Motion Newton’s laws of motion provide the foundation for the student’s understanding of basic aerodynamic principles. The instructor should develop multiple ways of explaining these laws to ensure that if the student does not comprehend one explanation, the instructor has an alternate explanation that relates to something that the student will understand. Begin with relating the laws to helicopter flight, such as the requirements for lift, thrust, and power to overcome the effects of the three laws and the energy state of the helicopter. If the student has a difficult time understanding flight examples, try using an example that is more familiar, such as a car or motorcycle. This helps the student better understand the laws when the instructor applies it to flight. First Law—the Law of Inertia A body at rest remains at rest, and a body in motion remains in motion at the same speed and in the same direction unless acted upon by some external force. The key point to explain is that if there is no net force resulting from unbalanced forces acting on an object (if all the external forces cancel each other out), then the object maintains a constant velocity. If that velocity is zero, then the object remains at rest. And, if an additional external force is applied, the velocity changes because of the force. A helicopter in flight is a particularly good example of the first law of motion. There are four major forces acting on an aircraft: lift, weight, thrust, and drag. If we consider the motion of an aircraft at a constant altitude, we can neglect the lift and weight. A cruising aircraft flies at a constant airspeed and the thrust exactly balances the drag of the aircraft. This is the first part sited in Newton’s first law; there is no net force on the helicopter and it travels at a constant velocity in a straight line. Now, if the pilot changes the thrust of the engine, the thrust and drag are no longer in balance. If the thrust is increased, the helicopter accelerates and the velocity increases. This is the second part sited in Newton’s first law; a net external force changes the velocity of the object. The drag of the helicopter depends on the square of the velocity. So, the drag increases with increased velocity. Eventually, the new drag equals the new thrust level and at that point, the forces again balance out, and the acceleration stops. The helicopter continues to fly at a new constant velocity that is higher than the initial velocity. We are again back to the first part of the law with the helicopter traveling at a constant velocity. |
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