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. In this example, only the motion of the helicopter in a horizontal direction is explained and as the student becomes comfortable with aerodynamics, further discussions should include the effects of the thrust on weight and on lift. For example, increasing the throttle setting increases the fuel usage and decreases the weight, and the increase in velocity increases the lift as well as the drag. Each of these changes effect the vertical motion of the helicopter. It is important to point out the role of engine power when explaining the law of inertia. Power is used to accelerate the helicopter, to change its velocity, and thrust is used to balance the drag when the helicopter is cruising at a constant velocity. When a helicopter is on a normal approach, the power demand is generally in the middle range and the total drag is at the lowest. As the aircraft decelerates to effective translational lift airspeed and terminates to a hover, the power demand is quite significant, generally the highest of all maneuvers. An airplane makes minimal power demands at the termination of its approach through the flare and landing. Second Law—The Law of Acceleration A change in velocity with respect to time. The force required to produce a change in motion of a body is directly proportional to its mass and rate of change in its velocity. For example, for a given helicopter, acceleration would be slower when loaded to maximum gross weight than when loaded to a lesser gross weight. During a normal takeoff, the power margin available between maximum torque available and hover power can be quite small based on helicopter weight and environmental factors. During the transition to forward flight and through effective translational lift airspeed, acceleration is limited until the aircraft is in smooth undisturbed air and the influence of induced drag begins to subside. Once the aircraft reaches its maximum endurance/ rate of climb airspeed, acceleration potential is increased as total drag is at its lowest point. [Figure 3-1, Point E] Total drag is the sum of parasite drag and induced drag as shown in Figure 3-1, Point A and C. The total drag curve can also be referred to as the thrust required curve because thrust is the force acting opposite drag. At the point where total drag [Figure 3-1, Point D] and thrust required are at a minimum, the lift-to-drag ratio is maximum and is referred to as L/DMAX. At L/DMAX, the entire airframe is at its most efficient, producing the most lift for the least drag. Maximum endurance is found at L/DMAX, because thrust required and thus fuel flow (fuel required) are at a minimum, giving maximum time airborne. Third Law—Action and Reaction For every action, there is an equal and opposite reaction. The instructor should relate the third law to the amount of power applied to the rotor system and the need for the antitorque or tail rotor to supply the equal and opposite reaction to the torque of the engine(s) applied to the main rotor. The rotor system of a helicopter accelerates air downward, resulting in an upward thrust. A single-rotor helicopter demonstrates this law perfectly. Consider a helicopter on floats that is not moored to a dock. As the main rotor begins to turn counterclockwise during aircraft start, the fuselage reacts by turning in a clockwise direction until the point at which the tail rotor has reached sufficient rpm to provide the thrust necessary to counteract that force. Torque effect is a result of Newton’s laws and an aspect of helicopter flight that a student must thoroughly understand. The turning of the helicopter’s main rotor blades in one direction causes the helicopter to turn in the opposite direction. In most helicopters, this is counteracted by the use of a second rotor (tail rotor) to provide the thrust to limit the rotation. Some helicopters use vectored air, while others use a counterrotating main rotor system. All have one thing in common—a method of counteracting the torque of the main rotor system. [Figure 3-2] At some point in training, the instructor should have the student bring the helicopter to a high hover and explain that work load is greater and an increased left pedal requirement exists to hold a constant heading. The opposite can be shown at a lower hover with a decrease in left pedal requirement to hold the same heading. Weight As weight increases, the power required to produce lift needed to compensate for the added weight must also increase. This is accomplished through the use of the collective. Most performance charts include weight as one of the variables and students must be aware of the importance of managing aircraft weight to obtain optimum performance. By reducing weight, the helicopter is able to safely take off or land at locations that would otherwise be impossible. Explain to students how maneuvers that increase the G loading such as steep turns, rapid flares, or pulling out of a dive create greater load factors and act as a multiplier of weight. The load factor is the actual load on the rotor blades at any time, divided by the normal load or gross weight. [Figure 3-3] At 30° of bank, the load factor is 1G, but at 60°, it is 1.8G, an increase of 80 percent. If the weight of the helicopter is 1,600 pounds, the weight supported by the rotor in a 30° bank at a constant altitude would be approximately 1,600 pounds. In a 60° bank, it would be 2,880 pounds and in an 80° bank, it would be 8,000 pounds. Emphasize to students that an additional cause of large load factors is rough or turbulent air. The severe vertical gusts produced by turbulence can cause a sudden increase in angle of attack (AOA), resulting in increased rotor blade loads that are resisted by the inertia of the helicopter. Thrust Thrust, like lift, is generated by the rotation of the main rotor system. Point out to the student that in a helicopter thrust can be forward, rearward, sideward, or vertical. The direction of the thrust is controlled with the cyclic. If cyclic control to produce thrust is too great, lift is lost and the aircraft descends. Conversely, if too little cyclic control is made, the aircraft begins a climb. Using visual aids, demonstrate how the resultant lift and thrust determines the direction of movement of the helicopter. [Figure 3-4] |
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