直升機(jī)教員手冊(cè) Helicopter Instructor’s Handbook
時(shí)間：2014-11-10 08:35來源：FAA 作者：直升機(jī)翻譯 點(diǎn)擊：次
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To view this page ensure that Adobe Flash Player version 9.0.124 or greater is installed. Explain to the student that the tail rotor also produces thrust. The amount of thrust is variable through the application of the antitorque pedals and is used to control the helicopter’s heading during hovering flight and trim during cruise flight. Drag No discussion of aerodynamics is complete without its defining the three types of drag, how drag is created, and its effect on the aircraft. A certificated flight instructor (CFI) must become intimately familiar with the drag chart and how it relates to airspeed and power demands. Demonstrate this during the performance planning phase as the student has actual torque values to compare. Then, when the student is flying the helicopter, apply the values that were computed and show the effect on the helicopter. A technique is to show how each flight control is affected by simple hover flight maneuvers. Demonstrate the change in torque that occurs between left and right pedal turns and explain why. Discuss how the cyclic is utilized to hold position over the ground, while the pedals rotate the fuselage and control heading. When excess power is available, demonstrate how the collective pitch can be applied to vary the hover height, or to accelerate the helicopter. It would be prudent to discuss here that if no excess power is available, application of the collective then may be used to control the rotor rpm. This is done by changing the pitch in the blades. Over application of the collective in a low power margin setting results in rotor rpm decay and a loss of lift. Rotor rpm is the key to sustaining the aircraft in a steady state profile and should never be allowed to decay below minimum operating levels. It is the key to life for a helicopter pilot! The types of drag are: 1. Parasite drag—drag created by the fuselage or any nonlifting components (e.g., strut, skin friction, interference). 2. Profile drag—caused by the frictional resistance of the rotor blades passing through the air. 3. Induced drag—results from producing lift. a. Blade tip vortices—pressure differential at tips of blades trying to equalize and produce a stream of vortices (turbulence). b. Induced flow—causes lift and total aerodynamic force to tilt further rearward on the airfoil. c. Total aerodynamic force tilted further backward at higher angles of attack. 4. Total drag—sum of induced, profile, and parasite. Use a graph that depicts drag/power relationship, and have the student identify the power requirements to overcome drag at various airspeeds. [Figure 3-1] The following describes the relationship of each of the different types of drag to the airspeed of the aircraft. 1. Parasite drag—lowest point at a hover, but increases with airspeed. The major source of drag at higher airspeeds. 2. Profile drag—remains relatively constant at low airspeed, but increases slightly at higher airspeed ranges. 3. Induced drag—major source of drag at a hover, but decreases with forward airspeed. 4. Total drag—the sum total of induced, profile, and parasite drag. a. Total drag decreases with forward airspeed until best rate of climb speed is reached. [Figure 3-1, Point E] b. Speeds greater than best rate of climb causes a decrease in overall efficiency due to increasing parasite drag. Once the student understands the forces acting on the helicopter, provide examples of balanced and unbalanced flight forces. For example, when hovering stationary in calm wind at a constant altitude, thrust is equal to drag and lift is equal to weight. The aircraft is not moving vertically or horizontally. The aerodynamic forces are balanced. [Figure 3-5] The student will also notice during hovering flight in a calm wind condition that with smaller American made helicopters like the Robinson R-22, Bell 206, and Schweizer 300, the left side of the aircraft will probably hang lower than the right. This is due to the direction of the tail rotor thrust and the engineered mast tilt to compensate for translating tendency. On much larger helicopters such as the BH-205, S-76, and BK-117, in which an additional gearbox is used to raise the tail rotor up to the main rotor plane, the tilting of the fuselage is not as prevalent. The pitch attitude will vary depending on the loading of the helicopter. Many helicopters when flown single pilot will be nose high at a hover. Conversely, they may be nose load when fully loaded. The center of gravity (CG) of the helicopter determines which portion of the landing gear will come off the ground first. The CFI must pay particular attention to the attitude of the helicopter as the student lifts it off the ground. If excess power is applied in other than a level attitude, the helicopter may proceed to roll beyond its dynamic rollover limits. When lifted off the surface correctly and safely, the pilot has the opportunity to lower the collective if a portion of the landing gear is attached or hung on the surface, thus preventing a rollover incident from occurring. It is imperative that the CFI closely monitor the attitude of the helicopter and not the actions of the student. This simple action may determine whether or not the helicopter is allowed to stray beyond the comfort level of the instructor to recover from a particular action by the student. Never allow a student to go beyond your comfort level. Several inputs are required simultaneously as the aircraft is brought to a hover. Stress to the student that these actions must occur without delay or coordinated flight will not occur. For example, as the collective is increased to lift the helicopter off the surface, the throttle must also be increased. Even if a governor accomplishes that action, the pilot still must monitor the power instruments to ensure that no limits are exceeded. With the increase in power, there is also an increase in torque and the tendency for the nose to turn to the right. The pilot must apply sufficient left pedal to maintain the helicopter heading. While this is occurring and the lift in the rotor system is changing, the pilot must apply cyclic to maintain position over the ground and not allow the helicopter to drift in any one direction. The helicopter bank attitude might not be level due to crosswinds and translating tendency. The pitch attitude might not be level due to tailwinds or CG. The pilot must ensure that the tail rotor is clear of all obstacles and is not allowed to hang so low that it impacts the ground or other objects. |
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