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旋翼机飞行手册 ROTORCRAFT FLYING HANDBOOK

时间:2011-04-05 11:32来源:蓝天飞行翻译 作者:航空 点击:

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NOTAR®
The NOTAR® system is an alternative to the antitorque rotor. The system uses low-pressure air that is forced into the tailboom by a fan mounted within the helicopter. The air is then fed through horizontal slots, located on the right side of the tailboom, and to a controllable rotating nozzle to provide antitorque and directional control. The low-pressure air coming from the horizontal slots, in conjunction with the downwash from the main rotor, creates a phenomenon called “Coanda Effect,” which produces a lifting force on the right side of the tailboom. [Figure 1-5]
LANDING GEAR
The most common landing gear is a skid type gear, which is suitable for landing on various types of surfaces. Some types of skid gear are equipped with dampers so touchdown shocks or jolts are not transmitted to the main rotor system. Other types absorb the shocks by the bending of the skid attachment arms. Landing skids may be fitted with replaceable heavy-duty skid shoes to protect them from excessive wear and tear.
Helicopters can also be equipped with floats for water operations, or skis for landing on snow or soft terrain. Wheels are another type of landing gear. They may be in a tricycle or four point configuration. Normally, the
1-4

 

There are four forces acting on a helicopter in flight. They are lift, weight, thrust, and drag. [Figure 2-1] Lift is the upward force created by the effect of airflow as it passes around an airfoil. Weight opposes lift and is caused by the downward pull of gravity. Thrust is the force that propels the helicopter through the air. Opposing lift and thrust is drag, which is the retarding force created by development of lift and the movement of an object through the air.
 
 

AIRFOIL
Before beginning the discussion of lift, you need to be aware of certain aerodynamic terms that describe an airfoil and the interaction of the airflow around it.
An airfoil is any surface, such as an airplane wing or a helicopter rotor blade, which provides aerodynamic force when it interacts with a moving stream of air. Although there are many different rotor blade airfoil designs, in most helicopter flight conditions, all airfoils perform in the same manner.
Engineers of the first helicopters designed relatively thick airfoils for their structural characteristics. Because the rotor blades were very long and slender, it was necessary to incorporate more structural rigidity into them. This prevented excessive blade droop when the rotor system was idle, and minimized blade twisting while in flight. The airfoils were also designed to be symmetrical, which means they had the same camber (curvature) on both the upper and lower surfaces.
 
Symmetrical blades are very stable, which helps keep blade twisting and flight control loads to a minimum. [Figure 2-2] This stability is achieved by keeping the center of pressure virtually unchanged as the angle of attack changes. Center of pressure is the imaginary point on the chord line where the resultant of all aerodynamic forces are considered to be concentrated.

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