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Army Flight Warrant Officer SIFT Test |Already Graded A+.
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Army Flight Warrant OfficerSIFT Test |Already Graded A+.
Helicopter Structure - -The typical structure of a helicopter is a fuselage (the
body of the craft) with a mast protruding upward out of it. The engine of the
craft drives power upward through the mast to the rotor blades, which are
controlled by the pilot. This main rotor is used to generate thrust. This thrust
is balanced by a second rotor, usually a tail rotor, which is intended to
account for the effects of torque. In the USA, UK and Germany, it is
conventional for the main rotor to rotate counterclockwise. The opposite is
true in other countries. Throughout this book, reference will be made only to
counterclockwise main rotors.
-Single Main Rotor Helicopter - -This is the most common design for a
helicopter -a single main rotor generating power, with a secondary rotor,
such as a tail rotor, used to offset the effects of torque. One disadvantage of
this design is that some power is used for countering torque, rather than
being used solely for lift.
-Co-axial Rotor Helicopter - -This type of helicopter features two sets of
blades rotating above and beneath each other. The two sets of blades are
mounted to the same mast, but rotate in opposite directions to each other.
This has the advantage of reducing payload and allowing for greater cargo
and passenger capacity as a result.
-Tandem Rotor Helicopter - -This helicopter design features two main
rotating blade sections: front and rear. Each set of blades rotates counter to
the other, meaning that torque is automatically offset. There is no need for
additional measures, such as a tail rotor, to deal with torque. Because of this,
all of the power from the engine can be used for lift. Consequently, tandem
rotor helicopters are some of the fastest and most powerful in existence.
-Thrust - -Thrust is created by the engine, or engines, of the helicopter. It is
transferred into motion via the mast and rotors. Sufficient thrust is needed to
overcome the drag and the weight of the helicopter and keep it moving
forward.
-Drag - -Drag is a force which acts against thrust. In the case of helicopters,
drag is caused by the shape and size of the aircraft disrupting the flow of air
as it travels. There are several types of drag which impact helicopters.
, -Profile Drag - -Profile drag is caused by the frictional resistance of
helicopter blades passing through the air. It has two components: form drag
and skin friction.
-Form Drag - -Form drag increases as the speed of the helicopter increases.
-Skin Friction - -Skin friction is caused by microscopic roughness of the
helicopter blades. Any roughness on the surface causes air to become
trapped and drag to increase.
-Induced Drag - -Induced drag is caused by airflow circulation around the
blades creating vortices. This type of drag increases when the helicopter is
flying at lower speeds, and decreases when it is flying at higher speeds.
During low speed flight, induced drag is the main cause of drag for
helicopters.
-Parasite Drag - -Parasite drag is the drag created by the helicopter
components that are external to the fuselage. For example, the landing gear,
cowlings and doors can all create parasite drag. Parasite drag slows down
the speed of a helicopter by opposing thrust.
-Total Drag - -Total drag is the sum total of the profile, induced and parasite
drag acting upon the helicopter at any given time. The point at which the
total drag is lowest is the preferable point for maximum endurance, best rate
of climb, and minimum rate of descent in autorotation.
-Weight - -Weight is a force which results from the combined mass of the
helicopter being acted upon by gravity. It is a downward force which acts
through the center of gravity of the helicopter. Weight is reliant upon more
than just the mass of the helicopter itself. When a helicopter is flying a
curved flight path, the angle at which the helicopter banks has an impact
upon the total weight.
The total weight at any given time is also impacted by the g force acting on
the craft. When the helicopter is flying a curved flight path, requiring the
pilot to bank the vehicle, then the g force increases. Weight results from
mass multiplied by negative g force. The more severe the banking, the
greater the rate of g force. As a result, if a helicopter is banking, or flying into
unexpected gusts, the weight acting on the craft increases. More thrust is
therefore needed to overcome this increase in weight and keep the craft
flying as the pilot intends. Weight acts counter to thrust.
-Lift - -Lift is the upwards force created by the spinning of the helicopter
blades allowing air to pass over them. Lift is necessary to give the helicopter
altitude by overcoming the weight acting upon it.
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