|Principle of Helicopter Flight|
( page 1 )
Helicopter, Lift is obtained by means of one or more power driven horizontal propellers which called
Main Rotor. When the main rotor of helicopter turns it produces lift and reaction torque. Reaction torque tends to make helicopter spin. On most
helicopters, a small rotor near the tail which called tail rotor compensates for this torque. On twin rotor helicopter the rotors
rotate in opposite directions, their reactions cancel each other.
The lifting force is produced by the main rotor . As they spin in the air and produced the lift. Each blade produces an equal share of the lifting force.
The weight of a helicopter is divided evenly between the rotor blades on the main rotor system. If a helicopter weight 4000 lbs and it has two blades, then
each blade must be able to support 2000 lbs.In addition to the static weight of helicopter ,each blade must be accept dynamic load as well . For example, if
a helicopter pull up in a 1.5 g manouver (1.5 time the gravity force), then the effective weight of helicopter will be 1.5 time of static helicopter weight or 6000 lbs.
due to gravitational pull.
The tail rotor is very important. If you spin a rotor with an engine, the rotor will rotate,but the engine and helicopter body will tend
to rotate in opposite direction to the rotor. This is called Torque reaction. Newton's third law of motion states , " to every action there is
an equal and opposite reaction" . The tail rotor is used to compensates for this torque and hold the helicopter straight. On twin-rotors helicopter ,
the rotors spin in opposite directions, so their reactions cancel each other.
The tail rotor in normally linked to the main rotor via a system of driveshafts and gearboxes , that means if you turn the main rotor ,
the tail rotor is also turn.Most helicopter have a ratio of 3:1 to 6:1 . That is, if main rotor turn one rotation , the tail rotor will turn 3 revelutions (for 3:1)or 6 revolutions
(for 6:1). In most helicopter the engine turns a shaft that connected to an input quill in the transmission gearbox. the main rotor mast out to the top and tail rotor
drive shafts out to the tail from the tranmission gear box.
|Dissymmetry of Lift |
All rotor systems are subject to Dissymmetry of Lift in forward flight . At a hover , the lift is equal across the
entire rotor disk . As the helicopter gain air speed , the advanceing blade develops greater lift because of the increased airspeed
and the retreating blade will produce less lift , this will cause the helicopter to roll (for example: if rotor speed = 400 km/hr , helicopter move forward=100 km/hr
then advancing blade will have speed=500 km/hr but the retreating blade will has moving speed of only 300 kr/hr ) . This has to be compensated for in some way .
|Blade Flapping |
Dissymmetry of lift is compensated for by Blade flapping. Because of the increased airspeed and lift
on the advancing blade will cause the blade to flap up and decreasing the angle of attack . The decreased lift on the retreating blade will cause the blade
to flap down and increasing the angle of attack . The combination of decreased angle of attack on the advancing blade and increased angle of attack on the
retreating blade through blade flapping action tends to equalize the lift over the two halves of the rotor disc.
next : principle page 2