The main rotor is responsible for both thrust and lift. There-fore, the force representing the total reaction of the airfoilsto the air maybe considered as being divided into two components.One component, lift, is the force required to support the weightof the helicopter. The other component, 2thrust, is the forcerequired to overcome the drag on the fuselage. We must make clearat this point that drag and weight are two forces separated fromeach other.

So much for vertical flight. Now we are going to investigate thethrust and drag forces acting on the fuselage during forwardflight.

In any kind of flight-vertical, forward, backward, sideward, orhovering- the lift forces of a rotor system are perpendicular tothe tip path plane (plane of rotation) . The tip path plane is theimaginary plane described by the tips of the blades in making acycle of rotation. When in vertical descent or hovering the tippath plane is horizontal and this resultant force acts verticallyupward.

To accomplish forward flight, the pilot tilts the tip path planeforward. The resultant force tilts forward with the rotor asshown in FIG 2-1 DETAIL C. The total force, now being inclinedfrom the vertical, acts both upward and forward; therefore, itcan be resolved into two components as shown in FIG 2-1 DETAILC. One component, thrust, acts in the direction of flight to movethe helicopter forward.

In rearward flight, the thrust and drag forces are similar tothose in forward flight but are reversed. The tip path plane istilted to the rear, the thrust component acts to the rear, andthe drag opposes the rearward motion of the aircraft. In sidewardflight, the pilot tilts the tip path plane in the desireddirection of flight, thrust is to the right or left in thedirection of flight, and drag acts in the opposite direction.

2.2 Torque

As a helicopter rotor turns in one direction, the fuselage tendsto rotate in the opposite direction. This torque effect is inaccord with Newton's third law of motion, which states, that, "To every action there is an opposite and equal reaction In thehelicopter, the reaction is in a direction opposite to that inwhich the rotor is driven by the engine and is proportional inmagnitude to the power being delivered by the engine. Provisionsmust be taken to counteract torque and for positive control overits effect during flight. Torque is a very important problem inhelicopters of the single main rotor configuration. In the singlemain rotor type, there is no torque reaction when the engine isshut off. Therefore, there is no torque reaction during autorota-tion.

The usual way to counteracting torque in a single main rotorhelicopter is by using an antitorque rotor. This auxiliary rotoris mounted vertically on the outer portion of the tail boom.Turning at a constant rpm usually higher than one-half enginespeed, the tail rotor produces thrust in a horizontal planeopposite in direction to the torque reaction developed by themain rotor.

Figure 2-2 shows the direction of the torque reaction and thedirection of tail rotor thrust for a helicopter in which the mainrotor turns from the pilot's right, to his front, to his left,and then to his rear. Most single rotor systems turn in thisdirection.

Since the torque effect on the fuselage is a result of the enginepower supplied to the main rotor, any change in engine powerbrings about a corresponding change in the torque effect.Furthermore, power requirements vary with flight conditions. Thismeans that there must be some provision for varying tail rotorthrust. Usually, a variable-pitch tail rotor is employed andrudder pedals are linked by cables with the pitch changemechanism in the tail rotor gearbox. This permits the pilot toincrease or decrease tail rotor thrust as required to neutralizethe torque effect.

The tail rotor and its controls have two operations. The firstone is to counteract torque, and the second one is to head thehelicopter in the desired direction of flight. Therefore, thetail rotor control pedals serve as rudder pedals. When the pilotwishes to maintain a constant heading, he keeps just enough pitchin the tail rotor to neutralize torque effect.

Although the tail rotor is the primary means of counteracting andcontrolling torque the tail rotor alone does not quite do thejob. Torque cannot be compensated for by a single force. The tailrotor alone would prevent rotation of the fuselage, but wouldcase translation of the helicopter, during hovering, in thedirection of tail rotor thrust.

Complete compensation for torque requires a couple of equalforces acting in the opposite directions. Tail rotor thrustconsistutes one of the forces. The second force is introduced byrigging the helicopter with the tip path plane tilted from 1 to2 1/2 to the left, depending on the helicopter.