An aircraft with two horizontal rotors that provide both the  lift and directional control is a dual rotor helicopter. The  rotors are counterrotating to balance the aerodynamic torque  and eliminate the need for a separate antitorque system.  [Figure 2-26] A fully articulated rotor is found on aircraft with more than  two blades and allows movement of each individual blade in  three directions. In this design, each blade can rotate about  the pitch axis to change lift; each blade can move back and  forth in plane, lead and lag; and flap up and down through a  hinge independent of the other blades.  [Figure 2-27]  The rigid rotor system is a rare design but potentially offers  the best properties of both the fully articulated and semirigid  rotors. In this design, the blade roots are rigidly attached to the  rotor hub. The blades do not have hinges to allow lead-lag or  flapping. Instead, the blades accommodate these motions by  using elastomeric bearings. Elastomeric bearings are molded,  rubber-like materials that are bonded to the appropriate parts.  Instead of rotating like conventional bearings, they twist and  flex to allow proper movement of the blades.  The semirigid rotor design is found on aircraft with two rotor  blades. The blades are connected in a manner such that as  one blade flaps up, the opposite blade flaps down. One of the differences between a helicopter and a fixed-wing  aircraft is the main source of lift. The fixed-wing aircraft  derives its lift from a fixed airfoil surface while the helicopter  derives lift from a rotating airfoil called the rotor.  During hovering flight in a no-wind condition, the tip-path  plane is horizontal, that is, parallel to the ground. Lift and  thrust act straight up; weight and drag act straight down. The  sum of the lift and thrust forces must equal the sum of the  weight and drag forces in order for the helicopter to hover.  During vertical flight in a no-wind condition, the lift and  thrust forces both act vertically upward. Weight and drag both  act vertically downward. When lift and thrust equal weight  and drag, the helicopter hovers; if lift and thrust are less than  weight and drag, the helicopter descends vertically; if lift  and thrust are greater than weight and drag, the helicopter  rises vertically.  For forward flight, the tip-path plane is tilted forward, thus  tilting the total lift-thrust force forward from the vertical. This  resultant lift-thrust force can be resolved into two components:  lift acting vertically upward and thrust acting horizontally in  the direction of flight. In addition to lift and thrust, there is  weight, the downward acting force, and drag, the rearward  acting or retarding force of inertia and wind resistance.  In straight-and-level, unaccelerated forward flight, lift equals  weight and thrust equals drag. (Straight-and-level flight is  flight with a constant heading and at a constant altitude.)  If lift exceeds weight, the helicopter climbs; if lift is less  than weight, the helicopter descends. If thrust exceeds drag,  the helicopter increases speed; if thrust is less than drag, it  decreases speed. 2-18 Figure 2-27.  Articulated rotor head. Figure 2-26.  Dual rotor helicopter. Forces Acting on the Helicopter Dual Rotor Helicopter Semirigid Rotor Rigid Rotor Types of Rotor Systems Fully Articulated Rotor Pitch change axis Drag hinge Flipping hinge