the lift is generated by the rotation of the assembly, which creates a flow of air over the blades. A helicopter is lifted into the air by the aerodynamic forces on the rotor and not pushed up by the downwash. Some helicopters have twin rotors in tandem at either end of the fuselage; but most have a single, main rotor with  a  tail  rotor  mounted  at  right  angles.  A  few  have tandem intermeshing rotors. TAIL ROTOR.—The  tail  rotor  is  used  for directional control and stability. It is mounted at right angles to the main rotor to counteract the torque of that system. By varying the pitch of the tail rotor blades, the pilot controls yaw. Helicopter  engines  are  connected  to  the  rotor shaft(s) by a transmission, which may be disengaged. That permits the engine(s) to be operated on the ground without engaging the rotor system and also permits a mode  of  flight  known  as  autorotation.  If  the  engines should stop while in flight, they can be disengaged; the freewheeling  action  of  the  rotor  will  allow  a  slower descent. AIRCRAFT MODEL DESIGNATIONS All aircraft have tri-service designations; that is, a given aircraft has the same alphanumeric identification symbol,  regardless  of  which  service  uses  the  aircraft. Look at table 8-2. Here, you can find the four basic parts of an aircraft model designation. 8-34 Figure 8-40.—Rotary-wing aircraft. Mission/type modification symbol Basic mission/type symbol Aircraft series number Model series letter A Attack A Attack These numbers are assigned sequentially within each basic mission category. The number is separated from the basic mission symbol by a dash. This letter, added to the series number, indicates an improvement or alteration of the basic mode. These are assigned in sequence; for example: F-4A, F-4B, F-4C, and so forth. C Cargo/transport B Bomber D Drone control C Cargo/transport E Special electronics E Special electronics H Search and rescue F Fighter K Tanker H Helicopter L Cold weather operations K Tanker M Missile capability O Observation O Observation P Patrol Q Drone S Antisubmarine R Reconnaissance T Trainer S Antisubmarine U Utility T Trainer V Vertical takeoff and landing (VTOL)/short takeoff and landing (STOL) U Utility X Research V Staff transport W Weather reconnaissance Table 8-2.—Aircraft Model Designations