modulated as engine power levels are changed. This is done  by changing the pitch of the tail rotor blades. This, in turn,  changes the amount of countertorque, and the aircraft can be  rotated about its vertical axis, allowing the pilot to control  the direction the helicopter is facing. Similar to a vertical stabilizer on the empennage of an  airplane, a fin or pylon is also a common feature on rotorcraft.  Normally, it supports the tail rotor assembly, although  some tail rotors are mounted on the tail cone of the boom.  Additionally, a horizontal member called a stabilizer is often  constructed at the tail cone or on the pylon.  A Fenestron$^{®}$ is a unique tail rotor design which is actually a  multiblade ducted fan mounted in the vertical pylon. It works  the same way as an ordinary tail rotor, providing sideways  thrust to counter the torque produced by the main rotors.  [Figure 1-101] A NOTAR$^{®}$ antitorque system has no visible rotor mounted  on the tail boom. Instead, an engine-driven adjustable fan  is located inside the tail boom. NOTAR$^{®}$ is an acronym  that stands for “no tail rotor.” As the speed of the main  rotor changes, the speed of the NOTAR$^{®}$ fan changes. Air  is vented out of two long slots on the right side of the tail  boom, entraining main rotor wash to hug the right side of the  tail boom, in turn causing laminar flow and a low pressure  (Coanda Effect). This low pressure causes a force counter  to the torque produced by the main rotor. Additionally, the  remainder of the air from the fan is sent through the tail  boom to a vent on the aft left side of the boom where it is  expelled. This action to the left causes an opposite reaction  The controls of a helicopter differ slightly from those found  in an aircraft. The collective, operated by the pilot with the  left hand, is pulled up or pushed down to increase or decrease  the angle of attack on all of the rotor blades simultaneously.  This increases or decreases lift and moves the aircraft up  or down. The engine throttle control is located on the hand  grip at the end of the collective. The cyclic is the control  “stick” located between the pilot’s legs. It can be moved in  to the right, which is the direction needed to counter the main  rotor torque.  [Figures 1-102] Figure 1-101.  A Fenestron or “fan-in-tail” antitorque system.  This design provides an improved margin of safety during ground  operations.  Figure 1-100.  A tail rotor is designed to produce thrust in a direction  opposite to that of the torque produced by the rotation of the main  rotor blades. It is sometimes called an antitorque rotor.  Controls Figure 1-102.  While in a hover, Coanda Effect supplies  approximately two-thirds of the lift necessary to maintain directional  control. The rest is created by directing the thrust from the  controllable rotating nozzle. $_{ }$B $_{l}$a d e r o t a ti o n  B l a d e r o t a t i o n Tail rotor thrust Resultant torque from main rotor blades T o r q u e T o r q u e Air jet Downwash Lift Air intake Rotating nozzle Main rotor wake 1-46