Translating Tendency or Drift Coriolis Effect (Law of Conservation of Angular  Momentum)  Ground Effect During hovering flight, a single main rotor helicopter tends to  drift or move in the direction of tail rotor thrust. This drifting  tendency is called translating tendency.  [Figure 2-32]  To counteract this drift, one or more of the following features  may be used. All examples are for a counterclockwise rotating  main rotor system. downwash from a purely vertical motion to a combination  of vertical and horizontal motion. As the induced airflow  through the rotor disk is reduced by the surface friction, the  lift vector increases. This allows a lower rotor blade angle for  the same amount of lift, which reduces induced drag. Ground  effect also restricts the generation of blade tip vortices due to  the downward and outward airflow making a larger portion  of the blade produce lift. When the helicopter gains altitude  vertically, with no forward airspeed, induced airflow is no  longer restricted, and the blade tip vortices increase with  the decrease in outward airflow. As a result, drag increases  which means a higher pitch angle, and more power is needed  to move the air down through the rotor. Ground effect is at its maximum in a no-wind condition over  a firm, smooth surface. Tall grass, rough terrain, and water  surfaces alter the airflow pattern, causing an increase in rotor  tip vortices.  [Figure 2-33] When hovering near the ground, a phenomenon known  as ground effect takes place. This effect usually occurs at  heights between the surface and approximately one rotor  diameter above the surface. The friction of the ground  causes the downwash from the rotor to move outwards from  the helicopter. This changes the relative direction of the  The Coriolis effect is also referred to as the law of  conservation of angular momentum. It states that the value  of angular momentum of a rotating body does not change  unless an external force is applied. In other words, a rotating  body continues to rotate with the same rotational velocity  until some external force is applied to change the speed of  rotation. Angular momentum is moment of inertia (mass  times distance from the center of rotation squared) multiplied  by speed of rotation. Changes in angular velocity, known as  angular acceleration and deceleration, take place as the mass  of a rotating body is moved closer to or further away from  the axis of rotation. The speed of the rotating mass increases  or decreases in proportion to the square of the radius. An  excellent example of this principle is a spinning ice skater.  The skater begins rotation on one foot, with the other leg  and both arms extended. The rotation of the skater’s body is  relatively slow. When a skater draws both arms and one leg  inward, the moment of inertia (mass times radius squared)  becomes much smaller and the body is rotating almost faster  than the eye can follow. Because the angular momentum  must remain constant (no external force applied), the angular  velocity must increase. The rotor blade rotating about the  rotor hub possesses angular momentum. As the rotor begins  to cone due to G-loading maneuvers, the diameter or the  disk shrinks. Due to conservation of angular momentum,  the blades continue to travel the same speed even though the  blade tips have a shorter distance to travel due to reduced  disk diameter. The action results in an increase in rotor rpm.  Most pilots arrest this increase with an increase in collective  pitch. Conversely, as G-loading subsides and the rotor disk  flattens out from the loss of G-load induced coning, the  blade tips now have a longer distance to travel at the same  Figure 2-32.  A tail rotor is designed to produce thrust in a direction  opposite torque. The thrust produced by the tail rotor is sufficient  to move the helicopter laterally. •  The main transmission is mounted at a slight angle to  the left (when viewed from behind) so that the rotor  mast has a built-in tilt to oppose the tail rotor thrust.  •  Flight controls can be rigged so that the rotor disk is  tilted to the right slightly when the cyclic is centered.  Whichever method is used, the tip-path plane is tilted  slightly to the left in the hover.  •  If the transmission is mounted so the rotor shaft is  vertical with respect to the fuselage, the helicopter  “hangs” left skid low in the hover. The opposite is  true for rotor systems turning clockwise when viewed  from above.  •  In forward flight, the tail rotor continues to push to  the right, and the helicopter makes a small angle with  the wind when the rotors are level and the slip ball is  in the middle. This is called inherent sideslip.  B l a d e r o t a t ion  Bla d e r o t a t i o n Drift Tail rotor thrust Tail rotor downwash T o r q u e T o r q u e 2-22