Figure 2-33.  Air circulation patterns change when hovering out of ground effect (OGE) and when hovering in ground effect (IGE). Figure 2-34. To ascend vertically, more lift and thrust must be  generated to overcome the forces of weight and drag. Figure 2-35.  The power required to maintain a straight-and-level  flight and a stabilized airspeed.  No wind hover Downwash pattern equidistant 360° Out of Ground Effect (OGE) In Ground Effect (IGE) Blade tip vortex Large blade tip vortex D r a g W e i g h t Lif t Thr ust Vertical ascent Helicopter movement Thrust Drag Weight Lift Resultant Resultant tip speed. This action results in a reduction of rotor rpm.  However, if this drop in the rotor rpm continues to the point  at which it attempts to decrease below normal operating  rpm, the engine control system adds more fuel/power to  maintain the specified engine rpm. If the pilot does not reduce  collective pitch as the disk unloads, the combination of engine  compensation for the rpm slow down and the additional pitch  as G-loading increases may result in exceeding the torque  limitations or power the engines can produce. Hovering is actually an element of vertical flight. Increasing  the AOA of the rotor blades (pitch) while keeping their  rotation speed constant generates additional lift and the  helicopter ascends. Decreasing the pitch causes the helicopter  to descend. In a no wind condition, when 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 ascends vertically.  [Figure 2-34] In steady forward flight with no change in airspeed or vertical  speed, the four forces of lift, thrust, drag, and weight must  be in balance. Once the tip-path plane is tilted forward, the  total lift-thrust force is also tilted forward. 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 force opposing the  motion of an airfoil through the air).  [Figure 2-35] Forward Flight Vertical Flight 2-23