These simple sheet metal extensions simply raise the return air intakes closer to the ceiling of the server room, where they can pull in only return (hot) air, and not supply (cold) air. Reducing air leaks from around cabling helps direct more cold air to the IT equipment. In addition, cutting cables and power cords to the correct length provides more room for air to flow away from the back of the rack. If the pitch angle is increased, the AOA increases if the pitch angle is reduced, the AOA is reduced.See Figures 1 and 2 below.) A single 12 inch blanking panel can reduce rack temperatures by 20F. Pilots adjust AOA through normal control manipulation of the pitch angle of the blades. Flapping is the primary means of compensating for dissymmetry of lift.

The flapping action alone, or along with cyclic feathering, controls dissymmetry of lift. No flapping occurs when the tippath plane is perpendicular to the mast. It occurs in response to changes in lift due to changing velocity or cyclic feathering. By flexing, the blades themselves compensate for the forces which previously required rugged hinges. The rigid rotor system has no vertical or horizontal hinges so the blades cannot flap or drag, but they can flex. The semi-rigid system does not have a hinge but flap as a unit. Flapping is the up and down movement of rotor blades about a hinge on a fully articulated rotor system. Most of the changes in AOA come from change in airspeed and rate of climb or descent others such as flapping occur automatically due to rotor system design. This action changes AOA, which changes coefficient of lift (CL), and affects overall lift of the rotor system. Collective feathering changes angle of incidence equally and in the same direction on all rotor blades simultaneously. Feathering is the rotation of the blade about its spanwise axis by collective/cyclic inputs causing changes in blade pitch angle. Collective and cyclic feathering help to make these changes. The pilot has little direct control over AOA except indirectly through the flight control input. Several factors may change the rotor blade AOA. Any increase in the AOA beyond this point produces a stall and a rapid decrease in lift.

Increasing the AOA increases lift until the critical angle of attack is reached. The turbulence results in a large increase in drag and loss of lift in the area where it is taking place. At this point, the airflow begins to separate from the airfoil and enters a burbling or turbulent pattern. As the AOA is increased further, it becomes more difficult for air to flow smoothly across the top of the airfoil. When the AOA is increased, air flowing over the airfoil is diverted over a greater distance, resulting in an increase of air velocity and more lift.

Effective airflow vs relative airflow