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CHAPTER 3
HELICOPTER AERODYNAMICS WORKBOOK
To balance the tip vortices, another vortex is formed at the blade root which writhes around
erratically through and around the main rotor system. This root vortex has an equal and opposite
effect on the tip vortex, which the pilot must correct. This usually manifests itself in heading
changes as the root vortex impinges on the tail rotor and generally occurs when within one rotor
diameter of the ground. Main rotor vortices can also affect the tail rotor during specific wind
conditions which are covered in the TH-57 NATOPS Manual.
GROUND EFFECT
While the helicopter is in a hover and in other flight conditions close to the ground, it
encounters ground effect (figure 3-13), a favorable aerodynamic phenomenon which requires
less power. Less power is required because there is less induced drag to overcome while "in
ground effect."
Figure 3-13
Since all of the induced velocities are reduced in ground effect and the velocity of air which
flows through the rotor system and reaches the ground goes to zero, induced drag is reduced and
less engine power is required (figure 3-14). As the helicopter moves vertically from the ground
to a distance out of ground effect (approximately one rotor diameter), the blades "see" a greater
induced velocity because the flow of air in the wake below the rotors is unimpeded. Combined
with rotational velocity, the resultant velocity is pointed slightly more downward, tilting the lift
vector aft, increasing the induced drag and power required to hover. The power savings can
amount to as much as 20%.
3-12 HELICOPTER POWERED FLIGHT ANALYSIS


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