At a hover, the rotor system requires a great volume of air upon which to work. This air must be
pulled from the surrounding air mass; this is an expensive maneuver that takes a great deal of
horsepower. The air delivered to the rotating blades is pulled from above at a relatively high velocity,
forcing the rotor system to work in a descending column of air.
The main rotor vortex and the recirculation of turbulent air add resistance to the helicopter while
hovering. Such an undesirable air supply requires higher blade angles of attack and an expenditure of
more engine power and fuel. In addition, the main rotor is operating in air filled with abrasive materials
that cause heavy wear on helicopter parts while hovering in ground effect.
3.13.
GROUND EFFECT
Figure 3.9 illustrates air flow in and out of ground effect. Notice the information in A and B that is
printed close to the sketches. Ground effect is a condition of improved performance found when
hovering near the ground. The best height is approximately one-half the main rotor diameter.
The improved lift and airfoil efficiency while operating in ground effect are due to the following
effects. First and most important, the main rotor-tip vortex is reduced. When operating in ground effect,
the downward and outward airflow reduces the vortex. A vortex is a flow involving rotation about an
axis or center. This makes the outward portion of the main rotor blade more efficient. Reducing the
vortex also reduces the turbulence caused by recirculating the vortex swirl, as shown in figure 3.9A.
Second the angle of the air is reduced as it leaves the airfoil, as illustrated in figure 3.9B. When the
airfoil angle is reduced the resultant lift is rotated slightly forward, making it more vertical. Reduction
of induced drag permits lower angles of attack for the same amount of lift and reduces the power
required to drive the blades.
3.14.
TRANSLATIONAL LIFT
The efficiency of the hovering rotor system is improved by each knot of incoming wind gained by
forward motion of the helicopter or by a surface headwind. As the helicopter moves forward, as shown
in figure 3.10, fresh air enters in an amount sufficient to relieve the hovering air-supply problem and
improve performance. At approximately 18 knots, the rotor system receives enough free, undisturbed air
to eliminate the air supply problem. At this time, lift noticeably improves; this distinct change is
referred to as translational lift. At the instant of translational lift, and
28
Previous Page
