translational
liftThe efficiency of the hovering
rotor system is improved with each knot of incoming wind gained by horizontal
movement or surface wind. As the incoming wind enters the rotor system,
turbulence and vortexes are left behind and the flow of air becomes more
horizontal. All of these changes improve the efficiency of the rotor system and
improve aircraft performance.
Improved rotor efficiency
resulting from directional flight is called translational lift. The
following picture shows an airflow pattern at airspeeds between 1-5 knots:
Note how the downwind vortex is
beginning to dissipate and induced flow down through the rear of the rotor disk
is more horizontal than at a hover.
This next picture shows the
airflow pattern at a speed of 10-15 knots. Airflow is much more horizontal than
at a hover. The leading edge of the downwash pattern is being overrun and is
well back under the helicopter nose. At about 16 to 24 knots (depending upon the
size, blade area, and RPM of the rotor system) the rotor completely outruns the
recirculation of old vortexes, and begins to work in relatively clean air:
The air passing through the
rotor system is nearly horizontal, depending on helicopter forward air speed.
As the helicopter speed
increases, translational lift becomes more effective and causes the nose to
rise, or pitch up (sometimes called blowback). This tendency is caused by the
combined effects of dissymmetry of lift and transverse flow. Pilots must correct
for this tendency in order to maintain a constant rotor disk attitude that will
move the helicopter through the speed range where blowback occurs. If the nose
is permitted to pitch up while passing through this speed range, the aircraft
may also tend to roll to the right.
When the single main rotor
helicopter transitions from hover to forward flight, the tail rotor becomes more
aerodynamically efficient. Efficiency increases because the tail rotor works in
progressively less turbulent air as speed increases. As tail rotor efficiency
improves, more thrust is produced. This causes the aircraft nose to yaw left if
the main rotor turns counter clockwise. During a takeoff where power is constant,
the pilot must apply right pedal as speed increases to correct for the left yaw
tendency.
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