HELICOPTER AERODYNAMICS WORKBOOK
Forward cyclic input in proportion to degree of blowback must be used to maintain a
constant rate of acceleration. Aft cyclic will be required during deceleration.
As the helicopter transitions to a hover from a decelerating glide slope as in a normal
approach, it often experiences an uncommanded nose-up tendency -- not nose-down as described
above. This is referred to as Pendulum Effect, and it occurs in response to increased collective
pitch. Although collective blade pitch is increased proportionally, forward flight dissymmetry of
lift is augmented. This overrides the effects of decelerating rotor blowback and causes the nose
of the aircraft to pitch up (figure 3-20).
TRANSVERSE FLOW AND CONING
Another phenomenon which occurs at about 15-20 kts is a non-uniform induced velocity
flow pattern across the rotor, or transverse flow. The wake vortices behind the rotor create
nearly twice the induced velocity at the trailing edge of the rotor disk as compared to the leading
edge, where it is approximately zero (figure 3-21).
This causes the blade over the nose to see an increase in AOA and, coupled with phase lag,
makes the rotor flap up on the left side. A sudden left cyclic input during acceleration may be
necessary to counteract this flapping. This fore-and-aft asymmetry of lift continues in forward
flight due to coning (figure 3-22), a steady upward flapping due to blade lift and centrifugal
force. In slow forward flight, coning causes the component of inflow to be more "up" in the
blade over the nose in comparison to the component of inflow over the tail.
3-18 HELICOPTER POWERED FLIGHT ANALYSIS