Figure 3-20

CHAPTER 3

HELICOPTER AERODYNAMICS WORKBOOK

Figure 3-20

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).

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