Figure 3-19

HELICOPTER AERODYNAMICS WORKBOOK

CHAPTER 3

PHASE LAG

The advancing blade will encounter its highest rotational speed 90 prior to a position over

the nose of the aircraft, but does not experience the highest degree of flapping at this point

(figure 3-19). In fact, this maximum flapping occurs over the nose, 90 later, due to a principle

of a dynamic system in resonance. A system in resonance receives a periodic excitation force

sympathetic with the natural frequency of the system. The flapping frequency of a centrally

hinged system is equal to the speed of rotation. Therefore, maximum response occurs 90 after

maximum periodic excitation. This is termed phase lag. In order for a helicopter in forward

flight to roll into a left turn, maximum lift must be realized at the right "wing" position and

minimum lift must be realized at the left "wing" position. Therefore, maximum AOA must

occur at the 180 position and minimum AOA must occur at the 360 position. To obtain the

appropriate response 90 after maximum excitation, logic tells us forward cyclic is the

appropriate input to initiate a left turn. No wonder helicopters are such a challenge to fly! Well,

they are challenging, but not for this reason. Inputs are translated 90 prior mechanically, thanks

to some design engineers who had a little foresight.

Figure 3-19

BLOWBACK

Let's get back to dissymmetry of lift. As the aircraft moves forward, the advancing blade

"sees" a higher airspeed, and the resultant dissymmetry of lift causes the blades to flap to a

maximum 90 later due to phase lag. This extra lift generated over the nose causes the nose to

pitch up. Conversely, the nose will tend to pitch down as the aircraft decelerates. The combined

effect of dissymmetry of lift and reduced induced velocity defines this transition to a more efficient

flight regime, called translational lift. The pitch-up tendency of the aircraft as it accelerates and

the pitch-down tendency as it decelerates are known as rotor blowback (figure 3-20).

HELICOPTER POWERED FLIGHT ANALYSIS 3-17