Figure 4-2

HELICOPTER AERODYNAMICS WORKBOOK

CHAPTER 4

-- the length of time you will remain airborne in the windmill brake state is simply a function of

terminal velocity.

Comparing the diagrams of conditions at the blade element (figure 4-1), we can observe

collective pitch required to maintain a constant thrust changes, due to the net flow through the

rotor disk. Additionally, in a climb, the flow causes the lift vector to tilt back, thus increasing the

power required. The opposite happens in the windmill brake state and low rates of descent.

During vortex ring state, the conditions are similar to those conditions in a climb, so collective

pitch setting and power required must be high to maintain vortex ring state. Therefore, reducing

collective setting to reduce pitch is a recovery technique for this condition.

AUTOROTATION

Continuing to lower the collective to minimum pitch transitions the helicopter from vortex

ring state to vertical autorotation state. A majority of the flow will be upwards through the rotor

system, but due to the presence of induced downflow, one may still classify it as being in vortex

ring state (figure 4-2).

Figure 4-2

There are differences, though. The lift vector becomes tilted forward (figure 4-3), providing

enough power to drive the tail rotor and gearboxes without the engine. Drag of the blades is also

overcome.

Figure 4-3

Blade Element in Autorotation

AUTOROTATION 4-3