Figure 4-5

CHAPTER 4

HELICOPTER AERODYNAMICS WORKBOOK

stored rotational energy. All that is left is to put in a little forward cyclic to level the aircraft and

use that last rotational energy by pulling collective to cushion the landing.

If one chose to arrive at flare altitude at minimum rate of descent airspeed or less, there is

little or no forward speed to trade off for this advantageous high rotor RPM. Forward speed is

already low, and if too much flare is combined with an improperly timed flare (too high),

forward speed may reduce to zero at a high altitude. This condition is known as becoming

"vertical," and since the rotor system already has little stored energy, there will not be enough

thrust available with collective increase to slow rate of descent at touchdown to a non-destructive

level.

Figure 4-5

BLADE ELEMENT AND THRUST DURING STEADY STATE AUTO AND FLARE

AIRSPEED AND Nr CONTROL

Lets go back to the point where the pilot had the choice of minimum rate of descent or max

glide airspeeds. Now we understand the practical side of his choices, lets explore what is

happening at the blade a little more closely.

In a steady state autorotation, the induced flow has been reversed. It works with rotational

flow to create relative wind from beneath the blade, which sustains the blades' rotation. One

look at the blade element diagram shows in-plane drag exists; therefore, not all of the blade is

producing thrust -- some of the blade is counterproductive to autorotative flight. The region

breakdown is shown in figure 4-6. The pro-autorotative (auto) region represents about 45% of

4-6 AUTOROTATION