Practiced this manoeuvre a few times when I was an instructor. An excellent way to demonstrate this movement of the air pocket effect is to enter the autorotation at the nominal speed, enabling the air pocket to develop below the aircraft. Begin pitching the nose up to bleed off the speed at a rate that allows the differential pressure to secure the air pocket below the A/C at a perpendicular angle. When zero speed is accomplished, the air pocket has now had time to anchor itself below the body as well as several “pockettes” being formed, at the latter stage, below the horizontal stabilisers. Due to the zero airspeed, the
wind then blows these pocki’s to the rear of the foresaid surfaces allowing the aircraft to sit in what is essentially an inverted parachute effect. Company policy was then to engage the rotor brake allowing Nr to drop to around 10% to further emphasis this phenomenon as well as demonstrating aerodynamic forces play no role in this manoeuvre. On successful completion of the demonstration, Nr was then increased to 100% whilst maintaining zero airspeed at a ROD of approximately 4000 fpm. Introduction of pitch on the blades then induces the essential vortex ring, which is necessary for the displacement of the air pockets and enabling a safe and gentle set down.
Hope this clears that up!
