To: Vfrpilotpb. .. .One of the reasons for not using a smaller diameter rotor such as the Proprotor on the V-22 on a helicopter is its’ inability to sustain lift during autorotation. On a conventional helicopter at least those that I worked on the rate of descent is around 1,400 feet per minute with a forward speed of around 70 miles per hour. If you place the V-22 into an autorotation in the helicopter mode the rate of descent is 4-6000 feet per minute and it can’t be arrested like in a conventional helicopter.. .. .It is my personal belief that this is the reason the FAA has elected to certify this type of aircraft as a powered lift aircraft and not under the helicopter certification requirements.