Hello Norbert

I did see a design once for a simpler method of obtaining a jump take-off.

Essentialy each blade can rotate ( exactly like pitch change on a helicopter ) except that they are sprung to remain at flight rpm. Through the centre of the rotor shaft passes the driveshaft which ends with a spider that has ( for 3 bladed roter ) 3 legs. These legs when driven push against the spring tension holding the flight pitch rpm causing the pitch to reduce to 0. Soon as the drive is removed they snap back to flight rpm. If that doesnt make sense I will attempt to draw it .

Sean