Hi,
We do operate Arenson Drive units with piercing 5 blades piercing propellers. The propellers are manufactured by Rolla. We do have some data on the forces and I will provide you some next week. Regards, Morel.
The Mach number is defined as the ratio of the velocity of the fluid to the velocity of sound. Ma=U/c. For air st standart conditions a flow can be considered incompressible if the velocity is less than about 100 m/s.