drag regime. At the same time, the speed of the retreating blade never approaches the stall point. Likewise, changes in drag are even more reduced, to the point of being negligible. This allows the Derschmidt rotor to be a rigid design, eliminating the complex series of bearings, flexible fittings and linkages used in conventional rotors. Since the motion in the Derschmidt rotor follows the natural change in drag through the rotation, the force applied to the blades to move them into position is quite small. Of the several designs he presented in his early patents, most used a very small linkage from a bell crank on the inner side of the blade attached to a small pushrod for operation. These rods were attached to a disk set eccentrically to the centre of rotation, which drove the blades into their proper locations. Last in the series of designs was a different approach that used a single counterweight for each blade, geared so its motion was mechanically amplified. The weight was selected to create a harmonic pendulum at the rotor's design speed. There was no mechanical attachment between the blades, and the entire assembly sat outside the hub, leaving ample room for maintenance. Bo 46 Bölkow had been interested in high-speed rotor flight for some time, and had drawn up several experimental concepts based on tip jet systems. Later they took on the job of developing a glass-fibre composite blade that was much stronger than the existing metal designs. When Derschmidt received his first patent in 1955, Bölkow took up the concept and started work on the Bölkow Bo 46 as an experimental testbed, paid for by a Ministry of Defence contract. The basic Bo 46 design was finalized in January 1959. The five-bladed rotor system was initially tested in a wind tunnel and turned in impressive results. These suggested that the Bo 46 would be able to reach speeds up to ; even advanced designs of the era were limited to speeds around . Construction of three highly streamlined fuselages started at Siebel. There were powered by an 800 hp Turboméca Turmo turboshaft driving a five-bladed Derschmidt rotor. The design originally featured a louvred fenestration for the anti-torque rotor that could be closed in high speed flight, but this was removed from the prototypes and the six-bladed rotor was conventionally mounted on the left side of the tail. The maximum speed