Bolkow Bo 46


The Bolkow Bo.46, three experimental prototypes of which were built for the German Ministry of Defense, served as a flying test-bed for the Derschmidt semi-rigid rotor. The first model, built jointly with SIAT, began test flights in January 1964. The Derschmidt rotor underwent a long series of wind tunnel tests, for each blade had a hinge about half-way along its radius, enabling the outer section of the blade to rotate 40° backwards or forwards in relation to the inner section connected to the hub. This enabled the movement of the advancing blade to be "delayed," thereby considerably reducing the tip speed. Conversely, by "accelerating" the movement of the outer portions, quite high rotational speeds could be achieved. However the complexity of this system prevented the Bo.46 from passing the experimental stage.

G.Apostolo "The Illustrated Encyclopedia of Helicopters", 1984

The Bölkow Bo 46 was an experimental helicopter built to test the Derschmidt rotor system that aimed to allow much higher speeds than traditional helicopter designs. Wind tunnel testing showed promise, but the Bo 46 demonstrated a number of problems and added complexity that led to the concept being abandoned. The Bo 46 was one of a number of new designs exploring high-speed helicopter flight that were built in the early 1960s.

Helicopter rotors operate in a much more challenging environment than a normal aircraft propeller. To start with, helicopters normally use the main rotor both for lift and manoeuvrability, whereas fixed-wing aircraft normally use separate surfaces for these tasks. Pitch and yaw are operated by changing the lift on different sides of the rotor, using a system of bell cranks to adjust the blades to different angles of attack as they rotate. To roll to the left, the blades are adjusted so there is slightly more angle of attack on the right and slightly less on the left, resulting in a net upward lift on the right side that rolls the aircraft. 

In forward flight, the rotor system is subject to various forms of differential loading. Imagine a rotor system where the tips of the blades rotate at 300 km/h relative to still air. When that helicopter is hovering, the blades see the same 300 km/h relative wind throughout their rotation. However, when the helicopter starts to move forward its speed is added to the speed of the blades as they advance towards the front of the aircraft, and subtracted as they retreat. For instance, if the helicopter is flying forward at 100 km/h, the advancing blades see 300 + 100 km/h = 400 km/h, and for the retreating ones its 300 - 100 km/h = 200 km/h.

In this example, the relative airspeed changes by a factor of two during every rotation. Lift is a function of the angle of the airfoil to the relative airflow combined with the speed of the air. To counteract this change in lift, which would normally roll the aircraft, the rotor system has to dynamically adjust the angle of the airfoils to ensure they generate a steady amount of lift throughout their motion. This adjustment is in addition to any that is being applied deliberately to manoeuvre. Since every control system has some mechanical limit, as the aircraft speeds up it loses manoeuvrability.

Drag is a function of the square of airspeed, so the same changes in speed cause the drag to vary by a factor of four. To reduce the net force as much as possible, helicopter blades are designed to be as thin as possible, reducing their drag, although this makes them inefficient for lift. In the 1950s, helicopter blades were made in much the same fashion as fixed-wing aircraft wings; a spar ran the length of the rotor blade and provided most of the structural strength, while a series of stringers give it the proper aerodynamic shape. This method of construction, given the materials of the era, placed enormous stresses on the spar.

To lessen the loads, especially the rapid changes, the rotor hubs included a system of bearings that allow them to move forward or back in response to drag, and up and down in a flapping motion in response to changing speed. These were in addition to the system used to change the angle of attack to provide control; rotor hubs tended to be very complex.

There is a limit to the rotor's ability to adjust to these changing loads, and this places a limit on the maximum speed of the helicopter.

All wings have a critical angle of attack where increases to the angle do not result in additional lift. This point is better known as the stall point. If a given helicopter airfoil design has a stall point at 100 km/h, which is not unusual, then when it is mounted to the hypothetical design above, the helicopter cannot travel any faster than 200 km/h; at that speed the retreating blades will be moving at their stall speed.

One solution to this problem is to spin the rotor faster; this maximizes the speed difference between the rotor tips and the fuselage, thereby increasing the aircraft speed where the rearward moving blades are nearing the stall point. However, this process also has its limits. As any airfoil approaches the speed of sound it encounters a problem known as wave drag that significantly increases drag, dominating efforts to add more power and sharply reducing efficiency. If the speed of the hypothetical design were doubled to 600 km/h, the advancing blades would start reaching these speeds when the aircraft reached about 200 km/h forward speed.

So the maximum speed of a helicopter is constrained by two factors. Increasing the rotational speed of the rotor decreases the forward speed where wave drag becomes a problem, but decreasing the speed of the rotor decreases the speed where the stall point becomes a problem. In practice, there are additional dynamic forces and limits to motion that limit helicopter designs to speeds far below the limits imposed above.

The basic problem inherent in rotor design is the difference in airspeed for the advancing and retreating blades. Among the many effects this causes is one of interest; the blades rotate forward and backward around the hub as drag increases and decreases. Consider a blade as it reaches the rear of the aircraft and starts to rotate forward; during this time the relative airspeed starts increasing rapidly, and the blade is pushed further and further back by the increasing drag. This force is absorbed in a drag bearing. During the brief period while it rotates around this bearing, the overall speed of the blade is decreased, slightly offsetting the speed due to forward motion.

Derschmidt's rotor design deliberately exaggerates this rotation to offset the increase and decrease in speed throughout the blade's rotation. At the same point of rotation as the traditional blade above, a Derschmidt rotor has advanced the blade considerably to an angle of about 40 degrees compared to its rest position straight out from the hub. As the blade continues advancing, a linkage swings the blade from 40 degrees forward to 40 degrees rearward, slowing the tip by about 1/2 the rotational speed. This process is reversed as the blade reaches its forward-most position, increasing the speed of the blade as it retreats.

The resulting motion helps smooth out the relative airspeed seen by the blade. Since the effects of the forward motion of the helicopter are reduced, or even eliminated at lower speeds, the rotor can be spun at a high speed without fear of reaching the wave 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.

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 500 km/h, whereas even advanced designs of the era were limited to speeds around 250 km/h. 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 was not limited by rotor considerations, but the maximum power of the engine. Adding separate engines for additional forward thrust was expected to allow speeds as high as 700 km/h.

During the early 1960s the company also outlined several production designs, most using twin rotors, the largest of these was the Bo 310. This was powered by two T55 or T64 engines, each of which drove both a Derschmidt rotor and a forward-facing propeller for additional forward thrust. Several versions of the Bo 310 were modelled, mostly passenger transports, but also attack helicopter versions. The Bo 310 would have a cruise speed of 500 km/h.

Initial test flights with the rotors locked started in the autumn of 1963. In testing a series of unexpected new types of dynamic loads were encountered, which led to dangerous oscillations in the rotor. These did not appear to be inherent to the design itself, but they could only be cured through additional complexity in the rotor. During the same period, rotor design was moving to composite blades that were much stronger than the older spar-and-stringer designs, which eliminated the need for the complex bearing system that relieved loads. Although the Derschmidt rotor still improved performance, it appeared the added complexity was not worthwhile.

Interest in the system waned, but research flights continued. The Bo 46 was eventually equipped with two Turboméca Marboré engines, allowing a speed of 400 km/h. The fibreglass bladed rotor proved to be workable however, and would go on to see wide service in the Bölkow Bo 105.

A preserved example of the Bo 46 is on public display at the Hubschrauber Museum, Bückeburg.

General characteristics

  • Crew: one pilot
  • Capacity: 1 passenger/observer
  • Main rotor diameter: 10.00 m (32 ft 10 in)
  • Main rotor area: 78.5 m2 (845 ft2)
  • Gross weight: 2,000 kg (4,400 lb)
  • Powerplant: 1 × Turboméca Turmo IIIB, 597 kW (800 hp)


  • Maximum speed: 320 km/h (200 mph)