Item 1742

OTHER: Helicopter - Inside - Interleaving - Derschmidt High-speed Rotor Idea



Ref. Sketch of Bolkow with Derschmidt High-speed Rotors:

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Bo-46 Three prototypes of thc Bo-46 are under construction. with fuselages manufactured by SIAT, to flight-test the remarkable Derschmidt high-speed rotor system. Each will be powered by an 800 s.h.p. Turbomeca Turm0 IIIB shaft-turbine. Driving a five-blade Derschmidt main rotor, and six-blade conventional tail rotor. Flight trials are scheduled to begin late this year.

Present helicopter speed limitations are due essentially to the fact that forward and rotational speeds add up on an advancing blade and subtract on a retreating blade. The Derschmidt rotor employs blades that lag behind in the advancing stage and sweep forward in the retreating stage of each revolution. The controlled lead-lag movement diminishes asymmetrical flow conditions and delays compressibility effects on the tips of the advancing blades, and wind-tunnel tests have indicated that this will make possible helicopter speeds of up to 310 mph,

The Derschmidt rotor incorporates a relatively large star-shaped hub assembly and articulated glass-fibre blades sweeping to each side of neutral. The blades function in the same manner as a resonant pendulum, so that the blade-swinging mechanism requires power only to initiate and synchronize angular blade movement. Projected future applications of the Derschmidt rotor include a 24-passenger winged rotorcraft designated Bolkow P31O. This would have a 3.200-3.400 shp engine (T55 or 'T64) at each wing-tip, driving forward-facing propellers and intermeshing Derschmidt rotors. Estimated performance Includes a range of 430 miles at 310 m.p.h. with 20min reserve fuel.

Bolkow have abandoned their single-seater but are pressing on with a rigid rotor and with the Derschmidt lead-lag rotor system, the latter now aimed at the twin-rotor P-310 passenger transport- a remarkable concept, to say the least. In the meantime, because the P-310 is scheduled for the late 1960s, Bolkow are shortly to begin ground tests of the five-blade Derschmidt rotor of the Bo46 research helicopter for the German Defense Ministry. The Bo46 is powered by a 700 hp Turmo IIIB free turbine and will fly, initially with the lead-lag system locked, in the autumn. Highspeed flights will begin next year. It is hoped that speeds of 250kt will be reached, though in a dive because of limited engine power.

The lead-lag system gives a speed ratio between advancing and retreating blades of 3 : 1 and is claimed to afford a 175kt airspeed increase over a rotor designed for lOOkt without any increase in required power. The lead-lag linkage is needed only to hold the blades in phase and bears virtually no stresses. In the wind tunnel one blade was actually disconnected and continued to lead-lag by itself.

The big P-310 will have side-by-side intermeshing Derschmidt rotors with connecting shaft and two turbines, either Lycoming T55s or GE T64s, driving both cross-shaft and tractor propellers optimized for high-speed flights. These will be kept in zero pitch in the hover and provide thrust in forward flight. Speeds up to 300kt are said to be predicted. Although the horizontal pylons form a wing, the rotors operate under power at all speeds, and the wing will provide only 10 or 20 per cent of the total lift. The P-310 will carry 18 to 20 soldiers. The complexity of the mechanical system is not viewed adversely, because there is already a good deal of experience with connecting shafts and gearboxes, and there will be much more by the time the aircraft is built. At the moment it is only a project.

The Bolkow rigid rotor, a three-blade, 20ft-diameter version of which is now running on a test stand, has entirely plastics blades, the only metal being the balance weights. Rigid rotors, now being flown by several companies in the USA, have become feasible with the much stronger rotor heads and linkages, which can now be provided. The rotor has gyroscopic stability and very fast reaction to control inputs, possibly calling for some form of control damping to avoid overstressing. Because of the increased stresses there seems to be a size limitation, presently set at the UH-1, a rigid-rotor version of which Bell have now flown at 150kt; but anything may develop in this exciting field.

In about 18 months' time, Bolkow are likely to fly their rigid rotor in the B6105, an experimental four-seater to be powered by two of the new 250 h.p. BMW 6022 turbines, or by equivalent Continental or Boeing turbines. The Wankel engines proposed last year have been abandoned, because development is going very slowly. The rotary-piston Wankel engine is nevertheless very intriguing for small aircraft, because of its great compactness and quietness. Bolkow estimate that two 120 h.p Wankels driving a single propeller would fit under the cowling at present required for the single 180 hp. Lycoming O-360 of the 207, as noted earlier.


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Initially displayed: June 15, 2009~ Latest revision; March 9, 2012