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Aviation History
1967
1967 - 0090.PDF
90 FLIGHT International, 19 January 1967 THE NGTE RIGID ROTOR . . . augmentation factor, defined as lift generated divided by jet momentum, and may be of the order of 20. The momentum is small by comparison with lift generated, and this differs from the jet flap principle in which, for a Ci, of 4, the augmenta- tion factor is slightly less than 4. Original work was done with elliptical-section rotors using two slots, symmetrically disposed with respect to the direction of the incoming airflow, and was an extension of the jet-flap principle now being investigated on the BAC H.I26 research aeroplane. By varying the air velocity of one slot with respect to the other the lift coefficient could be changed at will. The change to a single slot was due to Dr I. C. Cheeseman, now Head of Special Projects at NGTE. This slot was located 30° behind the tangential airflow direction on the upper surface. With the adoption of a circular shape, and the realisation of very high CL values, the application to helicopter- type vehicles was immediately seen; and models were there- fore built for wind tunnel tests which substantiated the theoretical work. A full-scale working model was built and this is the subject of the artist's drawing. Because of the necessity to duct air into the rotor head for slot-blowing, it was decided for convenience to increase the mass flow and use a tip-drive system for rotor propulsion. The cross-section of the rotor was circular and quite large in order to accommodate the air mass flow needed for rotor pro- pulsion and slot-blowing without excessive duct losses, and this permitted the use of a mechanically rigid structure. Trials were made at low tip speeds, because of the low critical Mach number for bodies of circular cross section; but it was found at an early stage that the power necessary for rotor drive was greater than expected, owing to the large induced power requirement at these low speeds. This effect was found to decrease with increase in tip speed, and thus demonstrated the need for higher rotor speeds for greater efficiency (induced power required is analogous to induced drag: for a given aerofoil moving at changing airspeeds, induced power required varies inversely as the airspeed changes). Wind tunnel models were built with a number of slots, the purpose of which was to improve the generation of lift at high CL values across the span in accordance with the local rotor speed. A high coefficient is required adjacent to the rotor hub, but has to be decreased across the span, since the critical Mach number decreases inversely with increase in CL. The ideal situation is that CL distribution which gives constant downwash over the rotor. In order to assess the rigid rotor from a commercial aspect NGTE decided to investigate a number of projects of short- range VTOL transport aircraft. So as to simulate as realistically as possible aircraft struc- ture and weights, a design study of a commercial aircraft of about the sijK of the Hunting 107 (later rescaled into the BAC One-Eleven) was made, and known as the Mk 1 model. This model featured a single, fuselage-mounted, two-bladed rigid rotor which could be stowed fore-and-aft in the fuselage for flight above stalling speed. Gross weight was assumed to be 40,0001b. Further study of the Mk 1 concept revealed a number of disadvantages. It was necessary to extend and retract the rotor head system to provide, respectively, airframe clearance during V/STOL and stowage during cruise. This entailed a protrusion into the cabin, objectionable on grounds of both pressure-hull integrity and loading ability, particularly for military transport purposes. For these and other reasons this project has now been discarded. A second project was initiated (known as the Mk 2 model! designed around the BAC One-Eleven airframe with a high wing on which the two Spey engines are mounted. The gross weight is about 73,0001b. Each engine drives its own rotor via an interconnecting shaft which not only ensures load-sharing in the one-engine-out case but constrains the rotors to remain correctly phased at 90° to each other. The nacelles have a duct flap arrangement and the airflows through the by-pass and engine sections are not allowed to mix. During vertical flight the flap is closed so that hot gas from the engine is diverted through a free power turbine which absorbs power to drive the rotor through a gearbox system, while cool gas from the by-pass is ducted into the rotor head to energise the slots. To make a comparison between tip drive versus mechanical drive, an alternative Mk 2 model design was evolved, using the Mk 1 tip drive but otherwise identical, and two parallel studies were made. Comparisons between the two demonstrated the great superiority of the latter, due both to the far greater efficiency of the mechanical drive and to the lack of torque problems associated with the twin-rotor layout, which is now agreed upon as being the best solution. The rotors, of circular cross-section out to half span and tapering to 20 per cent elliptical at the tip, are thick by com- parison with helicopter rotor blades of the same span. The gust response is very low, because of the symmetrical cross-section; and the parking problem—which is severe in other compound- rotor projects—is therefore virtually non-existent The rotors of the Mk 2 project would not in fact be stowed at all, but merely rotated after transition so that they lay parallel with the airflow for minimum drag. Comparative studies between the tip-driven and mechani- cally driven Mk 2 models has shown that, in the former case, four Spey engines were needed to meet the relevant civil safety requirements with one engine out, while with the latter only two engines are necessary. With two Speys the aircraft This "Flight" drawing shows the full-scale model mounted on a tes1 vehicle, as demonstrated at Farnborough last September. The Rolls- Royce Avon engine provides power for rotor propulsion and slot blowing' and also helps to accelerate the vehicle to 75 m.p.h. Strain-gauge balances record lift, drag, roll and pitch moments on the rotor 1 Driver 2 Supernumerary seat 3 Rotor systems operator 4 Periscope 5 Operators panel 6 Ultra-violet quick-look recorder 7 Signal conditioning cabinet 8 Signal acquisition cabinet 9 Digital recording cabinet 10 Tape recorder 11 ASI pitot 12 Rolls-Royce Avon intake 13 Rolls-Royce Avon 14 Compressor bleed 15 Spill valves 16 Rotor propulsion air 17 Slot air 18 Force-measuring strain gauges 19 Swivelling thrust nozzle 20 Sub-frame 21 Sub-frame pneumatic cushion 22 Levelling unit 23 Controllable dampers 24 Tower with altitude adjustments © Iliffe Transport Publications Ltd 1967
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