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Aviation History
1967
1967 - 0094.PDF
92 FLIGHT International, 19 January 1967 A powerplant layout, suggested by NGTE, which makes use of the unmixed flow through a turbofan engine to provide both rotor drive and propulsion. A, gas turbine; 6, rigid rotor; C, by-pass air for circulation control; D, engine efflux for propulsion or rotor drive through free turbine; E, oil cooler; F, gearbox THE NGTE RIGID ROTOR . . . would be able to hover at gross weight on the power of only one engine, while sufficient power is still available for tran- sition (with increase of forward speed, power take-off to drive the rotor decreases as the latter become off-loaded). The power required to energise the slots would be 1001b air per second for each engine in vertical flight. Much of the present work is concerned with rotor/wing interference. The download on the wing from the rotor is partly offset by the ground effect provided by the wing. The penalty was assumed to require 10 per cent rotor thrust to counteract this effect, which is possibly pessimistic. The normal e.g. shift (assumed to be 18in about the datum) is compensated by modulating the slot air to provide the equivalent of cyclic-pitch control for lift variation as required. The e.g. shift is transmitted to the rotor mounting by a torsion component in the wing. The 90° relative phasing of the rotors may result in excitation of undesirable structure modes; this aspect is being studied, but it is not thought to present any great obstacle. The BAC One-Eleven configuration is aimed at the short- haul market, which is an obvious first choice for VTOL vehicles. Sites need not necessarily be at city centres, but could be adjacent to a rail system, though the comparatively low noise level would probably not rule out operation from centres of urban population. With present limiting noise requirements at London Heathrow Airport pegged at 110 PNdb by day and 102 PNdb by night, that due to the Mk 2 model would be about at 95 pndb at 500ft. Most of the noise is in fact due to the engine inlet and a 3001b penalty per engine has been assumed for a noise-reduction system. Control of the Mk 2 model in VTOL is exercised in the following manner: pitch, cyclic control on both rotors; roll, differential collective pitch; yaw, differential control of the direction of jet deflection from the engines after passage through the rotor-drive turbines. Transition is made by tilting the plane of the rotor, thus tilting the lift vector (as with a helicopter) while forward thrust from the engines is progres- sively phased in by opening the duct flap. The working model constructed at NGTE and demonstrated at the SBAC Display is the most tangible evidence of rigid- rotor research. The Avon engine provides air for rotor pro- pulsion through a tip drive system, for slot blowing, and also for vehicle propulsion. The present rotor is a circular tube of 12ft span with a constant diameter of 5.6in. It is shortly to be fitted with extensions, each 24in long, which will change the tip section from circular to 20 per cent elliptical, necessitating a mechani- cal drive (tip drive duct losses would otherwise be prohibitive in the thin section) but still allowing the required quantity of slot air to be transferred without excessive losses. One of the advantages of the rigid rotor is that cyclic-lift variation may be achieved by modulating the slot air through a swashplate type of system, which does not rotate (unlike the cyclic/collective pitch mechanism of a conventional heli- copter rotor head), thereby saving both complexity and weight. A further advantage of this system is that cyclic pitch har- monics may be introduced with beneficial effect on the reduc- tion of vibration. This cannot be done with conventional rotors, beca'use of rotor inertia and mechanical complexity. The model as tested in the RAE 24ft low-speed wind tunnel has given the following spot values for a rotor speed of 400 r.p.m. and forward airspeed of 90ft/sec; rotor tip speed, 377ft/min; CL at 0.7 rotor radius, 4.75; power to drive rotor (tip-propulsion) 135 b.h.p.; power required for energising slots, 58 b.h.p.; lifting force, 1,4701b weight. The high augmenta- tion factor (greater than 30) is responsible for the relatively low power required for slot-blowing. The significance of this quantity is that, if the slot air were to be channeled through a duct of maximum efficiency, the propulsive force measured would be less than one-thirtieth of the lift force .obtained by using it to energise a slot The normal helicopter disc loading of 4Ib/sq ft to 61b/sq ft may be doubled with the rigid rotor. Thus, for a given vehicle weight, the rotor diameter may be decreased to about 70 per cent of that of a conventional rotor. What are the penalties? Clearly one cannot expect to have VTOL ability without a corresponding trade-off elsewhere in the performance envelope. The Mk 1 model suffered from very high fuel consumption—as a consequence of the low tip speed—and consequently high induced power requirements. The Mk 2 rotor drive is far more efficient, in exactly the same way that a turbo-fan engine is more efficient than a turbojet at low speeds. Under VTOL conditions the engine with its turbine take-off drive and rotor can be regarded as a turbofan engine, the fan in this case being the rotor. In comparison with a conventional aircraft the penalty arises from the extra weight represented by the shut-off valves, turbine arrangement, cross-shafting, pitch-control system, rotors and control systems, and possibly by extra structure in the centre-section region, which may need reinforcing. NGTE are not at present giving figures for the weight penalty; their philosophy is that with so much research work to be done (not only on aerodynamics at NGTE, but also in the field of materials), present figures may well be pessimistic. Also, the whole project is at present run purely by NGTE and the particular configurations now under consideration may not be those finally adopted by users. The cyclic/collective pitch mechanism, rotor and the general application of circulation control of aerofoils are, incidentally, all subjects of NGTE patents. The Mk 2 project, in comparison with the 200 Series BAC One-Eleven from which it is derived, and with the same payload, will be able to cruise at 500 m.p.h. over a range of 200 n.m. with full reserves, including a total of lOmin VTOL at the beginning and end of the flight and 45min stacking with stopped rotors. The Future The rigid rotor has proved itself in theory; but a lot more work and analysis is needed to assess the "second-order" effects of vibration, interference and other factors. Eh- Cheese- man envisages a further 18 months' research at NGTE, by which time the principle will have been verified in depth, and will be ready too for further work on applications by industry. What will be the next step? Wind tunnel models will have demonstrated the practicability of the elliptical rotor; and a working powerplant with the necessary power take-off for rotor drive and slot-blowing would be desirable for investiga- tion in this all-important area and for integration with a rotor fox full-scale ground testing. Funds permitting, a high- wing aircraft such as a Herald could be modified by the sub- stitution of a modified wing and powerplant installation for flight trials. In any event, assuming that the project is adequately supported (and a number of airlines are closely interested) no in-service date is foreseen before 1975. This timescale is seen by Dr Cheeseman as being of about the right order from both the technical and economic aspect. M.W.
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