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Aviation History
1969
1969 - 0810.PDF
716 V/STOL SURVEY ... HAWKER SIDDELEY INTER-CITY V/STOL . . . External noise levels The advance of low-noise technology has in recent years been rapid. Improvements have been made and are still being made on both enclosed turbofans and on external rotors. In the former, the move away from the pure-jet to the low by-pass engine and then to the high by-pass turbofan is now being reflected in lift unit design. The Rolls-Royce advanced lift turbofan has a by-pass ratio about twice that of the RB.211 and both jet velocity and fan tip-speeds are reduced to low values. Estimates indicate that turbofan-propelled V/STOL trans ports could actually lead to a significant Improvement in the community noise levels compared to conventional aircraft. Central London provides a significant example. Conventional aircraft are normally routed over at around 2,500ft to intercept the ILS westwards into Heathrow. At this altitude a typical large jet imposes a maximum noise level on the ground of nearly 95 PNdb; over an area a mile-and-a-half wide and about 15 miles long the noise exceeds 85 PNdb. But 100-seat turbofan VTOL aircraft, out of any one of a number of alternative Central London sites, would reduce community noise levels at all points to less than 90 PNdb. The total area experiencing more than 85PNdb would be limited to the vicinity of transition. In the conventional post-transition ffyout, this type of aircraft gains a considerable advantage by being able to fly in a relatively clean low-drag configuration. In the Central London environment discussed above, the ground noise level below the flight path would not exceed 75-80 PNdb. FLIGHT Internationa/, I May 1969 Conclusions The turbofan V/STOL formula using separate lift units for take-off and landing is a natural stablemate to the simple vectored-thrust concept pioneered by the Harrier. It is the natural formula for V/STOL aircraft with modest propulsive thrust requirements—just as the lift/thrust powerplant system is ideal for aircraft with high in-flight thrust requirements. The comprehensive research background enables the jet or turbofan V/STOL to be designed, as are conventional jet aero planes, with configurations best suited to the particular role. Unlike other V/STOL systems there is no unique and rigid interrelation between lift system and airframe. The lift jet engine is being developed to minimum weight and volume and is directly applicable to those roles where these qualities are of greatest value. Where minimum noise and/or extended hovering or low-speed flight is required, the jet-lift formula van be adapted, with little change of eon- figuration, to the new "fourth generation" lift unit such as the Rolls-Royce ALF. Compared with the low-noise levels and good s.f.c. in prospect for this type of lift unit, the alternative stopping-rotor concepts appear to offer little advan tage to offset their lack of full-scale research background (which shows particularly in the field of structural integrity). For a wide variety of roles, and particularly for the inter-city application, the all-turbofan V/STOL formula appears as one taking maximum advantage of the mainstream in technical development, not only in the efficiency, reliability and low- noise qualities of the modern high by-pass turbofan, but in the technical success of the British and European jet-lift flight test programmes of the past decade. The commercial and military prospects of this more civilised adaptation of the jet-lift formula is such that the initial direction taken by, in particular, British VTOL research in the mid-'fifties. can now be seen as inspired. West land inter-city VTOL By JOHN SPEECHLEY, DCaE, CE„0, FRAeS* SEEING AS INEVITABLE the overcrowding of airports, the jamming of surface traffic approaches and the great need for action to improve the elapsed journey time between major cities within the next decade, Westland realised the market potential that must arise for economic and socially acceptable inter-city VTOL aircraft to meet these needs. With close on a quarter of a century of helicopter experience behind it. the company decided that to obtain the transport efficiency required for inter-city VTOL from a pure helicopter, though far from impossible, meant that overcoming the low lift/drag ratio, and the inherent aerodynamic problems, could involve lines of development that would be both costly and protracted. Whilst the company decided to develop the helicopter further to meet the special cases where the pure helicopter is invalu able, it has chosen to compound the vertical flight capability of this kind of machine with the speed and economy of the fixed-wing aircraft to produce a viable inter-city VTOL airliner concept. In common with the rest of the world Westland has studied all possible methods of achieving this marriage^ This work, and the background of many years helicopter experience, led to the conclusion that the only form of compound VTOL that could be socially and economically acceptable was one based on the use of large diameter, lightly laden open rotors. Large rotors with low disc loadings provide the virtues of reduced tip speeds and lower rotor noise values, low downwash velocities, greater stability, ease of control and safety at low speeds. They also have the virtue of being the most efficient vertical-lift device in terms of power related to vertical per formance, and in fuel consumption at hover and during flight at very low forward speeds. With the studies of the various methods of compounding •Technical director (development). Westland Helicopters Ltd. rotary and the fixed-wings came the need to equate the benefits obtained with the complexity, added weight, and development costs of the mechanisms needed to effect conversion from one mode to another. Studies led Westland to decide that the lesser penalties lay in the direction of tilt-wing and tilt-rotor craft in which the large low-loaded rotors could be set to the horizontal plane for take-off and landing, and turned through 90 during a transition high-speed cruising flight in which the rotors would provide the traction required. Two projects were evolved at the beginning—they were known as WE0I and WE02: the former being a purely experimental six-seater to prove the principle, and the latter an extension of the priciple to an aircraft of over 100,0001b gross weight and to carry more than 100 passengers. Westland carried out a degree of private venture development of the smaller project including con structions of a full-scale mock-up and some wind tunnel testing. With the addition of the inter-city VTOL to the British Government's future programme for the aircraft industry, WE0I and WE02 became part of the research background and Westland progressively extended studies into a range of convertible rotorcraft. Throughout these studies Westland worked closely with Government and operational bodies and submitted a range of projects to meet the Government's proposals for inter-city VTOL airliners. Typical of these studies are a 20-seat feeder- liner variant and a 100-seat passenger version, the former having military and civil application. The basic parameters of these two types are:- Gross Stage Seats Speed (kt) weight (lb) length (n.m.) Feederliner or military transport (tilt rotor) .. 20 270 17.500 250/300 Civil passenger aircraft (tilt wingl . . 100 400 I 10.000 450/000
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