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Aviation History
1971
1971 - 0096.PDF
FLIGHT International, 21 January 1971 Stol fans 89 from Dowty By PETER MIDDLETON WITH SEVERAL HUNDRED thousand pounds of company money already spent on development of its variable-pitch fan, Dowty Rotol is far from merely experi menting with an attractive concept. By the mid-1970s the company fully intends and expects to have v-p fans in production. Propellers are unlikely to lay down and die for many years, but they do represent a declining market, and Dowty sees in the new fan a means of applying its propeller expertise to a potentially lucrative market, which could bring big business to its Cheltenham factory. Major noise reductions are obtainable with high bypass- ratio variable-pitch fans. This, together with their reverse- thrust and feathering capabilities, their very high ratio of take-off to cruise thrust and good thrust response, make the units particularly suitable for Stol aircraft. The 100- passenger types such as those BAC is studying would be the ideal vehicles for the principle v-p fan engine under consideration by Dowty and Rolls-Royce Bristol, which is an M45 derivative known as the RB.410. But the test pro gramme to date has made use of smaller engines—a Gnome and an Astazou—although a 6,000 s.h.p. Tyne is next in line for a variable-pitch fan. The Astazou and Gnome have now accumulated some 240hr of successful performance and noise assessment in collaboration with Rolls-Royce Bristol and the Ministry of Aviation Supply, who recently put modest support into the private venture programme which Dowty has been pursuing since 1965. The RB.410 results from taking an M45, removing the 3:1 bypass-ratio fan, substituting a variable-pitch 15-blade 68in, 173cm, diameter geared fan and adding another 1-p turbine stage. The result is an engine in the 15,0001b, 67kN take-off thrust class, with a bypass ratio of about 10:1. But cruise thrust increases by a much smaller amount because of the overall lower pressure ratio. Specific fuel consumption during cruise should drop by 20 per cent. The pace with which the Stol concept reaches maturity will depend on how the requirements of the system as a whole can be collated, but the RB.410 could be running in 1974 and be in production by late 1976. The design of the powerplant is fluid at present—it started life as a straight M45 with a gearbox and fan bolted on, but has begun to grow into a new engine. As the Government would be footing any major bills, and in view of the higher development cost of a new engine, there is increasing Profiles of gas generator intakes on engines (heading picture) which could be representative of the RBAI0 show a very small lip radius. It can be assumed, therefore, that when the fan is in reverse pitch, the gas generator intake air will pass through auxilHary slots. Similarly, the fan cowl ing will probably have auxilliary intakes for use in the reverse thrust mode. Diagram at right pressure for the design to revert towards the simpler and cheaper alternative. But it might only require about £112 million to get a 9,0001b, 40kN, thrust demonstrator rig running. A Rolls- Royce Tyne would drive (through an industrial gearbox, already available) a 54in, 137cm, fan based on Dowty Rotol YS-11 propeller blades. This would provide large- scale results on a "quiet engine" which the Ministry of Aviation Supply is known to have under consideration as a primary field for possible Government support. The climate of public opinion is becoming such that future generations of conventional large turbofans might use v-p fans as a means of reducing noise, and Dowty sees no reasons why an engine of RB.211 proportions should not be fitted with such a fan. Noise reduction would be the primary consideration in fitting a v-p fan to a conventional aircraft, where the performance advantages are less apparent than for Stol types. The very high ratio of take-off to cruise thrust offered by a v-p fan of 10:1 bypass ratio and its reverse-pitch capability are going to be of little importance for a jumbo jet with two miles of concrete from which to operate, and for such conventional
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