A Revolution in the Air

Dowty found that to make anyway? While red in aircraft noise you had to reduce prop tip speed, as this Is what generates the majority of noise from such aircraft. This could be done 1 year two means. Firstly, you could find a large, slow-revving propeller coupled to a very expensive gearbox. This reduces tip speed as prop revolutions decrease although It does necessitate an overly large, complex and expensive gearbox to be employed. In most cases, this would be con­ considered non-viable.

The second idea was the ducted propulsor. This had been tried before with varying degrees of success and though not at that time overly optimistic Dowty did realise why some of the earlier projects had failed to live up to expectations. With the DP we have the situation where a number of blades turn at lower rpm inside a specially? assigned conducted ceiling. This forces airflow over the streamlined engine nacelle more efficient fashion than normal propellers. After test bed running of a full-size unit coupled to a 300hp Continental engine Dowty were impressed with progress and so the decision to commence test flying the unit was made late in 1976. The aircraft chosen as a “prototype” was the Britten Norman Islander. This aircraft was chosen because it was in large-scale use throughout the world and is a high-wing design.

First flight was made in June 1977 and to date the statistics are nothing short of impressive. Noise has been drastically reduced; even far in excess of what Dowty envisaged. Normal Islander noise generation at 1000ft is 85dB (decibels) which the roughly comparable to normal conversation. Cabin noise in­ side the aircraft is also greatly reduced with only the mechanical noise of the engine being audible. Dowty and Britten Norman are now developing a sound-proofed engine nacelle and cowling to reduce this even further. Use of a small turbine would further enhance this aspect of operation and no doubt high-wing aircraft such as our own Nomad would greatly benefit from knowledge gained in this program.

Installation in the Islander was fairly simple and straightforward. The two 300hp Lycomings were mounted on jet-like pylons to allow extra height for the DP cowling. The engines are operated in the normal manner, the only real difference being that the noisy, and wasteful two-blade 80in. props have been replaced by seven 48in. diameter fans running within the JT9-like cowling.

Tip speed has been reduced from 942ft/sec to a whisper-quiet 565ft/sec. Overall thrust has been increased while improved engine cooling allows a leaner mix­ ture to be set thereby reducing exhaust pollutants and in­ increasing fuel efficiency.

As we go to press the DP Islander is completing its exten­ sive trials and a full decision will be made as to the future of this project before the end of this year.

Across the Channel, and acting in close cooperation with Dowty, a West German firm have also been dabbling in the DP principle, only in a slightly different mode. Rhein-Flugzeugbau (RFB), a subsidiary of VFW-Fokker, have spent the past half-decade flight testing a very stylish twin-seat civil aircraft named the Fanliner. RFB first flew their prototype Fanliner back in October 1973. Development was undertaken in partnership with Grumman American who provided the adhesive bonding technology for the airframe in addition to the complete wing and horizontal tail surfaces straight from their Grumman American Traveller aircraft.

Over the course of 1974/75 a variety of Wankel rotary engines were tested, each successively more powerful till the final pre-production test aircraft settled on one 150hp NSU­ Audi rotary unit which provided the Fanliner with a 155mph cruise over a range of 620 miles. At the time of writing flight testing was still underway and it was anticipated that’ a go­ ahead for production would not be given before the end of 1978.

Following on from the experience gained by RFB with the Fanliner project their test pilots quickly grasped the possibilities of producing a more powerful derivative for military training and jet flight simulation. And so was born the Fantrainer program. The RFB Fantrainer is a tandem two-seat aircraft using a Dowty seven-blade DP fan unit driven by two NSU-Audi 150hp rotary engines – this is the same engine as is currently installed in the Fanliner. In 1975 the Federal German govern­ ment provided 7. 5m Deutsche Marks to bring the project to prototype flight status, which will be completed by early 1979. Following a complete and exhaustive evaluation of the air­ craft and its claimed attributes a production decision will be made which could see upwards of 100 units entering German military service by 1980. While it was noise that had caused Dowty Rotol in the UK to look for an alternative form of propeller design, RFB’s con­ cern stemmed from the rapidly increasing cost of military pilot training. RFB recognised that it would cost 10 times as much per hour to operate an Alpha Jet, or similar, as com­ pared to a small piston-engined trainer. This constituted a tremendous increase in pilot training expenditure as aircraft became more sophisticated in design. RFB then sought to produce a small, relatively cheap aircraft that could fill theab­ initial stage in addition to taking some workload off the jet trainer. Thus, RFB developed the Fantrainer, which handling like a pure jet aircraft, would offer flight characteristics similar to those aircraft the pilot would later convert to. Or as RFB say – “jet training at piston prices”.

A similar situation now exists in the United States where the Beech T34C is capable of taking the student pilot further than normal on just a piston-engined trainer due to the jet­ feel of the turbine engine.

Ducted propulsors have made such advances in the past few years that we will no doubt see them as an integral part of most applicable designs in the ’80s. Lower noise, a more responsive aircraft, and greater fuel efficiency all make the ducted propulsor a winner that will be hard to beat.